THE UNIVERSITY OF ILLINOIS LIBRARY Digitized by the Internet Archive in 2015 https://archive.org/details/scientificdialogOOjoyc It jti.-il rr;^/'rsrn/'i//'i'n rTilir cnrfh ^ VVBLA S I J i: I ) J IV SCO T T . W I : li s T I ; K i- G I . A 1 n cf'the .rV OF ILL'' SCIENTIFIC DIALOGUES; INTENDED FOR THE INSTRUCTION AND EIVJTERTAINMENT OF YOUNG PEOPLE : IN WHICH THE FIRST PRINCIPLES OF NATURAL AND EXPERIMENTAL PHILOSOPHY ARE FULLY EXPLAINED. BY THE REV. J. JOYCE. NEW EDITION, COMPLETE IN ONE VOLUME, , WITH 185 WOOD CUTS. "Conversation, with the habit of explaining the meanm| o^^^^^^^^^ LONDON : PRINTED FOR SCOTT, WEBSTER, AND GEARY, (SUCCESSORS TO MR. DOVE) 36, CHARTERHOUSE SQUARE. PREFACE. i The Author feels himself extremely happy in the S opportunity which this publication afFords him of ^ acknowledging the obligations he is under to the ^ authors of * Practical Education/ for the pleasure and ^ instruction which he has derived from that valuable work. To this he is solely indebted for the idea of writing on the subject of Natural Philosophy for the ^ use of children. How far his plan corresponds with that suggested by Mr. Edgeworth, in his chapter on ^ Mechanics, must be left with a candid public to decide. The Author conceives, at least, he shall be justified ^ in asserting, that no introduction to natural and ex- 1023555 iv PREFACE. perimental philosophy has been attempted in a method so familiar and easy as that which he now offers to the public none which appears to him so properly adapted to the capacities of young people of ten or eleven years of age, a period of life which, from the Author's own experience, he is confident is by no means too early to induce in children habits of scientific reasoning. In this opinion he is sanctioned by the authority of Mr. Edgeworth. "Parents," says he, "are anxious that children should be conver- sant with mechanics, and with what are called the mechanical cowers. Certainly no species of know- ledge is better suited to the taste and capacity of youth, and yet it seldom forms a part of early in- struction. Every body talks of the lever, the wedge, and the pulley, but most people perceive that the notions which they have of their respective uses is unsatisfactory and indistinct, and many en- deavour, at a late period of life, to acquire a scientific and exact knowledge of the effects that are produced by implements which are in every body's hands, or that are absolutely necessary in the daily occupations of mankind." The Author trusts that the whole work will be found a complete compendium of natural and ex- PREFACE. V periraental philosophy, not only adapted to the understandings of young people, but well calculated also to convey that kind of familiar instruction which is absolutely necessary before a person can attend public lectures in these branches of science with advantage. " If," says Mr. Edgeworth, speaking on this subject, "the lecturer does aot communicate much of that knowledge which h3 endeavours to explain, it is not to be attributed either to his want of skill, or to the insufficiency of his apparatus, but to the novelty of the terms which he is obliged to use. Ignorance of the language in which any science is taught, is an insuperable bar to its being suddenly acquired ; besides a precise knowledge ol the meaning of terms, we must have an instantaneous idea excited in our minds whenever they are repeated ; and, as this can be acquired only by practice, it is impossible that philosophical lectures can be of much service to those who are not familiarly acquainted with the technical language in which they are deliveied." * * Mr. Edgeworth's chapter on Mechanics sh«uld be recommended to the attention of the reader, lut the author feels unwilling to refer to part of a wor:, the whole of which deserves the careful perusal of al per- sons engaged in the education of youth. A 2 PREFACE. It is presumed that an attentive perusal of these Dialogues, in which the principal and most common terms of science are carefully explained and illus- trated, by a variety of familiar examples, will be the means of obviating this objection, with respect to persons who may be desirous of attending those pub- lic philosophical lectures, to which the inhabitants of the metropolis have almost constant access. CONTENTS. MECHANICS. Of the Divisibility I. Introduction II. Of Matter. Matter III. Of the Attraction of Cohesion IV. Of the Attraction of Cohesion V. Of the Attraction of Gravitation VI. Of the Attraction of Gravitation VII. Of the Attraction of Gravitation VIII. Of the Attraction of Gravitation IX. Of the Centre of Gravity . X. Of the Centre of Gravity . XI. Of the Laws of Motion XII. Of the Laws of Motion XIII. Of the Laws of Motion XIV. Of the Mechanical Powers XV. Of the Lever XVI. Of the Lever . XVII. Of the Wheel and Axis XVIII. Of the Pulley . XIX. Of the Inclined Plane viii CONTENTS. Conversation Paffe XX. Of the Wedge 65 XXI. Of the Screw 67 XXII. Of the Pendulum .... 71 ASTRONOMY. I. Of the fixed Stars .... 74 II. Of the fixed Stars .... 77 III. Of the fixed Stars and Ecliptic . .81 IV. Of the Ephemeris . . . .85 V. Of the Solar System . . . .90 VI. Of the Figure of the Earth . . 94 VII. Of the diurnal Motion of the Earth . 98 VIII. Of Day and Night . . . .103 IX. Of the annual Motion of the Earth . 106 X. Of the Seasons 108 XI. Of the Seasons Ill XII. Of the Equation of Time . . .117 XIII. Of Leap Year 121 XIV. Of the Moon 124 XV. Of Eclipses 128 XVI. Of the Tides 132 XVII. Of the Harvest Moon . . .137 XVIII. Of Mercury 140 XIX. Of Venus 142 XX. Of Mars 146 XXI. Of Jupiter 149 XXII. Of Saturn 150 XXIII. Of the Herschel Planet . . .154 XXIV. Of Comets 155 XXV. Of the Sun 157 XXVI. Of the fixed Stars . . . . ib. CONTENTS. HYDROSTATICS. Conversation Page T Introduction » . • • • 162 TT 1J-. Of the \Veight and Pressure of Fluids loo TTT 111. Of the Weight and Pressure of Fluids 1/1 TV X V • Of the Lateral Pressure of Fluids 1 / D V « Of the Hydrostatic Paradox . I/O V 1. \Jl me JuLyQlOoLclllC XJciiuwa • • 183 V il. Of the Pressure of Fluids against the olQcS Ul V cisscla • • • • loo VTTT V ill* Of the Motion of Fluids 190 1 A. Of tViP Mntlnn nf Fluids iy4 , X. Of the Specific Gravity of Bodies lyy Ai. Of tha Snppifir* rrravitv of Bodies 201 XII. Of the Methods of finding the Specific Gravity of Bodies . . . • 2u5 XIII. Of the Methods of finding the Specific Gravity of Bodies ...» V • Of the Methods of finding the Specific Gravity of Bodies . . • « 213 XV. Of the Methods of finding the Specific Gravity of Bodies . . . . ZlD XVI. Of the Hydrometer .... XVII. Of the Hydrometer and Svi^imming OO/I 4Z'± XVIII. Of the Syphon and Tantalus's Cup . ZZi XIX. Of the Diver's Bell . . . . Zol XX. Of the Diver's Bell . . . . 234 XXI. Of Pumps . . . . . 237 XXII. Of the Forcing-pump— Fire-engine— Kope-pump — Chain-pump — and Water-press . . • i CONTENTS. PNEUMATICS. Conversation I. Of the Nature of Air . . .245 II. Of the Air-pump . . . .247 III. Of the Torricellian Experiment . . 252 IV. Of the Pressure of the Air . . . 255 V. Of the Pressure of the Air . . . 258 VI. Of the Weight of Air ... 261 VII. Of the Elasticity of Air . . . 265 VIII. Of the Compression of Air . . 270 IX. Miscellaneous Experiments on the Air- pump 274 X. Of the Air-gun and Sound . . 277 XI. Of Sound 281 XII. Of the Speaking Trumpet . . .285 Jilll. Of the Echo 288 XIV. Of the Echo 292 XV. Of the Winds 296 XVI. Of the Steam-engine . . . .302 XVII. Of the Steam-engine . . . .306 XVIII. Of the Steam-engine and Papin's Di- gester ...... 309 XIX. Of the Barometer . . . .313 XX. Of the Barometer, and its Application to the Measuring of Altitudes . .317 XXI. Of the Thermometer . . . .320 XXII. Of the Thermometer . . . .324 XX II I. Of the Pyrometer and Hygrometer . 327 X'vIV. Of the Kain-gauge, and Rules for judging of the Weather . . . 332 CONTENTS. xi OPTICS. Conversation Page I. Light : the Smallness and Velocity of its Particles 337 II. Kays of Light : — Reflection and Re- fraction . . . . . 341 IIL Refraction of Light . . .344 IV. Refraction and Reflection of Light . 348 V. Different Kinds of Lenses . . .352 VI. Parallel diverging and converging Rays 356 VII. Images of Objects. — Scioptric Ball, &c 360 VIII. Nature and Advantages of Light e 364 IX. Colours 367 X. Reflected Light and Plain Mirrors . 370 XI. Concave Mirrors . . . .373 XII. Concave Mirrors. — Experiments. . 376 XIII. Concave and Convex Mirrors . . 378 XIV. Optical Deceptions, Anamorphoses, &c. 382 XV. Different Parts of the Eye . . 385 XVI. Manner of Vision . . . .388 XVII. Spectacles, and their Uses . . . 392 XVIII. Rainbow 396 XIX. Refracting Telescope . . .400 XX. Reflecting Telescopes . . . 404 XXI. Microscope . . . . .407 XXII. Camera Obscura, Magic Lanthorn, and Multiplying Glass . . .413 MAGNETISM. I. The Magnet 418 II. Magnetic Attraction and Repulsion . 420 III. Methods of making Magnets , . 423 IV. Mariner's Compass . . , 427 xii CONTENTS. ELECTRICITY. Conversaaon ^ p^^^ I. Early History of Electricity . .431 II. Electrical Attraction and Repulsion . 433 III. Electrical Machine .... 438 IV. Electrical Machine .... 442 V. Electrical Attraction and Repulsion . 446 VI. Electrical Attraction and Repulsion . 452 VII. The Leyden Phial .... 454 VIII. Lane's Electrometer, and the Electrical Battery 459 IX. Experiments with the Battery . . 463 X. Miscellaneous Experiments . . 468 XI. Electrophorus. — Electrometer. — Thun- der-House, &c 472 XII. Atmospherical Electricity . . . 475 XIII. Of Atmospheric Electricity — of Falling Stars — Aurora Borealis — Water- Spouts and Whirlwinds — Earth- quakes 479 XIV. Medical Electricity .... 485 XV. Animal Electricity — of the Torpedo — of the Gymnotus Electricus — of the Silurus Electricus . . . .487 XVI. General Summary of Electricity, with Experiments . . . .491 GALVANISM. I. Of Galvanism; its Origin; Experiments — of the Decomposition of Water . 496 II. Galvanic Light and Shocks. . . 499 III. Galvanic Conductors — Circles — Tables — Experiments .... 503 IV. Miscellaneous Experiments . . 509 Glossary and Index 515 M E C H A N 1 C S. CONVERSATION I. INTRODUCTION. FATHER CHARLES — EMBIA, Charles. Father, you told sister Emma and me, that, after we had finished reading the Evenings at Home/' you would explain to us some of the principles of natural philosophy : will you begin this morning ? Father. Yes, I am quite at leisure ; and I shall, indeed, at all times take a delight in communicating to you the elements of useful knowledge ; and the more so in proportion to the desire vv^hich you have of col- lecting and storing those facts that may enable you to understand the operations of nature, as well as the works of ingenious artists. These, I trust, will lead you insensibly to admire the wisdom and goodness hy means of which the whole system of the universe is constructed and supported. Emma. But can philosophy be comprehended by children so young as we are 1 I thought that it had been the business of men, and of old men too. F. Philosophy is a word which in its original sense signifies only a love or desire of wisdom ; and you will not allow that you and your brother are too young to wish for knowledge. E, So far from it, that the more knowledge I get the better I seem to like it ; and the number of new ideas which, with a little of your assistance, I have obtained from the Evenings at Home,'' and the great .pleasure which I have received from the perusal of that work, will, I am sure, excite me to read it again and again. B 2 MECHANICS. F. You will find very little, in the introductory parts of natural and experimental philosophy, that will re- quire more of your attention than many parts of that work with which you were so delighted. C. But in some books of natural philosophy, which I have occasionally looked into, a number of new and uncommon words have perplexed me ; 1 have also seen references to figures, by means of large letters and small, the use of which 1 did not comprehend. F. It is frequently a dangerous practice for young minds to dip into subjects before they are prepared, by some previous knowledge, to enter upon them ; since it may create a distaste for the most interesting topics. Thus, those books which you now read with so much pleasure would not have afforded you the smallest en- tertainment a few years ago, when you must have spelt out almost every word in each page. The same sort of disgust will naturally be felt by persons who should attempt to read works of science before the leading terms are explained and understood. The word angle is continually recurring in subjects of this sort ; do you know what an angle is ? E. I do not think I do ; will you explain what it means ? F. An angle is made by the opening of two straight* lines. In this figure there are two straight a lines ah and cb meeting at the point b, and ^ ^-"'^ the opening made by them is called an y[o- 1. angle. °' C. Whether that opening be small or great, is it still called an angle 1 F. It is ; your drawing compasses may familiarize to your mind the idea of an angle; the lines in this figure will aptly represent the legs of the compasses, and the point /; the joint upon which they move or turn. Now you may open the legs to any distance you please, even so far that they shall form one straight line ; in tliat position only they do not form an angle. * Straight lines, in works of science, are usually de- nominated 7'ight lilies. INTRODUCTION. 