LIBRARY OF THE UNIVERSITY OF CALIFORNIA. GIFT OF MRS. MARTHA E. HALL1DIE. Class LECTURES ON EXPERIMENTAL PHILOSOPHY, LONDON PRINTEn BY THOMAS DAVISON. WHITEFRIARS. LECTURES ON EXPERIMENTAL PHILOSOPHY, ASTRONOMY, AND ,,' ; *>: .1. CHEMISTRY: :., INTENDED CHIEFLY dFor tfjc $fee of j&tutontg anD footing $ ergon*. BY G. OREGORY, D.D. LATE VICAR OF WEST HAM J DOMESTIC CHAPLAIN TO THE LORD BISHOP OF LLANDAFF; AND ADTHOR OF THE ECONOMY OF NATURE, &C. &C. OF THE UNIVERSITY TWO VOLUMES. VOL. I. SECOND EDITION, CORRECTED AND IMPROVED. LONDON: PRINTED FOR LONGMAN, HURST, REES, ORME,AND BROWN; BALDWIN, CRADOCK, AND JOY j SCATCHERD AND LETTERMAN; G. AND w. B. WHITTAKER; AND SHERWOOD, NEELY, AND JONES. 1820. PREFACE. THE object of this publication is, to afford a useful companion to such Stu- dents as may attend Lectures in the Universities, at the Royal Institution, or elsewhere ; and also to enable the Masters of private Seminaries, with a very moderate Apparatus, occasionally to indulge their pupils with a practical Course of Lectures on one or all of the important branches of Experimental Phi- losophy, Astronomy, and Chemistry. Having published some years ago " The Economy of Nature/' the author thinks it necessary to state, that both the plan and arrangement of that work are essentially different from those of the present. The Economy of Nature does not contain Astronomy, nor, in fact, Chemistry, as a distinct science ; on the other hand, a very large portion of that work is occupied with Mineralogy and Physiology, which in this are purposely omitted. Even the subjects which are common to both will be found to be dif- ferently treated in these Lectures. February 20, 1808. PREFACE SECOND EDITION. i THE first Edition of these Lectures having experienced a very extensive cir- culation, the Proprietors have thought it their duty to procure for the present such an entire and cautious revision as should render it still more worthy pub- lic favour. Th whole of the first volume, and so much^of the second as relates to Astro- nomy, has been carefully examined by a gentleman whose different works on Mathematics and several departments of Natural Philosophy have acquired a high reputation. He has made numerous ad- ditions and improvements, correcting errors, and carefully introducing as he went along, the most important dis- coveries both of English and of con- IV tinental Philosophers, down to the close of 1819- The chemical department has, in like manner, undergone the careful revision of a gentleman eminent in the science of Chemistry. So numerous and important have been the accessions to this region of human knowledge, in the course of the last twelve years, that a cautious revision has, in fact, included the entire re-composition of a considerable portion of the second volume. The Proprietors have every reason to believe that the improvements thus made to the Lectures will considerably aug- ment their utility : and they humbly yet confidently anticipate the reward of an enlightened public, for the expense they have incurred by engaging gentlemen of such acknowledged competence to make the volumes exhibit a correct yet popu- lar view of the present state of Experi- mental and Chemical Philosophy. July, 1820. DIRECTIONS FOR PLACING THE PLATES. VOL. I. Plate I. to face II. III. IV. VI. VIL VIII. IX. X. XL XII. XIII. XIV. XV. XVI. XVII. XVIII. Plate XIX. to face 224 18 XX. 228 22 XXI. 223 36 XXIL 242 41 XXIIL 253 49 XXIV. 255 54 XXV. 271 68 XXVI. 284 7K XXVII. 307 / O 102 XXVIII. 318 121 XXIX. 315 131 1 O 1 149 VOL. II. 166 Plate I. to face page 3 172 II. 12 185 III. 20 196 IV. 23 208 V. 108 219 VI. UQ 'lXKCXSTAT:! CS S.- H1HDMA [JJd CS. JTYMIO STATIC 3 J-r.irfuJun^del . ATTJ&ACTIO:N or C PL .Z. toil. lilllilllp 1 iillilill Jftrra/ Jun. ?deJ, :i'crttr sculp . PL. II. . OF THE r > v UNIVERSITY y HTEDHOSTATICS. S.rorlrr sculp. i:rv i)RAv h I r M o r TI c s Wrvttrsadp. OPTIC . P TI C S . PL. XT. S. Porter tculf. . E ]L1E C TJRI C ITT. n.. L\ . Itarat Jun ('. ATA'A \M ^ M , OPT! C , OPTIC o PTI r s, ASTRONOMY, rVl ASTJUXNTOMY. PL,.XXI. ^iiijF c i> fig-9 6 ' >ft a ? MECHANICS . PL.3IX. B A OPTIC S MECHANIC S , 'Fany Jim' del //,.. v.v. LIB),. Q F THE- >A ASTliOTSQIMY W/ U "X OF THE ^ffl UNlVER3| Ty OF 'iJPT CHEMISTRY. .V. VOL.H. />;i Y FL.in.voL.n. r /** <*il ASM 1 AODtfOMY. PL.I VOL II OF THE UNIVERSITY T>T. 7TXTX-. .. \ \i //. i I' It]. 1 1(> . j ' . .MY. I'L.n .VOI..H - . H: : PL. ir. VOL.11. M JJ-bra, I UNIVERSITY . VI. VOL. TT. V CONTENTS TO VOL. I. EXPERIMENTAL PHILOSOPHY. LECTURE I. Page General Objects and Principles 1 LECTURE II. Attraction 10 LECTURE III. Magnetism - - - - 21 LECTURE IV. Hydrostatics - 35 LECTURE V. Hydraulics - 48 VOL. I. b CONTENTS. Page LECTURE VI. Of Pneumatics - 6l LECTURE VII. The Phenomena of the Atmosphere - 78 LECTURE VIII. Electricity - - - - <# LECTURE IX. Electrical Phenomena and Galvanism - 10Q LECTURE X. Light - 129 LECTURE XI. The Refrangibility of Light . 148 LECTURE XII. Reflexibility of Light, or Catoptrics \64 LECTURE XIII. Vision and Optical Glasses - - - 178 LECTURE XIV. Colours - 199 LECTURE XV. The Laws of Motion - - - 216 CONTENTS. Page LECTURE XVI. The Mechanic Powers - - 232 ASTRONOMY. LECTURE XVII. System of the Universe - - 250 LECTURE XVIII. Of the Sun, and his real and apparent Motions 270 LECTURE XIX. The Primary Planets; the Mode of calculating their Distances, &c. - 280 LECTURE XX. The Secondary Planets - -312 LECTURE XXI. The Earth ... . 3? g LECTURE XXII. The Tides - - - - 343 LECTURES ON I ; ;, ; ^ EXPERIMENTAL PHILOSOPHY, fcc, , :. LECTURE I. EXPERIMENTAL PHILOSOPHY. GENERAL OBJECTS AND PRINCIPLES. You are, I presume, desirous, my young friends, of acquiring knowledge, of satisfying your cu- riosity, of storing your minds with useful ideas, of fitting yourselves for company and conversa- tion, and of enabling yourselves to proceed gra- dually in the paths of science, till you arrive at distinction and eminence. Suffer me to ask you, if you do not feel a strong curiosity to know the nature of all those objects that you see around you ; to be informed of the causes of those astonishing changes which you observe every day produce. You see the sun, which apparently rises every morning to give light and heat to the world. You will be surprised to be told, that it is not the sun that moves upon these occasions, but it is the earth VOL. i. B 2 Experimental Philosophy. [Lecture 1. on which you stand, that revolves upon an axis, and presents different parts of its surface to the sun at certain hours of the day. Or, when you :irc told tnis, do you not feel a wish to know the proofs and the reasons of it; and why the sun appears to n-iove, when in reality it is yourself, or rather the earth on which you stand ? Have patience, and you shall know all this; and it will be as clearly proved to you as any common fact, or as the result of any arithmetical operation. Again : You throw a stone, or shoot an arrow upwards into the air ; Why does it not go for- ward in the line or direction that you give it? Why does it stop at a certain distance, and then return back to you ? What force is it that presses it down to the earth again, instead of its going onwards ? On the contrary, Why does flame or smoke always mount upwards, though no force is used to send them in that direction ? And why should not the flame of a candle drop towards the floor, when you reverse it, or hold it downwards, instead of turning up, and ascending into the air ? You look into a clear well of water, or on the surface of a looking-glass, and you see your own face and figure, as if it were painted there, and even more correct than the best artist could draw it. Why is this? You are certain there is no such figure, either in the well or behind the looking-glass. You are told this is done by reflection. But what is reflection? It must be some property in light, which occasions its being General Objects and Principles. 3 thus thrown back to your eyes, and which causes you to see a figure as distinctly as if you looked upon the figure itself. This shall also be ex- plained to you ; as well as the reason why when you look upon the ground, at a wainscot, or on a rough unpolished table, you see nothing of the kind. When you look through some glasses you see things much bigger than they really are, or mag- nified; that is, made larger. When you look through others you see them less than they appear to your eyes, or diminished. What is there, then, in the one glass that it should cause things to appear larger than they do to your natural sight: or, in the other, that they should seem so diminished ? Yet this too will be explained ; and you may, by certain rules, be taught to calculate how much larger or smaller any glass will make an object appear, before you look through it. You cannot be unacquainted with that tre- mendous noise, which the ignorance of the an- tients considered as an indication that their god Jupiter was in a passion. We call it thunder. But what is thunder ? You have also probably seen fire descend in streams from the clouds, or pass instantaneously from one cloud to another ; and after darting first to one side, and then to the other, several times, come to the earth with a zig-zag kind of motion. This is lightning, and it proves fatal wherever it strikes: it kills 4 Experimental Philosophy. [Lecture 1 . men or cattle ; it sometimes levels to the ground the proudest edifices, and sets on fire the loftiest trees or buildings. You have probably never once thought what can be the cause of this thunder and lightning. But will you not be astonished to see it imitated on a smaller scale, the same noise excited, a rapid fire sent forth like that, and producing similar effects ? You see every day the clouds collected over your heads, and passing hither and thither, as directed by the wind. You see them assume different shapes and forms ; sometimes gathering into a large thick mass, at others breaking into small divisions. What are the clouds made of, think ye? Whence do they come? Why do they appear and disappear ? Why do not they fall down immediately upon the ground, as you see other bodies ? The clouds, you will probably guess, are water, because you see rain occasionally fall from them, and sometimes hail and snow. But how is water supported in the air? Why do the clouds at some times drop only rain, and at others hail or snow ? You will say hail and snow fall only in cold weather. But why is snow of that fine flaky consistence like feathers? And why is hail in little round balls ? All this may be ex- plained. You have doubtless observed that beautiful coloured arch in the heavens, which, from its appearance during rain, has been called the rain" General Objects and Principles. 5 bow, and which Almighty God has made the pledge, that he will not overflow the world with another deluge. But do you understand how this appearance is produced? It is, indeed, the action of light upon the drops of the falling rain ; but we can show you by what means this appear- ance, and these vivid colours, are produced; why it assumes the form of a bow ; why a se- cond bow is often seen accompanying the first or primary bow. We can measure the arch which it inscribes, and explain the whole of this wonderful spectacle. It must be well known to some of you from observation, and to most of you by the informa- tion of others, that the sea, at certain hours of the day, varying with the age of the moon, approaches, and overflows, to a certain height, the sandy beach by which it is surrounded. This flux and reflux of the ocean, as it is termed, is known by the common name of the TIDE. Antient tradition tells us, that a philosopher put himself to death, because he was unable to find out the cause ; but modern philosophy has laid open the whole theory of the tides, and can de- monstrate the nature of them upon irrefragable principles. In some parts of the world there are fountains of boiling water spouting from the earth. In others, the earth itself opens and emits flames and rivers of liquid fire, and throws out rocks and stones of an immense size, with a force and 6 Experimental Philosophy. [Lecture 1. velocity which are imitated in vain by the largest pieces of cannon. Whole countries have been swallowed up, and the proudest cities desolated and destroyed by earthquakes. What is the nature of these surprising operations? From what immediate cause are they produced? On what circumstances do they depend ? You will answer, they are produced by that Almighty Power which first created the universe. It is the hand of God that can alone direct or alter the course of nature. All this is true. Nothing is done, nothing can be done, without the agency, the direction of the Supreme Being. Yet Providence acts by determinate laws in all the arrangements of nature. It is not by chance, nor by an arbitrary disposal of things, that the operations of nature are effected. By the Divine Wisdom all things are disposed in weight and in measure; they are ordered on certain principles, and effected in certain constant and regular modes. These modes, in conformity with which the Divine Wisdom acts and governs the material universe, are termed the laws of nature. We cannot, it is true, account for every thing ; we cannot trace effects to their remotest causes ; but yet much is known by long observation, and the discoveries of learned and ingenious men from time to time. They have therefore referred what they call the laws of nature, to a few principles ; and these principles, when well understood, will General Objects and Principles. apply to the explanation of a long series nomena, that is, appearances, from the Greek word phainomai, to appear. It is principally by experiment that all the great discoveries of the moderns have^ been ac- complished. This, indeed, forms the grand line of distinction between the antient and the mo- dern philosophy, and this constitutes the sole merit and superiority of the latter. The antients reasoned and conjectured about the nature of things ; the moderns have submitted every thing to the direct and positive test of experience : this philosophy has therefore been termed experi- mental philosophy, because all its doctrines and principles are founded upon actual experiment, in opposition to that philosophy which is founded on fancy and conjecture. It is, I believe, to the old alchemists, or those who were engaged in the whimsical and visionary attempt to discover the philosopher's stone, or a method of converting other substances into gold, that we are ultimately indebted for this excellent philosophy. They engaged in various chemical processes, or experiments, in order to effect this grand discovery ; and from their patient and la- borious endeavours many useful inventions pro- ceeded, though often foreign from the particular discovery they were in quest of. Our country- man, Roger Bacon, a famous monk, who resided at Oxford in the twelfth century, was one of these; but one of the most rational and sagacious 8 Experimental PJiilosopJiy. [Lecture 1. of the whole sect. He was soon convinced of the difficulty of the research in which he was en- gaged, that of transmuting or changing other metals or substances into gold ; but he saw that experiment, and the mode of analysing or dividing bodies or substances into their constituen t parts, was the true mode of investigating nature. He there- fore ridiculed the idle conjectures and unmean- ing jargon of Aristotle and his followers. In the course of his researches he made that wonderful discovery, the composition and use of gunpowder. He had very nearly fallen upon that of air-bal- loons. He made a number of excellent experi- ments in chemistry and optics; and you know that his only reward was to be accounted a ma- gician by the ignorant age in which he lived, and even by the unenlightened part of mankind in succeeding times. To another Englishman, of the same name, the justly celebrated lord Bacon, philosophy is in- debted for its next great improvement. He fol- lowed the footsteps of his namesake and prede- cessor ; he reduced his principles to a system ; and laid it down as a maxim, that it was by experiment alone that any thing in philosophy could with certainty be known. He therefore traced out the way in which future experimental- ists might proceed, and afforded a variety of hints, on which they afterwards improved. The good and the illustrious Boyle, however, may be justly termed the father of modern phi- General Objects and Principles. 9 losophy. He adopted the Baconian principle of conducting all inquiries by experiment alone. He effected much in the analysing of bodies, and the examination into the principles of which they were composed. He is by many said to have invented that curious and useful instrument, the air-pump ; and his experiments on the nature of air have laid the foundation for ah 1 the modern doctrines concerning it. His discoveries on light and colours were an excellent introduction to the grand theory of Newton on that subject, and, possibly, served as the basis or foundation, of them. In short, there was scarcely a topic of natural philosophy to which he did not bend his attention, and scarcely one which he did not more or less improve : but still the facts educed were insulated. Such was the state of philosophy when Newton appeared. He reduced, into one grand scheme, all the scattered discoveries of his predecessors. He explained the motions of the heavenly bodies on a principle entirely new, and established that beautiful planetary theory which is now univer- sally received. He developed, with mathematical precision, all the phenomena of light and colours, the nature of vision, and the use of optical glasses and instruments, which last he greatly improved. In short, he gave body and consistency to natural philosophy, and made it, what it never was be- fore, a coherent system of truth, illustrated and proved by experiment. 1 LECTURE II. EXPERIMENTAL PHILOSOPHY. ATTRACTION. BEFORE we proceed to the higher branches of science, it will be necessary to explain what is usually meant by attraction, and the different kinds which have been distinguished by modern philosophers. In the first lecture I called your attention to the effect which follows when you throw a stone, or shoot an arrow upwards into the air. Instead of proceeding according to the direction in which you sent it, you see its force is quickly spent, and it returns to the earth with a velocity increasing as it descends. Now it is easy to conceive that the resistance of the air may stop it in its progress ; But why should it return ? Why should not the resistance of the air stop or impede it in its return ? The answer you will think very plain It is its weight that brings it back to the earth, you will say, and it falls because it is a heavy body. But what is weight? Or why is it heavy? It is, in truth, the earth which draws or attracts the stone or the arrow towards it ; this overcomes the force with which you sent it from you at first, and the resistance which the air would otherwise make to its falling. It is the force required to Attraction. 11 overcome this attraction, which causes a body to be heavy (gravis) ; and hence comes the verbal noun gravitation. To illustrate these matters, drop a little water or any other liquid on a table, and place upon the liquid a piece of loaf sugar, the water or fluid will ascend, or, in vulgar language, be sucked up into the pores of the sugar ; that is, the one is attracted by the other. Again, if you take two leaden bullets, and pare a piece off the side of each, and make the surface, where you have taken off the piece, exceedingly smooth, and then press the two balls together, you will find them adhere strongly together ; that is, they are mutually attracted by each other. If you take a piece of sealing-wax or amber, with a smooth surface, and rub it pretty quickly upon your coat sleeve till it becomes warm, you will find that if straws, feathers, hairs, or any very light bodies, are brought within the distance of from an inch to half an inch of it, these light bodies will be drawn to the sealing-wax or amber, and will adhere to it. Thus, in philosophical language, they are attracted by it. This last effect is very similar to what you have heard of the magnet or loadstone, or what many of you may have seen performed by the little artificial magnets, which afford a very rational and pretty amusement to young persons. You have seen needles, steel filings, or even knives or keys presented to the magnet, and at- 12 Experimental PhilosopJiy. [Lecture 2. traded by it. On this circumstance an amusing story in the Arabian Nights Entertainments is founded. A' rock of loadstone (adamant it is called by an error of the translator) is supposed to exist in a certain part of the ocean ; and when a vessel approaches it, all the iron bolts and nails are attracted by it, and the vessel consequently goes to pieces and is wrecked. But I can show you a still more surprising (and to most of you, I dare say, new) effect of attraction. I take two phials, which I number 1 and 2, filled each of them with a fluid perfectly colourless ; you see they appear like clear water : on mixing them together the mixture becomes perfectly black. I take another phial, No. 3, which contains a colourless fluid also, and I pour it into this black liquor, which again becomes perfectly clear, except a little sediment which re- mains at bottom. Lastly, I take the phial No. 4, containing also a liquid clear like water, and by adding a little of it, the black colour is re- stored. All this may appear to you like magic, but it is nothing more than an effect of attraction. Phi- losophy keeps no secrets, and I will explain it to you. The colourless liquor in the phial, No. 1, is water in which bruised galls have been steeped or infused ; that in No. 2, is a solution of sul- phat of iron, the name now given to the copperas or green vitriol of commerce. In plain terms, it is water in which common copperas or green Attraction. 13 vitriol is dissolved. The iron which this salt (green vitriol) contains, has a strong attraction for the gall water; and when they are mixed together they unite, and the mixture becomes black ; in fact, is made into ink. But when the phial, No. 3, which contains aqua fortis (the nitric acid, as it is called by chemists), is poured in, the iron, which has a stronger attraction for it than for the galls, unites with it, and having left the galls, the liquid is again clear. Again, the phial No. 4, contains potass, formerly called salt of tar- tar, or of wormwood. It is the vegetable alkali of chemists. The aqua fortis, or nitric acid, has a stronger attraction for this alkaline matter than it has for the iron ; it therefore drops the iron, which again unites with the matter of the galls, and the fluid resumes its black complexion. You may amuse yourselves with the same ex- periment in another way. If you write a few words with common ink (which you now know how to make) upon a thick paper, and let them dry,' and then take some aqua fortis diluted or weakened with water, and with a feather drop or rub it upon the letters, the writing will totally disappear. When this is dry, with another fea- ther smear it over with some of the solution of potass or salt of tartar, and the writing will be restored. These several kinds of attractions which I have now mentioned, philosophers have ranged under five distinct heads. The^r^, that, I mean, of 14 Experimental Philosophy. [Lecture 2. the stone or arrow falling to the ground, they have called the attraction of gravity, or gravity tion. The second, that of the two leaden balls adhering together, and of the water ascending into the pores of the sugar, they call the attrac- tion of cohesion, and also capillary attraction. The third is electrical attraction, because the sealing-wax, when chafed or warmed by rubbing, is in an electrified or excited state, like the glass cylinder of an electrical machine when rubbed against the cushion, and therefore attracts the hair, feathers, 8tc. The fourth is the magnetic attraction ; and the fifth is called chemical attrac- tion, or. the attraction of combination, because upon it many of the processes and experiments in chemistry depend; and because by this means most of the combinations which we observe in salts, the ores of metals, and other mineral bodies, are effected. On the two first of these species of attraction only I shall at present enlarge ; because it will be necessary to treat of the others when we come to investigate those branches of science to which they properly belong. First, therefore, of gravitation. It requires no experiment to show the attraction of gravity; for since the earth is in the form of a globe, it is manifest that it must be endued with a power of attraction to retain upon, its surface the various bodies which exist there, without their being hurjed away into the immensity of space in the Attraction. 15 course of its rotatory diurnal motion. The earth has therefore been compared to a large magnet, which attracts all smaller bodies towards its cen- tre. This is the true cause of weight or gravity (which are correlatives). All bodies are drawn towards the earth by the force of its attraction ; and this attraction is exerted in proportion to the quantity of solid matter which any body contains. Thus, when two bodies are placed in opposite scales, and we see one preponderate, we say it is heavier than the other ; in truth, that it contains a greater quantity of solid matter. For as every particle of matter is attracted by the earth, the greater number of such particles any body con- tains the more forcibly it will be attracted. The attraction of matter is universal : so that not only does the earth attract all bodies upon it, or near it ; but all such bodies reciprocally at- tract the earth. Nay, farther, the earth attracts all bodies in the universe, and they, again, all attract the earth. Every particle of matter exerts an attractive energy upon every other particle ; and each of the bodies into which particles are grouped attracts every other body. Thus, the sun attracts all the bodies in the planetary system ; and they, in their turn, attract the sun and each other. The fixed stars, again, attract each other, and our sun ; they also attract, and are attracted by, the several bodies to which they probably form distinct centres. The attractive forces of bodies upon each other, are directly proportional 16 Experimental Philosophy. [Lecture 2. to their quantities of matter, and inversely pro- portional to the squares of their distances. This is the first grand deduction of the Newtonian philosophy, established upon indubitable prin- ciples, and on which all the momentous facts of physical astronomy depend. The tides, the pre- cession of the equinoxes, the irregularities of the moon's motion, the mutual perturbations of the planets, and many other interesting phaenomena, all receive a satisfactory explication upon the principle of mutual and universal attraction. But to proceed : we know by experience that the weight or gravity of a body or thing is not in proportion to its bulk. A bullet of lead, of the same size as one of wood or of cork, will weigh considerably heavier, and one of gold would be heavier still. It is reasonable, there- fore, to suppose that the ball of gold or of lead contains a greater number of solid particles, which are united or pressed closer together than those of the wood or cork; the latter being more porous, and its particles lying less closely compressed or compacted together. One body containing more solid particles within a certain compass, size, bulk, or space, than another, gives origin to the terms specific gravity and density, which are greater or less in proportion as there are more or fewer constituent particles comprised within a given apparent bulk. II. The attraction of cohesion is observable in almost every natural object, since in reality it is Attraction. 17 that which holds their parts together. It has been already made evident in the experiment of the two leaden balls, and the same effect will be proved by pressing together the smooth surfaces of two pieces of looking-glass, particularly if a little moisture is dropped between them to ex- clude the air more perfectly. The adhesion or tenacity of all bodies is supposed to depend on the degree of this attraction which exists between their particles ; and the cohesive power of several solid substances has been ascertained by different courses of experiments, in which it was put to the test what weight a piece of each body of a certain diameter would sustain. In the following table the numbers denote the pounds avoirdupois, which, at a mean, are just sufficient to tear asunder a rod of each of the bodies, whose base is an inch square. Metals. Steel, bar 1 35,000 Ibs. Tin, cast 4,440 Ibs. Iron, bar 74,500 Bismuth 2,900 Iron, cast 50,100 Zinc 2,600 Silver, cast 41,500 Antimony 1,000 Copper, cast 28,600 Lead, cast 860 Gold, cast 22,000 Woods. Locusttree 20,100lbs. Teak, Orange 15,000 Ibs. Box 20,000 Alder 13,900 Jujeb 18,500 Elm 13,200 Ash 17,000 Mulberry 12,500 12,000 Ibs. Walnut 8,130 Ibs. 11,500 Mahogany 8,000 10,000 Poplar 5,500 9,800 Cedar 4,880 9,250 1 18 Experimental Philosophy . [Lecture 2. Fir Beech Oak Pear, Lemon The direct cohesive strength of a body is in the joint ratio of its primitive elasticity, of its toughness, and the magnitude of its section. Cohesion is also visible even in fluid substances, the particles of which adhere together, though with a less degree of tenacity than solid bodies. " The pearly dew*" is a well known phrase in poetical language, and the drops of rain or of dew upon the leaves of plants assume this round or pearly appearance by the attraction which the particles have for one another. In the same manner quicksilver, if divided into the smallest grains, will appear round, like small shot, because the particles attract each other equally in every direction, and thus each particle draws others to it on every side as far as its power extends. For the same reason two small drops of quicksilver, when brought near to each other, will seem to run together and unite. The attraction of cohesion exists between fluid and solid bodies. Thus a plate of glass or metal (Plate I. fig. 1.) which has been immersed in water or mercury, will retain some drops hanging to it, even when turned upside down, or inverted. Again, if two plates of glass, A. A. (fig. 2.), a little wetted on the surface, and separated on one Attraction. 19 side by any small interposing body B., about the thickness of a shilling, are immersed in water, the water will rise between them in the curve C. D. E., that is, highest on that side where the plates touch each other, and at a moderate height near the surface of the fluid. The same effect was instanced in the water or liquor rising in the piece of lump sugar ; and it may be seen every day, when a piece of blotting-paper is used to suck up a drop of superfluous ink. Another easy experiment will further illustrate die nature of this attraction. Suppose A. B. C. (fig. 3.) two glass plates a little moistened with oil of oranges, and placed upon each other, so as to touch at the end A. B. Let them be kept open at the other end by a small body C. If then a drop of the same oil is introduced at the end which is open, while the plates are kept in a horizontal position, the drop will proceed with an accelerated motion towards the end A. B. If the end A. B. is then a little raised, the drop will be suspended in its course, and, if raised to a considerable height, it will return, but slowly ; in which case the attraction of the plates is, in some degree, overpowered by the weight or gravity of the drop. This peculiar kind of attraction has received the name of capillary attraction, from the experiment having been made with small tubes as fine as a horse-hair (capillus^ Latin), in which the water will rise to a considerable height ; and upon the same principle, water or any other fluid will rise 20 Experimental Philosophy. [Lecture 2. in the cavities of a sponge. These experiments will succeed equally in a space which is void of air (such as the vacuum made by an air-pump) as in the open air ; so that the effect cannot pro- ceed from any pressure of the atmosphere, but must be caused by attraction alone. Some bodies, however, in certain circumstances, appear to possess a power the reverse of attrac- tion; and this is called, in philosophical lan- guage, repulsion. The repulsion of electricity and of magnetism will be evinced when we come to treat of those subjects ; and the -same feathers, which were at first attracted by the excited or electrified body, will be repelled or driven from it; the magnet will repel at one end the same bodies which it attracts at the other. Upon simi- lar principles, if a small piece of iron is laid on a bason of mercury, it will not sink, but will be supported by it, while the mercury will be de- pressed on each side ; and thus it is that a small needle will swim upon the surface of water. LECTURE III. EXPERIMENTAL PHILOSOPHY. MAGNETISM. IN my last lecture I endeavoured to make you acquainted with the nature of attraction in ge- neral. There is, however, scarcely any instance in which the principle of attraction is displayed in a more striking manner than in that of the MAGNET, or LOADSTONE; so called, as Mr. Adams conjectures, from load, the Saxon word for lead, that is, the leading-stone, from its proving a guide to seamen by means of the com- pass, or magnetic needle, which always points towards the north. The loadstone, or natural magnet, is an ore of iron, found more or less in every iron mine. Loadstones are of a dull brownish black colour, and most of them are sufficiently hard to afford sparks like a flint when struck with steel. They differ very much both in form and in weight. There was a very large one in the Leverian Mu- seum, but it did not appear to be very powerful. I observed in my second lecture, that the earth itself has been compared to a large loadstone ; and this opinion is countenanced by the immense quantity of iron which is contained within its bowels, or which indeed, more properly speaking, 22 Experimental Philosophy. [Lecture 3. is diffused through all nature. In a part of Vir- ginia there is a magnetic sand, the grains of which exhibit all the properties of larger load- stones, and indeed are loadstones in miniature. The great and distinguishing property of the magnet is its attraction for iron; and this at- traction is mutual between them. Thus, if a magnet and a piece of iron are placed each of them on a small piece of wood, in a bason or tub of water, so as to float on the surface, (see Plate II. fig. 4.) the magnet will approach the iron as well as the iron the magnet; and if either of them is held steady, the other will move to- wards it. Muschenbroek, by a series of experi- ments, endeavoured to ascertain the degree of force with which a magnet would attract at dif- ferent distances. He suspended a magnet two inches long, and sixteen drachms in weight, to one of the scales of an accurate balance, and under it he placed a bar of iron, while the weights were put in the opposite scale. At 6 inches it attracted 8 grains. 5 - - 31 4 4i 3 - - 6 2 - - 9 1 - 18 And in contact 87 From subsequent experiments, it has been proved that the magnetic force diminishes as the Magnetism. 23 square of the distance increases ; in this respect being analogous to gravity. Some natural magnets are much more power . ful than others; and it is remarked, that the smaller possess the power of attraction in a greater degree, in proportion to their size, than the larger. It indeed frequently happens, that a small load- stone, cut off from a large one, will lift a greater weight of iron than that from which it was cut off. This can only result from the large stone containing a considerable portion of matter not magnetic, which rather impedes the action of the magnetic part than otherwise. Loadstones have been found of not more than twenty or thirty* grains in weight, which would lift a piece of iron forty or fifty times heavier than themselves ; and we even read of one of only three grains, which lifted a weight of iron of seven hundred and forty-six grains, that is, two hundred and fifty times its own weight. This property, however, which is possessed by the natural loadstone, it will communicate to any piece of iron by only touching it ; and the piece of iron thus converted into a magnet will communicate it to others, and these again to other iron, without losing any part of, their magnetic virtue, which seems rather increased than dimi- nished by action. Magnets made by being touched by a loadstone, or by other iron which has been touched by it, are called artificial mag- nets, and are commonly sold in the shops of those 24 Experimental Philosophy. [Lecture 3. who deal in mathematical and philosophical in- struments. Soft iron acquires magnetism with more ease than hard iron or steel, but the latter will retain it much longer. A well tempered bar of steel will retain the magnetic virtue for many years without diminution. The magnet which has the strongest power of attraction does not always communicate it most freely to iron or steel. This circumstance has occasioned a distinction between the different kinds of magnet. Those which communicate most freely and in the greatest degree the mag- netic virtue, are called generous; those which raise the greatest weight in proportion to their size, are called vigorous magnets. The magnetic virtue is not diminished, but is rather increased, by communication. Though however it may be communicated by simply touching the bar of iron or steel, yet it is augmented by repeatedly touching or rubbing it with the magnet : but it must be always rubbed one way only, that is, either from left to right, or from right to left ; for if the magnet is drawn backward and forward on the iron the power will be destroyed, for rea- sons that will be hereafter explained, treating of the poles of the magnet. The magnetic virtue is found to be the most active at two opposite points of each magnet, which have been termed its poles, from their correspondence with the poles of the earth, as is found by placing the magnet on a small piece of Magnetism. 25 wood floating on water, or in any situation in which it may turn freely, when the magnet will arrange itself nearly in that direction, namely, from north to south. To find the poles of a magnet, place it under a smooth piece of glass, or a piece of white paper, and sift or shake some steel or iron filings on the paper or glass, and you will find them arrange themselves in beauti- ful curves, as represented in PL II. fig. 5. E E. At each pole, however, the filings will take a straight or rectilinear direction, as at A. B. and those which happen to be situated at a small di- stance from the poles will assume more or less of the curve in proportion to their distance from them. Some natural magnets are found to have more than two poles ; in which case they may be considered as two or more magnets united toge- ther, and, in fact, have been sometimes separated into so many distinct magnets. In England we call that the south pole of the magnet which points towards the north, and that is termed the north pole which is directed to the south. The foreign philosophers, on the con- trary, naii^e them according to the pole to which they point. That is, the north pole of the mag- net is that which is directed to the north or arctic region, and the contrary. The principle of repulsion is also very strik- ingly exemplified by the magnet; for if the same pole of two magnets is presented one to the other, that is, the north pole of one magnet to the north 26 Experimental Philosophy. [Lecture 3. pole of the other, they will mutually repel or drive away each other: if, on the contrary, the south pole of the one is presented to the north pole of the other, they will be mutually attracted. It is on this account that it is necessary, in mak- ing artificial magnets, to draw the magnet, with which they are rubbed or touched, always one way. It is most effectually done also by applying one of the poles of the magnet to the bar or piece of iron which is to be rendered magnetic, and drawing it slowly along several times. It is ex- traordinary that the end of the bar which is first touched with the magnet will have the contrary property to the end of the magnet with which it is touched or rubbed. If, for instance, the end with which the bar is touched is the north pole of the magnet, the end of the bar to which it is first applied will be a south pole, and the con- trary. It is obvious that the directive power of the magnet, or that which causes it, when placed so as that it can freely turn of itself, to take always a position nearly north and south, is the most useful property of the magnet. This is practi- cally applied by means of the mariner's compass, in which a fine needle, index, or piece of steel- wire, formed like the index or hand of a clock or watch, is so balanced as to turn horizontally with great ease on the prop which supports it. The needle or index is fixed in a box ; and under- neath it the points of the compass, or the different Magnetism. 27 quarters of the horizon, that is, east, west, north, and south, with their intermediate points, are marked on a card. As the magnetic needle al- ways points nearly towards the north, by observ- ing the course or direction of the ship, that is, which way her head is turned, it is easy to know to what point she steers ; and by keeping a regular account of the distance she traverses, the sea- man can go with considerable exactness from one place to another. Before this great and import- ant invention, seamen usually steered by ob- serving the fixed stars, and particularly the polar or north star. But as this could only be done in fine weather, and when the stars were visible, they frequently lost their way and suffered ship- wreck. Indeed few of them dared to sail out of sight of land. But when they had a tolerably cer j tain means of knowing one point of the heavens, it was easy to know the others ; and it became, after this invention, neither necessary to observe the stars, nor to be afraid of the open sea, out of sight of the shore. It was by means of the mariner's compass that Columbus was enabled to make the great discovery of the American continent, and by means of it subsequent voyagers have sailed quite round the globe. Though the position of the magnetic needle, when it comes to rest on a vertical pivot, is, as we have remarked, nearly north and south, or coincident with the meridian, yet it is not exactly so, nor is it the same at different places, or in the 28 Experimental Philosophy. [Lecture 3. same place at different times. In some parts of the North American continent, the needle now points north and south ; at others, it deviates or varies from this position, the variation or de- clination, as it is technically called, being in some places westerly, in others easterly. At London, the declination of the needle in the year 1580, was 11 15' towards the east. From that time the declination, easterly, gradually diminished until the year 1658, when the position of the horizontal needle at London was precisely north and south. From that period to the present the north end of the needle has deviated more and more from the true north towards the west, until now (in the autumn of 1819), the declination at London is 24 19' W. In like manner at Dublin, Edinburgh, Paris, Copenhagen, and other places, where the declination has been long observed, it is found to increase westerly: though in none of those places is the declination the same at it is at London. In all of them, however, it has increased but ittle during the last ten or fifteen years. In 1800, the declination at London was 24 3' ; hence, during the last nineteen years, the declination has not, on the average, varied a minute in a year : and, it is exceedingly probable, that it has nearly, if not quite, attained its greatest western limit in England. Besides this constant variation in the decima- tion, as referred from year to year, there are minor variations in different parts of the year, Magnetism. 29 and, indeed, in different parts of the day. Mr. Gilpin found by a mean of twelve years, from 1793 to 1805, that the declination" appeared to increase, or go westward, from the winter solstice to the vernal equinox O'.SO ; to diminish, or go eastward, from the vernal equinox to the summer solstice 1'.43; to increase again, from the summer solstice to the autumnal equinox, 2'. 43 ; and to decrease only OM4 from thence to the winter solstice. These minute changes were observed to take place at London : corresponding mutations have been noticed in different parts of the conti- nent of Europe. With regard to the diurnal variation, Colonel Beaufoy, whose observations have been carried on for some years, at Bushey-heath, near Stanmore, finds the maximum variation to occur at about half an hour past one o'clock in the afternoon. The mean of his observations for May, 1819, give, at 8h. 37m. A. M. 24 32' 42" W. at 1 h. 24 m. P. M. 24 41' 22", at 7h. 26m. P.M. 24 34' 10''. The mean for June, 1819, give, at 8h. 40 m. A. M. 24 31' 28" W. at Ih. 29m. P.M. 24 41' 41". at 7h. 47 ra. P. M. 24 35' 09". No satisfactory theory of these variations has yet been adduced. Magnets, while they attract other bodies, appear to be themselves subject to the attraction of the 30 Experimental Philosophy. [Lecture 3. earth ; for the magnetic needle, when it is so sus- pended as to move freely in a vertical plane, ge- nerally assumes a position with one of its poles elevated and the other depressed. This, how- ever, varies in different latitudes: near the equator it is in a position almost horizontal ; as it ap- proaches the northern regions, the south pole is depressed, or drawn towards the earth ; and on the other side of the equator, in the southern la- titudes, the. north pole is depressed. This is called the dip of the needle, and is subject to periodical variations. In 1720, the dip at Lon- don was 75 10'; in 1775, it was 72 30'; in 1805, 70 20' ; now, in 1819, it is 70" 32'. Iron may acquire the magnetic virtue by other means than communication with a magnet. 1st. If a bar is kept for a long time in a vertical po- sition, or, still better, in the direction of the dipping needle. Thus old iron bars in windows are often found strongly magnetic. 2d. If iron is heated and suffered to cool quenched in water, holding it in the position of the dipping needle, the same effect is produced. 3d. If it is rubbed hard in the same position by any steel instru- ment. 4th, A few strokes of a hammer, first at one end of a bar, and then at the other, while held in the position of the dipping needle, will pro- duce the effect. 5th. A shock of electricity passed through the bar will gf ten render it magnetic. Many entertaining experiments are performed by means of magnetism. In the shops, little Magnetism. 31 swans made of tin, or more properly of iron tinned over, are sold, which, when put to swim in a basin of water, will, when one end or pole of an arti- ficial magnet is presented to them swim after it ; and when the other end or pole is turned towards them, they may be chased round the bason. If a small piece of bread is stuck on the end of the magnet which attracts them, an igno- rant person will suppose that they are following the bread as if to eat it. A small fish may also be made in the same manner to swim in a basin of water, and will follow a magnetic hook, or be lifted out of the water by it. Sometimes an artificial pond is made, about an inch in depth, and seven or eight in diameter, with the hours of the day marked about its edge. One of the magnetic swans is then put to swim in the pond ; and if a watch is placed underneath, with a small magnet fixed to the end or point of its hour hand, the swan, guided by the magnet beneath, will then swim to the hour, and show the company the time of day. But there are not any of the magnetic experi- ments more interesting or entertaining than that of the divining circles. They are drawn on paper, pasted on the top of a thin box, fig. 6. Pi. II. The index a, is fixed on 'the axle of the toothed wheel c, which works into the pinion d. On the axle of d is another pinion of the same numberof teeth, that puts in motion the wheel g, of the 32 Experimental Philosophy. [Lecture 3. same size and number of teeth as the wheel c. On the axle of g is fixed the bar magnet qq, and they turn together. Over this axle (but inde- pendent of it) is fixed a point in the top of the box for th^ loose needle xx to turn upon, and which is the centre of the pasted circle F. In the compartments of this circle are written an- swers to the questions asked in the compartments of the circle G. A circle of strong paper, of the size of F, should cover the pasted circle, and turn easily on the centre ; it should have one of the triangular pieces cut out, in order to see the answers. If then the needle xx is taken off its point, and a person wishes to ask some of the questions on the carton G, the person must turn the index to the question, and then place the needle on its point, giving it a whirl round, when it will stop over the answer. The open part of the loose circle being turned to that place, will exhibit the answer. Itinerant jugglers often attract considerable notice by exhibiting a number of these experi- ments ; and there are several very amusing toys constructed upon magnetic principles, and sold in the shops of the makers of mathematical in- struments. After all, however, the theory of magnetism is but imperfectly developed ; nor, indeed, have its leading phenomena been very cautiously traced. Very imposing formulae have been published, especially by continental mathematicians, includ- Magnetism. 33 ing, as is pretended, all the phenomena of terres trial magnetism in different latitudes ; but when applied to recently ascertained facts, their in- accuracy is at once detected. There is reason to hope that the cloud which has long hung over this department of science will speedily be dis- pelled. Hitherto the effect of magnetic attraction has only been stated in very general terms, and no attempt has been made to estimate the quantity of 'that effect under different circumstances. Mr. Barlow, of the Royal Military Academy, was the first who undertook a regular series of experiments with a view to this determination, and he soon found that there were three distinct conditions to be attended to, viz. the position of the needle and compass, with respect to the attracting body, the mass, or rather the surface of that body, and the distance at which the ac- tion took place. With respect to position, he discovered that a plane may be conceived to be drawn through the centre of attraction of any mass of iron, inclining from north to south at an angle equal to the complement of the dip, in which plane the iron has no effect on the needle ; that is, while the pivot of the compass is found in this plane, the needle will have its true magnetic bearing the same as if no iron were in its vicinity. He also discovered the law of deviation out of that circle, showing it to depend upon the angle which the compass formed with the above plane, 34 Experimental Philosophy. [Lecture 3. and another passing vertically through the north and south points: helikewise found that atdifferent distances, the position being the same, the tangents of the angles of deviation were inversely propor- tional to the cubes of the distances, and directly proportional to the cubes of the diameter of the attracting ball. But the most remarkable result obtained in the course of these experiments (with the excep- tion of the discovery of the plane of no attraction above referred to) was, that the poicer of an at- tracting body is independent of the mass of that body ; a simple tin spherical shell of any given dimension, acting equally as powerful as a solid iron ball of the same diameter ; which is another striking instance, in addition to many others, of the analogy that subsists between the mag- netic and electric attractions. Mr. Barlows ex- periments, we understand , are not yet completed : but it is hoped he will soon lay his most interest- ing results before the woVld; as they will, doubt- less, admit of an important practical application, to the magnetism of iron in ships, and its effect upon the direction of the needle in the ship's compass. LECTURE IV. EXPERIMENTAL PHILOSOPHY. HYDROSTATl CS. THE word which stands as the title of this lec- ture, implies simply the science which relates to the weight of water compared with that of other bodies ; but the science, as now taught and cul- tivated, treats not only of the weight and pres- sure, but of every thing relative to the action and mechanical properties of the dense or in- compressible fluids, such as water, &c. Though water is generally regarded as in- compressible, yet it is not entirely so, since it is capable of transmitting sound, which proves that it is elastic, and every elastic body must be com- pressible. To prove the fact, however, the Floren- tine academicians filled a globe of gold perfectly full with water, and afterwards closed the orifice by a tight screw. The globe was then put into a press of considerable force ; it was a little flat- tened at the sides by the force of the press, but was proportionably extended in other parts of its surface, so that it was concluded that the water did not occupy less space than before. On press- ing it still harder, the water was made to exude through the 'pores of the gold, and adhered to 36 Experimental Philosophy. [Lecture 4. its surface like drops of dew. From this expe- riment it may be inferred, that if water is indeed capable of compression, it is so only in a very slight degree, since, instead of yielding to the force of pressure, it found its way out through the pores of the metal. The same has been proved more scientifically by subsequent philo- sophers. The first principle that may be laid down with respect to the pressure of fluids is, that the sur- face of all waters which have a communication whilst they are at rest will be perfectly level. To explain this more fully, observe the three united tubes (Plate III. fig. 7). It will be seen that if water is poured into the perpendicular tube A, it will run through the horizontal tube C, and rise in the opposite perpendicular tube B to the same height at which it stands in A. Hence appears the reason why water, con- veyed under the earth through conduit-pipes, will always rise to the level of the reservoir whence it is drawn. It is in this manner that the cities of London and Westminster are sup- plied with water, either from London Bridge water-works or the New River. In the former case, water is raised from the Thames by immense pumps worked by wheels, which are turned by the tide, to the highest part of the town whither water is to be conveyed by pipes ; and, in the latter, it is well known that the reservoir of the New River stands on a rising ground near Isling- Hydrostatics. 37 ton, which is higher than any of the places where the pipes terminate. It is surprising that the antients should have been totally ignorant of so simple a principle as that of water rising to its level ; yet it is to this ignorance that we owe those stupendous works of art, the antient aqueducts, the ruins of which we still behold with admiration. Thus, for in- stance, in Plate V. fig. 19 5 an arch or arches would have been built to carry the water from the spring head at the side , across the valley, to supply the house on the other side; whereas a simple pipe of lead, iron, or wood, carried under ground across the valley, will answer every pur- pose, and supply the house and ponds about it as amply as if an aqueduct had been constructed on the antient plan. The reason why water thus rises to its level, is because fluids press equally on all sides : thus (in fig. 7.) if the tube B were taken away, the water would still press at b with equal force as before; and if the tube C were taken away, the water would press against the part a as forcibly as it would if it had remained. Thus, if with the thumb we stop the end of the crooked tube b (fig. 8.) at a, when full of water, the water will press against the thumb with a force pro- portioned to the height of the water in the tube above a; and, if we remove the thumb, it will run over at a, and fall in b to the level of a. To explain this in a popular way, without the 38 Experimental PhilosopJiy. [Lecture 4. aid of mathematical theory, fluids have been sup- posed to be constituted of small globules, as re- presented in fig. 10. If therefore any one of the columns, 1, 2, 3, 4, or 5, be removed, its place will be immediately supplied by a number of small globules, which will roll from, the other columns and fill up the vacancy, and consequently the superficies of the whole will presently sink to the same level; as will be found to be the case in a vessel filled with shot, with bullets, or any small round and smooth bodies. On the other hand, supposing these particles to have a very smooth and slippery surface, so as to move with very great ease upon one another, if the vessel which contained them were not full, and any ad- dition were made to the quantity, this addition would displace a number of other particles, which would roll round, and restore the level at the surface. Thus, in fig. 9, we will suppose a per- pendicular pressure to be made by the column ik, opposite to the point d; but as it, can proceed no further than that point, because of the bottom of the vessel, the pressure will be directed late- rally towards the sides efof the vessel, in such a manner that, if there were any aperture then in the vessel, the fluid would flow out : as that however is not the case, the particles g and h being restrained by the side of the vessel, those which compose the lateral column force them- selves between these particles g and h, and h will be raised towards the surface of the fluid, Hydrostatics. 39 unless a column equal to i k press against it, and keep it in its place. Since therefore the particle h would be raised towards the top of the vessel, unless restrained by a pressure quite equal to the column i A;, it follows, that two columns of water, to be in equilibrium, must be perfectly on a level at their surface. On this principle we are enabled to account for springs, which are sometimes found on the tops of mountains. They, in fact, come from some waters which are situated upon mountains higher still, and flow through canals or natural pipes, which proceed under ground, perhaps for the distance of miles. It is upon these facts the maxim is founded, which has led to the hydrostatic paradox, and that is, that the pressure of fluids is not in pro- portion to their quantity, but in proportion to their perpendicular height; and from this the supposed paradox follows, that a given quantity of water may exert a force two or three hundred times greater or less, according to the manner in which it is employed. To make this plain, we will take three vessels of the same height, and the same base, though differing materially with respect to their forms, and the quantities they contain, viz. A, B, C, D, %. 13. E, F, G, H, fig. 11. L, M, N, O, P, Q, fig. 12. Now it may very easily be understood, that the vessel fig. 15, is pressed at the bottom B, C, by the whole mass of water it contains, and 40 Experimental Philosophy. [Lecture 4. that the pressure there must be equal at every part. The vessel fig. 1 1, however, is of a differ- ent shape, and will hold more than three times the quantity of water ; yet the pressure at the base is still the same as in the former instance, because the bottom F, G, supports only the column of water I, F, G, K, which is the same as that contained in the vessel fig. 6. All this may be easily comprehended ; but the great difficulty lies in understanding how the very small tube in fig. 12. can exert a pressure at the bottom or base of the vessel equal to that in the preceding. Here it will be necessary to remember the maxim that was laid down, That the pressure of fluids is in proportion to their height, and not to their quantity. Thus we may observe the column of water in fig. 12. is equal in height to the columns in fig. 11. and 13; and if we advert to what was said, when speaking of fig. 9, we shall perceive that the small column L, M, P, Q. displaces a quantity of water contained in the lower part of the vessel M, P, N, O, and forces it to rise to the top of the vessel at s, for instance, which, if strong enough, will cause a re-action equal to the pressure of a column of water M, P, r, s. The same will take place at the other side, and at every part of the vessel which is covered, so that in effect the pressure at the bottom N, O, will be the same as if the column of water were equal in size from the bottom N, O, to the top of the tube, as shown by the dotted lines. All this may Hydrostatics. 41 be proved by experiment, having a false bottom to each of the vessels supported by an iron rod fixed to a balance, as in fig. 13 ; in which case it will be found that the same weight, at the oppo- site end of the balance, is necessary to support the bottom in each. The hydrostatic bellows is a very pleasing machine, constructed upon this principle. It consists of two strong boards, united by leather, almost in the manner of a common bellows, only that for convenience its form is round (see Plate IV. fig. 14.) In this figure a is a pipe, which goes into the inside of the bellows, and u is a weight laid upon the upper board. If then water is poured into the pipe , the weight will be lifted up ; and if the pipe was still taller, a greater weight would be raised. By a very small force exerted in this manner, that is, by water conveyed through a very small perpendi- cular tube, Dr. Goldsmith relates that he has seen a very strong hogshead burst in pieces, and the water scattered about with incredible force. To show that this principle in hydrostatics is not without practical utility, it is only necessary to mention, that upon the plan of the hydro- static bellows a press has been constructed of immense power, see fig. 15, in which a is a strong cast iron cylinder, ground smooth on the inner side, and e is a piston or moveable plug, fitting very tight within it. c is a common forcing pump, in which the water ascends through a 42 Experimental Philosophy. [Lecture 4. valve at its lower end, and is forced through at o into the cylinder. This forms a pressure at m, by the action of one man working at s, which squeezes cotton bags, hay, or other packages, into twenty times less compass than they would otherwise occupy. The effect would be the same if c\ instead of a pump, were a slender tube, pro- vided it was long in proportion to the pressure which was required. From all these experiments it is easy to con- ceive why the banks of ponds, rivers, and canals blow up, as it is called. If water can insinuate itself under a bank or dam, even to the thick- ness of a shilling, the pressure of the water in the canal will force it up. In fig. 1 8, a is the sec- tion of a river or canal, and c is a drain running under one of its banks. Now it is evident that if the bank g is not heavier than the co- lumn of water de, that part of the bank must infallibly give way. This eifect is prevented in artificial canals, by making the sides very tight with clay heavily rammed down, or by cut- ting a trench, n, from two feet to eighteen inches wide along the bank of the river or canal, and a little deeper, which being filled up with earth or clay well moistened with water, forms a kind of wall when dry, through which the water cannot penetrate. Another maxim in hydrostatics, of equal im- portance with the former, is, that every body lighter than water, or, in other words, which Hydrostatics. 43 swims in it, displaces exactly as much of the water as is equal to its own weight. This fact is proved by a very easy experiment. Put a small boat, #, (fig. 17.) in one scale, and balance it with water in the opposite scale, b. If then the boat is put into the basin, fig. 16, exactly filled with water, it will be found that a certain quantity of the water will run over the brim of the basin, which water, on taking out the boat, you will find will be exactly replaced by the water which before balanced the boat in the op- posite scale, b, fig. 17. Hence it is plain, that a boat or other vessel sailing upon the water, displaces exactly as much of the fluid as is equal to the vessel and its lad- ing, and, if more weight is added, it will sink deeper in the same proportion, or, in other words, a weight of water equal to the added lading will be displaced ; whence a laden ship is said to draw more water , that is to sink deeper, than when it is light or unloaded. Every body, on the other hand, which is hea- vier than water, or which sinks in it, displaces so much of the water as is equal to the bulk of the body sunk or immersed in the water. Thus it is plain, that if a leaden bullet is dropped into a vessel of water, it will take up just as much room as a small globe of water of equal dimen- sions. On this principle are computed the tables of specific gravities, by means of what is called the hydrostatic balance ; for since every body 44 Experimental Philosophy. [Lecture 4. that sinks displaces a quantity of water exactly equal to its own bulk, it follows, that every body when immersed in water loses so much of its weight as is equal to the weight of an equal bulk of water. Thus, if the body, when weighed in air, is two ounces in weight, and an equal bulk of water is one ounce, it will of course lose, when weighed in water, one ounce of its apparent weight. It is by this means .that adulterated metals or coins are distinguished from the true ones : thus copper is bulk for bulk heavier than tin, and gold is heavier than copper or brass, which last is a mixture of copper and zinc. If therefore a brass counter is offered for a guinea, if of the same weight, though it may not to the eye appear much larger than a real guinea, yet you may depend upon it that it is so in fact. We will then take a guinea, which we are sure is real gold, and weighing it first in air, and then in water, we shall find it loses about one-nineteenth of its weight in the latter. We then weigh the brass counter in the same way, and find it loses about one-eighth, which we find is much more, and therefore we cannot doubt but the coin is made of base metal. When we look at tables of specific gravities, we see the specific gravity of gold put down at about nineteen one-half, of mercury at about thirteen one-half, lead eleven one-quarter, silver ten one-quarter, copper eight one-half, iron seven one-half, tin seven one- quarter, &c. ; that is, gold is nineteen times one- Hydrostatics. 45 half heavier than its bulk of water, and conse- quently loses more than one-nineteenth of its weight in that fluid. This mode of ascertaining the standard value of metals was invented by the famous philosopher Archimedes, who made use of it to detect a fraud in the golden crown of Hiero, king of Syracuse. This king had given a certain weight of gold to be made, by a goldsmith of that place, into a crown ; the weight of the crown was exactly the same as the weight of the gold he had received ; but Hiero still suspecting an imposition, Archi- medes was requested to detect the fraud ; and he was led to make the trial in this way, without melting the crown, or destroying the workman- ship, from the resistance which he found was made by the water to his own body upon his going into the bath. A quantity of fine gold was therefore brought, and equally balanced in a scale against the crown ; but when both came to be weighed in water, it was found that the crown was much lighter ; whence not a doubt could re- main but that it was made of adulterated metal. It is upon the same principles that the density of different fluids is put to the test. It might, it is true, be ascertained by weighing them against each other in different scales ; but it may be done in a more easy and expeditious manner upon the hydrostatic plan, since the same body that will sink in one fluid will swim in another, and the same body will sink to different depths in 46 Experimental Philosophy. [Lecture 4. different fluids. Thus I have known good house- wives in the country try the body of their mead and other liquors, by observing whether an egg will swim in them, which, we know, will sink in common water. The exact relative weight of fluids may be ascertained by suspending from one end of an accurate balance (such as that fig. 17.) either a small globe, or a conical piece of glass. Its weight in water being previously ascertained, which suppose to be two hundred and twelve grains ; if it is immersed in a fluid heavier than water, some weights must be added in the opposite scale ; as for instance, if it is sea water, then ten grains must be added, which will make the relative weight of sea-water to common water as four hundred and twenty-two to four hundred and twelve. If, on the con- trary, it is immersed in brandy, which is less dense, and consequently lighter than water, you will find it necessary to take out of the opposite scale about forty grains, and then the relative weight of brandy to water will be as three hun- dred and seventy-two to four hundred and twelve, or about one-tenth lighter. A very convenient instrument is made use of by excisemen, officers of the customs, and all whose business it is to ascertain the density* or strength of liquors. It is called an hydrometer, and is nothing more than a small hollow globe of glass or metal with a stem to it, like the han- dle of a teetotum, but longer, which stem is Hydrostatics. 47 marked or graduated. The instrument is made so that the ball sinks in water, but not entirely, and therefore a part of the stem is always above the surface. If it is immersed in a fluid lighter than water it will sink, and less of the stem will be above the surface ; if in a heavier fluid, it will rise higher, and more of the stem will be visible. This instrument is fully described, and its theory explained more at large in the first vol. of Gre gorv's Mechanics. LECTURE V. EXPERIMENTAL PHILOSOPHY. HYDRAULICS. HYDROSTATICS, we have seen, is that science which relates to the weight and pressure of fluids ; the science of hydraulics teaches us what respects the motion of fluids, and the means of raising them by pumps, and conducting them by pipes or aqueducts from one station to another. This branch of science is, also, called Hydrodynamics. It was laid down as a principle, in the preced- ing lecture, that of all waters which communicate with each other, the surface will be level, or, in common language, that water will rise to its level, or to the same height as its source. The reason of this was not fully assigned then, because it was not necessary ; it was observed, that fluids press equally on all sides; but another reason which partly operates to produce the level surface of water is the pressure of another fluid, that is, the air or atmosphere, which, as it bears equally on all points of the earth's surface, must equally press the source from which water is derived and the orifice of the tube or pipe in which it rises, as was evidenced in the three united tubes, which were exhibited as explanatory of this fact. That a reservoir of water, less than S3 feet in Hydraulics. 49 height, will not flow unless exposed to the pres- sure of the atmosphere, will be plain from filling a cask or other vessel full of this fluid. If the bung is perfectly tight, and there is no aperture above for the air to press upon it and force it out, it is in vain that we shall attempt to draw it off by opening a passage for it below. Hence the use of vent-holes, and vent-pegs in casks: by raising the vent-peg air is admitted, which forces the liquor to flow out at the cock or faucet, where- as if the vent-peg were kept tight no liquor whatever could be obtained. The Valencia is a common instrument made of tin, the lower part of which is in the figure of an inverted cone, (see PI. V. fig. 22.) with an orifice at the bottom , and one at the top b. It is used for taking sam- ples of liquors out of the bung-holes of casks. In order to use it, the operator puts it into the bung-hole with both orifices open, and the liquor rises through the orifice at bottom to the top of the instrument ; he then puts his thumb on the hole or aperture at top, so as to exclude the air' completely, and the liquor will not run out at the bottom till the air is admitted by the thumb be- ing removed, which is done in order to let it flow into the cup or vessel which is to receive it. Thus it is plain that fluids, circumstanced as above, are put in motion, or caused to flow, by the pressure of the atmosphere ; and it will be shown, that whenever that pressure is removed, they will rise above their natural level, and flow VOL. i. D 50 Experimental Philosophy. [Lecture 5, where they otherwise would not. The syphon or ' crane, is a bent tube, of which one leg is longer than the other (fig. 1). With this instrument we want to draw off the fluid contained in the vessel D, which we will suppose immoveable, as a well or a heavy cistern. We know that if the instrument is put into the vessel, without some particular management the fluid can never be made to flow over the bent part B ; for the air which presses on the surface of the fluid will also press through the bore of the tube, and prevent its pursuing that course. In order to use it, there- fore, we fill the syphon with water or some other fluid, and stopping both ends, immerse the shorter leg in the vessel D. The stoppage be- ing removed, the water will flow out at the leg C by its own gravity, and, by the pressure of the atmosphere on the surface, will continue to flow while there remains any fluid in the vessel. If a vacuum is made in the syphon, by drawing out the air with one's mouth, or in any other way, the same effect will take place. The syphon fountain is a beautiful example of the effect from the pressure of the atmosphere. In fig. 20, a is the long or outer leg of the syphon, which is inserted by a brass or wooden cap in the glass vessel c; the inner leg b also passes through the cap,and terminates in a spout- ing pipe of an extremely small bore. To make it act, we must first put it in a position the reverse of what it stands in at present, and through the Hydraulics. 51 leg a pour in at d a quantity of water, which will force the air out of the vessel through the leg b. We then stop both orifices with the finger, as in the common syphon, and immerse the leg b in the vessel e filled with water. The water in the glass will then flow out through the leg a ; and the glass being vacant of air, the water from the vessel e will ascend through the leg 5, and form a most beautiful jet or fountain within the glass vessel. The syphon may be disguised in such a man- ner as to produce many entertaining effects. The cup fig. 23, is called Tantalus's cup, from the celebrated fable of Tantalus, who is represented by the ancients as suffering continual thirst, and though he is in the midst of water, is unable to assuage it " E'en in the circling Hoods refreshment craves, And pines with thirst amidst a sea of waves; And when the water to his lips applies, Back from his lips the treach'rous water flies.'' In the cup there is a figure of Tantalus, and if we pour water into it, so that it shall nearly reach to the lips of the image, the water immediately sinks, and is drawn off again. The truth is, there is a syphon concealed within the image; and when the water is poured into the cup, so as nearly to reach the lips, the fluid is then raised above the bend of the syphon, which of course then begins to act, and the water is drawn off by the longer leg in the manner already described. 52 Experimental Philosophy. [Lecture 5. Sometimes the syphon is concealed in the handle of the cup (see fig. 23.) in such a manner, that when a person raises it to his lips to drink out of it, the fluid which it contains shall be carried over the bend of the syphon, and it will then be drawn off by the longer leg, so that the person shall not only be disappointed of his draught, but will have his clothes well splashed, to the great amusement of the by-standers. In some parts of the world there are what are called intermittent springs, or wells which seem to ebb and flow like the tides. This we shall perceive is usually caused by a natural syphon. In fig. 24, A is a well of this nature, B is a ca- vity or reservoir of water under ground, with which it communicates, by means of the pipe or syphon C. It is obvious, that unless the water in the reservoir rises above the height of the bend of the syphon C, the well cannot be filled ; but if by considerable rains, or any other cause, the reservoir should become full, then the syphon will begin to act, and the water will run into the well as long as there remains any in the reservoir. It will then cease to receive any more, and the drain from the well will empty it in its turn. At Gravesend there is a pond of this kind, which ebbs while the tide is coming into the adjacent river, fills after the tide has risen to its height, and all the time that it is ebbing in the river. At Larntown, in Worcestershire, there is also a brook which, in summer, has a stream sufficient to turn Hydraulics. 53 a mill, and the greater part of the winter is desti- tute of water. This probably communicates by a syphon with some cavity in the earth, which is filled by the melting of the snow to a certain height, and after that it will continue to be drawn off by the brook, so as to furnish a stream till the reservoir is entirely emptied. It is by the pressure of the atmosphere that the common or sucking pump is enabled to act. It is said to have been invented by a mathemati- cian of the name of Ctesebes, about one hundred and twenty years before Christ ; but the principle on which it acted was unknown till the 17th cen- tury. Mankind, perfectly ignorant that the air had weight, attempted to account for these effects by a maxim not only unfounded, but even desti- tute of meaning. This was, " that Nature ab- horred a vacuum." What they meant by Nature is as little to be understood as when the same word is used by those ignorant sciolists who affect to deny the existence of a God. Absurd, how- ever, as this maxim was, it remained uncontra- dicted till within one hundred and sixty years, when it met with a practical refutation. About that time some workmen were employed by the duke of Florence, to raise water by a common sucking pump to the height of fifty or sixty feet. A pump was accordingly constructed for that purpose; but, after all their efforts, they were unable to raise it above the height of thirty-two feet. It was then found either that Nature had 54 Experimental Philosophy. [Lecture 5. not this horror of a vacuum, or at least, that it , was a very limited kind of a horror; for why should Nature have a horror of a vacuum at one height and not at another ? The matter was re- ferred to the famous astronomer and philosopher Galileo ; but in his time philosophical knowledge was not sufficiently advanced to solve the diffi- culty. The difficulty is, however, now explained, through principles furnished by Galileo's pupil Torricelli. We knoAv that a pump is a hollow piece of timber or metal, to the bore of which a piston, bucket, or sucker, is exactly fitted. That the piston has a valve in it made with leather, like the clapper of a bellows. When the piston is pushed down, therefore, the air, or any fluid contained in the pump, will force it open ; and when the piston is drawn up, the pressure of the air or water, which has been admitted in that way, will keep the valve down. But to make the matter perfectly clear, let us represent the opera- tion in a glass model. In PI. VI. fig. 25, is a pump constructed on the plan of a common, or as it is usually called sucking pump. Let this pump then, D, C, B, L. be immersed in water at K ; in which case you will see the water rise as high as L in the pipe or body of the pump. G is the piston, sucker, or bucket, as it is sometimes called, in which a is the valve ; and at H is a box made similar to the bucket G with a valve in it , with this difference, that the box H is immov- Hydraulics. 55 able, and fills the bore of the pump. D is tli rod (which is generally of iron) by which the piston is raised. When, therefore, by drawing up the rod B the piston or bucket is raised from B to C, the valve and pisjton being perfectly or nearly air-tight, it is obvious that a vacuum is created, that is, there is a space from B to C, from which the air is drawn out. This, however, is in some measure supplied by the air from below, which enters through the valve b, which it opens by its force. It is evident, however, that this air must be exceedingly dilated, by its now occupying so much more space than it did before. The force or spring of the air, within the pump, is so much weakened, that it is not able to resist the pres- sure of the external air upon the water. The ex- ternal air, therefore, pressing upon tjbe surface of the water, forces it to ascend through the notched foot of the pump A, perhaps as high as e in the body or bore of the pump. By another stroke of the piston G, or by causing it to descend, the upper valv,e a is again opened by the force or spring of the air, and the valve below (b) is shut by the same pressure. Thus by the descent of the piston, all the air which was included be- tween the box H and the space C, to which the piston was before raised, will rise above the valve a in the piston, and by drawing it up, the valve a will again be shut, and a second vacuum created as before, which again will be filled by the air from below, ascending through the lower 56 Experimental Philosophy. [Lecture 5. valve b. The spring of the air being thus weak- ened by this second motion, the pressure of the atmosphere without the pump will cause the water again to ascend within it, we will suppose to F. By the next stroke the air will be almost entirely exhausted, and the water will rise in the body of the pump above the boxll, perhaps as high as B. On forcing down the piston or bucket again, the valve b in the box H will be shut by the pressure as before, and the valve a in the piston G will be opened by the same pressure, and consequently water instead of air will now be raised by the elevation of the piston. When the piston is thus raised, it is evident that a vacuitm will again be produced between the box H and the piston C, which will instantaneously be filled up by the water flowing through the valve b, as before described. Thus, by the continual work- ing of the pump, the water will be raised by the piston into the wider space, and caused to flow through the spout I. Every time the piston or bucket is raised, the valve b is lifted up by the water beneath, and every time the piston or bucket is forced down, the valve a rises, and the valve b is depressed. For the easiness of work- ing in common pumps, the rod D is fixed to a handle, which acts as a lever, and turns on a pin in the body of the pump. We have not yet, however, explained the diffi- culty respecting the pump of the duke of Flo- rence ; and you do not yet understand why the Hydraulics. 