3 In every other situation an angle is made by the open- ing of these legs, and the angle is said to be greater or^less, as that opening is greater or less. An angle is another word for a coriier, E. Are not some angles called right angles ? F, Angles are either right, acute, or obtuse. When the line ah meets another line cd in such a manner as to make the angles aha and ahc equal to one another, then those angles s l_A are called right angles. And the line ab is said to be perpendicular to cd. Hence 5* ; to be perpendicular to, or to make right angles with, a line, means one and the same thing. C. Does it signify how you call the letters of an angle 1 , F. It is usual to call every angle by three letters, and that at the angular point must be always a the middle letter of the three. There are cases, however, where an angle may be deno- minated by a single letter ; thus the angle abc may be called simply the angle for there X,^^^ is rio danger of mistake, because there is but a single angle at the point b, ,n\ C, I understand this ; for if, m the second figure, i were to describe the angle by the letter b only, you would not know whether I meant the angle abc or abd, F. That is the precise reason why it is necessary in most descriptions to make use of three letters. An acute angle (Fig. 1.) ahc is less than a right angle ; and an obtuse angle (Fig. 3.) abc is greater than a Hght angle. E. You see the reason now, Charles, why letters are placed against or by the figures, which puzzled you before. C, I do ; they are intended to distmguish the sepa- rate parts of each in order to render the description of them easier both to the author and the reader. E. What is the difference, papa, between an angle and a triangle 1 4 MECHANICS. F. An angle being made by the opening of two lines, and as you know that two straight lines cannot enclose a space, so a triangle ahc is a space bounded by three straight lines. It takes ^ X X r its name from the property of containmg '"y.^ ^ three angles. Inhere are various sorts of ° triandes, but it is not necessary to enter upon these particulars, as I do not wish to burden your memories with more technical terms than we have occasion for. C A triangle, then, is a space or figure containing three ancrles, °and bounded by as many straight Imes. F. Yes, that description will answer our present purpose. CONVERSATION II. OF MATTER. OF THE DIVISIBILITY OF MATTER. F. Do you understand what philosophers mean when they make use of the word matter? E. Are not all things which we see and feel com- posed of matter 1, . r • F. Every thing which is the object of our senses is composed of matter differently modified or arranged. But m a philosophical sense matter is defined to be an extended, solid, inactive, and moveable substance. C If by extension is meant length, breadth, and thickness, matter, undoubtedly, is an extended sub- stance. Its solidity is manifest by the resistance it makes to the touch. E. And tlie other properties nobody will deny, tor all material objects are of themselves ^^athout motion ; and vet it may be readily conceived, that, by applica- tion of a proper force, there is no body which cannot be moved. But I remember, papa, that you told us something strano-e about the divisibility of matter, which vou said mii,dit be continued without end. " F I did, some time back, mention this curious and interesting subject, and this is a very fit time tor me to explain it. DIVISIBILITY OF MATTER. 5 C. Can matter indeed be infinitely divided ; for I suppose that this is what is meant by a division with- out end ? T 1 • 1 F. Difficult as this may at first appear, yet I thmk it very capable of proof. Can you conceive of a par- ticle of matter so small as not to have an upper and under surface ? C. Certainly every portion of matter, however mi- nute, must have two surfaces at least, and then I see that it follows of course that it is divisible ; that is, the upper and lower surfaces may be separated. jP. Your conclusion is just; and, though there may be particles of matter too small for us actually to divide, yet this arises from the imperfection of our instruments ; they must nevertheless, in their nature, be divisible. E. But you v^^ere to give us some remarkable m- stances of the minute division of matter. F, A few years ago a lady spun a single pound of wool into a thread 168,000 yards long.^ And Mr. Boyle mentions that two grains aM a half of silk was spun into a thread 300 yards in length. If a pound of silver, which, you know, contains 5,760 grains, and a single grain of gold, be melted together, the gold will be equally dififused through the whole sil- ver, insomuch, that if one grain of the mass be dis- solved in a liquid called aqua forth, the gold will fall to the bottom. By this experiment, it is evident that a grain may be divided into 5,761 visible parts, for only the 5,761st part of the gold is contained in a single grain of the mass. The goldbeaters, whom you have seen at work in the shops in Long-acre, can spread a grain of gold into a leaf containing 50 square inches, and this leaf may be readily divided into 500,000 parts, each of which is visible to the naked eye : and by the help of a microscope, which magnifies the area or surface of a body 100 times, the 100th part of each of these be- comes visible ; that is, the 50 millionth part of a grain of gold will be visible, or a single grain of that 6 MECHANICS. metal may be divided into 50 millions of visible parts. But the gold which covers the silver wire used m making what is called gold lace, is spread over a much larger surface, yet it preserves, even if exammed by a microscope, an uniform appearance. It has been calculated that one grain of gold, under these circum- stances, would cover a surface of nearly thirty square yards. The natural divisions of matter are still more sur- prising. In odoriferous bodies, such as camphor, musk, and assafoetida, a wonderful subtilty of parts is perceived ; for, though they are perpetually filling a considerable space with odoriferous particles, yet these bodies lose but a very small part of their weight in a great length of time. . Again, it is said by those who have examined the subject with the best glasses, and whose accuracy may be relied on, that there are more animals in the milt of a single cod-fish, than there are men on the whole earth, and that a single grain of sand is larger than four millions of these animals. Now if it be admitted that these little animals are possessed of organised parts, such as a heart, stomach, muscles, veins, arte- ries, &c. and that they are possessed of a complete system of circulating fluids, similar to what is tound in larger animals, we seem to approach to an idea of the infinite divisibility of matter. It has indeed been calculated, that a particle of the blood of one ot these animalculse is as much smaller than a globe one-tenth of an inch in diameter, as that globe is smaller than the whole earth. Nevertheless, if these particles be compared with the particles of light, it is probable that they would be found to exceed them m bulk as much as mountains do single grains of sand. I mioht enumerate many other instances ot the same kind, but these, I doubt not, will be sufficient to convince you into what very minute parts matter is capable of being divided. , , i Captain Scoresby, in his Account ot the Greenland Seas, states, that, in July 1818, his vessel sailed for ATTRACTION OF COHESION. 7 several leagues in water of a very uncommon appear- ance The surface was variegated by large patches of a yellowish-green colour. It was found to be pro- duced by animalculse, and microscopes were applied to their examination. In a single drop of the water, examined by a power of 28,224 (magnified superfi- cies), there were 50 in number, on an average, in each square of the micrometer glass of l-340th ot an inch in diameter; and, as the drop occupied a circle on a plate of glass containing 529 of these squares, there must have been in this single drop of water, taken at random out of the sea, and in a place not the most discoloured, about 26,450 animalculae. How inconceivably minute must the vessels, organs, and fluids, of these animals be ! A whale requires a sea to sport in : a hundred and fifty millions oj these would have ample scope for their evolutions in a tumbler oJ water! CONVERSATION III. OF THE ATTRACTION OF COHESION. F Well, my dear children, have you reflected upon what we last conversed about? Do you com- prehend the several instances which I enumerated as examples of the minute division of matter] E. Indeed, the examples which you gave us very much excited my wonder and admiration, and yet, from the thinness of some leaf gold which I once had, I can readily credit all you have said on that part of the subject. But I know not how to conceive of such small animals as you described ; and I am still more at a loss how to imagine that ariimals so minute ^ should possess all the properties of the larger ones, such as a heart, veins, blood, &c. , , , ^ . F I can, the next bright morning, by the help ot my solar microscope, shew you very distinctly, the circulation of the blood in a flea, which you may get from your little dog; and with better glasses than those of which 1 am possessed, the same appearance g MECHANICS, might be seen in animals still smaller than the flea, perhaps even in those which are themselves invisible to the naked eye. But we shall converse more at large on this matter, when we come to consider the subject of optics, and the construction and uses of the solar microscope. At present we will turn our thoughts to that principle in nature, which philoso- phers have agreed to call gravity, or attraction. C. If there be no more difficulties in phdoso- phy than we met with in our last lecture, 1 do not fear but that we shall, in general, be able to under- stand it. Are there not several kinds of attraction 1 F. Yes, there are ; two of which it will be sufficient for our present purpose to describe ; the one is the attraction of cohesion; the other, that of gravitation. The attraction of cohesion is that power which keeps the parts of bodies together when they touch, and prevents them from separating, or which inclines the parts of bodies to unite, when they are placed suffi- ciently near to each other. C. Is it then by the attraction of cohesion that the parts of this table, or of the penknife, are kept to- gether ? F. The instances which you have selected are ac- curate, but you might have said the same of every other solid substance in the room ; and it is in propor- tion to the different degrees of attraction with which different substances are affected, that some bodies are hard, others soft, tough, &c. A philosopher in Hol- land, almost a century ago, took great pains in ascer- taining the different degrees of cohesion which be- longed to various kinds of wood, metals, and many other substances. A short account of the experiments made by M. Musschenbroek, you will^ hereafter find in your own language, in Dr. Enfield's Institutes of Natural Philosophy. C. You once shewed me that two leaden bullets having a little scraped from the surfaces, would stick together with great force ; you called that, 1 believe, the attraction of cohesion ? ATTRACTION OF COHESION. 0 F. I did : some philosophers, who have made this experiment with great attention and accuracy, assert, that if the flat surfaces, which are presented to one another, be but a quarter of an inch in diameter, scraped very smooth, and forcibly pressed together with a twist, a weight of a hundred pounds is tre- quently required to separate them. As it is by this kind of attraction that the parts ot solid bodies are kept together, so, when any substance is'separated or broken, it is only the attraction of co- hesion that is overcome in that particular part. E. Then, when I had the misfortune this morning at breakfast to let my saucer slip from my hands, by which it was broken into several pieces, was it only the attrabtion of cohesion that was overcome by the parts of the saucer being separated by its fall on the ground ] F. Just so ; for whether you unluckily break the china, or cut a stick with your knife, or melt lead over the fire, as your brother sometimes does, in order to make plummets ; these and a thousand other instances which are continually occurring, are but examples in which the cohesion is overcome by the fall, the knife, or the fire. E. The broken saucer being highly valued by mamma, she has taken the pains to join it again with white lead ■ was this performed by means of the at- traction of cohesion ? F. It was, my dear ; and hence you will easily learn that many operations in cookery are in fact no- thing more than different methods of causing this attraction to take place. Thus flour, by itself, has little or nothing of this principle, but when mixed with milk, or other liquids, to a proper consistency, the parts cohere strongly, and this cohesion in many in- stances becomes still stronger by means of the heat applied to it in boiling or baking. C. You put me in mind of the fable of the man blowing hot and cold ; for, in the instance of the lead, lire overcomes the attraction of cohesion; and the B2 10 MECHANICS. same power, heat, v/hen applied to puddings, bread, &c. causes their parts to cohere more powerfully. How are we to understand this ? F. I will endeavour to remove your difficulty. Heat expands all bodies without exception, as you shall see before we have finished our lectures. Now the fire applied to metals, in order to melt them, causes such an expansion, that the particles are thrown out of the sphere, or reach, of each other's attraction ; whereas the heat communicated in the operation of cookery, is sufficient to expand the particles of flour, but is not enough to overcome the attraction of co- hesion. Besides, your mamma will tell you, that the heat of boiling would frequently disunite the parts of which her puddings are composed, if she did not take the precaution of enclosing them in a cloth, leaving them just room enough to expand without the liberty of breaking to pieces ; and the moment they are taken from the water, they lose their superabundant heat, and become solid. £. When Ann the cook makes broth for little bro- ther, it is the heat then which overcomes the attrac- tion which the particles of meat have for each other, for I have seen her pour off the broth, and the meat is all in rags. But will not the heat overcome the attraction which the parts of the bones have for each other ? jP, The heat of boiling water will never effect this, but a machine was invented several years ago, by Mr. Papin, for that purpose. It is called Papin's Digester, and is used in taverns, and in many large families, for the purpose of dissolving bones as completely as a lesser degree of heat will liquefy jelly. On some future day I will shew you an^engraving of this ma- chine, and explain its different parts, which are ex- tremely simple.* * See Pneumatics, Conversation XVIII. ATTRACTION OF COHESION. 11 CONVERSATION IV. OF THE ATTRACTION OF COHESION. F. I will now mention some other instances of this great law of nature. If two polished plates of marble, or brass, be put together, with a little oil between them to fill up the pores in their surfaces, they will cohere so powerfully as to require a very considerable force to separate them. — Two globules of quicksilver, placed very near to each other, will run together and form one large drop. — Drops of water will do the same. — Two circular pieces of cork placed upon water at about an inch distant will run together. — Balance a piece of smooth, board on the end of a scale beam ; then let it lie flat on water, and five or six times its own weight will be required to separate it from the water. If a small globule of quicksilver be laid on clean paper, and a piece of glass be brought into contact with it, the mercury will adhere to it, and be drawn away from the paper. But bring a larger globule into contact with the smaller one, and it will forsake the glass, and unite with the other quicksilver. C. Is it not by means of the attraction of cohe- sion, that the little tea which is generally left at the bottom of the cup instantly ascends in the sugar when thrown into it 1 F, The ascent of water or other liquids in sugar, sponge, and all porous bodies, is a species of this at- traction, and is called capillary * attraction : it is thus denominated from the property which tubes of a very small bore, scarcely larger than to admit a hair, have of causing water to stand above its level. C. Is this property visible in no other tubes than those the bores of which are so exceedingly fine ? F, Yes, it is very apparent in tubes whose diame- ters are one-tenth of an inch or more in length, but the smaller the bore, the higher the fluid rises ; for it * From capillus, the Latin word for hair. 12 MECHANICS. ascends, in all instances, till the weight of the column of water in the tube balances, or is equal to, the at- traction of the tube. By immersing tubes of different bores in a vessel of coloured water, you wil see t.iat the water rises as much higher in the smaller tube, than in the larger, as its bore is less than that of tlie larger. The water will rise a quarter of an inch, and there remain suspended in a tube, whose bore is about one-eip-hth of an inch in diameter. This kind of attraction is well illustrated, by taking two pieces of glass, joined together at jr- — the side be, and kept a little open at k J^^ the opposite side ad, by a small piece ^^^^^^ of cork e. In this position immerse f ^^m ^ them in a dish of coloured water Jg, j -j /^, and you will observe that the at iv"5 traction of the glass at and near be n- • will cause the fluid to ascend to />, whereas about the parts d, it scarcely rises above the level of the water in the vessel. , , , C I see that a curve is formed by tlie water. F There is, and to this curve there are many cu- rious properties belonging, as you will hereafter be able to investigate for yourself. E Is it not upon the principle of the attrac- tion of cohesion, that carpenters glue their work to- p^ether 1 . , , ^ i ° F It is upon this principle that carpenters and cabinet-makers make use of glue; that braziers tm- men, plumbers, &c. solder their metals; and that smiths unite different bars of iron by means of heat. These and a thousand other operations of which we are ''continually the witnesses, depend on the same principle as that which induced your mamma to use ihe white lead m mending her saucer. And you ought to be told, that though white lead is frequently used as u cement for broken china, glass, and earthenware, vet if the vessels are to be brought agam into use, it is not'a proper cement, being an active poison besides^ one much stronger has been discovered, 1 believe, by ATTRACTION OF COHESION, 13 a very able and ingenious philosopher, the late Dr. Ingenhouz ; at least I had it from him several years ago ; it consists simply of a mixture of, quick-lime and Glocester cheese, rendered soft by warm water, and worked up to a proper consistency. E. What! do such great philosophers, as I have heard you say Dr. Ingenhouz was, attend to such trifling things as these 1 F. He was a man deeply skilled in many branches of science ; and I hope that you and your brother will one day make yourselves acquainted with many of his important discoveries. But no real philosopher will consider it beneath his attention to add to the con- veniences of life. C. This attraction of cohesion seems to pervade the whole of nature. F. It does, but you will not forget that it acts only at very small distances. Some bodies indeed appear to possess a power the reverse of the attraction of co- hesion. F, What is that, papa ? F. It is called repulsion. Thus water repels most bodies till they are wet. A small needle carefully placed on water will swim : flies walk upon it without wetting their feet : the drops of dew which appear in a morning on plants, particularly on cabbage plants, assume a globular form, from the mutual attraction between the particles of water ; and upon examination it will be found that the drops do not touch the leaves, for they will roll off in compact bodies, which could not be the case if there subsisted any degree of at- traction between the water and the leaf. If a small thin piece of iron be laid upon quicksd- ver, the repulsion between the different metals will cause the surface of the quicksilver near the iron to be depressed. n • i • i The repelling force of the particles of a fluid is but small ; therefore, if a fluid be divided it easdy unites again. But if a glass or any hard substance be broken, the parts cannot be made to cohere without 14 MECHANICS. being first moistened^ because the repulsion is too great to admit of a re-union. The repelling force between water and oil is like- wise so great, that it is almost impossible to mix them in such a manner that they shall not separate again. If a ball of light wood be dipped in oil, and then put into water, the water will recede so as to form a small channel around the ball. C. Why do cane, steel, and many other things, bear to be bent without breaking, and, when set at liberty again, recover their original form 1 F. That a piece of thin steel, or cane, recovers its usual form after being bent, is owing to a certain power, called elasticity, which may, perhaps, arise from tlie particles of those bodies, though disturbed, not being drawn out of each other's attraction ; there- fore, as soon as the force upon them ceases to act, they restore themselves to their former position. — But our half hour is expired ; I must leave you. CONVERSATION V. OF THE ATTRACTION OF GRAVITATION. F. We will now proceed to discuss another very important general principle in nature ; the attrac- tion of gravitation, or, as it is frequently termed, gravity, which is that power by which distant bodies tend towards each other. Of this we have perpetual instances in the falling of bodies to the earth. C. Am I, then, to understand that whether this marble falls from my hand, or a loose brick from the top of the house, or an apple from the tree in the orchard, that all these happen by the attraction of gravity 1 F. It is by the power which is commonly expressed under the term gravity, that all bodies whatever have a tendency to the earth ; and, unless supported, will fall in lines nearly perpendicular to its surface. E. But are not smoke, steam, and other light ATTRACTION OF GRAVITATION. 