57 water would rise in it no higher than thirty-two feet. We must recollect what was said respect- ing the cause of the water's rising in the body of the pump. We know it was the pressure of the atmosphere on the surface of the exterior water that forced it to rise. From this circumstance it is evident that the air has weight. But again, as the atmosphere, or that mass of air which surrounds the globe, is only of a limited height (supposed about forty -five miles) and that of gradually diminishing density, it follows that its weight or pressure must be limited also ; and it is found that a column of water of thirty- two or thirty-three feet high is, at a medium, equal in weight to a column of air of the same diameter or thickness the whole height of the atmosphere. Consequently the pressure of the atmosphere can never force water through any vacant space higher than about thirty-three feet. By the ac- tion of a common pump of four inches bore and thirty feet high, a single man can discharge twenty-seven gallons and a half of water in a minute ; if the pump is only ten feet above the surface of the well, the quantity discharged in that time may be eighty-one gallons six pints. The forcing pump is upon a different plan. Here the piston is without a valve, and the water which rises through the valve in the box is forced out by the depression of the solid piston. Thus, in fig. 29, when the piston or plunger g* is lifted up by the rod D, the water beneath forces D 5 58 Experimental Philosophy. [Lecture 5. up the valve b in the box H, and rises into the body or barrel of the pump above H. When the piston g, therefore, (which we must observe has no hole or valve in it) is depressed to H, the valve b being closed by this action, the water in the barrel of the pump, finding no other vent, is forced into the pipe M M, and so up through the pipe. If there is no occasion for a continued stream of water, the pipe M is continued to any given height, and then the water would be thrown out like a jet-d'eau at every stroke of the piston. But to make a continued stream a further con- trivance is necessary. To this end an air vessel, such as K K, is an- nexed to the pipe M, and into this air-vessel the water is forced by each stroke of the piston. When therefore the water, by this action conti- nued, gets above the lower end of the pipe GHI, which is fixed air-tight, in the top of the vessel, the air in the upper part is proportionably con- densed. The action of the pump being then continued, in proportion as the vessel K K is filled with water, the air above it is compressed, and in return presses on the surface and drives out the water through the pipe at the orifice in its end in a continual stream, and with great force. It is upon this principle that the famous and truly useful invention of the fire-engine is found- ed. It consists of two forcing pumps, and a large air vessel which communicates with the pipe. In fig. 27, A B is the body of the engine, in which 59 the water is contained ; D and E are two forcing pumps, wrought by the lever FG, moving on the centre h. The easiest mode of supplying the en- gine with water, is that which is usually employed in London in cases of fire, when a leather pipe communicates with the orifice of one of the pipes which supplies the city with water. When this cannot be done, the water is poured by .-buckets into the vessel AB, and being strained through the wire grating N, is, by the pressure of the atmosphere, raised (as before described in treat- ing of the forcing pump) through the valves at the lower end of the barrels D and E, when either of the forcers ascend, and at their descent it will be forced through the other valves alter- nately, into the air vessel C : the air, therefore, in this vessel being very strongly compressed, by its spring it will force the water up through the metal pipe within the air vessel; the part Q of which being flexible, its end may be directed to any part of the building where the flames predo- minate. By the means of forcing pumps water may be raised to any height above the level of a stream or spring, provided the machinery is sufficiently powerful to work them. The London Bridge water-works, which supply the city of London with water, consist of a certain number of forcing pumps, which are worked by large wheels turned by the tide. There is also a beautiful engine of this kind at the duke of Marlborough's at Blen- heim. 60 Experimental Philosophy. [Lecture 5. The most powerful forcing pumps, however, are wrought by steam engines, for steam is one of the strongest powers in nature. The steam engine consists of a large cylinder or barrel, in which is nicely fitted a solid piston, like that of a forcing pump. The steam is supplied from a large boiler close by, and is admitted into the cylinder by an orifice, which can be occasionally shut. The force of the steam lifts the piston, to . the top of which is affixed a long lever to work a forcing pump, or for any other purpose; and when the piston is lifted a certain height, it opens a small valve in the bottom of the cylinder, through which a small quantity of cold water be- ing admitted the steam is condensed, and thus a vacuum being created, the piston again descends, and is again lifted up by the force of the steam. For a detailed description of this invaluable en- gine, however, our readers must consult the En- cyclopaedias or Pantologia, and our best treatises on Mechanics. LECTURE VI. EXPERIMENTAlTPHILOSOPHY. OF PNEUMATICS. THE air we breathe is an heterogeneous mix- ture, that is, a matter composed of different sub- stances, and not of particles of perfectly the same nature. This is one of the secrets which the wonderful discoveries of modern chemistry have revealed to us. According to this system, caloric, or the matter of fire, is the basis of all fluidity, and therefore air may be considered as consisting of very minute particles, which swim, or are suspended in a mass of that very subtile and active fluid. The properties of caloric are not, however, perceptible in this mixture ; for on account of the attraction which subsists between those particles of which air is composed, and those of caloric, the latter is rendered latent, as Dr. Black expresses it, or, in other words, in- active. The matter of atmospheric air is therer fore composed of caloric as its basis, and some other matters. Or the other matters may be considered as dissolved and floating in the mass of fire, like salt, or gum, or any other substance in water. The nature of these matters will be explained in the chemical lectures, and would be 62 Experimental Philosophy. [Lecture 6. improper at present, since it is of the general properties of air of which I am now to treat, or rather of its mechanical and not its chemical pro- perties. Fluids are divided into two classes ; the incom- pressible, and the elastic. That branch of science which is called hydrostatics treats of all the known qualities of the former, and that of pneu- matics embraces all which respects the general properties of the elastic fluids. The elastic fluids are again divided into two classes, those which are condensible, such as vapour, which is easily condensed by cold; and the permanently elastic fluids, of which there are many, such as oxygen air or gas (the word gas being an old German term signifying spirit * ), nitrogen or azotic gas, or phlogisticated air, as it was first called, carbo- nic acid gas or fixable air, hydrogen gas or in- flammable air ( that which is used to inflate bal- loons), nitrous gas, hepatic gas, &e. But of their general or mechanical properties the com- mon air will serve to give a perfect idea. The properties of air of which the science of pneumatics particularly treats, are its weight, pressure, and elasticity or spring. That air, like all other bodies, is possessed of weight or gravity many obvious facts will serve to convince us ; and, in truth, it may be reduced to the simplest of ah 1 experiments, for air may be ac- * Whence our word ghost. Pneumatics. 63 tually weighed. If, for instance, a bottle which holds a wine quart is emptied of its air, either by the action of the air pump, or by filling it with quicksilver, and emptying the quicksilver out, taking care that, in corking it, no air is suffered to enter, it will be found to be sixteen grains lighter than it was before it was emptied of its air. A quart of air, therefore, weighs just sixteen grains ; a quart of water weighs fourteen thou- sand six hundred and twenty-one *., which, di- vided by sixteen, gives a result in round numbers of nine hundred and fourteen ; so that water at a medium is nine hundred and fourteen times heavier than air. This, however, is only to be understood of air near the surface of the earth ; for, in fact, as air is a body possessed of gravity, that which is near- est the earth sustains a greater pressure, and is consequently more dense or compact ; and it is rarer or more thin and light in the higher regions of the atmosphere, being less pressed with the weight of air which is above. The atmosphere, I observed in my last lecture, is that mass of air which surrounds the globe, and which is gene- rally computed to be about forty-five miles in height. If altitudes in the air are taken in arith- metical proportion, the rarity of the air will be in geometrical proportion ; and therefore sup- posing that the atmosphere extended to the height * A quart of water is generally calculated at two pounds, bat it is in fact something less. 64? Experimental Philosophy. [Lecture 6. of five hundred miles, it has been computed that one cubic inch, such as the air we breathe, would be so much rarefied at that height, that it might fill a hollow sphere equal in dimensions to the orbit of Saturn. We need not, however, have recourse to cal- culations to prove a fact so generally understood. All persons who have visited the tops of high mountains know by experience that the air is thinner or rarer at those altitudes than below. -As they ascend their breathing becomes quicker, the atmosphere is clearer, neither clouds nor va- pours can rise to such heights ; and it is common in these situations to see the lightning below pass from one cloud to another, while all above is clear and serene. Ulloa, who went to take the mea- sure of a degree upon the earth's surface, informs us, that while he stood on the top of one of the Andes in Peru, the clouds, which were gathered below the mountain's brow, seemed like a tem- pestuous ocean, all dashing and foaming, with lightnings breaking through the waves, and some- times two or three suns were reflected from its bosom. " In the mean time he enjoyed a cloud- less sky, and left the war of the elements to the unphilosophical mortals on the plain below him." The reason of all this must be evident. The clouds are vapour, that is, water rarefied by heat ; vapour is lighter than air near the surface of the earth, but in the higher regions the air is thinner Pneumatics. 65 and lighter than these vapours, and consequently they can only ascend to a limited height. What Ulloa observed on the Andes, has been confirmed by the adventurers in balloons, and particularly by Mr. Baldwin, who ascended from Chester in the year 1785. The earth was entirely hid from his view by the immense mass of vapours : he compares them to a sea of cotton, tufting here and there by the action of the air, and soon after the whole became an extended floor of white cloud. To prove the weight and pressure of the atmo- sphere I shall mention an easy experiment, which the student may make himself, without any phi- losophical apparatus. If we nearly fill a com- mon saucer with water, and then take a tea-cup, and burn in it a piece of paper ; while the paper is yet burning, turn down the cup, paper and all into the saucer, we shall soon see that the pres- sure of the air upon the water contained in the saucer will force it up into the cup. To under- stand the nature of this experiment it is necessary to anticipate in some degree what will be the subject of future lectures. Heat, caloric, or fire, is now known to be a real substance; when, therefore, the paper is burned in the tea- cup, the air is driven out by another fluid (viz. caloric) taking its place. Caloric, however, pe- netrates all substances ; and therefore when the flame is extinguished, it is dissipated through the pores of the cup, leaving almost a perfect vacuum^ 66 Experimental Philosophy. [Lecture 6. to fill which the water is pressed up, as before de- scribed. It would rise, if there were no impedi- ment, to the height of thirty-two feet, because, as I explained in my last lecture, a column of the atmosphere is at a medium equal in weight to a column of water of that height. The weight of the air, or rather of the atmo- sphere, is, however, exactly determined by the following experiment. Take a glass tube about three feet long, open at one end ; fill it with quicksilver, putting the finger upon the open end, turn that end down- ward, and immerse it into a small vessel of quick- silver, without admitting any air : then take away the finger, and the quicksilver will remain sus- pended in the tube twenty-nine inches and a half above its surface in the vessel ; sometimes more, and at other times less, as the weight of the air is varied by winds, vapours, and other causes. That the quicksilver is kept up in the tube by the pressure of the atmosphere upon that in the bason, is evident ; for, if the bason and tube are put under a glass, and the air is then taken out of the glass, all the quicksilver in the tube will fall down into the bason ; and if the air is ad- mitted again, the quicksilver will rise to the same height as before. The air's pressure therefore on the surface of the earth, is equal to the weight of twenty -nine inches and a half depth of quicksilver all over the earth's surface, at a mean rate. A square column of quicksilver, twenty-nine Pneumatics. 67 inches and a half high, and one inch thick, weighs just fifteen pounds, which is equal to the pressure of air upon every square inch of the earth's surface ; and one hundred and forty-four times as much, or two thousand one hundred and sixty pounds upon every square foot ; be- cause a square foot contains one hundred and forty-four square inches. At this rate a middle- sized man, whose surface may be about fourteen square feet, sustains a pressure of thirty thou- sand two hundred and forty pounds, when the air is of a mean gravity ; a pressure which would be insupportable, and even fatal to us, were it not equal on every part, and counterbalanced by the spring of the air within us, which is diffused through the whole body, and re-acts with an equal force against the outward pressure. Now, since the earth's surface contains, in round numbers, 200,000,000 square miles, and every square mile 27,878,400 square feet, there murst be 5,575,680,000,000,000 square feet oa the earth's surface ; which, multiplied bv 2,160 pounds, (the pressure on each square foot) give* 12,043,468,800,000,000,000 pounds for the pres- sure or weight of the whole atmosphere. The above experiment on the quicksilver, which is called the Torricellian experiment, after its inventor Torricelli, who made it about tin? year 164r5, is the foundation of that instrument which is called the barometer, so useful in fore- telling changes of the weather. In the common 68 Experimental Philosophy. [Lecture 6, barometer the quicksilver in the ball below is left open to the pressure of the atmosphere, which, according as it increases in weight or density, presses on the surface of the quicksilver, and forces it into the vacuum in the glass above. When the air is dense or heavy it supports the clouds and vapours ; when it is rarefied and thin it is unable to support them, and they fall in the form of mists, rain, hail, or snow. When, there- fore, the quicksilver rises in the glass, we say it is a sign of fair weather, when it falls it prognos- ticates foul. That the air is elastic is easily seen from various experiments, particularly when it is confined in a bladder or any flexible substance, we then find it may be compressed by force into a narrower com- pass, and that it will expand again when that force is removed. But of all instruments for showing the elasticity as well as all the other properties of the air, the air-pump is the most complete. It was invented nearly simultaneously by our illustrious countryman, Mr. Boyle, and a celebrated German, Otto Guericke. Whoever is acquainted with the construction of a common water-pump, can have no difficulty in comprehending the nature and action of the air-pump ; the principle is exactly the same, and we may therefore, without further preface, refer immediately to the Plate VII. fig. 28, to explain its operation. Having put a wet leather on the plate L L of Pneumatics. 69 the air-pump, place the glass receiver M upon the leather, so that the hole i in the plate may be within the glass. Then, turning the handle F backward and forward, the air will be pumped out of the receiver; which will then be held down to the plate by the pressure of the external air or atmosphere. For, as the handle F is turn- ed backward, it raises the piston d in the barrel B K, by means of the wheel E and rack D : and, as the piston is leathered so tight as to fit the barrel exactly, no air can get between the piston and barrel ; and therefore all the air above d in the barrel is lifted up towards B, and a vacuum is made in the barrel from b to d, upon which, part of the air in the receiver M, by its spring, rushes through the hole i, in the brass plate L L, along the pipe G, which communicates with both bar- rels by the hollow trunk I H K, and pushing up the valve 6, enters into the vacant jplace b d of the barrel B K. For wherever the resistance or pressure is taken off, the air will run to that place, if it can find a passage. Then, if the handle F is turned forward, the piston d will be depressed in the barrel ; and, as the air which had got into the barrel cannot be pushed back through the valve b 9 it will ascend through a hole in the piston, and escape through a valve at d, and be hindered by that valve from returning into the barrel, when the piston is again raised. At the next raising of the piston, a vacuum is again made, in the same manner as before, between b and d ; upon which 70 Experimental Philosophy. [Lecture 6. more of the air that was left in the receiver M gets out thence by its spring, and runs into the barrel B K, through the valve b. The same thing is to be understood with regard to the other bar- rel A I ; and as the handle F is turned backward and forward, it alternately raises and depresses the pistons in their barrels, always raising one wffile it depresses the other. A vacuum being made in each barrel when its piston is raised, the particles of air in the receiver M push out one another by their spring or elasticity, through the hole i, and pipe G, into the barrels ; until at last the air in the receiver becomes so much dilated, and its spring so far weakened, that it can no longer get through the valves, and then no more can be taken out. Hence there is no such thing as making a perfect vacuum in the receiver; for the quantity of air taken out at any one stroke will always be as the density of it in the receiver: and therefore it is impossible to exhaust it entire- ly, because, supposing the receiver and barrels of equal capacity, there will be always as much left as was taken out at the last turn of the handle. There is a cock & below the barrels, which being turned, lets the air into the receiver again ; and then the receiver becomes loose, and may be taken off the plate. There is also a glass tube m n (fig. 29.) open at both ends, and about thirty-four inches long ; the upper end communicating with a hole in the Pneumatics. 71 pump-plate, and the lower end immersed in quicksilver at n in the vessel N. To this tube is fitted a wooden ruler m m, called the gage, which is divided into inches and parts of an inch, from the bottom at n (where it is even with the sur- face of the quicksilver), and continued up to the top, a little below, to thirty or thirty-one inches. As the air is pumped out of the receiver M, it is likewise pumped out of the glass tube m n, be- cause that tube opens into the receiver through the pump-plate; and as the tube is gradually emptied of air, the quicksilver in the vessel N is forced up into the tube as in a barometer, by the pressure of the atmosphere. And if the receiver could be perfectly exhausted of air, the quick- silver would stand as high in the tube as it does at that time in the barometer : for it is supported by the same power or weight of the atmosphere in both. The quantity of air exhausted out of the re- ceiver on each turn of the handle, is always pro- portionable to the ascent of the quicksilver on that turn ; and the quantity of air remaining in the receiver, is proportionable to the defect of the height of the quicksilver in the gage, from what it is at that time in the barometer. By means of the air-pump all the mechanical properties of air are, as before observed, most completely ascertained. Thus the weight and pressure are clearly proved by a very easy and ob- 7 Experimental Philosophy. [Lecture 6. vious experiment. If we take a vessel of a long or cylindrical shape, (fig, 30.) which is open at the top, and place it on the pump, where the receiver stands in fig. 28, then press it on the top with the hand so as to exclude the external air, we shall find, as the vessel begins to be exhausted of air, a considerable pressure on the back of the hand ; and if the operation is conti- nued, that pressure will even become painful, and we shall perceive it impossible to remove the hand. This evinces that the weight of that co- lumn of air which is above must be considerable, and that the calculation above stated, of the weight which a man's body usually bears, is not overrated. If, instead of the hand, a piece of bladder is tied over the open top of the vessel, we shall see the bladder gradually sunk in like a jelly-bag, and at length burst with considerable force by the pressure of the external air ; a flat piece of thin glass, placed in the same situation, will be broken in pieces. Why then is the glass receiver, which, we see, is placed on the pump in fig. 1, not broken ? The reason of this is, first, the shape of the glass, which is globular or arched at top, and this is found, by long experience, to be the best form for supporting a weight ; secondly, these receivers are generally made of thick glass, and with particular care, so as to sustain a greater pressure than that of fifteen pounds on a square inch without any danger of breaking. A beautiful experiment to evince the pressure Pneumatics. 73 of the air, is this. Let a metallic cup be provided, in whose bottom shall be fixed a cylinder of thorn, or some other wood, about three inches long; and let this cup and attached cylinder be placed at the top of the receiver of the air-pump, so as to exclude all external air. Then let quicksilver be poured into this cup, and let a glass to re- ceive it be placed within the receiver. Then, as the rarefaction of the interior air proceeds, the quicksilver will be forced, by the external pres- sure, through the pores of the wood, and will be seen to descend in a beautiful shower. Various facts in nature are explained by under- standing the pressure and force of the air. The word suction is founded on a vulgar error, for, in fact, there is no such thing. In all cases where suction is supposed, a vacuum or void is created, and the pressure of the atmosphere forces the fluid to fill up this void. Thus when children suck at the breast, the mouth and lips of the child act as an air-pump. The child swallows the air in his mouth, while he holds the nipple fast in his lips, so that none can come in that way. A vacuum, of course, is created, and the external air pressing on the breasts of the mother, squeezes the milk into the infant's mouth. The action of cupping glasses is explained on the same principle. The air is driven out of the cupping glass by means of heat, (as in the expe- riment with the tea-cup,) that part of the body where the glass is applied has therefore no pres- VOL. I. E 74 Experimental Philosophy. [Lecture 6, sure of air upon it, and the fluids of the body are driven to that part where there is least resistance. By the air-pump we are also convinced more clearly of the elasticity and compressibility of the air. Take a bladder from which the air is almost totally exhausted, and which appears (juite flaccid and compressed, tie the neck of it tight as it was when full, and put it in an air- pump. As the air is exhausted we shall see the bladder gradually inflate, till, at length, it will be puffed out to the full size it was before we had expelled the air. Mr. Boyle relates that, by means of the air-pump, he had rarefied common air so as to make it fill nearly fourteen thousand times the space it did before. A similar effect would take place with a blad- der, by carrying it to the higher regions of the atmosphere, where, as before explained, the air is thinner and lighter, and consequently its pres- sure less. If a bladder half full is carried up to the top of a high mountain, it will gradually di- late to its former size. If, instead of a bladder almost empty, a full- blown bladder, or a thin glass bubble filled with air. and closely stopped, is put into the ah -pump, as soon as the air is exhausted, the bladder or the bubble will burst in pieces. The air is also capable of being rarefied by heat. If a bladder, half blown and tightly tied at the neck, is held to the fire, we shall find that it will dilate to nearly its full size ; and if either a Pneumatics. 75 full-blown bladder or a thin glass bubble filled with air is held close to a strong fire, it will burst. That air is a compressible fluid must be evident, when we consider that it is elastic ; and it must be further evident from what was said in the last lecture on the use of the air vessel an- nexed to the forcing pump and common fire engine. There is, however, a beautiful experi- ment expressive of the effects from compressed air, which, with the aid of the plate, I shall endeavour to describe. It is a kind of artificial fountain, which is made to send out a stream or jet of water by means similar to those employed in the fire engine, that is, by a body of compressed air forcing the water contained below it through a small pipe, and out of the jet or orifice of the pipe. In Plate VIII. fig. 31, ABCD, is a copper vessel, which may be made of any con- venient form ; within the vessel is a small pipe or tube N O open at bottom, and with what is called a stop cock *, such as R, at the upper end to keep in the air when it is necessary. To make the fountain play, we first fill it about two- thirds full, with water, then screw in the pipe, which must be made air-tight by oiled leather. The air contained between the surface * A stop cock is exactly like the common cocks used in beer barrels, &c. When turned one way there is an orifice through the stopple, which then admits the air, or any fluid ; when turned the other way it is solid, and stops the passage. 76 Experimental Philosophy. [Lecture 6. of the water and the top of the vessel is then of the same density with that of the atmosphere. We then take the condensing syringe, fig. 32, and screw it above the stop cock, and force a quantity of air into the vessel, which, as it can- not return, forces its way through the water into the upper part of the fountain, where it remains in a condensed state ; while the air in the foun- tain or vessel is condensing, we turn the stop cock R to prevent the escape of the water. We then screw on a jet or pipe with a small aperture at top, and when we turn the stop cock again, the condensed air above, by its expansion, forces the water through the pipe, and out at the jet, in a beautiful fountain. The condensing syringe, fig. 32, is made like a common squirt or syringe ; but it has a valve at bottom, which, instead of opening inwards as the valve of a pump, opens outwards at R. Near the top of the syringe there is a small hole P. When, therefore, the condensing syringe is screwed on the vessel, if we draw up the piston (which is solid, as in a squirt, and not with a valve, like the piston of a pump) there will be a vacuum left between that and the valve, till we draw up the piston as far as the little hole P, near the top. When it gets past the hole, the exter- nal air will rush in and fill up the vacuum ; when we push the piston down again, by which ac- tion the valve below is opened, and the air forced into the vessel the valve shuts, and re- strains the air from returning. Pneumatics. 77 Air, it is said, may be thus compressed into fifty thousand times less compass than its natural bulk, provided the apparatus is strong enough. On this principle of condensed air is constructed the air-gun, a very dangerous and destructive in- strument. It was formerly a very complex ma- chine, from having the chamber for containing the condensed air within the body or rather the butt end of the gun. That which is how in use was invented by the late ingenious Benj. Martin : see fig. 33. It is in shape exactly like a common gun. Just below the lock, a copper ball A, fig. 34, screws on, which is charged or filled with condensed air by a condensing syringe, ex- actly as we charge the brass fountain, only that the ball contains no water; the ball has a stop cock a, which is turned or shut when it is not on the gun : the bullet is rammed in a<= w ~i - -- musket, V* ~"^ nt me barrel very exactly. By drawing the trigger, a small valve is opened at the bottom of the barrel, and it is so contrived as to let out only one charge of condensed air at each pull of the trigger ; the bullet is discharged with a force sufficient to kill an animal at the distance of sixty or seventy yards. The copper ball con- tains about ten charges. There are generally two of these to each gun, and that which is not immediately in use may be carried in the pocket. In the next lecture we shall treat of the atmo- spherical phenomena. LECTURE VII. EXPERIMENTAL PHILOSOPHY. THE PHENOMENA OF THE ATMOSPHERE. THE word phenomenon, the plural of which stands at the head of this lecture, and which we shall frequently have occasion to use, means simply an appearance. It is derived from the Greek verb PHAINOMAT, which signifies to ap- pear; but it is generally used to imply any striking or remarkable appearance. The atmo- sphere was before explained t'o mean that mass of air which surrounds the earth. Various con- iectures have been made with respect to the neigm 01 me CILUIOO^U.^ . nn( ^ as we know to a certainty the relative weight of a column of ti*e atmosphere by the height to which its pressure will raise water or mercury in an empty tube, so different calculations have been founded on these data, to ascertain its extent as well as its density at different heights. If the air of our atmosphere were indeed every where of an uniform density, the problem would be very easily solved. We should, in that case, have nothing more to do, than to find out the proportion between the height of a short pillar of air, and a small pillar of water of equal weight ; and having compared The Phenomena of the Atmosphere. 79 the proportion the heights of these bear to each other in the small, the same proportion will be certain to hold in the great, between a pillar of water thirty-two feet high, and a pillar of air that reaches to the top of the atmosphere, the height of which we wish to know. Thus, for instance, we find a certain weight of water reaches one inch high, and a similar weight of air reaches seventy-two feet high : this then is the proportion two such pillars bear to each other in the small. Now, if one inch of water is equal to seventy-two feet of air, to how much air will thirty-two feet of water be equal ? By the common rule of proportion we readily find, that thirty-two feet, or three hundred and eighty-four inches of water, will be equal to three hundred and thirty-one thousand seven hundred and seventy-six inches, which makes something more than five miles, which would be the height of the atmosphere, were it homogeneous, or its density every where the same as at the earth's surface, where seventy-two feet of air were equal to one inch of water. But this is not really the case ; for the air's density is not every where the same, but de- creases as the pressure upon it decreases; so that the air becomes lighter and lighter the higher we ascend ; and in the upper regions of the atmosphere, where the pressure is scarcely any thing at all, the air, dilating in proportion, must be expanded to a surprising degree; and 80 Experimental Philosophy. [Lecture 7. therefore the height of the atmosphere must be much greater than has appeared by the last cal- culation, in which its density was supposed to be every where as great as at the surface of the earth. In order, therefore, to determine the height of the atmosphere more exactly, geometri- cians have endeavoured to determine the density of the air at different distances from the earth. The following sketch will give an idea of the method which some have taken to determine this density. If we suppose a pillar of air to reach from the top of the atmosphere down to the earth's sur- face ; and imagine it marked like a standard by inches, from the top to the bottom ; and still further suppose, that each inch of air, if not at all compressed, would weigh one grain. The topmost inch, then, weighs one grain, as it suffers no compressure whatsoever ; the second inch is pressed by the topmost with a weight of one grain, and this added to its own natural weight or density of one grain, now makes its density, which is equivalent to the pressure, two grains. The third inch is pressed down by the weight of the two inches above it, whose weights united make three grains ; and these added to its natu- ral weight, give it a density of four grains. The fourth inch is pressed by the united weight of the three above it, which together make seven grains ; and this added to its natural weight gives it a density of eight grains. The fifth inch, being The Phenomena of the Atmosphere. 81 pressed by all the former fifteen, and its own weight added, gives it a density of sixteen grains; and so on, descending downwards to the bottom. The first inch has a density of one, the second inch a density of two, the third inch a density of four, the fourth inch of eight, the fifth of sixteen, and so on. Thus the inches of air increase in density as they descend from the top, at the rate of one, two, four, eight, sixteen, thirty- two, sixty- four, &c. which is called a geometrical progres- sion. Or if we reverse this, and begin at the bottom, we may say, that the density of each of these inches becomes less upwards in a geometri- cal progression. If, instead of inches, we sup- pose the parts -into which this pillar of air is divided to be extremely small, like those of air, the rule will hold equally good in both. So that we may generally assert, that the density of the air, from the surface of the earth, decreases in a geometrical proportion. This being understood, should we now desire to know the density of the air at any. certain height, we have only first to find out how much the density of the air is diminished to a cer-. tain standard height, and thence proceed to tell how much it will be diminished at the greatest heights that can be imagined. At small heights the diminution of its density is by fractional or broken numbers. We will suppose at once that at the height of five miles, or a Dutch league, the air is twice less dense than at the surface of. 8 Experimental Philosophy. [Lecture 7. the earth : at two leagues high, it must be four times thinner and less dense, and at three leagues eight times thinner and lighter, and so on. In- stead of Dutch leagues, suppose we took a Ger- man league of seven miles, and that it was four times less dense at the height of the first German league, then it would decrease in the same pro- portion, and be four times less dense than the first at the second league, that is, sixteen times; and four times less dense than the second at the ,third league, that is, sixty-four times ; and four times less dense than the third at the fourth league, that is, two hundred and fifty-six times less dense than at the surface. In short, what- ever decrease it received in the first step, it will continue to have the same proportion in the second, third, and so on, and this, as was ob- served, is called geometrical progression. Upon the same principle it was attempted to calculate the height of the atmosphere. By carry- ing a barometer to the top of a high mountain, the density of the air at two or three different stations was easily ascertained. But, alas ! so feeble are human efforts in endeavouring to com- prehend and measure the works of the Creator, that this theory was soon demolished. It was found that the barometrical observations by no means corresponded with the density which, by other experiments, the air ought to have had ; and it was therefore suspected that the upper parts of the atmosphere were not subject to the The Phenomena of the Atmosphere. 83 same laws or the same proportions as those which were nearer the surface of the earth; or that, changes of temperature might operate with other causes to change the law. Another ingenious method was subsequently devised. Astronomers know, to the greatest exactness, the place of the heavens in which the sun is at any one moment of time : they know, for instance, the moment in which it will set, and also the pre- cise time in which it is about to rise. However, upon awaiting his appearance any morning, they always see the light of the sun before its body, and the sun itself appears some minutes sooner above the mountain top, than it ought to do from this cal- culation. Twilight is seen long before the sun ap- pears, and that at a time when it is eighteen degrees lower than the apparent horizon, or verge of the sky. There is then, in this case, something which deceives our sight ; for we cannot suppose the sun to be so irregular in his motions as to vary every morning : for this would disturb the regularity of nature. The deception actually exists in the at- mosphere. By looking through this dense, trans- parent substance, every celestial object that lies beyond it is seemingly raised up, n a way similar to the appearance of a piece of money in a bason filled with water. Hence it is plain, that if the atmosphere were away, the sun's light would not be brought to view so long in the morning before the sun itself actually appears. The sun, without the atmosphere, would appear one entire blaze of light the instant it rose, and leave us in total 84 Experimental Philosophy. [Lecture 7. darkness the moment of its setting. The length of the twilight, therefore, at a given time, is in proportion to the height of the atmosphere : or let us invert this, and say, that the height of the atmosphere is in some proportion to the length of the twilight. This consideration led to an inves- tigation (to which we shall recur when we treat of astronomy) from which it has been inferred that at the height of 45 miles, the atmosphere has sufficient density to bend the rays of light. At greater altitudes, the density is not sufficient to occasion any perceptible effects. The density of the air, however, depends not merely on the pressure it sustains, but on other circumstances ; so that it varies even at the same height in different parts, and in the same place at different times, as is seen by the mercury in the barometer rising to different heights, according to the state of the weather. Heat in particular was mentioned as a very powerful cause in rarefy- ing the air. From this circumstance arises one of the most striking and formidable of the atmo- spherical phsenomena the WIND. Wind is no- thing but a strong current or stream of air. Whenever the air is heated by the sun, or by any other means, it will be rarefied, and less able to resist the pressure of the adjacent air, which will consequently rush in "to restore the equilibrium," to speak in the technical language of philosophy, or, in plain terms, to reduce the rarefied part to an uniform density with the other. This current of air is sensibly felt near the door of a glass-house, The Phenomena of the Atmosphere. 