15 bodies, which we see ascend, exceptions to the gene. ral rule ? , , • r i F, It appears so at first sight, and it was formerly received as a general opinion, that smoke, steam, &c. possessed no weight: the discovery of the air-pump has shewn the fallacy of this notion, for m an ex- hausted receiver, that is, in a glass jar from which the air is taken away by means of the air-pump, smoke and steam descend by their own weight as com- pletely as apiece of lead. When we come to con- verse on the subjects of pneumatics and hydrostatics you will understand that the reason why smoke and other bodies ascend is simply because they are lighter than the atmosphere which surrounds them, and the moment they reach that part of it which has the same gravity with themselves they cease to rise. C. Is it, then, by this power that all terrestrial bo- dies remain firm on the earth 1 F By gravity, bodies on all parts oi the earth (which you kaow is of a globular form) are kept on its surface, because they all, wherever situated, tend to the centre ; and, since all have a tendency to the centre, the inhabitants of New Zealand, although nearly opposite to our feet, stand as firm as we do in Great Britain. C. This is difficult to comprehend ; nevertheless, if bodies on all parts of the surface of the earth have a tendency to the centre, there seems no reason why bodies should not stand as firm on one part as well as another. Does this power of gravity act alike on all bodies ? . . • n F, It does, without any regard to their figure or size ; for attraction or gravity acts upon bodies in pro- portion to the quantity of matter which they contain that is, four times a greater force of gravity is exerted upon a weight of four pounds than upon one of a single pound. The consequence of this prmciple is that all bodies at equal distances from the earth fall with equal velocity. E. What do you mean, papa, by velocity r 16 MECHANICS. F, I will explain it by an example or two : if you and Charles set out together, and you walk a mile in half an hour, but he walk and run two miles in the same time, how much swifter will he go than you ? E. Twice as swift. F. He does, because, in the same time, he passes over twice as much space ; therefore, we say his velo- city is twice as great as your's. Suppose a ball, fired from a cannon, pass through 800 feet in a second of time, and in the same time your brother's arrow pass through 100 feet only, how much swifter does the cannon-ball fly than the arrow ? E. Eight times swifter. F. Then it has eight times the velocity of the ar- row ; and hence you understand that swiftness and velocity are synonymous terms ; and that the velocity of a body is measured by the space it passes over in a given time, as a second, a minute, an hour, &c. E. If I let a piece of metal, as a penny-piece, and a feather, fall from my hand at the same time, the penny will reach the ground much sooner than the feather. Now how do you account for this if all bo- dies are equally affected by gravitation, and descend with equal velocities, when at the same distance from the earth ? F. Though the penny and feather will not, in the open air, fall with equal velocity, yet if the air be taken away, which is easily done, by a little appara- tus connected with the air-pump, they will descend m the same time. Therefore the true reason why light and heavy bodies do not fall with equal velocities, is, that the Jonner, in proportion to its weight, meets with a much greater resistance from the air than the latter. C. It is then, I imagine, from the same cause that, if I drop the penny and a piece of light v.ood into a vessel of water, the penny shall reach the bottom, but the wood, after descending a small way, rises to the surface. F. In this case, the resi!>ting medium is water in- stead of air, and the copper, being about nine times ATTRACTION OF GRAVITATION. 17 heavier than its bulk of water, falls to the bottom without apparent resistance. But the wood, bemg much liohter than water, cannot sink in it; therelore, thouP-h by its jnomentum'' it sinks a small distance, yet, as soon as that is overcome by the resisting medium, it rises to the surface, being the lighter substance. CONVERSATION VI. OF THE ATTRACTION OF GRAVITATION. E. The term momentum, which you made use of yesterday, is another word which I do not understand, F. If you have understood what I have said re- specting the velocity of moving bodies, you will easily comprehend what is meant by the word momentum . The momentum, or moving force, of a body, is its weight multiplied into its velocity. You may, for instance, place this pound weight upon a chma-plate without any danger of breaking, but, if you let it lall from the height of only a few inches, it will dash the china to pieces. In the first case, the plate has only the pound weight to sustain ; in the other, the weiglit must be multiplied into the velocity, or, to speak in a popular manner, into the distance of the height from which it fell. ^ If a ball a lean against the obsta- ^.^^,,3^ cle h, it will not be able to overturn e JlJJ^ g it, but if it be taken up to c, and suf- j^- fered to roll down the inclined plane de against b, it will certainly overthrow it; in the former case, b would only have to resist the weight of the ball a, in the latter it has to resist the weight mul- tiplied into its motion, or velocity. C. Then the momentum of a small body, whose velocity is very great, may be equal to that of a very large body with a slow velocity. * The explanation of this term wilt be found in the next Conversation. 18 MECHANICS. F. It may, and hence you see the reason why im- mense battering-rams, used by the ancients in the art of war, have given place to cannon balls of but a few pounds weight. C. I do, for what is wanting in weight, is made up ^ by velocity. F. Can you tell me what velocity a cannon ball of 28 pounds must have, to effect the same purposes, as would be produced by a battering ram of 15,000 pounds weight, and which, by manual strength, could be moved at the rate of only two feet in a second of time ? C. I think I can :— the momentum of the battermg ram must be estimated by its weight, multiplied into the space passed over in a second, which is 15,000 multiplied by two feet, equal to 30,QP0 ; now if this momentum, which must also be that of the cannon ball, be divided by the weight of the ball, it will give the velocity required ; and 30,000 divided by 28, will give for the quotient 1072 nearly, which is the num- ber of feet which the cannon ball must pass over in a second, in order that the momenta of the battering ram and the ball may be equal, or, in other words, that they may have the same effect in beating down an enemy's wall. E, I now fully comprehend what the momentum of a body is, for if I let a common trap-ball accident- ally fall from my hand upon my foot, it occasions more pain than the mere pressure of a weight several times heavier than the ball. F. If you let a pound, or a hundred pounds, fall on the floor, only from the height of an inch and a quar- ter, it will strike the floor with a momentum equal to double its weight : and if you let it fall from four times that height, or five inches, it will have double that effect ;— and if it fall nine tmies that height, or eleven inches and a quarter, it will have treble the effect ; — and by falling sixteen times the height, or twenty inches, it will have four times the effect, and so on. Hence it is plain, that if you let the ball diop ATTRACTION OF GRAVITATION. 19 from your hand at the height of twenty inches only, it will have eight times more effect in causing pain than the mere pressure of the ball itself. C. If the attraction of gravitation be a power by which bodies in general tend towards each other, why do all bodies tend to the earth as a centre ? F. I have already told you that by the great law of gravitation, the attraction of all bodies is in proportion to the quantity of matter which they contain. ^ Now the earth, being so immensely large in comparison of ail other substances in its vicinity, destroys the effect of this attraction between smaller bodies, by bring- ing them all to itself. — If two balls are let fall from a high tower at a small distance apart, though they have an attraction for one another, yet it will be as nothing when compared with the attraction by which they are both impelled to the earth, and consequently the tendency which they mutually have of approaching one another will not be perceived in the fall. If, however, any two bodies were placed in free space, and out of the sphere of the earth's attraction, they would in that case assuredly fall toward each other, and that with increased velocity as they came nearer. If the bodies were equal, they would meet in the middle point between the two; but if they were unequal, they would then meet as much nearer the larger one, as that contained a greater quantity of matter than the other. C. According to this, the earth ought to move to- wards falling bodies, as well as they move to it. F, It ought, and, in just theory, it does: but when you calculate how many million of times larger the earth is than any thing belonging to it ; and if you reckon the small distances from which bodies can fall, you will then know that the point where the falling bodies and earth will meet, is removed only to an in- definitely small distance from its surface ; a distance much too small to be conceived by the human imagi- nation. We will resume the subject of gravity to-morrow. 20 MECHANICS. CONVERSATION VII. OF THE ATTRACTION OF GRAVITATION. E. Has the attraction of gravitation, papa, the same eflPect on all bodies, whatever be their distance from the earth 1 F. No ; this, like every power which proceeds from a centre, decreases as the squares of the distances from that centre increase. E. J fear that I shall not understand this unless you illustrate it by examples. F, S appose you are reading at the distance of one foot from a candle, and that you receive a certain quantity of light on your book ; now if you remove to the distance of two feet from the candle, you will, by this law, enjoy four times less light than you had be- fore ; here then though you have increased your dis- tance but two-fold, yet the light is diminished four- fold, because four is the square of two, or two multi- plied by itself. If, instead of removing two feet from the candle, you take your station at 3, 4, 5, or 6 feet distance, you will then receive, at the different dis- tances, 9, 16, 25, 36 times less light than when yoi. were within a single foot from the candle, for these, as you know, are the squares of the numbers, 3, 4, 5 and 6. The same is applicable to the heat imparted by a fire ; at the distance of one yard from which, a per- son will enjoy four times as much heat, as he who sits or stands two yards from it ; and nine times as much as one that shall be removed to the distance of three yards. C. Is then the attraction of gravity four times less at a yard distance from the earth, than it is at the surface 1. F, No ; whatever be the cause of attraction, winch to this day remains undiscovered, it acts from the centre of the earth, and not from its surface, and hence the difference of the power of gravity cannot be dis- ATTRACTION OF GRAVITATION. 