85 or wherever there is a large fire. A current of air is also to be perceived rushing through the key-hole, or any chink or crevice, into a heated room. This may serve to give a general idea of the causes of winds. This principle we consequently find realised on a great scale, in what are called the trade winds, which blow constantly from east to west near the equator. When the sun shines intensely upon any part of the earth, it is plain that, by the immense accession of heat, the air must be greatly rarefied. The cold air will therefore rush from the adjacent parts to that where there is little resistance, and consequently cause a stream or current of air, in other words, a wind, towards that quarter. The sun rises in the east, and sets in the west, consequently the air will be heated gradually from east to west, and the wind will blow in that direction. Near the equator, there- fore, where the surface of the earth is heated in succession from east to west, there will be a constant wind from the east, but on the north side of the line it will incline a little to the north, and on the south side a little to the south, for an obvious reason, because it is colder towards each pole, and therefore the mass of cool air will be principally drawn from these quarters. The same cause will explain, in a popular way, the land and sea breezes in the tropical climates. In islands, and small tracts of land which run into the sea in those regions, it will generally be found 86 Experimental Philosophy. [Lecture 7. that, during the day, there is a current of air to- wards the sea, and at evening the current sets in from the sea to the land. The reason of this is, that water is always of a more even temperature, that is, of a more equal heat, than land. During the day, therefore, the land becomes considerably heated, and the air is rarefied ; the consequence is, that in the afternoon a breeze sets in from the sea, which is less heated. On the contrary, dur- ing the course of the night the land loses its heat, while that of the sea continues more nearly the same. Towards morning, therefore, a breeze re- gularly proceeds from the land towards the ocean, where the air is warmer, and consequently more rarefied, than on shore. The monsoons are periodical winds which blow between the tropics, and which, though the theory of them is rather more complicated, originate in the same cause. They depend, in- deed, upon large tracts of territory being heated during the warm season, by which the general course of the trade winds is partially interrupted. Thus, when the sun approaches the tropic of Cancer, the soil of Persia, Bengal, China, and the ad joining countries, is so much more heated than the sea towards the southward of these countries, that instead of the usual trade wind, the current of air proceeds at that season from the south to the north, contrary to what it would if no land was there. But as the high moun- tains in Africa, during all the year, are extremely The Phenomena of the Atmosphere. 87 cold, the low countries in India to the east- ward of it become hotter than Africa during the summer, and the air is naturally drawn thence to the eastward. From the same cause the trade wind in the Indian ocean blows, from April to October, in a north-east direction, contrary to the general course of the trade wind in the open sea in the same latitude ; but when the sun re- tires behind the tropic of Capricorn, these north- ern parts become cooler, and the general trade wind assumes its natural direction. In the north- ern tropic the monsoons depend upon similar causes. In our climate the winds are more variable, because the rarefactions which take place in the air are here more partial, more frequent and sud- den, than in the tropical regions. I have suf- ficiently explained, that whatever dilates or rare- fies the air in any part must produce a wind or current of air towards that part. Among the most pewerful causes of winds, therefore, we must account the electricity of the atmosphere, which (as will be explained hereafter) is the cause of thunder and lightning. A thunder storm, therefore, is commonly either preceded or followed by a smart gale of wind. The rays of the sun are also sometimes partially interrupted by clouds or mists in particular places, conse- quently the earth will be more strongly heated in one part than another, in which case there will always be a current of air from the colder to the 88 Experimental Philosophy. [Lecture 7. warmer region. The fall of rain too, and many other circumstances, may produce an alteration in the temperature, which will be followed by a change in the wind. The velocity of the wind has been frequently measured with great accuracy, and varies under different circumstances. It has been said of swift horses, such as Childers and Eclipse, that they outstripped the wind, and so they did at its mean rate. But we ourselves can even go faster than the wind in some states ; for in calm weather, when its motion is just perceptible, its velocity is not more than one or two miles in an hour, and even a brisk wind Joes not travel at the rate of more than 15 or 20 miles an hour. Childers, on the contrary, is known (o have run at the rate of nearly one mile in a minute, that is at least 50 in the hour, which is equal to the ve- locity of a storm. The storms which we experience in these happy climates are as nothing when compared with those dreadful convulsions of nature which are occa- sionally felt in warmer latitudes, where the fruits of a whole year's labour are often destroyed by a single hurricane. These terrible phenomena happen in the West Indies, generally in the rainy season, about the month of August. They are always preceded by an unusual calm ; but the storm comes on suddenly, commonly accom- panied with rain, thunder, and lightning, and sometimes with an earthquake. Whole towns The Phenomena of the Atmosphere. 89 are made a heap of ruins by one of these hurri- canes ; fields of sugar-canes are whirled through the air ; the strongest trees are torn up by the roots and tossed like stubble ; nor can any build- ing be constructed strong enough to afford a shelter from the beating of the storm, and the deluge of wet with which it is accompanied. The island of Jamaica was visited in the year 1780 by this fatal calamity, and the damage which ensued is not to be calculated. The hur- ricanes in the West Indies have been attributed, with great probability, to some occasional ob- struction in the usual and natural progress of the equatorial trade winds. The harmattan is a wind which prevails oc- casionally during the months of December, January, and February, in the interior parts of Africa, and always blows towards the Atlantic ocean. There are generally three or four returns of it every season; it blows with a moderate force, not quite so strong, indeed, as the sea breeze. A fog or haze always accompanies the harmattan, so that the sun is concealed the greater part of the day, and the largest building cannot be seen at a quarter of a mile distance. The particles which constitute this fog are deposited on the leaves of trees, and on the skins of the negroes, making them appear white. But the most extraordinary property of this wind is its extreme dryness. No dew falls during its con- tinuance (on the average about a week), and the 90 Experimental PhilosopJiy. [Lecture 7. grass is parched up like hay. Household furni- ture is cracked and destroyed, the pannels of wainscots split, the joints of a well-laid floor of seasoned wood will be opened so as to admit the breadth of a finger between them, and the covers of books, though shut up in a close chest, are bent as if they had been exposed to the fire. Nor does the human body escape ; the eyes, nostrils, lips, and palate are parched up, and made very uneasy. Though the air is cool, there is a prick- ling heat all over the skin ; and if the harmattan continues four or five days, the scarf skin peels off*. . This wind, though fatal to vegetable life, is said to be conducive to the health of the human body. It stops -all epidemics; indeed no infec- tion can be communicated, even by inoculation, during its continuance. It relieves patients la- bouring under fevers, and is remarkable for the cure of ulcers and cutaneous diseases. The sirocco is as deleterious as the harmattan is salubrious. It is common in Italy and the south of France. In the former it is called the sirocco, from a common opinion that it blows from Syria ; in the latter it is called the Levant wind. The medium heat of the atmosphere while it it blows, is one hundred and twelve degrees. It is fatal to vegetables, and often destructive to the human species. It depresses the spirits in an un- usual degree ; it suspends the power of digestion, so that those who eat a heavy supper, while it continues, are often found dead in their beds in The Phenomena of the Atmosphere. 91 the morning. The sick, at that afflicting period, commonly sink under the pressure of their dis- eases ; and it is customary in the morning, when this wind has blown a whole night, to inquire who is dead. The sarnie^ or mortifying wind of the deserts near Bagdat, is also dreadful in its effects. At its approach the camels instinctively bury their noses in the sand, and travellers throw themselves as close as possible to the ground till it has passed by, which is commonly in a few minutes. As soon as those who have life dare to rise up, they examine how it fares with their companions, by plucking their arms and legs ; for if they are struck by the wind they are often so mortified that their limbs will come asunder. The fatal effects of this wind must depend upon a quantity of putrid vapour with which it is charged, pro- bably from passing over stagnant lakes, or marshes loaden with putrid matter. Whirlwinds, which are so sportive in their appearance in this country, carrying up straws and other light bodies a considerable height in the air, have been known in the tropical countries to produce most tremendous effects. It is probably a description of them which is known there by the name of t^wiados ; these carry up with them the whole materials of a cottage, or even large trees, with the same velocity as our whirlwinds do straws and the lightest bodies. A whirlwind at land is a water-spout at sea ; at 92 Experimental Philosophy. [Lecture 7. least, botli seem to proceed from the same cause. Wherever the air is suddenly rarefied in a par- ticular spot, from electricity or any other cause,* a kind of vacuum is created, and the circum- ambient air rushing at once from every quarter, a conflict of winds takes place, and the circular motion, already noticed, ensues. It is to be ob- served that, in water-spouts at sea, the water ascends, and does not descend (according to the vulgar notion) from the cloud, which is formed at the extremity of the spout. The water in this case rises, where the vacuum is created by the whirlwind, by the pressure of the atmo- sphere, as in a common pump. Only the vacuum not being quite perfect, it rises in small drops, and forms the cloud at the upper extremity of the phenomenon. An artificial water-spout may be made in a very easy way. In a stiff paper or card make a hole just wide enough to insert a goose quill, then cut the quill off square at both ends ; place the card at the top of a wine glass or tumbler filled with water to within about a quarter of an inch of the lower orifice of the quill. Then apply the mouth to the upper part of the quill, and draw out the air. The water in the glass will then be seen raised in the form of an in- verted cone like a water-spout, and not in a con- tinued stream, but broken into drops, and mingled with particles of air. It is by the agency of the air that water is raised in vapour from the earth to form clouds. The Phenomena of the Atmosphere. 93 You need not be told,. I. presume, that clouds are water in a suspended state, and so is the common smoke which ascends from our chim- neys, the columns of which, in fact, are so many clouds. Vapour is water expanded by heat or fire to the state of an elastic fluid, and it rises in the atmosphere*, because va- pour is lighter or less dense than our common air (it is, in fact, fourteen hundred times lighter than the water of which it is composed, whereas common air is only about nine hundred times lighter than water) ; and it is a rule in philosophy, depending on the principle of gravitation, that when two fluids of different densities are brought together, the lighter will always rise to the sur- face. It is, however, only near the surface of the earth that the air is denser and more heavy * There is a constant process of evaporation going on from all bodies on the surface of the earth which contain moisture. In a dry atmosphere the evaporation from the human body is very considerable, but the heat which that carries ofi" is continually recruited by the vital principle, which is wonderfully adapted to resist, to a certain extent, the eflecti both of a hot and a cold medium, keeping the blood in either, very nearly at the same temperature. When, however, this principle is roused by exercise, and a warm and moist air, or a spasm on the skin obstructs the free passage of the perspirable matter, the blopd becomes over-heated, and we feel oppressed. On the other hand, exposure to a keen dry wind, without sufficient exercise, endangers delicate persons, from the too great cooling of the blood. 94* Experimental Philosophy. [Lecture 7. than water. The vapours, therefore, can only rise to a limited height; and it is generally agreed that there are no clouds at the height of four or five miles in the atmosphere : their usual height, indeed, seldom exceeds a mile, nor very often half a mile. Vapour, by coming in contact with a cold body, can be deprived of its heat, and is suddenly condensed into water again, as in the refrigeratory of a still, where the vapour, confined in a spiral tube, is made to pass through cold water, and is condensed, as in the steam engine, which was noticed in a former lecture. If, therefore, "the vapours in the atmosphere, by ascending into the colder regions of the air, by electricity, or by meeting with cold winds, are deprived of the heat which keeps them in the vaporific state, they will of course be con- densed to clouds, and will fall down in the form of ram. Perhaps the attraction of the earth, when they approach it, may, in many cases, serve to draw off the superfluous heat, or electricity, and condense the vapours; which may account for its generally raining on the tops of mountains, and for the changes of the weather predicted by the barometer. For when the air is so far rarefied as not to be able to support the column of mer- cury to a certain height in the tube of the ba- rometer, it is generally regarded as a sure pro- gnostic of rain. The air in the higher regions being sometimes The Phenomena of the Atmosphere. 95 intensely cold, the vapours immediately after condensation are frozen, and the frozen particles in their slow descent unite at a determinate angle, forming the beautiful feathery flakes of snow, each of which is, in fact, a very compli- cated group of little crystals. Hail is sometimes an entire drop frozen in its descent through a colder region, or by means of a rapid evaporation, in which case it is a transparent globule; but much more frequently a common snow flake rolled up in a manner by whirling between two cur- rents forming an opake nucleus, which by its ex- treme coldness encrusts itself with clear ice out of the vapours it meets with in falling. These rolled snow flakes often fall unencrusted before a severe frost. Angular hailstones are the frag- ments of larger spheres which have broken in their fall, probably by the expansion of air en- veloped in the spongy nucleus. The dew, which falls in a summer evening, is part of the vapour which is raised in the course of the day by the sun's heat ; but not being com- pletely dissolved or dispersed in the atmosphere, it is condensed, and falls with the evening's cold. In cool nights the dew often becomes frozen in the form of hoarfrost. The atmospherical phenomena will be further explained when we treat of electricity. LECTURE VIII. EXPERIMENTAL PHILOSOPHY. ELECTRICITr. IP the electrical fluid is not caloric, or the matter of fire, it resembles that element in so many of its phsenomena and effects, that there is reason to believe it a combination of it with some other substance. But of the nature of that combina- tion we are at present ignorant. To mortify the pride of man, philosophy leaves some things unexplained : the really ignorant are those who think they can penetrate into every secret of nature ; whereas the truly wise will see that there is much placed out of the reach of human com- prehension, and many things yet left to be disco- vered by the industry and the patience of man. The electric matter resembles caloric or fire in its most usual effects, the power of igniting or setting on fire inflammable bodies; in melting metals; in the emission of light; and in the velocity of the electric spark. Friction, which is known to produce heat and fire, is also the most powerful means of exciting electricity; heat also extends itself most rapidly in humid bodies and metals, and these are the best con- ductors of electricity ; and as caloric is the most Electricity. 97 elastic of all fluids, and perhaps the great cause of repulsion, so the electrical repulsion may, perhaps, be referred to the same principle. On the contrary, there are some facts which seem to prove that the electric matter is some- what different in its nature from caloric. The electric matter affects the organs of scent; its progress may also be arrested by certain sub- stances which, on that account, are called non- conductors; glass, in particular, which admits the passage of both heat and light, stops the course of the electric matter: on the contrary, the electric fluid will adhere most tenaciously to some other bodies, without diffusing itself even to those which are in contact with them : thus an electric spark has been drawn by a wire- through the water of the river Thames, and has set fire to spirit of wine on the opposite side. The principal phenomena of electricity are first, The electrical attraction and repulsion. Secondly, The electrical fire rendered visible: and, thirdly, The power which certain substances possess of conducting the electrical matter; whence arises the distinction between con- ductors and non-conductors, or non-electric and electric bodies. The electric are those which are capable of being excited, such as glass, amber, &c, but do not conduct; the non-electrics are such as conduct the electric matter, but cannot be excited to produce it, such as metals, stones, and all fluids. VOL i. r 98 Experimental Plnlosopliy. [Lecture 8. These phaenomena were not, however, all dis- covered at once ; on the contrary, it was by slow degrees that philosophy became acquainted with the properties of this surprising fluid. It was, however, long known that amber* and some other matters, when rubbed on a soft and elastic substance, had a power of attracting feathers, straws, or other light bodies. We may, without either pains or cost, make the experiment: by taking a piece of sealing-w r ax, and rubbing it quickly upon a coat sleeve, or any piece of woollen cloth, we shall find that it will readily attract hair, feathers, chaff, &c. A smooth bubble of glass will answer still better. Sulphur is also a body that is capable of exercising this power of attraction ; and to observe more perfectly its effects, Otto Guericke, burgo- master of Magdebourg (the same who is men- tioned in a preceding lecture, as having afforded hints for the construction of the air-pump), made a large globe of sulphur, which he fixed in a wooden frame, and, by whirling it about rapidly, and rubbing it at the same time with his hand, he was enabled to perform several experi- ments. This may be regarded as the first elec- trifying machine. He observed that a body which was attracted by his globe was afterwards repelled by it, but that if it touched another body, it became after that capable of being attracted again. Thus he was able to keep a feather sus- * Amber, electron in Greek, whence the name electricity. Electricity? 99 pended over his globe ; but if he drove it near a linen thread, or the flame of a candle, it in- stantly recovered its propensity to approach the globe again. This fact is now explained; the feather, by being attracted by the globe, and especially when in contact with it, becomes charged, or loaded with the electric matter; when it touches or comes very near a body which is not charged with electricity, it parts with its share to that body, and returns again to receive a fresh supply, if " within the sphere of attrac- tion," that is, within those limits whither the attractive powers of the globe extend. This philosopher was enabled to remark the hissing noise which a stream of the electric mat- ter produces, and he had a glimpse of the elec- tric light ; but Dr. Wall, an English philosopher, observed it more clearly. By rubbing amber upon a woollen cloth in the dark, he found that light was produced, attended by a hissing or rather a crackling noise. Mr. Hawksbee, another of our countrymen, observed the same thing of glass ; and he constructed a kind of machine, which enabled him to put a glass cylinder in motion. Thus the electric attraction and the electric light were proved by experiment; but it was reserved for Mr. Grey, a pensioner of the Char- ter-house, to make the distinction between those bodies which are capable of being excited to electricity, and those which are only capable of receiving it from others. After attempting in 100 Experimental Philosophy. [Lecture 8. vain to give the power of attraction to metals, by rubbing, hammering, and heating, he conceived a suspicion, that as a glass tube, when rubbed in the dark, communicated its light to other bodies, it might possibly be made to communicate also its power of attraction. He provided himself, therefore, with a glass tube three feet five inches long, and near an inch and one-fifth in diameter. The ends of the tube were stopped with cork, . and he found that when the tube was excited by friction, a feather was attracted as powerfully by the cork as by the tube itself. To convince him- self more fully, he procured a small ivory ball, which he fixed to a stick of deal four inches long, and thrust into the cork ; and he found that it attracted and repelled the feather even with more vigour than the cork itself. He afterwards fixed the ball to a longer stick, and even to a piece of wire, with the same success. Lastly, he attached it to a piece of packthread, and hung it from a high balcony, where he found that, by rubbing the tube, he enabled the ball to attract light bodies in the court below. His next attempt was to examine whether this power acted as well horizontally as perpendicu- larly. With this view he made a loop of cord, which he hung to a nail in one of the beams of the ceiling, and ran his packthread, which had the ivory ball at the end, through the loop ; but in this state he found, to his utter mortification, that his ball had totally lost the power of attraction. On Electricity. 101 mentioning his disappointment to a friend, it was suggested, that the cord which he employed for the loop, through which the pac-kthf^cl rah, might be so coarse as to intercept the electric power. To remedy this, they made , the hWp of silk, which they considered as stronger, in pro- portion to its thickness, than the former. . With this apparatus they succeeded beyond expectation. As they attributed their success entirely to the fineness of the silk of which the loop was made, they thought they would perform still better by supporting the packthread by a very fine brass or iron wire ; but to their utter astonishment, the electric virtue was entirely lost; while, on the contrary, when the apparatus was supported by the silk loops, they were able to convey the power of attraction along a packthread of seven hundred and sixty-five feet in length. It was evident, therefore, that these effects depended upon some quality in the silk, which disabled it from conducting away the electric power, as the hempen cord and the wire had done; and, by subsequent experiments, this hypothesis was amply confirmed. This little narrative may serve to give a tole- rably competent idea of non-conducting and con- ducting bodies; and we must remember, that those bodies which do not conduct the electric fluid are most capable of exciting it, and are sup- posed to be naturally charged or loaded with a quantity of it. They have, therefore, been called 102 Experimental Philosophy. [Lecture 8. electrics ; such are amber, jet, sulphur, glass, and all precious stones ; all resinous substances ; and t)-e clvjj-1 parts of animals (except the bones), such ( as ,ha.ir, wool, silk, &c. On the contrary, stony sijbst2f hours, as there would be eclipses after the moment of conjunction. But this does not happen : for the spectator at D sees the ter- mination of the eclipse about sixteen minutes later than the calculation predicts ; so that, in all the intermediate positions between C and D, the difference as far as this limit has been con- tinually increasing. Now C D, the rectilinear distance between these two positions, is equal to the diameter of the earth's orbit, that is, to about 190 millions of English miles. This space, therefore, is passed over by light in 16 minutes; so that, assuming it to move uniformly, we find, by an easy proportion, the space passed Experimental Philosophy. [Lecture 10. over by light in a second to agree with what we have just stated. This discovery we owe to Roemer, a Danish astronomer, and it is extremely interesting and important. Such, then, is the rapidity with which these rays are darted forward, that the journey they per- form thus in less than eight minutes, a ball from the mouth of a cannon would not complete it in several weeks. But here it may be said, If the velocity of light is so very great, how is it that it does not strike against objects with a mon- strous force? If the finest sand (the objector may continue to observe) was thrown against our bodies with the hundredth part of this velocity, each grain would be as fatal as the stab of a stiletto : How then is it, that we expose, without pain, not only other parts of our bodies to the in- cursions of light, but our eyes, which are a part so exquisitely sensible of every impression ? To answer this objection, experiment will inform us, that the minuteness of the parts of light is still several degrees beyond their velocity ; and they are therefore harmless, because so very small. A ray of light is nothing more than a constant stream of minute parts still flowing from the luminary, so inconceivably little, that a candle, in a single second of time, has been said to dif- fuse several millions of particles of light. The sun furnishes them, and the stars also, without appearing in the least to consume by granting us the supply. Musk, while it diffuses its odour. Light. 133 wastes as it perfumes us; but the sun's light is diffused in a wide sphere, and seems inex- haustible. That the motion of light is inexpressibly rapid you may easily convince yourselves, by only giving attention to the firing of a cannon at a con- siderable distance, and observing the time that elapses between your seeing the flash and hearing the sound. It has been calculated from some very accurate experiments, that sound travels at the rate of one thousand one hundred and forty-two feet, or three hundred and eighty yards, in a second of time ; and if you remark, as was before observed, the time which intervenes between your seeing the flash and hearing the noise of the cannon, you will soon perceive how infinitely more rapid light must be in its motions than sound. II. It is a principle in mechanics, that the force with which all moving bodies strike is conjointly in proportion to the size of those bodies, or the quantity of matter which they contain and the velocity with which they move. Now if we con- sider the amazing velocity of light, it is evident, that if the separate particles of it were not in- finitely smaller than we can conceive, they would be destructive in the highest degree. To illus- trate this by a plain examplej: A few grains of shot, fired out of a musket or fowling-piece, will deprive a large animal, or even a man, of life. How is this? If the shot were thrown by the hand, 134 Experimental Philosophy. [Lecture 10. it would hurt neither the man nor the animal. It is the velocity, the swiftness, with which it is impelled by the force of the powder, that enables it to penetrate solid substances. Now it has been demonstrated that light moves at least two millions of times faster than a cannon-ball; and conse- quently if the particles of light were only equal in size to the two millionth part of a grain of sand, we should be no more able to withstand their force than we should that of sand shot point blank from the mouth of a cannon. How in- finitely small must these then be, when it is more than probable they are not equal to a twentieth that size, that is, not equal to theforty millionth part of a grain of sand ! What an idea does this give us of the works of our infinite Creator, and how little must we seem in our own eyes ! O Phi- losophy, how is it that thou dost not always teach mankind humility ! But we have other proofs not less decisive than this, of the extreme minuteness of the particles of light. When we observe with what facility they penetrate the hardest bodies, glass, crystal, precious stones, and even the diamond itself, through all which they find an easy passage, or those bodies could not be transparent, How ex- tremely small must these particles be ! When a candle is lighted, if there is no obstacle to ob- struct its rays, it will fill a space of two miles round with luminous particles in an instant of time, and before the least sensible part of the Ligttt. 135 substance is lost by the luminous body. If the whole space were filled with men, every eye would see the candle the moment it was posited in a visible situation. Farther, how small must the particles of light be, when they pass without re- moving the minutest particles of microscopic dust that lie in their way, and even these minute par- ticles are rendered visible, by reflecting back the particles of light that strike against them ! Small as the particles of light are, it is highly probable that, though diffused through all space, they are separated from each other by distances of much more than a thousand miles. This may be inferred as follows : It is an obvious fact, that the effect of light upon our eyes is not instantane- ous, but that the impression remains for some time. You may easily satisfy yourselves of this, by shutting your eyes after having looked for some time on a candle, a star, or any other lumi- nous body, when you will perceive that a faint picture will remain of the object for some time. The smallest division of time, that we can well conceive, will be the one hundred and fiftieth part of a second. If, therefore, one lucid part of the sun's surface emits one hundred and fifty par- ticles of light in a second of time, we may con- ceive that these will be amply sufficient to afford light to the eye without any intermission. You will remember, then, that light travels at the rate of about one hundred and seventy thousand miles in a second ; so that, the sun emitting one hun- 136 Experimental Philosophy. [Lecture 10. dred and fifty particles in that space of time, each particle must be more than one thousand miles distant from the other*. Indeed it is reasonable to suppose that they must be at great distances asunder, or they could not pass so continually as they do in all directions, without interfering with each other. If, in fact, light were not thus thinly diffused it must be extremely injurious to our organs, since we find that when it is condensed or com- pressed, as in the focus of a burning-glass, there is no substance that can withstand its force. Gold, when exposed to its influence, is instantly melted, and even the diamond itself, which re- sists a very intense chemical heat, is suddenly dissolved. To show, however, still more de- cisively, that the particles of light are naturally in this extremely rare or diffused state, or, in other words, follow each other at an immense dis- tance, it is a well-known fact, that the rays of light, even when collected in the focus of the strongest burning-glass, will not inflame spirit of wine, or any other combustible matter, while they merely pass through it. To make you com- * This is, in truth, quite an extreme estimate. It ap- pears from the accurate experiments of M. D'Arcy (Mem. Acad. Par. 1?65), that the impression of light upon the retina continues two minutes and forty seconds : and as a particle of light would move thirty-two millions of miles during that interval, constant vision would be maintained by a succession of luminous panicles, thirty-two millions of miles distant from each other. Light. 137 prebend this fact more clearly, I must observe, that whatever light passes through is called a me- diiim, and those substances which do not reflect the rays, but which may be seen through, are called transparent ; those, on the contrary, which intercept or reflect the rays, are called opaque. Now a phial in which spirit of wine is contained is a transparent medium, and in that state the spirit will not be set on fire : if, on the other hand, the spirit is poured forth into a spoon, or any opaque vessel, which, in fact, intercepts the rays of light, stops them in their progress, and thus collects them in a mass, it will immediately be inflamed. This, I think, proves, that the par- ticles of light must follow each other at a great distance, and that they must be in the first place compressed together by the force of the burning- glass, and then stopped and condensed by an opaque body, to enable them to produce a consi- derable degree of heat. That light may be exceedingly diluted, as well as condensed, we may easily perceive ; for the light of the glow-worm, of rotten wood, and of what are called the solar pJwsphori, can never be condensed by any burning-glass, so as to pro- duce the slightest degree of heat. The expe- riment has also been made with the light of the moon, and that has been found too faint and rare to be condensed into a burning focus. The principle upon which the rays of light are collected in the focus of a burning-glass NY ill be 138 Experimental Philosophy. [Lecture 10. explained hereafter, when we treat of lenses, and of mirrors. But I do not wish to pass over any thing that I mention, without an attempt to render it clear to your comprehension. I men- tioned the solar phosphor i , of which it is pro- bable that very few of you have heard before. They are certain substances which, when ex- posed for a little time to the strong rays of the sun, are found to imbibe a large quantity of light, so that they will shine, or appear luminous, if immediately carried into a dark place. The most remarkable of these is the Bolognian phos- phorus. It was accidentally discovered by a shoemaker of Bologna. This man had collected together some stones of a shining appearance at the bottom of Mount Peterus, and being in quest of some chemical secret (probably the philosopher's stone, which was to turn every thing into gold), he put them into a crucible to calcine them, or reduce them to the state of a cinder. Having taken them out of the crucible, they were exposed to the light while he was examining them, and afterwards he happened to carry them into a dark place, probably to throw them away; when, to his utter surprise, he observed that they possessed a self-illuminating power. Baldwin, of Misnia, another chemist, observed some time after, that chalk, dissolved in aqua fortis (after the aqua fortis had been evaporated by heat, and the matter reduced to a perfectly dry state), exactly resembled the Bo- Light. 139 lognian stone in its property of imbibing light, and emitting it after it was brought into the dark, whence it has been termed Baldwin's phosphorus. In truth, .the same effect may be produced from calcined oyster-shells, and from all the varieties of that mineral called ponderous spar, of which the Bolognian phosphorus is a species. Diamonds ateo, and some emeralds, and other precious stones, will emit light when carried out of^a light into a dark place. The light emitted by these phosphor! always bears an analogy to that which they have imbibed. In general it is reddish ; but when a weak light only has been admitted to them, or when it has been received through white paper, the light which they give out is pale or whitish. III. Notwithstanding the rarity of light, how- ever, and the smallness of its particles, it is not destitute of force or momentum. To prove this, a most ingenious experiment was made by the late Mr. Mitchell. He constructed a small vane in the form of a common weathercock, of a very thin plate of copper, about an inch square, and attached to one of the finest harpsichord wires, about ten inches long, and nicely balanced at the other end of the wire by a grain of very small shot. The vane was supported in the manner of the needle in the common mariner's compass, so that it could turn with the greatest ease; and to prevent its being affected by the vibrations of the air, it was enclosed in a glass 140 Experimental Philosophy. [Lecture 10. case, or box. The rays of the sun were thrown upon the broad part of the vane, or copper plate, by a burning-glass of two feet diameter, in con- sequence of which it was observed to move re- gularly at the rate of about one inch in a second of time. Upon this experiment a very curious calculation is founded. The instrument or vane weighed about ten grains, and the velocity with which it moved was at the rate of one inch in a second. The quantity of matter therefore con- tained in the rays of light which struck against the vane in that time amounted to about the twelve hundred millionth part of a grain: the velocity of light exceeding the velocity of the instrument in about that proportion. The light in this experiment was collected from a surface of about three square feet, and as it was from a concave mirror *, only half the quantity was re- flected. The quantity of light therefore incident upon a square foot and half of surface is no more than one twelve hundred millionth part of a grain. But the density of the rays of light at the surface of the sun is greater than at the earth, in the proportion of forty-five thousand to one. From one square foot of the sun's sur- face, therefore, there ought to issue, in the space of one second, one forty thousandth part of a grain of light to supply the consumption. More than two grains a day therefore is, according to * Mirrors or looking-glasses reflect about half the light that fulls on them perpendicularly. Light. 141 this hypothetical computation, expended from the sun's surface, or six hundred and seventy pounds in six thousand years, which would have shortened his diameter about ten feet, if it were formed of matter of the density of water only. From all this you will conclude that I have adopted the common theory, that the sun is the great source of light ; and if his diameter is rightly calculated (of which there can be no doubt) at eight hundred and seventy-eight thousand eight hundred and eight miles, we see there is no ground for any apprehensions that the sun will speedily be exhausted by the waste or consump tion of light. The matter will not be widely dif ferent, if we imagine, as is now generally believed, that the particles of light are emitted from a luminous atmosphere which surrounds the body of the sun. IV. Another principle to which I proposed to call your attention is, that light always moves in straight lines. This is evident from an experi- ment which any person may easily make, viz. that of looking through a bent tube, when no light whatever will be apparent. As a further proof it is only necessary to mention, that when light is intercepted by any intervening body, the shadow is bounded by straight lines. It is generally supposed, according to this principle, that those bodies only are transparent whose pores are such as to permit the rays of light to pervade them in a rectilinear direction ; Experimental Philosophy. [Lecture 10. and they act like a straight tube, which allows them a free passage ; and those bodies are opake whose pores are not straight, and which there- fore intercept the rays, like ,the bent tube already mentioned. If the rays of light proceed in straight lines, it is obvious that they must be sent from every visible object in all directions. It is however only by those rays which enter the pupil of our eye that they are rendered visible to us ; but, being sent in all directions, it is evident that some rays from every part must reach the eye- Thus the object ABC (pi. XI. fig. 46) is rendered visible to an eye in any part, where the rays Aa, Ab, Ac, Ad, Ae, Ba, Bb, Be, Bd, Be, Ca, Cb, Cc, Cd, Ce, can come ; and these affect our sight with the sense of different colours and shades, according to the properties of the body from which the light is reflected, as will be ex- plained when we come to treat of colours. Of the refraction and refaction of light I shall hereafter treat more at large ; but, ip the mean time, it will greatly facilitate the study of optics, if you will carefully peruse, and still more if you will commit to memory, the following principles and definitions. 1. Light is a substance, the particles of which are extremely minute, which, by striking on our visual organs, gives us the sensation of seeing. 2. The particles of light are emitted from what are called luminous bodies, such as the sun, a Light. 143 fire, a torch, or candle, &c. &c.: they are re- flected or sent back by what are termed opdke bodies, or those which have no power of affording light in themselves. 3. Light, whether emitted or reflected, always moves in straight or direct lines ; as may easily be proved by looking into a bent tube, which evidently obstructs the progress of the light in direct lines ; and proves that the theory of recti- linear emission is free from the objections which lie against the hypothesis of the undulatory mo- tion of light. 4. By a ray of light is usually meant the least particle of light that can be either intercepted or separated from the rest. A beam of light is ge- nerally used to express something of an aggregate or mass of light greater than a single ray. 5. Parallel rays are such as proceed equally distant from each other through their whole course. The distance of the sun from the earth is so immense, that rays proceeding from the body of that luminary are generally regarded as parallel. 6. Converging rays are such as, proceeding from any body, approach nearer and nearer to each other, and tend to unite in a point. The form of rays thus tending to a union in a single point has been compared to that of a candle-ex- tinguisher ; it is in fact a perfect cone. 7. Diverging rays are those which, proceed- 144 Experimental Philosophy. [Lecture 10. ing from a point, continue to recede from each other, and exhibit the form of an inverted cone. 8. A small object, or a small single point of an object, from which rays of light diverge or indeed proceed in any direction, is sometimes called the radiant, or radiant point. 9. Any parcel of rays, diverging from a point, considered as separate from the rest, is called a pencil of rays. 10. r Fhe focus of rays is that point to which converging rays tend, and in which they unite and intersect or cross each other. It may be considered as the apex or point of the cone ; and it is called the focus (or fire-place), because it is the point at which burning-glasses burn most intensely. 11. The virtual or imaginary focus is that supposed point behind a mirror or looking-glass, where the rays would have naturally united, had they not been intercepted by the mirror. 12. Plane mirrors or speculum? are those re- flecting bodies, the surfaces of which are per- fectly plain or even, such as our common look- ing-glasses. Convex and concave mirrors are those the surfaces of which are curved. 13. An incident ray is that which comes from any body to the reflecting surface ; the reflected ray is that which is sent back or reflected. 14. The angle of incidence is the angle which is formed by the line which the incident ray glit. 145 describes in its progress, and a line drawn per- pendicularly to the reflecting surface; and the angle of reflection is the angle formed by the same perpendicular and the reflected ray. Thus, in fig. 47, AB is the reflecting surface, CG is a line drawn perpendicularly to that surface, e is a ray of light incident at G, and reflected tof; and the angle CGe of incidence is evidently equal to the angle CGjfof reflection. 15. By a medium, opticians mean any thing which is transparent, such as void space, air, water, or glass> through which consequently the rays of light either may or do pass in straight lines. 16. The refraction of the rays of light is their being bent, or attracted out of their course in passing obliquely from one medium to another of a different density, and which causes objects to appear broken or distorted when part of them is seen in a different medium. It is from this property of light that a stick, or an oar, which is partly immersed in water, appears broken. 17. A lens is a transparent body of a different density from the surrounding medium, com- monly of glass, and used by opticians to collect or disperse the rays of light. Lenses are in gene- ral either convex, that is, thicker in the middle than at the edges, which collect and, by the force of refraction,, converge the rays, and consequent- ly magnify; or concave, that is, thinner in the middle than at the edges, which by the refrac- VOL. i. H 146 Experimental Philosophy. [Lecture 10. tion disperse the rays of light, and diminish the objects that are seen through them. The va- rieties of these will be described in a subsequent lecture. 18. Vision is performed by a contrivance of this kind. The crystalline humour, which is seated in the fore part of the human eye, imme- diately behind the pupil, is a perfect convex lens. As therefore every object is rendered visible by beams or pencils of light which proceed or di- verge from every radiant point of the object, the crystalline lens collects all these divergent rays, and causes them to converge on the back part of the eye, where the retina or optic nerve is spread out ; and the points where each pencil of rays is made to converge on the retina, are exactly correspondent to the points of the object from which they proceed. As, however, from the great degree of convergence which this con- trivance will produce, the pencils of light pro- ceeding from the extreme points of the object will be made to cross each other before they reach the retina, the image on the retina is always inverted. 19. The magnitude of the image painted on the retina will, therefore, it is evident, depend on the greatness or obtuseness of the angle under which the rays proceeding from the extreme points of the object enter the eye. For it is plain, that the more open or obtuse the angle is, the greater is the tendency of these rays to meet Light. 147 in a point and cross each other: and the sooner they cross each other, after passing the crystal- line lens, the larger will be the inverted image painted on the retina. The visual angle, there- fore, is that which is made by two right lines drawn from the extreme points of any object to the eye ; and on the measure of that angle the apparent magnitude of every visible object will depend/ 20. The prism used by opticians is a piece of fine glass, in form of, a geometrical triangular prism ; it has the power of separating the rays of light. LECTURE XI. EXPERIMENTAL PHILOSOPHY. a THE REFRANGIBIL1TY OT LIGHT. THE natural progress of light, we have already seen, is in straight lines; yet it is found to be subject to the laws of attraction, as well as all other bodies; and, under the impulse of that power, it is sometimes turned out of its direct course. This only happens when it passes out of one medium into another of a different den- sity, as from air into water or glass, or from water or glass into air; and this property of light is called refraction. A very easy experi- ment will show you what is meant by refraction ; for. if you put one end of a straight stick into water, it will appear at the surface as if it were broken, that is, refracted, from the Latin verb refrangv, to break. It is evident that this effect can only arise from the rays of light being drawn or attracted out of their direct course ; and this I shall prove by a very common and a very easy experiment. Put a shilling, or any other conspicuous but small object, into a bason or other vessel, and then re- tire to such a distance, as that the edge of the vessel shall just hide it from your sight. If, then, you remain motionless while the vessel is filled with Rtfrangibility of Light. 149 water, you will find that the shilling will be ren- dered perfectly visible, though in fact neither you nor it have changed places in the slightest degree. Let it be remembered, that it is only the rays which fall obliquely that are thus refracted; for a ray which falls perpendicularly is equally attract- ed on all sides, and therefore suffers no refrac- tion at all. To illustrate this by the experiment which has just been mentioned. You must know that it is by light reflected from it to your eye that any object is rendered visible : you see the shilling in the bason, therefore, by rays of light which are reflected from its surface. Now the angle of incidence and the angle of reflection are equal ; and as you stand in an oblique direction to the shilling, you see it, while the bason is empty, by rays of light which fall upon it in a direction exactly as oblique as that in which your eye is situated towards it. The shilling, then, which before was hid from your sight, is ren- dered visible by pouring in the water, because the rays of light, which serve to render it then visi- ble, are bent out of their course. Thus the ray of light, AC, pi. XII. (fig. 48), which passes ob- liquely from the air into water at C, instead of continuing its course to B, takes the direction 4, .and consequently an object at a would be Tendered visible by rays proceeding in that direc- tion, when they would not have touched it, had .they proceeded in their direct course, 150 Experimental Philosophy. [Lecture 11. By this figure you will understand that the angle of refraction PCa is not so large as the angle of incidence pCA, but bears a certain pro- portion to it ; and this proportion or ratio varies with respect to different mediums. Thus, when a ray passes from air into water, the angle of incidence is to that of refraction in the ratio of about four to three ; from air into glass nearly as three to two; from air into diamond nearly as five to two; and the contrary proportion holds in passing back again ; as when light passes from water into air, the ratio is as three to four, &c. From all this you will clearly understand, that the more obliquely a ray falls, the greater is the refraction. It is also necessary that you should remember, that light is refracted or drawn towards the perpendicular, (as in fig. 48), when it passes out of a rare into a denser medium ; and it is re- fracted from the perpendicular, or in a more ob- lique direction, when it passes from a dense me- dium into one which is rare ; and the denser the medium, the greater is the refraction : thus the diamond is found to refract most powerfully. This principle will explain several of the com- mon phsenomena of nature. Mr. Walker ob- serves, that " many a school-boy has lost his life by supposing the bottom of a clear river to be within his depth, as (when he stands on the bank) the bottom will appear one-fourth nearer the surface than it really -is. w In this case, the Refrangibillty of Light. 151 rays proceeding out of the denser medium (the water) into the rarer (the air), they are bent out of their course more obliquely towards the eye of the spectator. Have you ever seen a skilful marksman shoot a fish in the water with a bullet? If you have, the sportsman could tell you that he took his aim considerably (perhaps a foot) below the fish as it appeared, because it seemed much nearer the top of the water than it really was. The distortion of objects through a wrinkled or crooked pane of glass, arises also from the unequal refraction of the rays that pass through it. When light passes out of pure space into air, it is also refracted ; and therefore the sun is visible, by means of the refraction of our atmo- sphere, some minutes before he rises above the horizon in the morning, and some minutes after he sets below it in the evening. It has been cal- culated that, in looking through the common glass of a window, objects appear about one-thir- tieth part of an inch out of their real place by means of the refraction. But the most excellent use to which this prin- ciple has been applied is the construction of op- tical glasses ; for, by grinding the glass thinner at the edges than in the middle, those rays of light, which would strike upon it in a straight line, or perpendicularly if it were plain, strike upon it ob- liquely, and consequently suffer a refraction, and are made to converge ; and ? on the contrary, by making the glass thinner in the middle than at 152 Experimental Philosophy. [Lecture 11. the sides, the rays are refracted the contrary way, and are made to diverge. The reason of this will be sufficiently evident, if it be recollected that all curves or segments of a circle may be conceived as formed of a number of straight lines infinitely short, and inclining to each other like the stones in the arch of a bridge, or the bricks at the top of an arched window- frame. It is evident, therefore, that in fig. 49, where parallel rays are supposed to strike a sur- face of this form, those only which enter the mid- dle part will go in a straight direction, whereas those which strike the sides will strike them ob- liquely, and will consequently be refracted. If the surface, then, be a perfect curve, as in fig. 50, it is plain that only the ray which strikes the centre point of the curve will enter it in a straight direction, and consequently all the rest which strike it obliquely will be more or less refracted, according to the degree of obliquity, and will consequently be made to converge. Glasses are usually ground for optical purposes into seven different shapes (see fig. 51). First, the glass may be flat on both sides, as the com- mon pane of a window, No. 1, Or, secondly, it may be flat on one side and convex on the other, plano-convex. No. 2. Or, thirdly, it may be convex on both sides, like our ordinary reading-glasses, No. 3. Or, fourthly, it may be flat on one side and concave on the other, plano- concave, as No. 4. Fifthly, it may be concave Refrangibility of Light. 153 on both sides, like the glasses near-sighted peo- ple generally use, as No. 5. Sixthly, it may be concave on one side and convex on the other, like the crystal of a watch, though not in such a degree, as No. 6 ; this is usually called a menis- cus. Seventhly, it may have one side, which must be convex, ground into little facets, like those of some jewels, while the other side is plain. Children know it by the name of a multiplying- glass, as Ncu 7. The effects of these different glasses will be easily understood from what has been premised. A ray entering the plain glass, No. 1, will indeed be refracted by the glass, but it will suffer another refraction on going out of it, which will nearly rectify the former; the place of the object will, therefore, as was before stated,, be a little changed, but its figure will remain unaltered. If^ again, several parallel rays enter the glass, No. 2, plain on one side and convex on the other, as in figure 50, they will be differently refracted, in proportion to the obliquity with which each of them falls upon the surface. The middle ray, for instance, which passes perpendicularly through, will not be refracted at all, but go on straight forward. All the other rays, howeyer, will suffer refraction. The ray CE., fig. 50, will be refracted upwards to F ; the ray A D will be refracted downwards to the same point. There they will cross, and then go onward, diverging or separating from each other for ever ; that which H5 154? Experimental Philosophy. [Lecture II. came from the bottom going upward, and that which came from the top downward. The figure given there is flat, but it must be supposed spherical, the glass being represented edgeways. If so, therefore, the collected bundle of rays, passing through the glass, unite and form a cone, or a figure like a candle extinguisher, the bottom of which is at the glass, and the point at F. This point, as I once before had occasion to mention, . is called the focus of the glass. From a calcula- tion in geometry, we learn that the distance from this point is always equal to the diameter of the circle which the glass would make if its convexity were continued. When the rays of the sun fall directly upon a glass DE, (see fig. 52) equally convex on both sides, they will be refracted still more abruptly, and meet sooner in a point or principal focus at f. The distance of this focus is, we are informed by the same calculation, equal to the semi-dia- meter of the circle, which the convexity of the glass continued would make. Either this glass or the former, as they collect the rays of the sun into a point, will burn at that point, since the whole force of the rays is concentrated there. The broader the glass in these instruments, the greater will be its power, from its collecting a greater number of rays. It is to be observed, that they are only parallel rays ? or those which proceed in a direct line to the surface of the glass, that are thus converged Ref i eligibility of Light. 155 to a point or focus; the rays of the sun, how- ever, come from so great a distance, that they are always regarded as parallel. Divergent rays, such as proceed from a point, as the flame of a candle, will be refracted parallel. If, therefore, we place a candle exactly at a focal distance from one or both of these glasses, as at^J its rays will, upon going through the glass, all run parallel to each other. If the candle is placed nearer the glass than its focal distance, the rays, after passing through the glass, will no longer run parallel, but separate or diverge : if it is placed farther off, the rays will then strike the glass more parallel, and will therefore, upon passing through it, con- verge or unite at some distance behind the glass. After the rays have united or converged to a focus, they will cross each other, and form an inverted picture of the flame of the candle, as may be seen on a paper placed, at the meeting of the rays behind. How the image is inverted, therefore, is easy to apprehend ; for the upper rays, after refraction, were such as came from the under part of the luminous body ; and the under rays, on the contrary, came from its top : . so that the rays are turned upside-down, and So consequently is the image. It is very pleasing to view a picture of this kind thus formed, each ray preserving the colour it had in the luminous object with the utmost imitative precision. The shadings of the little piece are far beyond the reach of art, and the design far more correct 156 Experimental Philosophy. [Lecture 11. than that of the finest painter. I mention the candle as being an obvious luminary ; but if any object whatever is placed at the proper distance from a convex glass, its picture will be, in the same manner, thrown behind, and may be re- ceived upon paper, or any other body, in all its natural proportions and colourings. The nearer the natural object is to the refracting glass, the farther off will this picture be behind it ; be- cause, as was said before, the rays which form it do not then converge or unite, but at a great focal distance. The farther off the natural ob- ject is, the nearer will be the focal distance it makes, and consequently the nearer will be the picture behind the glass ; for, wherever the focus is, there will the perfect picture be. When however the rays come from several objects at a moderate distance, they may be considered as all parallel, and this difference of focus is then imperceptible. To put what has been said in other words. As the rays of the sun may be all considered as falling parallel upon every glass of the convex kind, so they must always unite behind it in a focal point. As all the rays flowing from other objects are not always parallel, when placed too near the glass, they separate after refraction, and run off divergent ; when placed at a proper dis- tance, they unite or converge in a focal point, and there imprint a picture, if there is any thing properly placed to receive it, in which the natural Refrangibility of Light. 157 figure will be represented, its motions, its colours and shadings. The whole of the preceding theory may be illustrated by means of a common reading-glass. If a candle is held so near it, as that the rays passing through shall strike the wainscot of the chamber with a bright spot, just as large as the glass itself, the candle is then at the focal dis- tance; and rays, striking the glass divergently, are refracted through it, parallel to each other, neither spreading nor drawing together as they proceed. If the candle is held nearer than the focal distance, the rays will fall then more di- vergent upon the glass, and will consequently be refracted more divergent, so that they will form a very broad spot of light upon the wainscot. If the candle is placed at a much greater distance than the focus, the rays fall upon the glass more nearly parallel, and consequently, when they are refracted will tend to unite and converge behind the glass, and will form but a small speck of vivid light on the wainscot. This speck, if closely examined, will appear a perfect picture of the candle. Every visible point, in any body whatever, may be considered as a candle sending forth its rays, which split and pencil out into several other rays before they arrive at the eye. Each body is as if composed of an infinite number of splendid points or candles, each point with its own radi- .ance, .and diffusing itself on every side. Instead 158 Experimental Philosophy* [Lecture 11. of one body, the eye, in fact, is impressed with thousands of radiant points sent out from that body, which being grouped at the bottom of the eye, imprint the picture of the object whence they flow. Each point sends forth its own rays. It is upon this principle the camera obscura is constructed. If we take a double convex glass and adapt it so as to fit a hole in the window- shutter of a darkened chamber, so that no light shall come into the room but through the glass ; then let us place a sheet of white paper behind it at the proper distance, we shall thus have a ca- mera obscura ; for a picture of every external ob- ject will pass through the glass, and be painted upon the paper in the most beautiful colours that imagination can conceive, and all the motions of those objects also. It is necessary, in this ex- periment, that the window should not be opposite to the sun ; for then we should see no image but that of his brightness : and yet it is necessary also, that while we make the experiment, the sun should shine and illuminate the objects strongly, which are to paint themselves within. Without this strong illumination, the rays will be sent so feebly from every object, that we shall have but a faint picture, if any at all. Painters and architects often make use of a similar contrivance, or portable camera obscura, to take a draught of landscapes or buildings : their glass is fixed in a box, and by means of a mirror, on which the diminished pictures fall, they are Refrangibility of Light. 159 reflected upon oiled paper or polished glass properly placed, upon which the artist sketches his draught. With regard to the contours, or outlines, which this picture gives, nothing can be more exact ; but, so far as respects the shading and colouring, the artist can expect but little as- sistance from it : for, as the sun is every moment altering its situation, so is the landscape every moment varying its shade; and so swift is this succession of new shades, that, while the painter is copying one part of a shade, the other part is lost, and a new shade is thrown upon some other object. If such a glass, that is, double convex, is so fitted to a hole in a dark lantern, that little pictures, painted in transparent colours on pieces of glass, may be passed successively along be- tween the gloss and the candle in the lantern, we shall thus have a magic lantern. The pictures, striking the glass very divergent, will be refracted very divergent also, and will be painted upon the wall of the chamber in all their colours, as large as we please to make them ; for the farther the wall is from the glass, the more room will the rays have to diverge. As these figures would be painted on the wall reversed, if the picture were held upright, it is necessary to turn them upside down, when we would exhibit the shadows on the wall erect. The same kind of contrivance is now employed, with great success, to elucidate the principal phenomena of astronomy. 160 Experimental Philosophy. [Lecture 11. In looking through a glass of this description, that is, a convex or double convex lens, the ob- jects which we look at will appear magnified; for it is a rule in optics, that we see cvi'ry tiling in the direction of that line In which the rays ap- proach us last. When I come to treat of the eye, the reason of this will be explained. Suffice it to say for the present, that the larger the angle under which any object is seen, the larger will any object appear. The convergence of the rays of the convex lens, therefore, enlarges greatly the angle of vision, as must be evident if we continue the lines/D,/E,/T, and/G, fig. 52, in the direction to which they point, and therefore in proportion to the distance the appearance of the objects will be enlarged. The jcommon spectacle-glasses and reading-glasses are of this description. The effects of the plano-concave and double concave lenses, No. 4 and 5, are directly op- posite to those of the convex lenses; for the thick parts of these glasses, you see, are towards the edge, and therefore their attractive and re- fractive powers are not towards the centre, but towards the circumference. Parallel rays, there- fore, striking one of these glasses are made to diverge, or are dispersed. Rays already divergent are rendered more so ; and convergent rays are made less convergent. Hence objects seen through these glasses appear considerably smaller than they really are. To prove this, let ab (fig. 53) Rcfrangibility of Light. 161 represent an arrow, which would be seen by the eye, if no glass were between, by the con- vergent rays, ca and db ; but if the concave lens D be interposed between the object and the eye, the line ac will be bent towards g*, and the line bd will be bent towards k 9 and consequently both will be useless, as they do not enter the eye. The object then will be seen by other lines, such as ao and 6r, which, on entering the glass, will be refracted, and bent in the directions oc and rd. According to the rule just now laid down, there- fore, every object is seen along the line which enters the eye last. The arrow is seen according to the angle or, which is much smaller than the angle db ; consequently it will appear considerably diminished, and at the distance of nm. The spectacles which are used by near or short- sighted persons consist of concave lenses; for the reason of short sight is, that, the form of the eye being too convex, the rays are made to con- verge before they reach the optic nerve ; and there- fore the concave glass, causing a little divergence, .assists this defeat of sight. But this matter will be still further explained when we treat of vision. The meniscus, No. 6, is properly like the crystal of a common watch, and it neither mag- nifies nor diminishes. Sometimes, however, it is made in the form of a crescent ; that is, thickest in the middle; and in that case it acts like a double convex lens. It is evident that all lenses, as to their surfaces,, 162 Experimental Philosophy. [Lecture 11. whether concave or convex, are segments of dif- ferent circles, the radii and diameters of which may vary almost to an infinite extent. The distance of the principal focus, or focus of parallel rays, that is, the point where all the parallel rays meet, as the point/; fig. 52, will vary in different lenses, according to their respective degrees of convexity. Hence, when opticians speak of the radius of a lens, when they say it is three or six inches, they mean that the convex surface of the glass is that part of a circle, the radius (that is, half the diameter) of which is three or six inches. The axis of a lens is a straight line drawn through the centre of its spherical surface. The principal focus, or focus of parallel rays, in convex lenses, is ascertained (as was before intimated) upon mathematical principles. It may however be found with sufficient accuracy for common purposes, by holding a sheet of paper behind the glass, when exposed to the rays of the sun, and observing when the luminous spot is smallest, and when the paper begins to burn. Or when the focal length does not exceed three feet, it may be found by holding the glass at such a distance from the wall opposite a window sash, as that the sash may appear distinct upon the wall. You will observe, that in a double convex lens the rays of light are twice refracted ; first, on entering the convex surface of the dense medium, the glass; and, secondly, on going out of the Tte/mngiUlity of Light. 163 same dense medium, and entering the rare me- dium, or the air, which, from the form of the glass, you know must present a concave surface. Now rays are equally converged by entering a convex surface of a dense medium, and a concave surface of a rarer medium. The focus of a double convex lens, then, is at only half the distance of the focus of one which has only one convex surface, that is, a plano-convex. The focus of a double convex lens, therefore, as you have already seen, fig. 52, is the length of the radius, or semi-diameter of that circle, which is formed by the convexity of either of its surfaces. That branch of optics which respects the re- frangibility of light is usually called dioptrics, from the Greek dia, through, and optomai, to *ee ; so that it means to see through. LECTURE XII. EXPERIMENTAL PHILOSOPHY. RF.FLEXIBIL1TY OF LIGHT, OE CATOPTRICS. THERE is no part of the science of optics more amusing, or indeed more astonishing, to un- scientific readers, than that which regards the reflection of light. How a looking-glass comes to reflect images without their touching it ; how the whole figure of a man, six feet high, shall be seen in a glass not above three feet ; how, when we look at some polished surfaces, as a watch- case, for instance, a man's face seems not bigger than his finger-nail ; while, if we look on other surfaces, the face shall be of gigantic size ; these are all wonders that the curious would wish to understand, and the inexperienced to examine. The property which polished surfaces possess of reflecting light, is referred by Newton to the principle of repulsion. For it is justly remarked by him, that those surfaces, which to our senses appear smooth and polished, are found, when viewed through a microscope, to be still rough and uneven. It will, however, suffice for our purpose, in describing the effects of reflection, if we consider every particle of light as rebounding from the surface of a mirror, like a tennis-ball from the wall of a tennis-court. ReflexiUlity of Light. 1 65 It is, in truth, by reflection that all objects are rendered visible. Even glass, crystal, and water reflect a part of the rays of light, or their forms and substance could not be distinguished ; but those bodies which transmit it copiously, are called clear or transparent ; those which do not transmit it, are termed opake. The whole of the light which falls upon bodies, is not, however, reflected. On the contrary, it is calculated that the smoothest and most polished surfaces do not reflect above half the light that falls upon them. Those bodies with polished surfaces, which re- flect most copiously the rays of light, are called mirrors; by the ancients they were made of metal, as iron, tin, or copper, and exquisitely polished ; those in general use among us are made of glass, rendered opake at the back part by an amalgam or mixture of tin and quicksilver, or mercury. Mirrors are made in various forms; plane, that is, with a smooth and level surface; convex, concave, or cylindrical. The most com- mon are the plane mirrors. A ray of light striking perpendicularly, in a direct line, upon a plane mirror, is reflected in exactly the same direction. Those rays which strike it obliquely, are reflected back in an op- posite direction, but^, with exactly the same degree of obliquity. Hence the great law of reflection is, that the angle of reflection's exactly equal to the angle of incidence. This was explained to 166 Experimental Philosophy. [Lecture 12. you in the tenth lecture, fig. 47, and it will serve to elucidate all the phsenomena of reflection. Lest you should, however, have attended to the maxims and definitions subjoined to that lecture less assiduously than you ought, I shall refer you to another figure. In PL XIII. fig. 54, ?io may be considered as a ray of light striking perpendicularly on the surface of the mirror a &, and it is consequently reflected back in the same line. The ray d o, coming from the luminous body d, strikes the mirror obliquely, and is re- flected to the eye in the line o e, in such manner, that the angle e o n is equal to the angle o d n ; in other words, the angle of reflection is equal to the angle of incidence. This, you will answer, is sufficiently clear ; but how comes it that I do not see the object at o, since it is there that the rays strike the mirror ? And why is it, that, on the contrary, the object appears behind the glass, and in the situation of s ? This has been partly explained by a rule which I formerly laid down ; namely, that we see every thing in that line in which the rays last approached us. Now an object is rendered visible, not by single rays proceeding from every point of its surface, but by pencils of rays, or collections of divergent rays issuing from every point, as was explained in the preceding lecture. These pencils of rays are afterwards, by the refractive powers of the eye, converged again to Reflexibility of Light. 167 points upon the optic nerve, which lies at the back of the eye ; and these points of convergent rays on the optic nerve, are correspondent to the points of the objects from which the rays diverged. Now the pencils of rays strike the mirror, while they are in their divergent state ; and as the angle of reflection is equal to the angle of in- cidence, they are reflected' back in the same state, and converge exactly as they would have done had they not been intercepted by the mirror. As, therefore, we always see objects in the line in which the rays approached us last, the two lines, viz. that which goes from the object towards the mirror, and the reflected line, are united in the mind of the spectator, and the object is con- sequently seen at s, at an equal distance behind the mirror, as the object was before it. To make this clear, however, I shall present you with another diagram. The lines D c, (fig. 55.) are the lines of incidence, c B are the lines of re- flection, and these form equal angles on the surface of the polished mirror; so that all the ray scorning from the object, and falling upon the mirror at c, will strike the eye at B, and the reflected image will thus become visible. Now no object can be seen that does not lie in a straight line from ttfe eye, or, at least, appear to do so. The body D, therefore, when it comes reflected to the eye, will appear to lie in the straight line AA, which, since the angle of in- cidence is equal to that of reflection, will be 168 Experimental Philosophy. [Lecture 12. exactly in the two lines D c and c B. The rays, therefore, going from D to c 9 will seem to have proceeded to A, and consequently the picture will be there. For, as the rays have diverged in going from the object at DD, and diffused them- selves upon the surface of the glass, they will be again converged into an equal focus, by the time they arrive at B 5, and they will therefore paint the object at A A. Hence we may learn, that if a man sees his whole image in a plane looking glass, the part of the glass that reflects his image, need be but one- half as long, and one half as broad as the man. For the image is seen under an angle, as large as the life ; the reflecting mirror is exactly half-way between the image and the eye, and therefore need be but half as large as the object, to sub- tend an angle as large as the image ; or, in other words, it is just half as large as the image, which is of the same size with the man. Thus the man AB, (see fig. 56) will see the whole of his own image in the glass CD, which is but half as large as himself. His eye, at A, will see the eye of the image at an equal distance behind the glass at E. His foot at B, will send its rays to D ; these will be reflected at an equal angle, and the ray will therefore seem to have proceeded in the direction of FDA, so that the man will see his foot at F ; that is, he will see his whole figure atEF. It is thus that plane mirrors reflect. The Reflexibility of Light. 169 nature of those which are convex or concave is a more difficult study, though the same law pre- vails with respect to them as with respect to the others. To understand the principles on which they act, it will be expedient to call to your recollection what was said in the former lecture on spherical surfaces. All curves or arches may be considered as composed of a number of small flat planes, lying obliquely to one another. Pa- rallel rays, therefore, striking an object opposed, to them in this position, will strike it more or less obliquely. Thus, in fig. 57, the rays a, 6, c, d, which would fall perpendicularly on a horizontal surface, strike obliquely upon those which are opposed to them ; and, instead of being reflected parallel, are reflected divergent. For the same reason, convergent rays would be reflected less convergent by such a mixed surface as this, and divergent rays would be rendered still more divergent. Fig. 58, you see, is the reverse of the preceding, and it serves very well to represent the effects of a concave mirror. By this you must perceive that the parallel rays a, b, c, d, which would have been reflected parallel by a plane mirror, are made to converge, because, instead of striking this mirror in a direct line, they strike it obliquely ; and you may easily conceive, that by the same rule, convergent rays will be reflected still more convergent, and divergent rays will be made to converge less. As by a mirror of the convex kind convergent VOL. I. I 170 Experimental Philosophy. [Lecture 12. rays are rendered less convergent, you will easily comprehend why objects are diminished by it. By the rays being made less convergent, the visual angle is diminished; for, you know, we see every object in the line in which the rays of light last approached the eye. By the same rule, a concave mirror magnifies or enlarges the image of an object ; for the visual angle is enlarged or rendered more obtuse, and consequently the image is magnified in proportion to the curvature of the . concave surface. To prove what I have just now laid down with respect to convex mirrors, in fig. 59, a b is a dart, which is seen in the convex mirror c d. Now, though rays issue from the object a b in all directions, as was explained in the tenth lecture, Plate XI. fig. 46, yet it is seen only by means of those which are included within the space between o and 7i, because it is only those which can be reflected to the eye at r. Now you will easily perceive that if these rays had gone forward in the direction in which they were proceeding, they would have united at p, and the object would have been seen of its full size. As it is, however, the rays are reflected less convergent than they were in their natural course, and the angle o r n, being less than the angle a p 5, the image at s appears smaller than the object, and nearer to the surface of the mirror. The reason of this last effect has been already explained, when I said that objects are rendered visible, not Reflexilility ofLigU. 