21 cerned at the small distances to which we can have access ; for a mile or two, which is much higher than, in general, we have opportunities of making experi- ments, is nothing in comparison of 4000 miles, the distance of the centre from the surface of the earth. But could we ascend 4000 miles above the earth, and of course be double the distance that we now are from the centre, we should there find that the attractive force would be but one-fourth of what it is here ; or in other words that a body, which at the surface of the earth weighs one pound, and, by the force of gravity, falls through sixteen feet in a second of time, would at 4000 mile^ above the earth weigh but a quarter of a pound, and fall through only four feet in a second * E. How is that known, papa, for nobody ever was there ? F. You are right, my dear, for Garnerin, who some years ago astonished all the people of the metropolis and its neighbourhood, by his flight in a balloon, ascended but a little way in comparison of the distance that we are speaking of. However, I will try to ex- plain in what manner philosophers have come by their knowledge on this subject. The moon is a heavy body connected with the earth by this bond of attraction ; and by the most accurate observations it is known to be obedient to the same laws as other heavy bodies are: its distance is also clearly ascertained, being about 240,000 miles, or equal to about sixty semi-diameters of the earth, and of course the earth's attraction upon the moon ought to diminish in the proportion of the square of this dis- * Ex. Suppose it were required to find the weight of a leaden ball at the top of a mountain three miles high, which, on the surface of the earth weighs 20lb. If the semi-diameter of the earth be taken at 4000, then add to this the height of the mountain, and say, as the square of 4003 is to the square of 4000, so is 20lb. to a fourth proportional : or as 16024009 : 16000000 : : 20 : 19'97 : or something more than 19lb. 15^03., which is the weight of the leaden ball at the top of the mountain. 22 MECHANICS, tance ; that is, it ought to be 60 times 60, or 3600 times less at the moon than it is at the surface of the earth. This is found to be the case, by the measure of the deviation of its orbit from a right line. Again, the earth is not a perfect sphere, but a spheroid, that is, of the shape of an orange, rather flat at the two ends called the poles, and the distance from the centre to the poles is about seventeen or eighteen miles less than its distance from the centre to the equa- tor ; consequently, bodies ought to be something hea- vier at, and near the poles, than they are at the equa- tor, which is also found to be the case. Hence it is inferred, that the attraction of gravitation vanes at all distances from the centre of the earth, in proportion as the squares of those distances increase.* C. It seems very surprising that philosophers, who have discovered so many things, have not been able to find out the cause of gravity. Had Sir Isaac New- ton been asked why a marble, dropped from the hand, falls to the ground, could he not have assigned the reason ? F. That great man, probably the greatest man that ever adorned this world, was as modest as he was great, and he would have told you he knew not the cause. The late learned Dr. Price, in a work which he published forty-five years ago, asks, " who does not remember a time when he would have wondered at the question, why does ivater run down hill? Uhat io-norant man is' there who is not persuaded that he understands this perfectly 1 But every improved nian knows it to be a question he cannot answer. For the descent of water, like that of other heavy bodies, depends upon the attraction of gravitation, the cause of which is still involved in darkness. E. You just now said that heavy bodies by the force of gravity fall sixteen feet in a second of time ; is that always the case ? F. Yes, all bodies near the surface of the earth full * See Conver. VI. on Astronomy. ATTRACTION OF GRAVITATION. 23 at that rate in the first second of time, but as the attrac- tion of gravitation is continually acting, so the velocity of falling bodies is an increasing, or, as it is usually called, an accelerating velocity. It is found, by very accurate experiments, that a body, descending from a considerable height by the force of gravity, falls 16 feet in the first second of time ; 3 times 16 feet in the next ; 5 times 16 feet in the third ; 7 times 16 feet in the fourth second of time ; and so on, continually in- creasing according to the odd numbers, 1, 3, 5, 7, 9, 11, &c. In our latitude the true distance fallen is 16 feet one-tvi^elfth ; but, by reason of the centrifugal force, this distance varies a little in different latitudes. But this shall be explained to you hereafter. CONVERSATION VIII. OF THE ATTRACTION OF GRAVITATION. E, Would a ball of twenty pounds' weight here, weigh half an ounce less on the top of a mountain three miles high ? F, Certainly ; but you would not be able to as- certain it by means of a pair of scales and another weight, because both weights being in similar situa- tions would lose equal portions of their gravity. E. How, then, would you make the experiment ? F. By means of one of those stee'l spiral-spring in- struments which you have seen occasionally used, the fact might be ascertained. C. I think, from what you told us yesterday, that with the assistance of your stop-watch, I could tell the height of any place, by observing the number of seconds that a marble or other heavy body would take in falling from that height. F, How would you perform the calculation 1 C. I should go through the multiplications accord- ing to the number of seconds, and then add them together. 24 MECHANICS. F. Explain yourself more particularly : — supposing you were to let a marble or penny-piece fall down that deep well which we saw last summer in the brick field near Ramsgate, and that it was exactly five seconds in the descent, what would be the depth of the well ? C. In the first second it would fall 16 feet ; in the next 3 times 16 or 48 feet ; in the third 5 times 16 or 80 feet ; in the fourth 7 times 16 or 112 feet ; and in the fifth second 9 times 16 or 144 feet : now if I add 16, 48, 80, 112, and 144 together, the sum will be 400 feet, which according to your rule is the depth of the well. But was the well so deep 1 F. I do not think it was, but we did not make the experiment ; should we ever go to that place again you may satisfy your curiosity. You recollect that at Dover Castle we were told of a well there 360 feet deep. Though your calculation was accurate, yet it was not done as nature effects her operations ; it was not performed in the shortest way. C. I should be pleased to know an easier method ; this however is very simple, it required nothing but multiplication and addition. F, True, but suppose I had given you an exam- ple in which the number of seconds had been fifty in- stead of five, the work would have taken you an hour or more to have performed ; whereas, by the rule which I am going to give, it might have been done in half a minute. C. Pray let me have it ; I hope it will be easily remembered. F. It will ; I think it cannot be forgotten after it IS once understood. The rule is this, *' the spaces described by a body falling freely from a state of rest increase as the squares of the times increase." Conse- quently you have only to square the number of seconds, that is, you know, to multiply the number into itself, and then multiply that again by sixteen feet, ATTRACTION OF GRAVITATION. 25 Use space which it describes in the first second, and you have the required answer. Now try the example of the welL C. The square of 5, for the time, is 25, which mul- tiphed by 16 gives 400, just as I brought it out before. Now if the seconds had been 50, the answer would be 50 times 50, which is 2500, and this multiplied by 16, gives 40,000 for the space required. F. I will now ask your sister a question, to try how she has understood this subject. Suppose you observe by this watch that tire time of the flight of your bro- ther's arrow is exactly six seconds, to what height does it rise ? E. This is a different question, because here the ascent as well as the fall of the arrow is to be con-* sidered. F. But you will remember that the time of the ascent is always equal to that of the descent ; for as the velocity of the descent is generated by the force of gravity, so is the velocity of the ascent destroyed by the same force. E. Then the arrow was three seconds only m fall- ing ; now the square of 3 is 9, which multiplied by 16, for the number of feet described in the first second, is equal to 144 feet, the height to which it rose. F, Now, Charles, if I get you a bow which will carry an arrow so high as to be fourteen seconds in its flight, can you tell me the height to which it ascends ? ... C. I can now answer you without hesitation ; — it will be 7 seconds in falling, the square of which is 49, and this again multiplied by 16 will give 784 feet, or rather more than 261 yards, for the answer. F. If you will now consider the example which you did the long way, you will see that the rule which I have given you answers very completely. In the first second the body fell 16 feet, and in the next 48, these added together make 64, which is the square of the 2 seconds muhiplied by 16. The same holds true oiUm 3 first seconds, for in the third second it fell 80 iect. C 26 MECHANICS. whicli added to the 64, give 144, equal to the square of 3 multiplied by 16. Again, in the fourth second it fell 112 feet, which added to 144, give 256, equal to the square of 4 multiplied by 16 ; and in the fifth second it fell 144 feet, which added to 256, give 400, equal to the square of 5 multiplied by 16. Thus you will find the rule holds in all cases, that the spaces described by bodies falling fvpelii from a state of rest increase os the squares of the times increase. C. I think I shall not forget the rule. I will also shew my cousin Henry how he may know the height to which his bow will carry. F. The surest way of keeping what knowledge we have obtained, is by communicating it to our friends. C. It is a very pleasant circumstance indeed, that the giving away is the best method of keeping, for 1 am sure the being able to oblige one's friends is a most delightful thing. F. Your sentiments are highly gratifying to me ; fain would I confirm them by adding to your stock of knowledge. And, in reference to this subject,. it may be necessary to guard you against the notion, that be- cause the spaces described by falling bodies are as the square^ of the times, the velocities increase in the same ratio. This is not the case. The velocity acquired by a body falling freely, at the end of the first second of its motion, is such as, if it continued uniform, would carry it over 32 feet in the next second. And in all succeeding intervals the velocities are as the times: that is, at the end of 2, 3, 4, and 5 seconds, the velocities acquired will be respectively, twice, thrice, four times, and five times 32 feet ; or, 64, 96, 128, and 160 feet. E, Before we quit this part of the subject, papa, let me try if I thoTouohly comprehend you. A fall- ing body having been in motion 4 seconds, will have descended 256^feet, and will then have a velocity of 128 feet ; but the motion still accelerates and causes tlie body to pass over nine times 16, or 144 feet, m the 5th 'second, making in all 400 feet : it will then CENTRE OF GRAVITY. 