171 by a single ray, but by pencils of divergent rays proceeding from every point of the object. Sup- pose, therefore, G (fig. 60) a radiant point of any object, from which a pencil of divergent rays proceeds, and falls on the convex mirror a b. These rays (agreeably to the rule laid down above, that convex mirrors cause divergent rays to diverge still more) will be rendered more divergent, and will have their virtual or imaginary focus at g, that is, much nearer to the surface of the mirror than if it were plane. For these reasons, a person looking at his face in a convex mirror, will see it diminished. Thus, in fig. 61, though rays proceed from every part of the face, it is only the rays that touch the mirror within the space between c and r that can, agreeably to the great law of reflection, (the angle of incidence being equal to the angle of reflection) be reflected to the eye. The rays c and r being therefore rendered less convergent (as in the former instance in fig. 59), he will see the chin along the line o r s, and the forehead along the line o c n, and the angle of vision being thus diminished, all the rest of the features will be proportionably reduced. Large objects, how- ever, placed near a convex mirror, will not only appear reduced, but distorted; because, from the form of the glass, one part of the object is nearer to it than another, and consequently will be reflected under a different angle. Convex mirrors are at present a very fashion- 172 Experimental Philosophy. [Lecture 12. able part of modern furniture, as they exhibit a large company, assembled in a room, in a very small compass. Globes lined with amalgam used to be formerly hung up in the middle of a room, by which the whole company were exhibited at one view, seated at a dinner-table, or dispersed about the room. The phenomena of concave mirrors are still different. By them convergent rays are ren- dered still more convergent, and consequently the visual angle is enlarged. Their general effect is therefore to magnify. This will be sufficiently exemplified by PL. XIV. fig. 62. In this, as in the former instance, a face is looking at itself; and I take the extreme of those rays which can be reflected to the eye, one from the forehead and one from the chin. These lines, ac, and mn, are reflected to the eye at o, which con- sequently sees the image in the lines of reflection, and in the angle odq, and therefore evidently magnified beyond the natural size, and at a small distance behind the mirror. This effect, however, will only take place when the eye is between the mirror and its prin- cipal focus, that is, the focus or point, where rays falling parallel or perpendicular on the glass, will unite after reflection ; the point where the rays of the sun (which are always considered as parallel) will unite and burn: for a concave mirror acts as a burning-glass. By the great law of reflection, the principal focus of a concave Reflexibitity of Light. 173 mirror, is at one-fourth of the diameter of that sphere, of which the concave surface is a section, which is therefore sometimes called the centre of concavity. At this point the rays reflected from the mirror, are converged and cross ; and if the spectator's eye is beyond this point or focus, he will not see the image behind the mirror, but before it, a shadowy form, suspended in the air ; but, from the crossing of the rays, it appears inverted. In fig. 63, a b is a concave mirror, cd is a hand held up before it. The image, therefore, you see is not placed behind the mirror, as happens in every other case, but the hand seems to hang suspended in the air at m. The reason of this very extraordinary and striking phenomenon is to be found in what was already intimated. Objects are rendered visible, not by single rays, but by pencils of divergent rays, proceeding from the different points of the object. If these pencils of divergent rays should happen by any cause to be united, the object will in that point cease to be visible. This happens in the focus of a concave mirror, where, by the law of re- flection, they are all united. If the eye, there- fore, is placed in that point, it will see nothing of the image. It must recede to a sufficient distance to permit the rays to cross and again becpme divergent. In that case the image will be seen, not behind the mirror at the virtual or imaginary focus, as it is in plane and convex mirrors, but 1 74 Experimental Philosophy. [Lecture 12. suspended in the air between the eye and the real focus, for every image is seen about that place, whence the pencils of rays begin to diverge. In plane mirrors the rays have only diverged from the luminous points of the object itself; and as the eye cannot see behind, it sees the image in a straight line, but joins the line of incidence and that of reflection together. The image there- fore appears at the same distance behind the glass, as the object stands before it. In concave mirrors the case is entirely diff erent ; for in them there is an actual focus, where the rays are con- verged to a point, and from which they begin gain to diverge. The image is therefore seen there, but in an inverted position, for reasons already given. Thus, in fig. 63, the rays c and d go diverging from the two opposite points of the object; by the action of the mirror they are again made to converge to a point at o S 9 where they cross, and again proceed divergent to the eye. It will, however, render this interesting part of optics still clearer, if I present you with an- other diagram, similar in some degree to the preceding. In fig. 64,'AcB is a concave mirror. The centre of concavity is at C. From the points of the dart D, we suppose a pencil of divergent rays emitted, which you see touch the mirror at AcB. These rays are reflected, according to the general law of reflection, (the angle of reflection being ecjeiial to the angle of incidence) which is Reflexibility of Light. 1 75 proved by drawing the dotted lines C A, Cc, CB, from the centre of concavity to the points whence these rays are reflected, which are therefore per- pendiculars to the surface of the mirror. The angle C Ad, or the angle of reflection, you see, is equal to DAC, the angle of incidence, and so you will find it of the rest. The reflected rays then, you see, converge to a point, and form the extremity of the dart (which is now inverted) at d. In the same manner every other pencil of rays emitted from the object, will be converged at or near the principal focus, and the image will be formed at e d. For you wih 1 perceive that if the rays E t /J T5g*, ' E A, were continued to the mirror, they would be reflected and converged at e^ forming the opposite extremity of the dart. When the object is further from the mirror than the centre of concavity C, the image wih 1 be nearer the mirror, and smaller than the object; when the object is nearer than the centre of con- cavity, the image will then be more remote, and larger. Thus, if e d was the object, DE would be the reflected image. It is not many years since a person derived considerable emolument from exhibiting in the metropolis some optical deceptions of this kind, with concave mirrors. A ghastly apparition was sometimes made to meet the ignorant spectator, and from its shadowy appearance it was evidently nothing human ; sometimes a hand was held out in the air, with every possible mark of friendship, 176 Experimental Philosophy. [Lecture 12. but when he approached to unite it with his own, a drawn sword was instantly presented to his breast. A nosegay, or a piece of fruit was offered, but when he attempted to seize it, a death's head snapped at him. I mentioned that concave mirrors were fre- quently used as burning-glasses, and a curious experiment may be made by means of them, to show that common culinary fire may be reflected in the same manner as the rays of the sun. If two large concave mirrors are placed opposite to each other, as in fig. 65, at almost any distance, and a red-hot charcoal is held in the focus of one at a, and a match, or any combustible matter, in the focus of the other at b, the match, &c. will be presently set on fire by the reflected flame of the charcoal. You have seen, I dare say, the distorted figures which are sometimes painted on boards, and ex- hibited in the shop- windows of opticians. They look like a mere splash of a painter's brush ; but when a mirror of a cylindrical or conical form is set in the middle of the board, a beautiful figure is reflected from it. This shows that what ap- pears to be a casual dash of paint on the board is, in fact, a figure drawn with the nicest mathe- matical precision. When the image is to be rectified by a cylindrical mirror, the lines are only extended, and, by the great law of reflection, the rays from the picture are reflected by the mirror less convergent, and the figure is con- Reflexibility ofLiglit. 177 sequently rectified. A little consideration on this subject, applying the principles which have been laid down in the course of this lecture, will easily enable you to see the theory on which these mirrors act, particularly if you have the objects before you : without which, indeed, an infinity of words must be expended in describing and explaining them. LECTURE XIII. EXPERIMENTAL PHILOSOPHY. VISION AND OPTICAL GLASSES. IT has already been explained, that objects are rendered visible not by single rays, but by small bundles of rays diverging from every point of the object, like an inverted cone, or like a painter's brush or pencil, and therefore called pencils of light. It has also been intimated, that these pencils of light are, by the refractive powers of the eye, again made to converge upon the back part of that organ, in points corresponding to those from which they proceeded, so as to form there a complete image of the object. In the tenth lecture, fig. 46, it was further shown, that pencils of light are sent forth in all directions, from every part of a visible object; so that an eye, when placed in any situation that light can travel to it from the object in a straight line, (whether above or below, or at either side) shall be able to perceive it. In describing the nature of refraction, enough has been said to show you that it is the property of every convex glass to cause the rays of light to converge. In this respect the eye is to be Vision and Optical Glasses. 179 considered as a convex lens, constructed with such admirable skill by the great Author of Nature, that the rays converge to a point exactly in the proper place ; so that if the humours were otherwise disposed, even to the breadth of a horse- hair, the effect would be totally destroyed. But you will understand the subject better, by con- sidering the structure of this curious organ ; in describing which, I shall adopt the simple, but expressive language of Mr. Ferguson. The eye is nearly of a globular form. It con- sists of three coats and three humours. (See fig. 66.) The part DHHG of the outer coat is called the sclerotica ; the rest, D E F G, the cornea. Next within this coat, is the choroides, which serves for a lining to the other, and joins with the iris mn, mn. The iris is that coloured circle which gives the character, as to colour, to the eye, and is composed of two sets of muscular fibres; the one of a circular form, which con- tracts the hole in the middle, called the pupil, when the light would otherwise be too strong for the eye ; and the other of radial fibres, tending every where, from the circumference of the iris, towards the middle of the pupil ; which fibres, by their contraction, dilate and enlarge the pupil when the light is weak, in order to let in more of its rays. The third coat is only a fine expansion of the optic nerve L, which spreads like net-work all over the inside of the choroides, and is there- fore called the retina ; upon which are painted 180 Experimental Philosophy. [Lecture 13. the images of all visible objects, by the rays of light which either flow or are reflected from them. Under the cornea is a fine transparent fluid, like water, which is therefore called the aqueous humour. It gives a protuberant figure to the cornea, fills the two cavities mm and nn, which communicate by the pupil P, and has the same refractive power as water. At the back of this lies the crystalline humour R, which is shaped like a double convex glass, and is a little more convex on the back than the forepart. It con- verges the rays, which pass through it from every visible object, to its focus at the bottom of the eye. This humour is transparent, like crystal, and is much of the consistence of hard jelly. It is inclosed in a fine transparent membrane, from which issue radial fibres, called the ligar mentum ciliare, all around its edge ; and join to the circumference of the iris. These fibres have a power of contracting and dilating occasionally, by which means they alter the shape or convexity of the crystalline humour, and also shift it a little backwards or forwards in the eye, so as to adapt its focal distance at the bottom of the eye, to the different distances of objects; without which provision, we could only see those objects distinctly, that were all at one distance from the eye. At the back of the crystalline lies the vitreous humour KK, which is transparent like glass, and Vision and Optical Glasses. 181 is the largest of all in quantity, filling the whole orb of the eye, and giving it a globular shape. It is much of the same consistence as the white of an egg, and very little exceeds water in its refractive power. As every point of an object ABC, sends out pencils of rays in all directions, some rays, from every point on the side next the eye, will fall upon the cornea between' E and F ; and by passing on through the humours and pupil of the eye, they will be converged to as many points on the retina or bottom of the eye, and will there form a distinct inverted picture cba of the object. Thus, the pencil of rays qrs, that flows from the point A of the object, will be converged to the point a on the retina ; those from the point B will be converged to the point b ; those from the point C will be converged to the point c ; and so on of all the intermediate points : by which means the whole image abc is formed, and the object made visible. That vision is effected in this manner, may l?e demonstrated experimentally. Take a bullock's eye while it is fresh, and having cut off the coats from the back part, quite to the vitreous humour, put a piece of white paper over that part, and hold the eye towards any bright object, and you will see an inverted picture of the object upon the paper. It has been a matter of inquiry among scientific persons, why the object appears in an upright 182 Experimental Philosophy. [Lecture 13. position, while the image on the retina is inverted. In truth, we know nothing of the connexion which exists between the thinking faculty and the organs of sensation. It may, however, suf- fice to answer the present question, if we say that the mind certainly does not look upon the image which is painted on the optic nerve. That nerve is sensible of the impression, from the rays of light being reflected upon it, as the organs of touch feel the impression of any external object, by coming in contact with it. Nor is there any reason why the mind should not perceive as ac- curately the position of bodies, if the rays reflected from the upper parts of those bodies are made to touch the lower parts of the eye, as if they had been directed to the upper parts. Suffice it, that such a correspondence is established between the parts of the eye to which the rays are converged, and the different parts of the object, that we do not find that persons blind from infancy, who have been restored to sight by the operation of couching, have been led into the smallest mistake as to this point*. To very perfect sight the three humours of the eye appear necessary. Yet by a very bold experiment (for such it undoubtedly was at first), it is found that we can see tolerably well, even though one of them should be taken away, par- * For an elaborate disquisition on this subject, the reader may consult the Rev. A. Horn's Essay on Vision. Vision and Optical Glasses. I8C ticularly if we assist the sight by glasses. It very often happens that the crystalline humour loses its transparency, and thus prevents the admis- sion of the visual rays to the back parts of the eye. This disorder is called by the surgeons a cataract. As we know that the crystalline hu- mour stands edgeways behind the pupil, all then that we have to do, is to make it lie flat in the bottom of the eye, and it will no longer bar out the rays that come in at the pupil. A sur- geon, therefore, takes a fine straight awl, and thrusting it through the coats of the eye, he de- presses the crystalline humour into the bottom of the eye, and there leayes it. Or sometimes he cuts the coats of the eye, the crystalline and the aqueous humour burst out together; in some hours the wound closes, a new aqueous humour returns, and the eye continues to see, by means of a glass, without its crystalline humour. This operation is called couching for the cataract. Cheselden once couched a boy who had been blind from his birth with a cataract. Being thus introduced, in a manner, to a new world, every object presented something to please, astonish, or terrify him. The most regular figures gave him the greatest pleasure, the darkest colours displeased, and even affrighted him. The first time he was restored, he thought he actually touched whatever he saw; but by degrees his experience corrected his numberless mistakes. More recently an interesting case of this kind 184< Experimental Philosophy. [Lecture 13. has been described in the Philosophical Trans- actions by Mr. Ware. The eye may be remedied when the crystalline humour onJy is faulty ; but when there happens to be a defect in the optic nerve, then the disorder is almost always incurable. It is called the gutta serena^ a disorder in which the eye is, to all ap- pearance, as capable of seeing as in the sound state ; but, notwithstanding, the person remains for life in utter darkness. The nerve is insensible, and scarcely any medical treatment can restore its lost sensations. This is the disorder so pathetically described by Milton in his lamenta- tions on his own blindness. In the course of the preceding lectures it was necessary to mention the angle of vision. But you will now be able better to understand why an object seen under a large angle, as near objects are, appears larger than the same object would at a distance. Thus men and women, when you meet them in the street, appear of their na- tural size, but if you look down upon them from the top of St. Paul's, they appear as small as puppets ; and thus if you look from one end towards the other of a long and straight row of trees, you will see them gradually diminish, as they are further removed from your eye, though on a near inspection you would find them all of an equal size. The reason of this can be no longer a secret. You are already informed, that rays (or rather pencils of rays) are sent forth Vision and Optical Glasses. 185 from every visible object, in all directions, some more and some less convergent. When you are near, therefore, you see the extreme points of any object by pencils of rays, which converge or meet in an angle more obtuse than when it is at a greater distance; and as the rays cross each other in the eye, a larger image is of course painted on the retina. Thus, in PL XV. fig. 67, the ob- ject ABC is seen by the eye at D, under the angle APC. and the image upon the retina cba is very large ; but to the eye at E, placed at double the distance, the same object is seen under the angle A/?C, which is only equal to half the angle APC. The image cba, therefore, is only half as large in the eye at E as in the eye at D ; and this will sufficiently explain why objects appear smaller in proportion to their distance from the ej/e. Observe, however, that this proposition will admit of some exceptions, where the judgment corrects the sense. Thus, if a man six feet high (and not far distant from the spectator) is seen under the same angle with a dwarf two feet high (say at the distance of three feet from the spectator), still the dwarf will not appear as tall as the man, because the sense is corrected by the judgment, which makes a comparison of both with sur- rounding objects of known size. These ex- ceptions will, however, in general, only take place with respect to near objects, and those with whose forms we are well acquainted. From what has been said of the structure of 186 Experimental Philosophy. [Lecture 13. the eye, you will also perceive the causes of distinct and indistinct vision. To see an object distinctly, it is necessary that every pencil of diverging rays, which reaches the eye from the object, should be converged to a point on the optic nerve, corresponding to that from which the rays have diverged. If, on the contrary they are brought in an unconverged state to the retina, you may easily conceive that the particles of light will be so scattered and dispersed, as to make an indistinct impression. This last defect takes place when the eye, by age or infirmity, is made flat, and consequently is not sufficiently convex to cause the rays to converge in their proper place ; persons with this defect can often see objects better at a great distance than very near. The opposite fault to this is when the eye is too convex, when the rays will be made to unite too soon, before they reach the retina ; persons with this defect, therefore, are called short sighted because they can only discern objects which are very near to the eye. I have seen a very pretty contrivance in the shop of an optician, illustrative of the causes of weak and short sight. Two eyes were made of glass, as fig. 68 and 69, and the pencils of diverg- ing rays, issuing from three points, were repre- sented by threads of silk of three different colours. Thus in fig. 68, which represents weak or in- distinct vision, you see the rays are not united in points when they reach the back of the eye, Vision and Optical Glasses. 187 where the retina is situated ; but if they were suffered to pass on without interruption, would converge in some part behind it. On the con- trary, in figure 69, you see that, from the great convexity of the cornea, the rays are made to converge too soon, and, in effect, the perfect and distinct image is formed in the midst of the vitreous humour, and before it reaches the retina. From what you have already learnt of the na- ture of lenses, you will be able to comprehend that the remedy for the former of these defects, that is, where the eye is too flat to cause the rays to converge in their proper place, is a double convex lens, the property of which is to increase the convergency of rays. The focus of this glass, however, must be exactly adapted to the wants of the eye for which it is intended. As therefore the eye grows flatter from age and infirmities, this will explain what is meant by " spectacles for all ages." Where the defect of sight is not great, as in younger persons, spectacles not very convex will suffice; but where the eye is very flat, as in old persons, glasses of a stronger mag- nifying power will be required. On the contrary, near sighted eyes (such as fig. 69) being too convex, it is necessary to pre- vent the rays from converging too soon, which can only be done by means of a concave glass, which renders convergent rays less convergent. This glass, however, must also be exactly adapted 188 Experimental Philosophy. [Lecture 13. to the necessity of the eye, otherwise the rays will not converge at the proper point. I cannot quit this subject without noticing the gross stupidity of the atheist. Can any persons in their senses conceive that so nice, so exquisite an organ as the eye should be formed by chance ! That by chance the humours should be disposed with the most perfect mathematical precision, so that a mistake to the breadth of a hair would be sufficient to defeat the purpose of vision ! Yet these are the men, my young friends, who without understanding any principle of any one science, have the impudence to call themselves philo- sophers* ! though in what their philosophy can consist, would require more than Newton pos- sessed to be able to discover. There is reason to believe, that the use of convex glasses, both as burning glasses and mag- nifiers, was not unknown to the antients ; and, in the twelfth century, Alhazen, an Arabic philo- sopher, treated at some length of the magnifying power of these glasses. He was followed by our * Why they have chosen to adopt this name no man can possibly devise. They might as well have called them- selves architects, heralds, antiquarians, or by any other de- nomination with which they have no connexion what- ever. Ask any of these pretended philosophers why a convex lens causes the rays of light to converge, or any similar question, and you will soon see whether they have any pretension to the name of philosophers, Vision and Optical Glasses. 189 truly illustrious countryman Roger Bacon, who demonstrated by experiment that a small segment of a glass globe would assist the sight of old persons. Thus he may be regarded as the person who first discovered the theory of spectacles, though they were not brought into use until the following century. The telescope was invented about the end of the sixteenth century, and the discovery is commonly supposed to have been casual. The account which is generally received is, that the children of Zacharias Jansen, a spectacle-maker of Magdeburgh, trying the effect of a convex and concave glass united, found that when placed at a certain distance from each other, they had the property of making distant objects appear nearer to the eye ; but the reason of this effect was not discovered till the time of Kepler. The microscope was also an invention of Jansen or his children: and as it is rather a simpler instrument than the telescope, it will serve to introduce you very properly to a knowledge of these kinds of glasses. You already know that the nearer any body is to the eye, the larger is the angle under which it will be seen ; but if placed too near, the image will be confused, because the divergence of the rays is then too great to admit of their being properly converged on the retina by the humours of the eye. In fact, an eye which is not near sighted cannot discern any object clearly at a shorter distance than six 190 Experimental Philosophy. [Lecture 13. inches ; and many objects are too small to be seen at that distance. This deficiency is supplied by the microscope. The single microscope is only a small convex glass cd, (fig. 70,) having the object ab placed in its focus, and the eye at the same distance on the other side ; so that the rays of each pencil, flowing from every point of the object on the side next the glass, may go on parallel in the space between the eye and the glass; and then, by entering the eye at C, they will be converged to as many different points on the retina, and form a large inverted picture AB upon it, as in the figure. If, as in fig. 71, which represents the effect of this microscope, the object AB is in the focus of the lens DE, and the eye is in the other focus F, as much of the object will be visible as is equal to the diameter of the lens ; for the rays AD and BE proceed through the extremities of the lens, and are united at F. Hence a maxim in optics that when an object is placed in one focus of a lens., and the eye in the other ^ any lineal dimen- sion of the object appears just twice as large as it would to the naked eye, whatever the size of the lens. For the lines FD and FE, if protracted as far as A and B, would form an image exactly twice as large. If the eye is nearer to the lens than the focus, it will see the object still larger; and if it is further off than the focus, it will not see it so large. Vision and Optical Glasses. 191 To find how much this glass magnifies, divide the least distance (which is about six inches) at which an object can be seen distinctly with the bare eye, by the focal distance of the glass ; and the quotient will show how much the glass mag- nifies the diameter of the object. The most powerful single microscopes are very small globules of glass, which any person may make for himself by melting the ends of fine glass threads in the flame of a candle. The double or compound microscope consists of an object-glass cd, (fig. 72,) and an eye-glass ef'. The small object ab is placed at a little greater distance from the glass cd than its principal focus, so that the pencils of rays flowing from the dif- ferent points of the object, and passing through the glass, may be made to converge and unite in as many points between g and h, where the image of the object will be formed: which image is viewed by the eye through the eye-glass ef. For the eye-glass being so placed that the image gli may be in its focus, and the eye much about the same distance on the other side, the rays of each pencil will be parallel, after going out of the eye- glass, as at e and^ till they come to the eye at A:, where they will begin to converge by the re- fractive power of the humours ; and after having crossed" each other in the pupil, and passed through the crystalline and vitreous humours, they will be collected into points on the retina, and there form the large inverted image AB. Experimental Philosophy. [Lecture 13. The magnifying power of this microscope is as follows. Suppose the image gh to be six times the distance of the object db from the object-glass cd ; then will the image be six times the length of the object : but since the image could not be seen distinctly by the bare eye at a less distance than six inches, if it is viewed by an eye-glass ef, of one inch focus, it will be brought six times nearer the eye ; and consequently viewed under an angle six times as large as before ; so that it will be again magnified six times ; that is, six times by the object-glass, and six times by the eye-glass, which multiplied into one another make thirty-six times ; and so much is the ob- ject magnified in diameter more than it appears to the bare eye; and consequently thirty-six times thirty-six, or one thousand two hundred and ninety-six times in surface. The solar microscope is constructed upon si- milar principles. Two convex glasses are in- closed at their proper distances in a brass tube. This tube being fixed in the window-shutter of a dark room, the object is put between the two glasses, when a very large inverted image of it will be exhibited on the opposite wall, pro- vided the sun shines sufficiently bright and clear upon the microscope. This instrument bears a strong analogy, therefore, to the camera obscura already described. Sometimes, three lenses are employed, and the magnifying power of the mi- croscope proportionally increased. Vision and Optical Glasses. 193 What microscopes effect upon minute bodies very near, telescopes effect with regard to great bodies very remote; namely, they enlarge the angle in the eye under which the bodies are seen ; and thus, by making them very large, they make them appear very near: the only difference is, that in the microscope the focus of the glasses is adapted to the inspection of bodies very near ; in the telescope, to such as are very remote. Sup- pose a distant object at A B (see fig. 73), its rays come nearly parallel, and fall upon the convex glass cd; through this they will converge in points, and form the object E at their focus. But it is usually so contrived, that this focus is also the focus of the other convex glass of the tube. The rays of each pencil, therefore, will now diverge before they strike this glass, and will go through it parallel ; but the pencils all together will cross in its focus on the other side, as at e, and the pupil of the eye being in this focus, the image will be viewed through the glass, under the angle geh, so that the object will seem at E under the angle DeC. This telescope inverts the image, and therefore is only proper for viewing such bodies as it is immaterial in what position they appear, as the sun, the fixed stars, &c. By add- ing two convex glasses, the image may be seen upright. The magnifying power of this, which is called the dioptric telescope, is found by dividing the focal distance of the object-glass by the focal VOL. I. 194 Experimental Philosophy. [Lecture 13. distance of the eye-glass, and the quotient ex- presses the magnifying power. The greatest inconvenience attending dioptric or refracting telescopes was found to be that which arises from what is called the aberration of light, which, when high magnifiers were used, that is, lenses much thicker in the middle than at the sides, produced often a confused, and sometimes a coloured image. This effect is the result of refraction, and it consists in different rays, according to their obliquity, uniting in dif- ferent foci, though proceeding through the same lens. This will be easily understood by fig. 74. Suppose, then, PP to be a convex lens, and E e an object, the point E of which corresponds with the axis of the lens, and sends forth the rays EM, EN, EA, EM, EN, all of which reach the surface of the glass, but in different parts. The ray EA, which penetrates the centre of the glass, suffers no refraction ; the rays EM, EM, which pass near EA, will be converged to a focus at F But the rays EN, EN, which strike more ob- liquely near the edges of the glass, will be differ- ently refracted, and will meet about G, nearer to the lens, where they will form another image Gg. In this manner several images will be formed in different foci ; and though to the eye which looks through the lens one image only will be apparent, yet that image, from being composed of so many combined, will be confused and distorted. Vision and Optical Glasses. 195 What is thus established in theory may be de- monstrated by experiment, and that experiment is easy to make. Cover one side of a glass globe or of a thick lens with a piece of brown paper, making a row of pin-holes across the diameter of the lens very accurately at equal distances. Let the light which passes through the lens fall upon a sheet of white paper, and you will find that when the paper is held near the lens the spots of light will be nearly at equal distances ; but if the paper is further removed, the intervals between the exterior spots become less than the intervals between the interior, and soon unite. But there is a still further aberration, which is productive of even a greater inconvenience than this which I have now specified. When I come to treat of the prism and the prismatic colours, you will find that each particle of light is suscep- tible of a different degree of refrangibility, and consequently that every lens (especially high magnifiers) acts in some degree as a prism in separating the different coloured rays Hence, if we suppose PP (fig. 75) to be a double convex lens, and oo an object at some distance from it, if the object oo were red, the rays proceeding from it would form a red image Rr ; if it were violet, an image of that colour would be formed at \v nearer the lens ; and if the object were white, or any other combination of different coloured rays, these rays would have their respective foci at dif- ferent distances from the lens, and form in fact 196 Experimental Philosophy. [Lecture 13, a succession of images, in the order of the pris- matic colours from Rr to Vv. As in the former case, these different images will form but one to the eye of the spectator ; but it will be imperfect and coloured at the edges, as well as the field of view. Various remedies were devised for this defect. At length Mr. Dollond, finding that flint and crown glass had different refracting powers, and that crown glass (the common window glass) dispersed the rays of light less than any other, adapted two convex glasses of crown glass to a double concave of flint glass (which has the great- est dispersive power), so as exactly to fit, and by that means made them counteract each other, so that the field of view is presented perfectly colour- less. These telescopes, therefore, are called achro- matic (or colourless) telescopes. The reflecting telescope accomplishes- by re- flecting the rays issuing from any object, what the last did by refracting them. Let ab, (PL XVI. fig. 76) be a distant object to be viewed ; parallel rays issuing from it, as ac and bd, will be reflected by the metallic concave mirror, cd to st y and there brought to a focus, with the image a little further and inverted, agreeably to the effect of a concave mirror on light, as formerly described. The hole in the mirror cd does not distort or hurt the image st 9 it only loses a little light ; nor do the rays stop at the image st ; they go on, and cross a little before they reach the small concave mirror en : from this mirror the rays are reflected nearly parallel through the hole O, in the large Vision and Optical Glasses. 197 mirror, to R ; there they are met by the plano- convex lens hi, which brings them to a conver- gence at S, and paints the image in the small tube of the telescope close to the eye. Having by this lens, and the two mirrors, brought the image of the object so near, it only remains to magnify this image by the eye-glass Jcr ; by which it will ap- pear as large as zy. To produce this effect, it is necessary that the large mirror should be ground so as to have its focus a little short of the small mirror, as at q ; and that the small mirror should be of such con- cavity as to send the rays a little converging through the hole o ; that the lens hi should be of such convexity as to bring those converging rays to an image at S ; and that the eye-glass Icr should be of such a focal length, and so placed in the tube, that its focus may just enter the eye through the small hole in the end of the tube. To adapt the instrument to near or remote ob- jects, or rather to rays, that issue from objects converging, diverging, or parallel, a screw, at the end of a long wire, turns on the outside of the tube, to take the small mirror nearer to, or fur- ther from, the large mirror ; and so as to adjust their foci according to the nearness or remoteness of the objects. The sun-glass at the end of the small tube should be unscrewed, when any other object, except the sun, is looked at. This pecu- liar construction of the reflecting telescope is called the Gregorian telescope, from the name of its inventor. 198 Experimental Philosophy. [Lecture IS. To estimate the magnifying power of the Gre- gorian telescope, multiply the focal distance of the large mirror by the distance of the small mirror from the image S ; then multiply the focal dis- tance of the small mirror by the focal distance of the eye-glass Tcr ; lastly divide these two products by one another, and the quotient is the magnify- ing power. Sir Isaac Newton formed his telescope upon a somewhat different principle from that of Gregory. In his instrument, still known by the name of the Newtonian telescope, instead of the small concave mirror en, there is placed diago- nally a plane mirror, on which the spectator looks through the side of the telescope by means of an eye-glass adapted to that purpose. The cele- brated Dr. Herschel commonly uses the New- tonian telescope on an improved principle, and through that makes most of his observations. Dr. HerschePs great telescope is however of a different construction. It has only one large con- cave reflector at the bottom of the tube ; and the spectator stands with his back to the object, and looks in upon the reflector through an eye-glass. The magnifying power of this is the same as that of a Newtonian telescope would be of the same sized reflector; but, there being only one re- flector, the quantity of light is less diminished. A minute description of this curious telescope is given under the word TELESCOPE in that uni- versal dictionary called the Pantologia* LECTURE XIV. EXPERIMENTAL PHILOSOPHY. COLOURS. I HAVE explained the nature of vision, and that it is by means of the rays of light which are sent from the different objects that sur- round us to our eyes that they are rendered visible. But you are yet at a loss to understand whence proceed the infinite variety of colours in which the whole creation is superbly arrayed. You must be rendered sensible of these colours by means of the light : but you will be surprised to learn that the colours are not in the things, but in the light itself; and that every beam or pencil of light is composed of particles of different colours. " The blushing beauties of the rose, the modest blue of the violet," says Goldsmith, "are not in the flowers themselves, but in the light that adorns them: odour, softness, and beauty of figure, are their own ; but it is light alone that dresses them up in those robes which shame the monarch's glory." You must have observed yourselves, that the colours of objects are essentially altered by the light in which they are seen. The colours of 200 Experimental Philosophy. [Lecture 14. various pieces of silk or woollen stuff are not the same by day as by candle light ; but there is a common experiment which will yet more forcibly illustrate what I have been observing, and prove that colour is not in the objects, but in the light by which they are seen. Let a pint of common spirit, the cheapest will answer as well as the best, be poured into a soup-dish, and then set on fire : as it begins to blaze, let the spectators stand round the table, and let one of them throw a handful of salt into the burning spirit (still keeping it stirred with a spoon). Let several handfuls of salt be thus successively thrown in ; the spectators will see each other frightfully changed, their colours being altered into a ghastly blackness. It is plain, then, that the solar rays are composed of matter different from the light which is emitted by this flame ; and the truth is, that the light of a candle is somewhat different from both. But the genius of Newton has enabled us to go still further in ascertaining the nature of light. He has analysed it with as much expert- ness as a chemist analyses any physical sub- stance, and has divided it into its component parts. To this noble discovery the great philo- sopher was led rather by accident than by de- sign ; but a mind such as Newton's was able to improve whatever hint chance submitted to his view. It was in attempting to rectify the errors arising from the aberration of light in the glasses Colours. 