27 have acquired a velocity of 5 times 32, or 160 feet in a second, which if it continued uniform for another 5 seconds, would carry the body over 800 feet, or just twice the space described by the body in the first 5 seconds, during which its motion was equably accele- rated by gravity. F. You have most accurately caught the distmc- tion I wished you to understand. With this we conclude our present conversation. CONVEKSATION IX. ON THE CENTRE OF GRAVITY. F. We are now going to treat upon -the Cenfre of Gravity, which is that point of a body in which its whole weight is as it were concentrated, and upon which, if the body be freely suspended, it will rest ; ; and in all other positions it will endeavour to descend ' to the lowest place to which it can get. C. All bodies then, of whatever shape, have a cen- I tre of gravity? F. They have ; and if you conceive a line drawn i' from the centre of gravity of a body towards the cen- ' tre of the earth, that line is called the line of direction, \ alono- which every body, not supported, endeavours ! to fall. If the line of direction fall within the base of I any body, it will stand ; but if it does not fall within the base, the body will fall. I If I place the piece of wood a on the edge ! of a table, and from a pin c at its centre of gravity be hung a little weight d, the line ' of direction cd falls within the base, and therefore, though the wood leans, yet it stands secure. But if upon a another piece I of wood b be placed, it is evident that the centre ot -i gravity of the whole will be now raised to e, at which I point if a weight be hung, it will be found that the line of direction falls out of the base, and therefore i the body must fall. 28 MECHANICS. . E. I think I now see the reason of the advice wkich you gave me, when we w^cre going across the Thames in a boat. F, I told you that if ever you were overtaken by a storm, or by a squall of wind, while you were on the water, never to let your fears so get the better of you as to make you rise from your seat, because, by so doing, you would elevate the centre of gravity, and thereby, as is evident by the last experiment, increase the danger : whereas, if all the persons in the vessel were, at the moment of danger, instantly to slip from their places on to the bottom, the risk would be ex- ceedingly diminished, by bringing the centre of gra- vity much lower within the vessel. The same prmci- ple is applicable to those who may be in danger of being overturned in any carriage whatever. E. Surely then, papa, those stages which load their tops with a dozen or more people, cannot be safe for the passengers. F. They are very unsafe ; but they would be more so were not the roads about the metropolis remarkably even and good ; and, in general, it is only withm twenty or thirty miles of London, or other great towns, that the tops of caaTiages are loaded to excess. C. I understand, then, that the nearer the centre of gravity is to the base of a body the firmer it will stand. , - , F. Certainly ; and hence you learn the reason why conical bodies stand so sure on their bases, for, the tops being small in comparison of the lower parts, the centre of gravity is thrown very low ; and, it the cone be uprio ht, or perpendicular, the line of direction talis in the middle of the base, which is another fundamen- tal property of steadiness in bodies. For the broader the base, and the nearer the line of direction is to the middle of it, the more firmly does a body stand ; but if the hue of direction fall near the edge the body is easily overthrown. , n j C. Is that the reason why a ball is so easily rolled along a horizontal plane 1 CENTRE OF GRAVITY. 29 F. It IS ; for m all spherical bodies the base is but a point, consequently almost the smallest force is snt- ficient to remove the line of direction out of it. Hence it is evident that heavy bodies situ- ated on an inclined plane v^^iil, while the line of direction falls within the base, slide down upon the plane; but they will roll when that line falls without the base. The body a will slide down the plane de, but the bo- dies b and c will roll down it. E, I have seen buildings lean very much out of a straight line ; why do they not fall 1 F, It does not follow, because a building leans, that the centre of gravity does not fall within the base. There is a high tower at Pisa, a town in Italy, which leans fifteen feet out of the perpendicular ; strangers tremble to pass by it ; still it is found by experiment that the line of direction falls within the base, and therefore it will stand while its materials hold together. A wall at Bridgenorth, in Shropshire, which I have ! seen, stands in a similar situation, for so long as a I line cb, let fall from the centre of gravity c of the i building* ab, passes within the base cd, it will remain I firm, unless the materials with which it is built go to { decay. 1 C. It must be of great use, in many cases, to know \ the method of finding the centre of gravity in different kinds of bodies. F. There are many easy rules for this with respect I to all manageable bodies : I will mention one, which j depends on the property which the centre of gravity I has, of always endeavouring to descend to the lowest point. If a body a be freely suspended on ^ a pin 6, and a plumb line be be hung by -hj-jj / A the same pin, it will pass through the Yj. centre of gravity, for that centre is not ^^^^ jL the lowest point, till it fall in the same °' * See the Vignette to this volume. 30 MECHANICS. line as the plumb line. Mark the line he ; then hang the body up by any other point, as d, with the plumb line J/, which will also pass through the centre of gravity, for the same reason as before ; and therefore, as the centre of gravity is somewhere in be, and also in some point of df, it must be in the point e where those lines cross. CONVERSATION X. OF THE CENTRE OF GRAVITY. C. How do those people who have to load carts and waggons with light goods, as hay, wool,&c. know where to find the centre of gravity ? F. Perhaps the generality of them never heard of such a principle ; and it seems surprising that they should nevertheless make up their loads with such ac- curacy as to keep the line of direction in or near the middle of the base. E. I have sometimes trembled to pass by the hop waggons which we have met on the Kent Koad. F. And without any impeachment of your courage, for they are loaded to such an enormous height, that they totter every inch of the road. It would indeed be impossible for one of these to pass with tolerable security along a road much inclined ; the centre of gravity being removed so high above the body of the carriage, a small declination on one side or the other would throw the line of direction out of the base. E. When brother James falls about, is it because he cannot keep the centre of gravity between his feet ? F. That is the precise reason why any person, whether old or young, falls. And hence you learn that a man stands much firmer with his feet a little apart than if they were quite close, for by separating them he increases the base. Hence also the difiiculty of sustaining a tall body, as a walking cane, upon a narrow foundation. E. How do rope and wire dancers, whom I have seen at the Circus, manage to balance themselves ? CENTRE OF GRAVITY. 31 F They generally hold a long pole, with weights at each end, across the rope on which they dance, keepino- their eyes fixed on some object parallel to the rope, by which means they know when their centre of gravity declines to one side of the rope or the other, and thus, by the help of the pole, they are enabled to keep the centre of gravity over the base, narrow as it is It is not, however, rope-dancers only who pay attention to this principle, but the most common ac- tions of the people in general are regulated by it. C. In what respects'? F We bend forward when we go up stairs, or rise from* our chair, for when we are sitting our centre of gravity is on the seat, and the line of direction falls behind our base ; we therefore lean forward to bring the line of direction towards our feet. For the same reason a man carrying a burthen on his back leans forward ; and backward if he carries it on his breast. If the load be placed on one shoulder he leans to the other If we slip or stumble with one foot, we natu- rally extend the opposite arm, making the same use of it as the rope-dancer does of his pole. This property of the centre of gravity always en- deavourincr to descend, will account for appearances, which are^sometimes exhibited to excite the surprise of spectators. £. What are those papa? F One is, that of a double cone, appearing to roll up two inclined planes, forming an angle with each other for as it rolls it sinks between them, and by that means the centre of gravity is actually descending. Let a body ej\ consisting of two equal cones united at their o^^^^^^^^^i bases, be pla ced upon the edges ^^r^;:^:^^— ^ of two straight smooth rulers /Q^^^^^^,,:^^...::^^^^^^ ah and cd, which at one end meet in an angle at a, and rest ^-^ on a horizontal plane, and at ° , i the other are raised a little above the p ane ; the body will roll towards the elevated end of the rulers, and 32 MECHANICS. appear to ascend ; the parts of the cone that rest on the rulers growing smaller as they go over a larger opening, and thus letting it down, the centre of gra- vity descends. But you must remember that the height of the planes must be less than the radius of the base of the cone. C. Is it upon this principle that a cylinder is made to roll up hill ? F. Yes it is, but this can be effected only to a small distance. If a cylinder of pasteboard, or very light wood, ah, having its centre of gravity at c, be placed on the inclined plane de, it will roll down the inclined plane, because a line of direction from that centre lies out of the base. If 1 now fill the little hole o with a Tig. 12. plug of lead, it will roll up the in- clined plane, till the lead gets near the base, where it will lie still : because the centre of gravity, by means of the lead, is removed from c towards the plug, and therefore is descending, though the cylinder is ascending. Before I put an end to this subject, I will shew you another experiment, which without understanding the principle of the centre of gravity cannot be ex- plained. Upon this stick a, which, of itself would fall, because its centre of gravity hangs over the table b, I suspend a bucket c, fixing another stick d, one end in a notch between a and e, and the other against the in- side of the pail at the bottom. Now you will see that the bucket will, in this position, be supported, though filled with water. For the bucket being pushed a little out of the perpendicular, by the stick d, the centre of gravity of the whole is brought under the table, and consequently supported by it. The knowledge of the principle of the centre of gravity in bodies, will enable you to explain the structure of a variety of toys which are put into the LAWS OF MOTION. 33 hands of children, such as the little sawyer, rope- dancer, tumbler, 4c. CONVERSATION XI. ON THE LAWS OF MOTION. C. Are you now going, papa, to describe those ma- chines, which vou call mechanical powers ? F. We must, I believe, defer that a day or two longer, as I have a few more general prhiciples with which I wish you previously to be acquamted. E. What are these, papal F, In the first place, you must well understand what are denominated the three general laws of mo- tion : the first of which is, that every body will con- tinue in its state of rest, or of unform motion, until it is compelled bu some force to change its state. _ I his constitutes what is denominated the inertia, or macti;- vity of matter. And it may be observed that, m all cases, the quantity of motion gained by one body is always equal to that lost by some other body. C. There is no difficulty of conceiving that a body, as this ink-stand, in a state of rest must always remain so, if no external force be impressed upon it to give it motion. But I know of no example which will lead me to suppose, that a body once put into motion would of itself continue so. , . i i F You will, I think, presently admit the latter part of the assertion as well as the former, although it cannot be established by experiment. ^ E. I shall be glad to hear how this is. F. You will not deny that the ball which you strike from the trap has no more power either to destroy its motion, or cause any change in its velocity, than it has to change its shape. ■, C. Certainly ; nevertheless, m a few seconds after I have struck the ball with all my force, it falls to the ground, and then stops. _ F Do YOU find no diflference in the time that is ^ C 2 31 MECHANICS. taken up before it conies to rest, even supposing your blow the same 1 C. Yes, if 1 am playing on the grass, it rolls to a less distance than when 1 play on the smooth gravel. F. You fmd a like difference when you are playing at marbles, if you play in the gravel court, or on the even pavement in the arcade. C. The marbles run so easily on the smooth stones in the arcade, that we can scarcely shoot with a force small enough. E. And I remember Charles and my cousin were last winter trying how far they could shoot their marbles along the ice in the canal ; and they went a prodigious distance, in comparison of that which they would have gone on the gravel, or even on the pave- ment in the arcade. F. Now these instances properly applied will con- vince you, that a body once put into motion would go on for ever, if it were not compelled by some ex- ternal force to change its state. C. I perceive what you are going to say : — it is the rubbing or friction of the marbles against the ground which does the business. For on the pavement there are fewer obstacles than on the gravel, and fewer on the ice than on the pavement ; and hence you would lead us to conclude, that if all obstacles were removed, they might proceed on for ever. 15ut what are we to say of the ball ; what stops that ? F. Besides friction, there is another and still more important circumstance to be taken into consideration, wiiich affects the ball, marbles, and every body in motion. C. I understand you, that is the action of gravitation, F. It is ; for from what we said when we con- versed on that subject, it appeared that gravity has a tendency to bring every body in motion to the earth ; con.se(|uently, in a few seconds, your ball must come to the ground by that cause alone ; but besides the at- traction of gravitation, there is the resistance which the air, through which the ball moves, makes to its passage . LAWS OF MOTION. E. That cannot be much, I think. F. Perhaps, with regard to the ball struck from your brother's trap, it is of no great consideration, be- cause the velocity is but small ; but in all great velo- cities, as that of a ball from a musket or cannon, there will be a material difference between the theory and practice, if it be neglected in the calculation. Move your mamma's riding-whip through the air slowly, and you observe nothing to remind you that there is this resisting medium ; but if you swing it with consider- able swiftness, the noise which it occasions will in- form you of the resistance it meets with from some- thing, which is the atmosphere. C. If I now understand you, the force which com- pels a body in motion to stop, is of three kinds ; 1. the attraction of gravitation ; — 2. the resistance of the air ; — and 3. the resistance it meets with from friction. F. You are quite right. C. I have now no difficulty of conceiving, that a body in motion will not come to a state of rest, till it is brought to it by an external force, acting upon it in someway or other. I have seen a gentleman, when skating on very slippery ice, go a great way without any exertion to himself, but where the ice was rough be could not go half the distance without making fresh efforts. F. I will mention another instance or two of this law of motion. Put a basin of water into your little sister's waggon, and when the water is perfectly stdl move the waggon, and the water, resisting the motion of the vessel, will at first rise up in the direction con- trary to that in which the vessel moves. If, when the motion of the vessel is communicated to the water, you suddenly stop the waggon, the water, in endeavouring to continue the state of motion, rises up on the oppo- site side. In like manner, if, while you are sitting quietly on your horse, the animal starts forward, you will be in danger of falling off backward ; but if, while you are galloping along, the animal stops on a sudden, you will be liable to be thrown forward. 36 MECHANICS. C. This I know by experience, but I was not aware of the reason of it till to-day. F. One of the first, and not least important, uses of the principles of natural philosophy is, that they may be applied to, and will explain,, many of the common concerns of life. We now come to the second law of motion ; which is, ''that the change of motion is proportional to the Jorce impressed, and in the direction of that force, ^' C. There is no difficulty in this ; for if, while my cricket-ball is rolling along, after Henry has struck it, I strike it again, it goes on with increased velo- city, and that in proportion to the strength which I exert on the occp.sion ; whereas, if, while it is rolling, 1 strike it back again, or give it a side blow, I change the direction of its course. F. In the same way, gravity, and the resistance of the atmosphere, change the direction of a cannon-ball from its course in a straight line, and bring it to the ground ; and the ball goes to a farther or less distance, in proportion to the quantity of powder used. The third law of motion is, " that, to every action of one body upon another, there is an equal and con^ trary re-action." If 1 strike tliis table, I communi- cate to it (which you perceive by the shaking of the glasses) the motion of my hand : and the table re- acts against my hand, just as much as my hand acts against the table. ^ If you press with your finger one scale of a balance, to keep it in equilibrio with a pound weight in the other scale, you will perceive that the scale pressed by the finger acts against it with a force equal to a pound, with v.'hich the other scale endeavours to descend. In all cases, the quantity of motion gained by one body is always equal to that lost by the other in the same direction. Thus, if a ball in motion strike another at rest, the motion communicated to the latter will be taken from the former, and the v elo- city of tiie former will be proportionally diminished. LAWS OF MOTION. 37 A horse drawing a heavy load, is as much drawn back by the load as he draws it forward. E. 1 do not comprehend how the cart draws the ^^^F!'But the progress of the horse is impeded by the joad, which is the same thing; for the force which the horse exerts would carry him to a greater distance in the same time, were he freed from the mcumbrance of the load, and, therefore, as much as his progress falls short of that distance, so much is he, in eliect, drawn back by the re-action of the loaded cart. Ao-ain, if you and your brother were in a boat, and if by means of a rope, you were to attempt to draw another to you, the boat in which you were would be as much pulled toward the empty boat as that would be moved to you ; and, if the weights of the two boats were equal, they would meet in a point halt way be- tween the two. . If you strike a glass bottle with an iron hammer, • the blow will be received by the hammer and the fvlass ; and it is immaterial whether the hammer be moved against the bottle at rest, or the bottle be moved against the hammer at rest, yet the bottle will be broken, though the hammer be not injured, because the same blow which is sufficient to break glass is not sufficient to break or injure a lump of iron. From this law of motion you may learn m what manner a bird, by the stroke of its wmgs, is able to support the weight of its body. C. Pray explain this, papa. F If the force with which it strikes the air below it is equal to the weight of its body, then the re-action of the air upwards is likewise equal to it ; and the bird, beinp; acted upon by two equal forces in contrary di- rections, will rest between them. If the force ot the stroke is greater than its weight, the bird will rise with the difference of these two forces ; and, it the stroke be less than its weight, then it will sink with the difference. 38 MECHANICS. CONVERSATION XII. ON THE LAWS OF MOTION. C. Are those laws of motion which you explained yesterday of great importance in natural philosophy 1 F. Yes, they are, and should be carefully commit- ted to memory. They were assumed by Sir I. New- ton as the fundamental principles of mechanics, and you will find them at the head of most books writ- ten on these subjects. From these also we are natu- rally led to some other branches of science, which, though we can but slightly mention, should not be wholly neglected. They are, in fact, but corollaries to the laws of motion. E. What is a corollary, papa? F. It is nothing more than some truth clearly de- ducible from some other truth before demonstrated or admitted. Thus by the first law of motion every body must endeavour to continue in the state into which it is put, whether it be of rest, or uniform motion in a straight line : from which it follows, as a corollary, that when we see a body move in a curve line, it must be acted upon by at least two forces. C. When I whirl a stone round in a sling, what are the two forces which act upon the stone 1 F. There is the force by which, if you let go the string, the stone will fly off in a right line ; and there is the force of the hand, which keeps it in a circular motion. E. Are there any of these circular motions in nature ? F, The moon and all the planets move by this law : — to take the moon as an instance. It has a constant tendency to the earth, by the attraction of gravitation, and it has also a tendency to proceed in a right line, by that projectile force impressed upon it by the Creator, in the same manner as the stone flies from your hand ; now, by the joint action of these two forces it describes a circular motion. LAWS OF MOTION. 