201 of the telescope, that his attention was directed to the wonderful effect which is produced by a prism. The prism of the opticians is a triangular pris- matic piece of glass, usually of the length of about three inches. If a small hole ~F r fig. 77, is made in the window- shutter, EG, of a dark chamber, and a beam of light, SF, proceeding directly from the sun (for the experiment will only succeed when the sun shines), is made to pass through the prism, ABC, an image of the sun, PT, will be represented on the sheet of paper, MN, fixed to the opposite wall. But you will observe two very extraordinary cir- cumstances attending this representation of the sun. The first, that the figure is not round but oblong; and, secondly, if you will observe the figure in the plate, you will see that it is intended to represent different colours, and in the real image these colours will be found extremely vivid. On measuring the image, which philo- sophers have agreed in calling a spectrum. Sir Isaac Newton found that, at the distance of eighteen feet and a half from the prism, the breadth of the image was two inches and a half, and its length ten inches and one quarter, that is, nearly five times its breadth. The sides were right lines distinctly bounded, and the sides were semicircular, as in the plate. From this it was evident that it was still the image of the sun, but elongated by some refractive power in the 202 Experimental Philosophy. [Lecture 14. glass. In the image PT the- colours succeeded in this order from the bottom at T, to the top at P, namely red, orange, yellow, green, blue, indigo, violet*. Unable as yet to account for the phenomenon, he was induced to try the effect of two prisms, and he found that the light, which by the first prism was diffused into an oblong, was by the second reduced to a circular form, as regularly as if it had passed through neither of them. After various conjectures and experiments, he had recourse, at length, to what he calls the experimentum crucis. At the distance of about twelve feet from the prism, which was close to tiie aperture F, he placed a board which might receive the image in the same manner as the sheet of paper MN. In this board there was also a small hole, through which some of the light might pass ; behind this hole, then, he placed a second prism, and, by moving the first prism, he made the several parts of the image cast by it on the board to pass successively through the hole, so as to be refracted again upon the wall by the second prism. He found then, that the different colours of the spectrum, when permitted to pass through the hole in the board, were incapable of further decomposition : * These, taken in an inverse order, are readily called to mind, by means of the word vilgyor, formed of the successive initials of violet, indigo, Mue green, yellow, orange, red. Colours. 203 that the red rays continued red, the orange the same, he. The cause of the phenomenon, therefore, was no longer a secret. It was plain that every beam of light consisted of particles different in colour, or which rather have the effect of producing different colours, and that all of them blended together formed white. It was further evident, that the particles of one colour were more refrangible than those of another ; and therefore those which formed the upper part of the image or spectrum suffered a much greater refraction than those at the bottom; in other words, were more under the influence of the at- tractive powers of the glass. Hence it was further evident why the figure or spectrum was of an oblong form instead of round ; for the particles of light, being differently refrangible, were spread out longitudinally by the action of the prism. Various experiments will convince you that white light is no more than a compound of these parti-coloured rays or particles. Thus, if, instead of the sheet of paper MN, you sub- stitute the large convex glass D, see fig. 78, in its place, the scattered rays will be converged and united at W, where, if the paper is placed to receive them, you will see a circular spot of a lively white. At W also the rays will cross each other ; and if the paper is removed a little further, you will see the prismatic colours again displayed as at RV, only in an inverted order, owing to the crossing of the rays. 204 Experimental Philosophy. [Lecture 14. To show further in what manner white is produced. Let two circles be drawn, as in fig. 79, on a smooth round board ABCDEFG, and the outermost of them divided into three hundred and sixty equal parts or degrees : then draw seven right lines, as A, B, &c. from the centre to the outermost circle; making the lines A and B include eighty degrees of that circle; the lines B and C forty degrees ; C and D sixty ; D and E sixty ; E and F forty-eight ; F and G twenty- seven; G and A forty-five. Then, between these two circles, paint the space AG red, in- clining to orange near G ; GF orange, inclining to yellow near F ; FE yellow, inclining to green near E ; ED green, inclining to blue near D ; DC blue, inclining to indigo near C ; CB indigo, inclining to violet near B; and BA violet, in- clining to a soft red near A. This done, paint all that part of the board black which lies within the inner circle; and putting an axis through the centre of the board, let it be turned very swiftly round that axis, so that the rays pro- ceeding from the above colours may be all blended and mixed together in coming to the eye; and then the whole coloured part will appear like a white ring, a little grayish ; not perfectly white, because no colours prepared by art are perfect. Any of these colours, except red and violet, may be made by mixing together the two con- tiguous prismatic colours. Thus, yellow is made by mixing together a due proportion of orange Colours. 205 and green ; and green may be made by a mixture of yellow and blue. The theory of colours is therefore now un- folded. Those bodies, or those parts of bodies, which have the property of reflecting only the red-making rays, will appear red; those which reflect the violet will be violet, &c. ; and those which reflect some rays of one colour and some of another will be the intermediate shade or colour between both ; and as white is a compound of all the seven primary colours, so black is an entire deprivation of them all; and when an object appears black, the light is completely absorbed, or at least not reflected by it. To prove, however, still more forcibly that colour is not in the objects, but in the light itself; no object whatever can reflect any other kind of light than that which is thrown upon it ; and when any one of the pri- mitive rays has been separated from the rest, nothing can change its colour. Send it through another prism, expose it in the focus of a burning glass, yet still its colour continues unaltered ; the red ray will preserve its crimson, and the violet its purple beauty ; whatever object falls under any of them soon gives up its own colour, though ever so vivid, to assume that of the prismatic ray. Place a thread of scarlet silk under the violet-making ray, the ray continues unaltered, and the silk instantly becomes purple. Place an object that is blue under a yellow ray, the object immediately assumes the radial colour. 206 Experimental Philosophy. [Lecture 14. In short, no art can alter the colour of a separated ray ; it gives its tint to every object, but will assume none from any ; neither reflec- tion, refraction, nor any other means can make it forego its natural hue ; like gold, it may be tried by every experiment, but it will still come forth the same. In whatever manner we consider the colour of a single prismatic ray, we shall have new cause to admire the beauties of nature. Whatever compositions of colouring we form, if examined with a microscope, they will appear a rude heap of different colours unequally mixed. If by joining, for instance, a blue with a yellow, we make the common green, it will appear to the naked eye moderately beautiful; but when we regard it with a microscopic attention, it seems a confused mass of yellow and blue parts, each particle reflecting but one separate colour : but very different is the colour of a prismatic ray ; no art can make one of equal brightness, and the more closely we examine it the more simple it appears. To magnify the parts of this colour would be but to increase its beauty. The red and orange rays, you have seen, are least subject to refraction, or are least turned out of their way by the interposition of the glass; they are therefore, we may conclude, either larger than the rest, or propelled with greater force ; in technical language, they have the greatest momentum. Agreeably to this we The Rainbow. 207 find, that when the eyes are very weak they can scarcely support a scarlet colour; its impres- sions are too powerful, and, next to the solar beam itself, dazzle and disturb the organ. On the contrary, the more refrangible the rays (the violet for instance), the less forcibly they strike the eye; and green, the intermediate colour, is the most agreeable, and is that in which Providence has chosen to array the meadows and the woods, in a delightful variety, the di- versities of green being greater than those of any other colour. Of all the objects of nature the rainbow ex- hibits the prismatic colours in the greatest per- fection. It is, indeed, a natural prism, and separates the component particles of light with the same accuracy and precision. The rainbow was one of those phsenomena which astonished and perplexed the antients; and, after many absurd and unsuccessful con- jectures, their best philosophers, Pliny and Plutarch, relinquished the inquiry as one which was above the reach of human investigation. In the year 1611 Antonio de Dominis made a con- siderable advance, however, to the true theory, by suspending a glass globe in the sun's light, when he found that, while he stood with his back to the sun, the colours of the rainbow were reflected to his eye in succession by the globe, as it was moved higher or lower. He was, however, unable to account for the pro- 208 Experimental Philosophy. [Lecture 14. duction of the different colours, as the experi- ments with the prism had not yet been made, and it was reserved for Newton to perfect the discovery. To begin, however, with the experiment of the former philosopher, let us suppose ourselves in his place. Let A, (PL XVII. fig. 80,) be a glass globe, and ScZ a ray from the sun, and falling on the globe at d ; it will, in that place, suffer a refraction, and instead of going on to c will be bent to n. From n a part of the light will be reflected (for a part will necessarily pass through), and falling obliquely at o, it will again be re- fracted. In this case you see that the globe, from its form, will act in some measure like a prism, ^and the ray will be separated into its component parts. An eye, therefore, situated at g, w 7 ill see the red rays at the line just above the orange, Sec. and so on to the violet. Now you wilf recollect, that in a shower of rain there are drops at all heights, and therefore the eye situated at g will see all the different colours. This will account for the first or primary bow, which you see is thus formed by two re- fractions and one reflection; but there is often a second bow on the outside of the other, which is rather fainter, and which is made by two refractions and two reflections. To ex- plain this, take a similar glass globe, B, fig. 81. Let the ray T in that enter at the bottom of the globe at r, where it is refracted, and part of the The Rainbow. 209 light will escape at *, and the rest, instead of escaping to w 9 will be reflected to t ; from this, part will escape to x, and part will be again re- flected to u 9 where it suffers another refraction, and is sent to the eye at g, where the violet rays will be first visible, and then the others in suc- cession. Now each drop of rain may be considered as a small globe, and within a certain range will refract and reflect the light in the manner above described. To make the matter still plainer, therefore, let us for the present imagine only three drops of rain, and three degrees of colours in the section of a bow (fig. 82). It is evident that the angle CFE is less than the angle BFE, and that the angle AFE is the greatest of the three. This largest angle then is formed by the red rays, the middle one consists of the green* and the smallest is the purple. All the drops of rain, therefore, that happen to be in a cer- tain position to the eye of the spectator, will reflect the red rays, and form a band or semi- circle of red; those again in a certain position will present a band of green, &c. If he alters his station, the spectator will still see a bow, though not the same bow as before; and if there are many spectators, they will each see a different bow, though it appears to be the same. The phsenomenon assumes a circular appear- ance, because it is only at certain angles that the coloured or refracted rays are visible to our eyes, 210 Experimental Philosophy. [Lecture 14 as is evident from the experiment with the glass globe, which will only refract the rays in a certain position. The least refrangible, or red rays, make an angle of forty-two degrees two minutes, and the most refrangible, or violet rays, an angle of forty degrees seventeen minutes. Now if a line is drawn horizontally from the spectator's eye, it is evident that angles formed with this line, of a certain dimension in every direction, will produce a circle, as will be evident by only attaching a cord of a given length to a certain point, round which it may turn as round its axis, and in every point will describe an angle with the horizontal line of a certain and determinate extent. From an analytical investigation (which, how- ever, it would not be consistent with our plan to introduce here* ) it results that the total breadth of the interior bow is 2 15', that of the exterior bow 5 40 7 , and the distance between them 8? 25'. We see a greater or a less part of the rainbow, according as the sun is more or less elevated above the horizon. When die luminary is near the plane of the horizon, then the axis of vision (as EF) which is at the same time, that of the cone formed by all the effectual rays, coincides with the horizon ; and the rainbow, in this case, is a emkarcle. In proportion as the sun is elevated, the axis EF sinks below its first position, and the It ma? be seen in a note at page 21 8, rol. ii. of Gregory's translation of Hauy's Philosophy. Colours. 211 bow regularly diminishes. Lastly, when the sun is 42 above the horizon, the axis being sunk the same number of degrees below that circle, the summit of the rainbow touches the horizon : so that, when the sun is higher than this no primary bow can be seen. A portion, however, of the exterior or secondary bow, may be seen, if the sun have any elevation between 42 and 54. If we stand on an eminence, when the sun is at the horizon, a rainbow exceeding a semicircle, (and, indeed, in favourable circumstances, ap- proaching to an entire circle), may be seen, As the cause of colours must be now apparent to you, and as it is evident that they must pro- ceed from some quality in bodies or their surfaces, which causes them to reflect rays of a particular hue, you will easily understand why some bodies, which are called semipellucid, afford one colour by transmitted, and another by reflected light. The truth is, the beam of light in passing through them is dissected and separated, and part of one colour is permitted to pass through, and part is sent back. If a solution of a wood called lignum nephriticum is put into a clear phial, when viewed only by the reflected light which falls upon it, the solution will appear blue ; but if held up against the light, and seen through, the colour will be a fine yellow. The same is found to be the case with some precious stones, and some glass compo- sitions. Thus, if a small quantity of arsenic is mixed in the composition of glass, the mass will Experimental Philosophy. [Lecture 14. appear bluish white by the reflected light, but orange by that which is transmitted through it. The blue colour of the sky may be accounted for upon this principle. The atmosphere may be considered as a semipellucid medium, which is loaded with small and light particles of va- pour ; and these particles may be compared with the particles of arsenic, which are mingled in the glass above mentioned. If the air is very heavily charged with these vapours, therefore, a large proportion of the light will be reflected, and that dusky whiteness appears which distinguishes mists and fogs ; but in a clear state of the atmo- sphere only the weaker and more refrangible rays, such as the blue, violet, &c. are reflected, and hence proceeds the blue colour of the sky. On the same principle depends the green colour of the sea. It is a mixed mass, charged with heterogeneous particles. All the more re- frangible rays, therefore, are reflected, while the stronger rays, the red, orange, &c. are trans- mitted. Thus Dr. Halley, in a diving-bell, sunk many fathoms deep in the sea, observed, that when he extended his hand out of the bell into the water, the upper part of it was red, and the lower part a blueish green. The redness was occasioned by the strong red rays, which in their progress through the mass of water were intercepted and reflected by his hand ; while, on the contrary, the heterogenous particles dis- persed through the water reflected only the re- Colours. 213 frangible rays, so as to afford the appearance of green. These principles applied to many other of the phenomena of nature will serve to explain their causes ; and if they excite you but to use your own understandings, and to think for your- selves, this sketch of the phenomena of light and colours may be of as essential service to you as the most laboured detail. Since the former editions of this work were published, philosophers have entered into a new field of investigation in the region of optics. Be- sides the properties of light indicated by the words reflection, refraction, and Inflection, there has recently been discovered another, denomi- nated polarization. Dr. Sebeck in Germany, Dr. Brewster in Scotland, and M. M. Malus and Biot in France, are the philosophers to whom we owe the principal discoveries in this new track of inquiry. When the particles of light traverse crystal- lized bodies, endowed with a double refraction (such, for example, as Iceland spar), they expe- rience about their centre of gravity divers mo- tions, which depend upon the nature of the forces which the particles of the crystal exercise upon them. Sometimes the effect of these forces is limited to disposing all the moleculae of the same ray similarly the one to the other, in such manner that their homologous faces are turned towards the same parts of space. This is the phenomenon to which Malus gave the name of polarization, Experimental Philosophy. [Lecture 14. assimilating the effect of the forces to that of a magnet, which should turn the poles of a series of magnetized needles all in the same direction. When this disposition obtains, the luminous par- ticles are retained in the whole extent of the crystal, and experience no farther motion about their centre of gravity. But there exist other cases where the particles which traverse the crystal are not fixed to a constant position. During all the time of their passage, they oscil- late about their centre of gravity with computa- ble velocities and periods. Sometimes, again, they turn upon themselves, as it were, with a continued motion of rotation. The various phenomena, thus briefly alluded to, are classified under the terms fixed and movea- ble polarization. The philosophers above named have established, illustrated, and confirmed them, by a great variety of striking experiments ; and some new instruments (such, for example, as the calorigrade, now sold by opticians) have ori- ginated from these researches. The train of dis- coveries connected with polarization is by no means completed. It has, however, already fur- nished a most striking confirmation of the New- tonian theory of colours, and of the rainbow, establishing their correct accordance with nature and truth, even in the minutest particulars. The best account which has yet been given to the world of the discoveries relating to polariza- tion, may be found in the fourth Vol. of Biot's Colour*. 215 Treatise on Natural Philosophy. This philoso- pher, however, has fallen into some strange errors in his explication : we, therefore, hope that Dr. Brewster, whose researches into the nature of polarization have been extensive, elaborate, and successful, will speedily favour the world with a connected view of the whole subject. LECTURE XV. EXPERIMENTAL PHILOSOPHY. THE LAWS OF MOTION. EVERY thing in mechanics depends upon very simple principles, and may be resolved ultimately into the power of gravity and the laws of mo- tion. In treating of gravitation, in our second lec- ture, it was shown to be that kind of attraction which subsists between the mass of the earth and all those bodies which are on its surface. It is that which, in the stated revolutions of this planet, prevents us, and all the bodies which surround us, from falling into infinite space ; and which draws so forcibly every thing whatever towards the centre of the earth. That this attraction is greater or less at different distances is generally allowed ; a body which at one semidiameter of the earth weighs one pound will have four times less weight at two semidia- meters, and nine times less at three. At small distances, however, we are not sensible of this difference in weight; for though we could be elevated a mile above the earth's surface, when we consider that its diameter is about eight thou- The Laws of Motion. 217 Band miles, we shall easily see that the small dif- ference which this would produce is scarcely to be estimated. Falling bodies, however, we know, acquire an accelerated or increased force, according to the height from which they are precipitated; but this mast be accounted for from different prin- ciples. Every man is sensible that the fall of a stone is to be dreaded in proportion to the height from which it descends. If it falls from only a foot above his head, it is not likely to be so fatal as if it fell from the parapet of a high house. The falling body, therefore, must of necessity acquire an increase of velocity in its descent ; and, in fact, it is said that a leaden bullet let fall from one of the steeples of Westminster Abbey ac- quired velocity sufficient to pierce through a deal board. This effect must therefore be referred to the law of acceleration conjointly with the first law of motion, as laid down by Sir Isaac Newton, which is, that " all bodies are indifferent to motion and rest : in other words, a body at rest will continue in that state, unless put in motion by some exter- nal impulse ; and a body in motion will continue that motion for ever, unless stopped by some ex- ternal obstruction." This property of matter is termed, in the technical language of philosophy, its vis intrtice. To apply this to the case immediately in point, it is evident that the bullet which is dropped VOL. i. L 218 Experimental Philosophy. [Lecture 15. from the steeple of Westminster Abbey, having, by the power of gravity, once acquired a certain degree of motion, would continue to fall, by the motion it had received by the first impulse, even if the cause were to cease. For instance, if when it had fallen halfway it were possible to deprive it of gravity, it would still, by the above law, continue its motion, and in the direction in which it was sent, as a stone continues to proceed, when thrown by the hand, without any new impulse. The power of gravity, however, does not cease, and therefore every inch the bullet falls it re- ceives an increase of motion. Thus, if in the space of one second it falls one pole (sixteen feet and a half), it will then have acquired as much swiftness or velocity as will carry it through three poles in the next second, through five in the third, through seven in the fourth, and nine in the fifth. This. will account for its accelerated motion, and for the increased force with which it falls near the bottom. Thus the time which bodies take in falling is easily calculated ; for, if they fall about one pole in the first second, which is what they nearly do by the force of gravity, they will then fall three in the next, and in five seconds they will fall about twenty-five poles, or three hundred feet. These spaces, how- ever, are a little diminished by the resistance of the air. As heavy bodies are uniformly accelerated in their descent, they are as uniformly retarded by The Laws of Motion. 219 the power of gravity in their ascent. Thus, if I were to throw the bullet up to the steeple of Westminster Abbey, I must give it just as much force as it acquired in its descent. Thus again, the body D in rolling down the inclined plane, A B (Plate XVIII. fig. 83) will acquire suf- ficient velocity by the time it arrives at B to carry it up nearly to C ; and if the plane were per- fectly smooth, and the air gave no resistance, it would carry it up quite to that point: it is upon this principle the pendulum is constructed. You all know, I conceive, that a simple pendulum consists of a bob or ball fixed to a small string or wire. If therefore the bob (fig. 84) is let go at a, it will fall to d, and by the velocity it acquires in the fall it will rise to c : this is called an oscil- lation ; and if a pendulum were put in motion in a space quite void of air, and free from all resist- ance from friction on the point of suspension, it would move for ever. Pendulums vibrate in pro- portion to the square roots of their lengths, and the vibrations of the same pendulum are always performed in the same space of time. Hence their great utility in measuring time ; for a pendu- lum of thirty-nine inches, one-fifth will vibrate an aliquot part of the time the earth is turning on its axis, that is, l-86400dth part, or sixty times in a minute. Near the equator, however, pendu- lums move slower than near the poles ; and they are also subject to variations and irregularities from heat and cold, which causes the metals, of 220 Experimental Philosophy. [Lecture 1 5, which the rods are usually formed, to lengthen of contract. It is from that sluggishness of motion, which is called the tis inertice of bodies, that there proceeds something like an endeavour in all bo- dies to preserve the state in which they are ; when at rest to continue in a state of rest, and when in motion to continue in motion. This position may seem abstruse, but it will admit of illustration by the most common facts. If I push a bowl of water with my hand, the water flies backwards over the edge upon my hand, for it endeavours to continue in the state of rest in which it was. But if I take the bowl in my hand, and run along with it, and suddenly stop short, the water flies forward the way I was run- ning, from its vis inertice, or tendency to continue in the same state of motion. In the same man- ner, if I am sitting in the front of a carriage, which, after going very fast, stops suddenly, I am jolted from my seat, and my head will, with- out care, drive through the front glass of the carriage. It is a plain and obvious principle, that the greater the quantity of matter is which any body contains, the greater will be its vis inertias. The heavier any body is, the greater is the power which is required, either to set it in motion or to stop it. So again, the swifter any body moves, the greater is its force ; as was sufficiently exem- plified in the case of a bullet, which was supposed The Laws of Motion. to fall from the steeple of Westminster Abbey. But to make the matter still plainer: if the roller a (fig. 85) leans against the obstacle b, it will be found incapable of overturning 1 L v but if a is taken up to c, and suffered to roll down the inclined plane against #, it will overturn it in- stantly. It is plain, therefore, that by its conti- nued motion the roller a has acquired a force which it had not in itself. The stroke which a strikes at b is called its momentum. Hence re- sults the well-known maxim in philosophy, which I have before had occasion to repeat to you " That the whole momentum, or quantity of force, of any moving body, is estimated by the quantity of matter multiplied by the velocity or swiftness with which it moves." When the pro- ducts, therefore, arising from multiplying the quantity of matter in any two bodies by their respective velocities, are equal, we say their mo- menta, or moving forces, are the same. Thus, if a body, which I call A, Aveighs forty pounds, and moves at the rate of two miles in a minute ; and another body, which I call B, weighs only four pounds, and moves at the rate of twenty miles in a minute, the entire force with which these two bodies will strike each other would be equal, and each of them would require an equal force to stop it. For forty multiplied by two gives eighty, the force of A ; and twenty multi- plied by four is eighty, the force of B. Upon this easy principle depends much of Experimental Philosophy* [Lecture 15. practical mechanics : and it holds universally true, that when two bodies are suspended on any machine, so as to act contrary to each other ; if the machine is put into motion, and the per- pendicular ascent of one body multiplied into its weight is equal to the perpendicular descent of the other body multiplied into its weight, those bodies, how unequal soever in their weights, will balance "one another in all situations : for, as the whole ascent of one is performed in the same time with the whole descent of the other, their respective velocities must be directly as the spaces they move through ; and the excess of weight in one body is compensated by the excess of velocity in the other. Upon this principle it is easy to compute the power of any mechanical engine, whether simple or compound; for it is but only finding how much swifter the power moves than the weight does (i. e. how much further in the same time), and just so much is the power increased by the help of the engine. The second law of motion laid down by Sir Isaac Newton is " That the alteration of the state of any body from rest to motion, or from one motion to another, is always in proportion to the force which is impressed, and in the direc- tion of that force." All motion is naturally rectilinear. A bullet projected by the hand, or shot from a cannon, would for ever continue to move in the same direction it received at first, if no other power The Laws dfMotfai. diverted its course. When therefore we see a body move in a curve of any kind whatever, we conclude it must be acted upon by two powers at least ; one putting it in motion, and another drawing it away from the rectilinear course in which it would otherwise have continued to move : and whenever that power, which bent the motion of the body from a straight line into a curve, ceases to act, the body will again move on in a straight line touching that point of the curve in which it was when the action of that power ceased. For example, a pebble moved round in a sling ever so long a time, will fly off the moment it is set at liberty, by slipping one end of the sling cord : and will go on in a line touching the circle it described before; which line would actually be a straight one, if the earth's attraction did not affect the pebble, and bring it down to the ground. This shows that the natural tendency of the pebble, when put into motion, is to continue moving in a straight line, although by the force that moves the sling it is made to revolve in a circle. From this maxim it will evidently appear, that when two forces act at once upon the same body, in different directions, it will go in neither, but in a course between both. If the billiard ball a (fig. 86) is struck at once by the two cues b and f, it will be impelled forward in the diagonal or middle line, whereas b would have impelled it in the line e, and c in the line d. Experimental Philosophy. [Lecture 15. Or if a boat (fig. 87) is drawn up the stream by two men on the opposite banks, it will follow the direction of neither exactly, but will proceed directly in the middle of the stream. Suppose again (PL XIX. fig. 88) the body A to represent a ship at sea ; and that it is driven by the wind, in the right line AB, with such a force as would carry it uniformly from A to B in a minute : then suppose a stream or current of water running in the direction AD, with such a force as would carry the ship through an equal space from A to D in a minute. By these two forces, acting together at right angles to each other, the ship will describe the line AEC in a minute ; which line (because the forces are equal and perpendicular to each other) will be the diagonal of an exact square. If the acting forces are equal, but at oblique angles to each other, so will the sides of the parallelogram be : and the diagonal run through by the moving body will be longer or shorter, according as the obliquity is greater or smaller. Thus, if two equal forces act conjointly upon the body A 3 (fig. 89) one having a tendency to move it through the space AB in the same time that the other has a tendency to move it through an equal space AD ; it will describe the diagonal AGC in the same time that either of the single forces would have caused it to describe either of the sides. If one of the forces is greater than the other ; then one side of the parallelogram will The Laws of Motion. 225 be so much longer than the other. For if one force singly would carry the body through the space A E, in the same time that the other would have carried it through the space A D, the joint action of both will carry it in the same time through the space A H F, which is the diagonal of the oblique parallelogram A D E F. If both forces act upon the body in such a manner, as to move it uniformly, the diagonal described will be a straight line ; but if one of the forces acts in such a manner as to make the body move faster and faster, then the line de- scribed will be a curve. And this is the case of all bodies which are projected in rectilinear direc- ^tions, and at the same time acted upon by the power of gravity, which has a constant tendency to accelerate their motions in the direction wherein it acts. This last is an observation of great importance, as it is the foundation of the beautiful system of Newton concerning the planetary motions. The force which impels these bodies forward in a rec- tilinear direction, is called the projectile or the centrifugal force, as driving them from the centre ; and the force which draws it towards the centre, or the power of gravity, is called the centripetal force. Thus, if the body A (fig. 90) is projected along the straight line A F H in open space, where it meets with no resistance, and is not drawn aside by any power, it will go on for ever with the same velocity, and in the same direction. But L5 226 Experimental Philosophy. [Lecture 1 5. if, at the same moment the projectile force is given it at A, the body S begins to attract it with u force duly adjusted*, and perpendicular to its motion at A, it will then be drawn from the straight line AFH, and forced to revolve about S in the circle ATW; in the same manner, and by the same law, that a pebble is moved round in a sling. And if, when the body is in any part of its orbit (as suppose at K), a smaller body, as L, within the sphere of attraction of the body K, is projected in the right line LM, with a force duly adjusted, and perpendicular to the line of attraction LK; then the small body L will revolve about the large body K in the orbit NO, and accompany it in its whole course round the yet larger body S. Here S may represent the sun, K the earth, and L the moon. But of this we shall treat more at large in the lectures on astronomy. These principles will serve to explain many facts which will come from time to time under your observation. Thus if a leaden ball is dropt from the mast-head of a ship, under swift sail, you would suppose, before the ball would reach the deck, the ship would be slid from under it, and that it would fall behind the ship into the sea, * To make the projectile force a just balance to the gravitating power, so as to keep the planet moving in a circle, it must give such a velocity as the pl.met would acquire by gravity, when it had fallen through half the femidiameter of that circle. The Laws of Motion. 227 This is not the fact ; for the ball falls down by the side of the mast, as if the ship were at anchor. Why? Because the ball is under the influence of two forces ; one horizontal, by the motion of the ship, which is the same as if you had sent it forwards from your hand with the same degree of velocity as the ship moves at ; the other force is perpendicular, by the power of gravity : so that though it appears to fall perpendicularly, it does not, but describes, in space, the same kind of semi-parabola as a ball shot from a gun. If I throw a log of wood into the Thames, when the wind is across the river, the log will not obey the current, by going down the river, nor the wind, by going across the river, but will go in an oblique direction made up of the two. The third law is, that " re-action is always equal to action." Thus, in consequence of this principle, the resistance of a body at rest, which is acted or pressed upon, acts against a moving body with a certain degree of power, and produces the same effects as an active force would have done in the same direction. Thus, if I strike an anvil with a hammer, the anvil exerts against the hammer the same force with which it is struck itself. Hence a common trick in the country, of a man lying on die ground with a large anvil on his breast, and suffering a strong man to strike it with a sledge hammer with all his might. If the anvil be very large, its vis inertke resists the force of the blow, and the man is 228 Experimental Philosophy. [Lecture 15. perfectly safe. If the anvil were very small, only the weight of a pound or two, the first stroke would kill the man. A pretty experiment of Mr. Walker's will serve also to illustrate this part of the subject. " Let a be a little cannon, (PI. XX. fig. 91.) and b a hollow piece of iron or brass, to slip on pretty tight upon c c, and of the same weight as a. Now if half a thimbleful of gunpowder be put in a, and b shut upon it, both being sus- pended by two strings ; if the powder is fired, the parts a and b will be thrown equally distant from r, the center where they hung; showing the re-action to be equal to the action. Hence a heavy gun seems to recoil less than a light one, on account of its greater vis inertice ; otherwise its re-action is the same, with the same charge." Hence it is evident, that when a load is drawn by a horse, the load acts against the motion of the horse, and the action of the animal is as much impeded by the load, as the motion of the load is promoted by his efforts. Many other illustra- tions of these laws may be seen in the larger treatises of mechanics. Before I proceed to the consideration of the six mechanic powers, it is necessary to say a few words on what is called the centre of gravity. The centre of gravity is that point of a body in which the whole force of its gravity or weight is united, and to which its action may usually be referred. Whatever, therefore, supports that The Laics of Motion. point, bears, in fact, the weight of the whole body ; and while it is supported the body cannot fall, because all its parts are in perfect equilibrium about that point. Thus, if I endeavour to balance my cane, by laying it across upon my finger, after some time I find a place where neither end will preponderate. The part, then, which rests upon my finger is the centre of gravity. An imaginary line drawn from the centre of gravity of any body towards the centre of the earth, is called the line of 'direction , and it is in this line all heavy bodies will descend. The difficulty of sustaining a tall body upon a narrow foundation will be evident, if you attempt to balance your cane with its small end upon your finger. Its centre of gravity is somewhere about the middle of the cane, and unless you have sufficient dexterity to keep the foundation on your finger perpendicular under the centre of gravity, it will undoubtedly fall. In this consists the great difficulty of posture-masters and rope- dancers. The dancer on the rope balances him- self by a long pole loaded at both ends with lead, and keeps his eye steadily on some point exactly in the line of the rope, by which he can see whether his centre of gravity is either on one side or the other of his slippery foundation, and if any irregularity takes place he rectifies it by his balancing pole. Every body stands firm on its base, when the Experimental Philosophy. [Lecture 15. direction falls within such base ; for in this /the body cannot be made to fall, without first raising the centre of gravity higher than it was before. Thus, the inclining body ABCD, (fig 92.) whose centre of gravity is E, stands firmly on its base CDIK, because the line of direction EF falls within the base. But if a weight, as ABGH, is laid upon the top of the body, the centre of gravity of the whole body and weight together is raised up to L; and then, as the line of direction ID falls without the base at D, the centre of gravity I is not sup- ported ; and the whole body and weight tumble down together. As a practical illustration of this, I shall mention that the tower of Pisa (fig. 93.) leans sixteen feet out of the perpendicular, and stran- gers are consequently afraid to pass under it. If, however, the materials will hold together, there is no necessity for any such apprehension. For if the plummet c is let fall from its centre of gra- vity, you will see that the line of direction is within its base or foundation, and therefore it has stood without a miracle these three hundred years. The nearer the centre of gravity and the line of direction coincide, the firmer any body stands upon a horizontal plane. If the plane is inclined a body will slide down it, if the line of direction falls within the base; but it will tumble down The Laws of Motion. 231 when that line falls without the base. Thus the body A (fig. 94.) slides down the inclined plane C D, while the body B rolls down upon it. The broader the base the firmer any body stands ; thus you find you stand firmer with your feet a little asunder than when close together ; and in the former case it will require a much greater force to push you down. Hence the advan- tage of walking with the feet rather wide asunder, on a slippery pavement in frosty weather. When- ever the line of direction, however, falls without the base of our feet, we necessarily fall ; " and it is not only pleasing," says Mr. Ferguson, " but even surprising, to reflect upon the various and unthought-of methods and postures which we use to retain this position, or to recover it when it is lost. For this purpose we bend our body forward when we rise from a chair, or when we go up stairs: and for this purpose a man leans forward when he carries a burden on his back, and backwards when he carries it on his breast ; and to the right or left side as he carries it on the opposite side." A thousand more in- stances might be added, but they will readily suggest themselves to the mind of reflecting persons. / LECTURE XVI. EXPERIMENTAL PHILOSOPHY. THE MECHANIC POWERS. MAN, considered as to his bodily structure, is but a feeble creature ; it is mind which gives him a superiority over other animals. Con- trivances to assist his natural powers we have rea- son to believe took place at a very early period of society, as we find few nations, even in the most savage state, which are entirely without them. It is philosophy, however, which explains their theory and uses, and which extends their appli- cation. When we survey the vast variety of complex machines, which one of our great manufactories, for instance, exhibits, we are struck with astonish- ment, and the creative genius of man appears to the greatest advantage ; but the surprise of the unscientific person will be increased, when he learns that this vast assemblage of mechanism is reduced into six simple machines or powers, from which, and their different combinations, the most stupendous works of human art are produced. These machines are ; 1. the lever ; 2. the wheel and axle ; 3. the pulley ; 4. the inclined plane ; 5. the wedge ; and 6. the screw. 1. The lever is, perhaps, the simplest of all Mechanic Powers. the mechanic powers, and was probably the first which was brought into use. It is a bar of iron or wood, one part of which is supported by a prop, and upon that prop all the other parts turn as on their centre of motion. You see the lever made use of in one form or other every day when a labourer takes a hand-spike, or large stake, and placing a stone under some part near the end, by putting the extremity under a cask, a piece of timber, or any other body, and attempts to move it, by pulling at the other end, he makes use of a lever. The handle of a pump is a lever also ; even the poker with which I raise the fire is a lever, the bar of the grate is the prop, and at the end which I hold in my hand is applied the strength or power. This is, however, not the only kind of lever, for in fact there are three different sorts or orders of these instruments. The first is that which I have been describing, viz. when the prop is placed between the weight to be raised and the power (see fig. 95.) In this figure ABC is the lever; D is the fulcrum or prop; and the part AB and BC, on different sides of the prop, are called the arms of the lever. It is demonstrable that in this instrument the nearer the prop is to the end A, and the longer the arm BC is, the less force will be required to effect any given purpose. This is, indeed, re- duced to a matter of experiment. For let P repre- sent a power, whose gravity is equal to one ounce; and W a weight, whose gravity is equal to twelve 534* Experimental Philosophy/. [Lecture 16. ounces. Then, if the power is twelve times as far from the prop as the weight is, they will ex- actly counterpoise ; and a small addition to the power P will cause it to descend, and raise the weight W; and the velocity with which the power descends will be to the velocity with which the weight rises, as twelve to one : that is, directly as their distances from the prop ; and consequently, as the spaces through which they move. Hence it is plain that a man who by his natural strength, without the help of any machine, could support a hundred weight, will by the help of this lever be enabled to support or rather raise twelve hun- dred. If the weight is less, or the power greater, the prop may be placed so much farther from the weight, and then it can be raised to a prpportion- ably greater height. For, universally, if the in- tensity of the weight multiplied into its distance from the prop is equal to the intensity of the power multiplied into its distance from the prop, the power and weight will exactly balance each other ; and a little addition to the power will raise the weight. Thus, in the present instance, the weight W is twelve ounces, and its distance from the prop is one inch ; and twelve multiplied by one is twelve; the power P is equal to one ounce, and its distance from the prop is twelve inches, which multiplied by one is twelve again ; and therefore there is an equilibrium between them. So, if a power equal to two ounces is ap- plied at the distance of six inches from the prop, Mechanic Powers. 35 it will just balance the weight W; for six multi- plied by two is twelve, as before. And a power equal to three ounces placed at four inches dis- tance from the prop would be the same ; for three times four is twelve; and so on, in pro- portion. The statera, or Roman steelyard, is a lever oif this kind, and is used for finding the weights of different bodies by one single weight placed at different distances from the prop or centre of mo- tion D. For if a scale hangs at A, the extremity of the shorter arm, AB, is of such a weight as will exactly counterpoise the longer arm EC ; if this arm is divided into as many equal parts as it will contain, each equal to AB, the single weight P (which we may suppose to be one pound) will serve for weighing any thing as heavy as itself, or as many times heavier as there are divisions in the arm BC, or any quantity between its own weight and that quantity. As for example, if P is one pound, and placed at the first division, one in the arm BC, it will balance one pound in the scale at A ; if it is removed to the second division at two, it will balance two pounds in the scale ; if to the third, three pounds ; and so on to the end of the arm BC. If each of these in- tegral divisions is subdivided into as many equal parts as a pound contains ounces, and the weight P is placed at any of these subdivisions so as to counterpoise what is in the scale, the pounds and odd ounces will by that means be ascertained. 236 Experimental Philosophy. [Lecture 16. To this kind of lever may be reduced several sorts of instruments, such as scissars, pincers, snuffers, which are made of two levers acting contrary to one another, their prop or centre of motion being the pin which keeps them together. The second kind of lever has the weight to be raised between the prop and the power. Thus, in raising the water-plugs in the streets of Lon- don, you will see the workman put his iron crow through the hole of the plug till he rests the fur- ther extremity of it on the ground, and making that his prop, he raises the lever or crow, and draws out the plug. In this lever, as in the for- mer, the longer the arm of the power is, or the greater the distance of the workman from the weight, the more is his natural force assisted by the machine. To estimate this, if A B (fig. 96.) is a lever on which the weight W of six ounces hangs at the distance of one inch from the prop G, and a power P equal to the weight of one ounce hangs at the end B, six inches from the prop, by the cord CD going over the fixed pulley E, the power will just support the weight ; and a small addition to the power will raise the weight one inch for every six inches that the power descends. This lever shows the reason why two men car- rying a burden upon a stick between them, bear unequal shares of the burden in the inverse pro- portion of their distances from it. For it is well known, that the nearer any of them is to the Mechanic Powers. 237 burden the greater share he bears of it ; and if he goes directly under it, he bears the whole. So if one man is at G, and the other at B, having the pole or stick AB resting on their shoulders; if the burden or weight W is placed five times as near to the man at G, as it is to the man at B, the former will bear five times as much weight as the latter. This is likewise applicable to the case of two horses of unequal strength to be so yoked, as that each horse may draw a part pro- portionate to his strength ; which is done by so dividing the beam they pull, that the point of traction may be as much nearer to the stronger horse than to the weaker, as the strength of the former exceeds that of the latter. To this kind of lever may be reduced oars, rudders of ships, doors turning upon hinges, cutting-knives which are fixed at the point of the blade, &c. The third kind of lever is when the power is placed between the weight arid the prop. An example of this kind of lever you see when a man raises a long ladder to place it against a wall. It is obvious that this kind of lever, so far from assisting human strength, requires a power much greater than the weight to be raised. For let E (fig. 97.) be the prop of the lever AB, and W, a weight of one pound, placed three times as far from the prop, as the power P acts at F by the cord C going over the fixed pulley 238 Experimental Philosophy. [Lecture 16. D ; in this case the power must be equal to three pounds, in order to support the weight. Disadvantageous as this kind of lever appears, it is upon this principle the human arm is con- structed ; for the muscle which moves the arm, and which is inserted in the bone below the elbow, may be considered as the power, which you see is placed between the weight to be raised by the hand and the prop, or place where the muscle is inserted above. To compensate for this disadvantage, these muscles are made unusu- ally strong, and we may judge of their immense power by the weights which athletic persons are enabled to wield. The same power exerted only on equal terms ought to raise a weight of ten thousand pounds. II. The wheel and axle (fig. 98.) is the next in order of the mechanic powers. The power is, in this machine, applied to the circumference of the wheel, and the weight to be raised is fastened to one end of a rope, of which the other end winds round an axle that turns with the wheel. This instrument is more commonly used with a handle : thus, to wind up a common kitchen jack, I turn the handle, which coils the cord round the axle in the middle : to wind a bucket from a well, I do the same thing ; to wind up my watch, the same : the handle in all these is in the place of a wheel, and the farther this handle is from the centre, the axle, on which the Mechanic Powers. 239 whole weight is sustained, the more powerful will it be. Or if it is a wheel, the more its dia- meter exceeds the diameter of the axle, the greater will he its power. Thus, if the diameter of the wheel is eight times as great as that of the axle, it will have eight times the power ; and a man who by his natural strength could only lift a hundred weight, by this machine will be en- abled to lift eight hundred. Of this kind are the machines called cranes, which you see employed at the water-side, for winding up bales of goods out of ships. The large circular crane, in which a man or horse walks and turns it horizontally, is also a machine of this nature; and the capstan^ which draws up the cables of ships, and is turned by hand-spikes inserted in holes at the end of the roller or cap- stan. The windlass, also used in warehouses for raising goods, is the wheel and axle ; and, indeed, many more complex machines may be resolved into this principle. The spokes of the wheel, or the winch which turns the axle, may be considered as levers, and therefore by some the wheel and axle are referred to the same principle. III. The pulley is usually considered as the third mechanic power, though, in truth, the single pulley AA (fig. 99.) gives no mechanical advantage, and only enables us to change the direction. This is evident from the figure, where the two equal weights W and P balance each 40 Experimental Philosophy. [Lecture 1(5. other as exactly as the arms of a balance or scale beam, which are of equal lengths. Thus it gives a man no advantage, except that he can apply his weight as well as his strength in rais- ing a body from the earth, and then he can lift more than his own weight. With a combination of pulleys, however, the case is different. For if a weight W hangs at the lower end of the moveable pulley D, and the cord GF goes under the pulley, and is fixed at the top of the hook H on one side, and nailed to the block C on the other ; it is evident that H and C between them support the whole weight W ; H supports one half, and C the other half. Now suppose I take the support of one of their halves upon myself, but merely change the direc- tion of my power, and instead of holding up the cord at C, throw it over the immoveable pulley fixed there, and exert my strength below at P; it will be evident that I support one half the weight W, and the hook H supports the other. If therefore I draw the cord at P, the weight W will continue to rise, but wherever it rises, I con- tinue to support but half its weight while H sup- ports the other. Thus, one single moveable pulley diminishes one half of the weight to be raised ; if we should add another, it would di- mmish the half of that which remained, and so on. For instance, if a weight of eight hun- dred pounds is to be raised, I use one moveable pulley, and that will lessen the weight one hah , Mechanic Powers. that is, to four hundred : I add another move- able pulley, and that will lessen the remaining four by one half, which is two hundred ; if I still add a third, that will lessen the remaining two by one hah , which is one ; so that if I use three moveable pulleys in raising eight hundred weight, I shall be able to raise it with as much ease as one hundred without them. As systems of pulleys have no great weight, and lie in a small compass, they are easily car- ried, and can be used in many cases where more cumbrous engines cannot. They have much friction, however, because the diameter of their axis bears a very considerable proportion to their own diameter, because they are apt to rub against each other, or against the sides of the block, and because the rope that goes round them is never perfectly pliant. Still they are highly useful, and their combinations may be varied at pleasure, to suit the case in hand, whe- ther at land or sea. IV. The inclined plane is very justly regarded as the fourth mechanic power, though some have rejected it altogether. The advantage of this machine (if you will admit of that term) is, that by means of it a heavy body may be made to ascend a given height with much less power than it would require to raise it the same height if it were perpendicular. This is a very common mode of assisting human strength ; you will every day see porters, when they have to roll a cask or VOL. i. M Experimental P?iilosophi/. [Lecture 16. bale up the step of a warehouse, place a board along from the step to the ground, which ren- ders the ascent gradual and easy. The power of the inclined plane is as great as its length exceeds its perpendicular height. For instance, let AB (PI. XXII. fig. 100) be a plane parallel to the horizon, and CD a plane inclined to it ; and sup- pose the whole length CD to be three times as great as the perpendicular height AC ; in this case the cylinder E will be supported upon the plane CD, and kept from rolling down upon it by a power equal to a third part of the weight of the cylinder. Therefore, a weight may be rolled up this inclined plane with a third part of the power which would be sufficient to draw it up by the side of an upright wall. If the plane were four times as long as high, a fourth part of the power would be sufficient ; and so on, in pro- portion. Or, if a weight were to be raised from a floor to the height AC, by means of the ma- chine ABCD, (which would then act as a half wedge, where the resistance gives way only on one side) the machine and weight would be in equilibrio when the power applied at AC was to the weight to be raised as AC to AB ; and if the power is increased, so as to overcome the friction of the machine against the floor and weight, the machine will be driven, and the weight raised ; and when the machine has moved its whole length upon the floor, the weight will be raised to the whole height from A to C. Mechanic Powers. 243 V. The wedge is nearly allied to the inclined plane ; indeed it may properly be considered as two equally inclined planes joined together. You know that its uses are to cleave or separate wood or stone, or any heavy bodies that adhere toge- ther. The power of the wedge is as its length to the thickness of its back. To show how we may calculate the force of a wedge, let a (fig. 101) be a wedge, which is interposed between the two cylinders c and w, which are pulled against the wedge by the two weights r and s, represent- ing the resistance to be overcome by the force of the wedge. If then r and s influence the cylin- ders each with a force equal to two pounds, the resistance to be overcome will be equal to four pounds. Now the length of the wedge a is twice the thickness of its back, and the weight o, suspended to it, is two pounds. Here, then, is a resistance equal to four pounds overcome by a weight of two pounds, by means of a wedge, the length of which is double the thickness of its back. This explains sufficiently what a wedge will be able to effect by simple weight or pres- sure ; but we see every day, where a hard stone or a piece of tough wood is to be cleft by a wedge, that a ton weight would not force it in, when a smart stroke of a hammer, which has not a for- tieth part of that weight, will effect it at once. In this case we are to have recourse to what was said in the last lecture on the momentum or force which is gained by the velocity of a moving 244 Experimental Philosophy. [Lecture 16. body, and consider that the momentum of a hammer consists of its weight multiplied by the velocity with which it moves (which is consi- derable), and then the effect will appear less ex- traordinary. It is by means of the momentum of the hammer striking with considerable ve- locity, that the wedge is driven in ; and then its friction keeps it from slipping out again. VI. The screw (fig. 102) may properly be con- sidered as an inclined plane wrapt round a cy- linder. The power of the screw is therefore as the length of each spiral or thread is to its height, or, in other words, as the circumference of the threads to their distance from one another. The screw, however, can only be wrought by means of a handle or winch, which is, in fact, a lever, and it may, therefore, be regarded as a com- pound machine. To estimate its force, then, let us suppose that I desire to screw down the press G upon B ; every turn I make once round with both handles, I shall drive the press only one spiral nearer to B ; so that if there are eleven spirals, I must make eleven turns of the handles, FL, before I come to the bottom. In pressing down the screw, therefore, I act with a force as much superior to the resistance of the body I de- sire to press, as the circumference of the circle, which my hands describe in turning the machine, exceeds the distance between two little spirals of the screw. For instance, suppose the distance between the two spirals to be half an inch, and Mechanic Powers. the length of both handles twelve inches. My hands placed upon them in going round will de- scribe a circle, which, upon calculation, will be found to be seventy-six inches nearly, and con- sequently this will be an hundred and fifty-two times greater than half an inch, which was the distance between two of the spirals. Thus, if a bodyjis to be pressed down with this machine, one man will press it, with this assistance, as much as an hundred and fifty-two men without it. Or if the screw were so contrived as to raise the weight instead of pressing it, which sometimes is the case, the human force would be assisted in the same proportion with the same instrument. But we here only speak as if the handles of the screw were but twelve inches across, and the spirals a whole half inch distant from each other ; what if we suppose the handles ten times as long, and the spirals five times as close ; the increase of the human force then would be astonishing. The power of the screw may, however, be still more correctly estimated by t what is called the perpetual screw. To explain this, let the wheel C (fig. 103) have a screw db on its axle, work- ing in the teeth of the wheel D, which suppose to be forty-eight in number. It is plain, that for every time the wheel C and screw ab are turned round by the winch A, the wheel D will be moved one tooth by the screw; and, there- fore, in forty-eight revolutions of the winch, the 246 Experimental Philosophy. [Lecture 16. wheel D will be turned once round. Then, if the circumference of a circle described by the handle of the winch A is equal to the circum- ference of a groove e round the wheel D, the velocity of the handle will be forty-eight times as great as the velocity of any given point in the groove. Consequently, if a line goes round the groove e, and has a weight of forty-eight pounds hung to it below the pedestal EF, a power equal to one pound at the handle will balance and sup- port the weight. To prove this by experiment, let the circumferences of the grooves of the wheels C and D be equal to one another ; and then if a weight of one pound is suspended by a line going round the groove of the wheel C 5 it will balance a weight of forty-eight pounds hanging by the line g ; and a small addition to the weight H will cause it to descend, and so raise up the other weight. If the line g, instead of going round the groove e of the wheel D, goes round its axle I, the power of the machine will be as much in- creased as the circumference of the groove e exceeds the circumference of the axle: which, supposing it to be six times, then one pound at H will balance six times forty-eight, or two hun- dred and eighty-eight pounds hung to the Jme on the axle ; and hence the power or advantage of this machine will be as two hundred and eighty-eight to one. That is, a man who, by Mechanic Powers. 247 his natural strength, could lift a hundred weight, will be able to raise two hundred and eighty- eight hundred weight, or 1 4 tons 8 hundred, by this engine. But the following engine is still more power- ful, on account of its having the addition of four pulleys ; and in it we may look upon all the mechanical powers as combined together, even if we take in the balance. For as the axle D of the bar AB (fig. 104) enters its middle at C, it is plain that if equal weights are suspended upon any two pins equi-distant from the axis C, they will counterpoise each other. It becomes a lever by hanging a small weight P upon the pin n 9 and a weight as much heavier upon either of the pins b 9 d y or e, as is in proportion to the pins being so much nearer the axis. The wheel and axle FG is evident ; so is the screw E which takes in the inclined plane, and with it the half wedge. Part of a cord goes round the axle, the rest under the lower pulley K, over the upper pulley L, under AT, over /, and then it is tied to a hook at M in the lower or moveable block, on which the weight W hangs. In this machine, if the wheel F have thirty teeth, it will be turned once round in thirty re- volutions of the bar AB, which is fixed on the axis D of the screw E : if the length of the bar be equal to twice the diameter of the wheel, the pins e and n at the ends of the bar will move sixty times as fast as the teeth of the wheel do ; 248 Experimental Philosophy. [Lecture 16. and, consequently, one ounce at P will balance sixty ounces hung upon a tooth q in the ho- rizontal diameter of the wheel. Then if the diameter of the wheel F be ten times as great as the diameter of the axle G, the wheel will have ten times the velocity of the axle ; and therefore one ounce P at the end of the lever AB will balance ten times sixty, or six hundred ounces hung to the rope H which goes round the axle. Lastly, if four pulleys are added, they will make the velocity of the lower block K, and weight W, four times less than the velocity of the axle ; and this being the last power in the machine, which is four times as great as that gained by the axle, it makes the whole power of the machine four times six hundred, or two thousand four hundred. So that if a man could lift one hun- dred weight in his arms by his natural strength, he would be able to raise two thousand four hun- dred times as much, or 120 ton weight, by this engine. But it is here as in all other mechanical cases ; for the time lost is always as much as the power gained, because the velocity with which the power moves will ever exceed the velocity with which the weight rises, as much as the intensity of the weight exceeds the intensity of the power. The friction of the screw itself is very consi- derable ; and there are few compound engines which will not, upon account of the friction of the parts against one another, require a third part more of power to work them when loaded. Mechanic Powers. 249 than what is sufficient to constitute a balance be- tween the weight and the power. Some philosophers have considered the wheel and axle, and the system of pulleys, as only mo- difications of the lever; and the wedge and the screw as (modifications of the inclined plane. If this be admitted, we shall then have, instead of six, only two mechanical powers. The mo- difications and combinations of these are, how- ever, almost endless, and wonders are performed, when to these means of increasing force are added the most powerful agents in nature, wind, water, and steam, as exemplified in the wind- mill, the water-mill, and, above all, the steam- engine. If the simple and obvious principles I have here elucidated shall assist the student in ' estimating the advantage of the more common machines, and stimulate him to pursue his re- searches into the manner of operation of the more complex engines to which I have just ad- verted, these explications will not have been given in vain. M 4 LECTURE XVII. ASTRONOMY. . SYSTEM OF THE UNIVERSE. ASTRONOMY is that science which treats of the heavenly hodies. It is by means of this science that we know the movement of those bodies, the duration of their revolutions, whether apparent or real, their position, their respective distances, &c. The origin of astronomy is very obscure, and appears to be also very antient. " There is no doubt," says Cassini *, " but that astronomy was known almost from the beginning of the world. It was not only curiosity which led man to the study of astronomy, but it may be said that ne- cessity itself obliged him to it. For if he did not observe the seasons which result from the apparent changes of the sun's place, it would be impossible to succeed in the practice of agri- culture and other useful arts." Astronomy, even if it could be considered as useless to man, derives from its very nature a cer- tain degree of dignity. But let it be remembered, that upon it navigation, geography, and chrono- logy greatly depend. By its aid man passes the * Memoirs of the Academy of Sciences, vol.vni. page 1. System of the Universe. 251 seas, and penetrates into foreign climates, be- comes acquainted with those which he inhabits, and regulates the dates of ages past. Hipparchus laid the principal foundations of a methodical system of astronomy one hundred and forty-seven years before Christ. On the ap- pearance of a new fixed star, he took occasion to make a general catalogue of the stars, assigning to each its place in the heavens, and its mag- nitude, so as to enable posterity to ascertain, whether any new star had appeared, or any of those which he had observed had suffered any change. Ptolemy, about two hundred and eighty years afterwards, added his observations to those of Hipparchus ; and by the natural advantage which he possessed over his predecessor, he was enabled to rectify greatly the observations of the former philosopher. Ptolemy was the last of the Greeks who made any considerable improve- ments in the science of astronomy. It was after- wards cultivated by the Arabians with great assi- duity, and success, but did not meet with any encouragement in Europe till about the middle of the 13th century. At this period Alphonsus the Tenth, king of Castile, became its zealous patron, and immortalized himself by a series of astronomical tables, which were published under his direction, and were distinguished by the name of the Alphonsine tables. It was not, however, till the sixteenth century that astronomy was placed upon its proper basis 252 Astronomy, [Lecture IT. as a science, by the system of Copernicus*, pub- lished at Nuremberg in 1543, and afterwards brought to perfection by Kepler, Galileo, and Newton : a system so bold and daring, that it produced general astonishment, and yet its truth has been confirmed by the observations of every succeeding age. The surface of the heavens seems to us to be studded with stars ; between the fixed stars and us there seem to be other stars which change their situations respectively one towards another, and these all astronomers have agreed in calling planets^ or wandering stars. The antient philosophers, who knew very little even of the movements of the planets, had no means of knowing the true disposition of their orbits ; and this is the reason they vary so greatly in their opinions. They supposed, at first, the earth to be immoveable, as the centre of the universe, and that all the celestial bodies turned about her; which, indeed, was natural for them to believe, without having discussed the proofs to the contrary. It is asserted, however, that the Babylonians, and afterwards Pythagoras and his disciples, considered the earth as a planet, and the sun as immoveable, and the centre of our planetary system. Plato is said to have been the reviver of the system of the immobility of the earth ; and many * Born at Thorn, in Royal Prussia, in 1472- System of the Universe. 255 philosophers followed his opinion ; among others was Claudius Ptolemy, the celebrated astronomer and mathematician of Pelusium in Egypt, already mentioned, who lived in the beginning of the second century of the Christian sera. It is, how- ever, incredible that, the true system of the world having been once discovered, the hypothesis by which the earth is supposed to be the centre of the celestial movements should have again pre- vailed ; for though this hypothesis accords with some of the most obvious appearances, and seems to agree at first with the simplicity of nature, yet it is impossible on that system to account for all the celestial phsenomena. Ptolemy, who has given the name to this system, endeavours to prove that the earth T ( PL XXIII. fig. 105) is immoveable as the centre of the universe ; and he places the other planets round about her in the following order, beginning with those which he believes the next to the Earth : the Moon D , Mercury $ , Venus ? , the Sun 0, Mars $ , Jupiter 1, and Saturn T? 5 till he comes at length to the fixed stars. When, however, astronomers had begun to observe the planets, they remarked that Mercury and Venus are sometimes nearer and sometimes farther from us than the Sun ; and that Venus never departs from the Sun more than about forty-seven de- grees and a half; and Mercury about twenty- eight degrees and a half, and sometimes much less. But it is evident that if these two planets Astronomy. ^Lecture 17, were turned about the Earth, as they supposed the Sun himself turned, they would sometimes appear opposite to the Sun, or more distant from him than one hundred and eighty degrees; which never happens. This is the reason why the Egyptians regarded these two planets as satellites of the Sun, and thought that they turned about him, their orbits being carried with him in his revolutions about the Earth. They therefore supposed the Earth T (fig. 106) im- mov cable, as the centre of the system ; and they supposed the other celestial bodies to turn round her : first, the Moon D ; secondly, the Sun ; about which they made Mercury $ and Venus to revolve, till they came to Mars $ , Jupiter i;, and to Saturn T?; and lastly to the fixed stars. At the present day, however, when we know the immense distance at which the stars are placed, both these systems become insupportable. They require that all the heavenly bodies should go through the whole course of their orbits in about 24 hours, which would give to the fixed stars a rapidity of motion that exceeds all belief: nay, the Sun himself would in a single second have to describe a space of more than two thousand five hundred miles. Copernicus, with a view of obviating the inconveniences of the imaginary systems that preceded him, commenced at first by admitting the diurnal motion of the Earth, or her motion System of the Universe. 255 round her own axis, which rendered useless that prodigious celerity in the motions of the heavenly bodies, of which I have just spoken, and by these means simplified the system. This motion once admitted, it was no violent step to admit of a second motion of the Earth in the ecliptic. These two motions explain, with the utmost faci- lity, the phenomena of the stations and motions of the planets. According to Copernicus, then, the Sun S (PI. XXIV.%. 107) is the centre of our planetary system, and the planets turn about him in the order following ; Mercury g , Venus ? , the Earth J, Mars , Jupiter 1, Saturn T? , (to which we may add Ceres, Pallas, Juno, Vesta, and the Georgium Sidus $) at a distance from the Sun, nearly as the numbers 4, 7, 10, 15, 52, 95, 191* The Moon, also, he supposed to be carried round the Earth in an orbit which goes along with the Earth in her annual revolution round the Sun. In like manner about Jupiter, Saturn, and the Georgium Sidus, are the -four satellites of the first, the five satellites of the second, and the two satellites of the third ; none of which, however, were known to Copernicus. Although the celestial phenomena explain themselves with the greatest facility according to the system of Copernicus, and though observa- tion and reason are equally favourable to it, yet it was rejected by an able astronomer who flou- rished soon after his own time. Tycho-Brahe, from the experiment that a stone thrown from a 256 Astronomy. [Lecture 17. high tower fell at its foot, argued that the Earth must be without motion, never reflecting that the Earth, in that case, is like a vessel in full sail, when if a stone is thrown from the mast, it would fall at the foot of that mast, provided the motion of the vessel was neither accelerated nor retarded during the fall. Tycho-Brahe, therefore, invented a system between that of Ptolemy and that of Copernicus. He supposed that the Earth was at rest, and that the other planets revolving round the Sun, turned also with him round the Earth in twenty-four hours. It was towards the end of the sixteenth century that he proposed his system. He placed the Earth (fig. 108) immoveable, as the centre, and made the Moon turn round her, as well as the Sun S, and the fixed stars : the other planets, viz. Mercury, Venus, Mars, Jupiter, and Saturn, turning round the Sun, in orbits which are carried with him in his revolution round the Earth. As the system of Tycho-Brahe requires the same rapidity of motion as that of Ptolemy and of the Egyptians, it is at once annihilated by the same arguments. Leaving, however, for the present the history of astronomical discoveries, I shall request your attention to the celestial phenomena. There are evidently two sorts of stars ; the one luminous of themselves, and throwing light on every object which surrounds them to a certain distance ; such as our Sun, and those which we System of the Universe. 257 call fixed stars. The others are opake bodies, as the Earth which we inhabit, not luminous of themselves, but which shine by a borrowed light ; in few words, luminous by reflecting that light which comes from a luminous star: such are the planets of the first and second order, and the comets. The stars of the firmament are said to be fixed, because they have been generally observed to pre- serve the same distance from each other : they do not all appear to us of the same magnitude, whether they are really different in size one from the other, or whether they appear so to us in consequence of their different distances. It is probable that both these causes operate to exhibit the fixed stars of such various magnitudes. Be this as it may, astronomers have agreed in distri- buting the fixed stars into six different classes, according to their relative magnitude, inde- pendent of those small stars which compose the white and brilliant spaces in the heavens, which are denominated nebulae, and that bright band which extends across our hemisphere, and which from its lucid appearance is termed the milky way. Those which are distinctly visible are fewer in number than might be supposed. The British catalogue, which, besides the stars visible to the naked eye, includes a great number which can- not be seen without the assistance of a telescope, contains no more than three thousand in both he- mispheres. The number of stars discoverable,, 258 Astronomy. [Lecture 17. in either hemisphere, by the naked eye, is not above a thousand. From what we are able to judge by computation and observation, it is con- cluded that none of the fixed stars can be at a less distance than 32,000,000,000,000 of miles from us, which is further than a cannon-ball would fly in 7,000,000 of years. The famous French astronomer Lalande, indeed, makes the distance by a late computation to be 7,086,760,000,000 leagues. Though the number of the fixed stars is less than common observers might imagine, yet it is still too great, from their resemblance to each other, to enable us to distinguish them by giving each a particular name, as has been done with regard to the planets. Astronomers therefore have found a commodious method of arranging them under various figures, called constellations. They have given to these constellations the names and figures of various personages celebrated in antiquity, and even of many animals or of inani- mate bodies, as instruments, machines, &c. which fable has feigned to have been carried from earth to heaven. Ptolemy has enumerated forty-eight constellations; and there are upon our globes about seventy. On Senex's, Jones's, and Gary's globes Bayer's letters are inserted* ; the first in * In the best of Jones's and Gary's globes, the double, triple, quadruple, and nebulous stars are indicated by appropriate characters. System of ike Universe. the Greek alphabet being put to the largest star in each constellation; the second to the next, and so on ; by which means every star is as easily found as if a name were given to it. Thus if the star a, in the constellation of the ram, is mentioned, every astronomer knows as well what star is meant, as if it were pointed out to him in the heavens. The constellations which surround the ecliptic, or apparent annual path of the Sun, and which fill that zone of the heavens which is called the zodiac, are the twelve following : Aries, or the ram,