39 E. And what would be the consequence, supposing the projectile force to cease ? F. The moon must fall to the earth ; and if the force of gravity were to cease acting upon the moon, it would fly off into infinite space. Now the projectile force, when applied to the planets, is called the cen- trifugal force, as having a tendency to recede or fly from the centre ; and the other force is termed the ceiitripelal force, from its tendency to some point as a centre. C. And all this is in consequence of the inactivity of matter, by which bodies have a tendency to con- tinue in the same state they are in, whether of rest or motion ? F, You are right, and this principle, which Sir Isaac Newton assumed to be in all bodies, he called their vis ineriice, which has been referred to before. C. A few mornings ago you shewed us that the attraction of the earth upon the moon* is 3600 times less, than it is upon heavy bodies near the earth's sur- face. Now as this attraction is measured by the space fallen through in a given time, I have endeavoured to calculate the space which the moon would fall through in a minute, were the projectile force to cease. F, Well, and how have you brought it out ? C. A body falls here 16 feet in the first second, consequently in a minute, or 60 seconds, it would fall 60 times 60 feet, multiplied by 16, that is 3600 feet, which is to be multiplied by 16 ; and as the moon would fall through 3600 times less space in a given time than a body here, it would fall only 16 feet in the first minute. F. Your calculation is accurate. I will recall to your mind the second law, by which it appears, that every motion, or change of motion, produced in a body, 7nust be proportional to, and in the direction of, the force impressed. Therefore, if a moving body receives an impulse in the direction of its motion, its velocity * See Conversation IV. 40 MECHANICS. vviil be increased ; — if in the contrary direction, its velocity will be diminished ; — but if the force be im- piessed in a direction oblique to that in which it moves, then its direction will be between that of its former motion, and that of the new force impressed. C. This I know from the observations I have made with my cricket-ball. F, By this second law of motion, you will easily understand, that if a body at rest receive two im- pulses at the same time, from forces whose directions do not coincide, it will, by their joint action, be made to move in a line that lies between the direction of the forces impressed. E. Have you any machine to prove this satisfac- torily to the senses ? F. There are many such invented by different per- sons, descriptions of which you will hereafter find in various books on these subjects. But it is easily un- derstood by a figure. If on the ball a a ^ force be impressed sufficient to make it ^ — f move with an uniform velocity to the | point b, in a second of time ; and if an- ^ other force be also impressed on the ball. Fig. 14. which alone would make it move to the point c, in the same time ; the ball, by means of the two forces, will describe the line ad, which is a diago- nal of the figure, whose sides are ac and ab. C. How then is motion produced in the direction rftheforcel According fo the second law, it ought to be in one case in the direction ac, and in the other in that of ab, whereas it is in that of ad. F. Examine the figure a little attentively, carrying this in your mind, that for a body to move in the same direction, it is 7iot necessary that it should move in the same slrainht line ; but that it is sufficient to move either in that line, or in any one parallel to it. C. I perceive then that the ball when arrived at d, has moved in the direction ac, because bd is parallel to ac; and also in the direction ab, because cd is parallel to it. LAWS OP MOTION. 41 F. And in no other possible situation but at the point d could this experiment be conformable to the second law of motion. When bodies move in a curve, it must be kept in mind that there must be a continued action of external force ; otherwise, if that action were to cease at any point, the body would continue its motion in a straight line. CONVERSATION XIIT. OF THE LAWS OF MOTION. F. If you reflect a little upon what we said yester- day on the second law of motion, you will readily de- duce the following corollaries. (Fig. 14.) 1. That if the forces be equal, and act at right angles to one another, the line described by the ball will be the diagonal of a square. But in all other cases, it will be the diagonal of a parallelogram of some kind. 2. By varying the angle, and the forces, you vary the form of your parallelogram. C. Yes, papa, and I see another consequence, viz. that the motions of two forces acting conjointly in this way, are not so great as when they act separately. F. That is true, and you are led to the conclusion, I suppose, from the recollection, that in every triangle any two sides taken together are greater than the re- maining side ; and therefore you infer, and justly too, that the motions which the ball a must have received, had the forces been applied separately, would have been equal to ac and ab, or, which is the same thing, to ac and cd, the two sides of the triangle adc, but by their joint action, the motion is only equal to ad, the remaining side of the triangle. Hence then you will remember, that in the compo- silion, or adding together of forces (as this is called), motion is always lost : and in the resolution of any 42 MECHANICS. one force, as ad, into two others, ac and ah, motion is gained . C. Well, papa, but how is it that the heavenly bodies, the moon for instance, which is impelled by two forces, performs her motion in a circular curve round the earth, and not in a diagonal between the du-ection of the projectile force, and that of the attrac- tion of gravity to the earth ? F. Because, in the case just mentioned, there was but the action of a single impulse in each direction, whereas the action of gravity on the moon is con- tmual, and causes an accelerated motion, and hence the line is a curve. C. Supposing then, that a represent the moon, and «c the sixteen feet through which it would fall in a minute by the attraction of gravity towards the earth, and ab represent the projectile force acting upon it for the same time. If ab and ac acted as sinde impulses, the moon would in that case describe the diagonal ad ; but since these forces are constantly acting^ and that of gravity is an accelerating force also, therefore instead of the straight line ad, the moon will be drawn into the curve line aed. Do I understand the matter right ? F. You do; and hence you easily comprehend how, by good instruments and calculation, the attrac- tion of the earth upon the moon was discovered. The ih'ird law of motion, viz. that action and re- action are equal and in contrary directions, may be illustrated by the motion communicated by the per- cussion of elastic and non-elastic bodies. E. \Vhat are these, papa? F. Elastic bodies are those which have a certain spring, by which their parts upon being pressed in- wards, by percussion, return to their former state ; this property is evident in a ball of wool or cotton, or in sponge compressed. Non-elastic bodies are those which, when one strikes another, do not rebound, but move together after the stroke. LAWS OF MOTION. Let two equal ivory balls a and b be sus- pended by threads ; if a be drawn a little out of the perpendicular, and let fall upon b, it will lose its motion by communicating it to b, which will be driven to a distance c, _^ ^ _ equal to that through which a fell; and ^1^15, hence it appears that the re-action of b was equal to the action of a upon it. E. But do the parts of the ivory balls yield by the stroke, or, as you call it, by the percussion 1 F. They do ; for if I lay a little pamt on a, and let it couch b, it will make but a very small speck upon it ; but if it fall upon h, the speck will be much larger ; which proves that the balls are elastic, and that a little hollow, or dint, was made in each by colli- sion. If now two equal soft balls of clay, or glazier s putty, which are non-elastic, meet each other with equal' velocities, they would stop and stick together at the place of their meeting, as their mutual actions destroy each other. , C. I have sometimes shot my white alley against another marble so plumply, that the marble has gone off as swiftly as the alley approached it, and that re- mained in the place of the marble. Are marbles, therefore, as well as ivory, elastic ? F. They are. — If three elastic balls a, b, c, be hung from adjoining centres, and u be drawn a little out of the perpendicu- lar, and let fall upon 6, then will a and b become stationary, and q will be driven ^, to d, the distance through which a fell O o O upon b. ^^ig- If you hang any number of balls, as six, eight, &c. so as to touch each other, and if you draw the outside one away to a little distance, and then let it fall upon the others, the ball upon the op- posite side will be driven off, while the rest remain stationary, so equally is the action and re-action of the stationary balls divided among them. In the same manner, if two are drawn aside and suffered 44 MECHANICS. to fall on the rest, tl>e opposite two will liy off, and the others remain stationary. There is one other circumstance depending upon the action and re-action of bodies, and also upon the vis inerti(t of matter, worth noticing : by some authors you will find it largely treated upon. If I strike a blacksmith's anvil with a hammer, action and re-action being equal, the anvil strikes the hammer as forcibly as the hammer strikes the anvil. If the anvil be large enough, I might lay it on my breast, and suffer you to strike it 'with a sledge hammer with all your strength without pain or risk, for the vis ijievtia: of the anvil resists the force of the blow. But if the anvil were but a pound or two in weight, your blow would probably kill me. CONVERSATION XIV. ON THE MECHANICAL POWERS. C. Will you now, papa, explain the mechanical powers ? F, I will, and I hope you have not forgotten what the momentum of a body is. G. No, it is the force of a moving body, which force is to be estimated by the weight, multiplied into its velocity. F. Then a small body may have an equal momen- tum with one much larger ? C. Yes, provided the smaller body moves as much swifter than the larger one, as the weight of the latter is greater than that of the former. F, What do you mean when you say that one body moves swifter, or has a greater velocity, than another ? C. That it passes over a greater space in the same time. Your watch will explain my meaning : the minute-hand travels round the dial-plate in an hour, but the hour-hand takes twelve hours to perform its course in, consequently, the velocity of the miiiute- MECHANICAL POWERS. 45 hand is twelve times greater than that of the hour- hand ; because, in the same time, viz, tvi^elve hours, it travels twelve times the space that is gone through by the hour-hand. F. But this can be only true on the supposition that the two circles are equal. In my watch, the minute-hand is longer than the other, and conse- quently, the circle described by it is larger than that described by the hour-hand. C. I see at once that my reasonmg holds good only in the case where the hands are equal. F. There is, however, a particular point of the longer hand, of which it may be said, with the strictest truth, that it has exactly twelve times the velocity of the extremity of the shorter. C. That is the point at which, if the remainder were cut off, the two hands would be equal. And, in fact every different point of the hand describes different spaces in the same time. F. The little pivot on which the two hands seem to move (for they are really moved by different pivots, one within another) may be called the centre of motion, which is a fixed point ; and the longer the hand is, the greater is the space described. C. The extremities of the vanes of a wind-mill, when they are going very fast, are scarcely distin- guishable, though the separate parts, nearer the mill, are easily discerned ; this is owing to the velocity of the extremities being so much greater than that of the other parts. E. Did not the swiftness of the round-abouts, which we saw at the fair, depend on the same principle, viz. the length of the poles upon which the seats were fixed ! F. Yes ; the greater the distance, at which these seats were placed, from the centre of motion, the greater the space which the little boys and girls travelled for their halfpenny. E. Then those in the second row, had a shorter ride for then money than those at the end of the pol^s ? 4G MECHANICS. F. Yes, shorter as to space, but the same as to time. In the same way, when you and Charles go round the gravel -walk for half an hour's exercise, if he run while you walk, he will, perhaps, have gone six or eight times round, in the same time that you have been but three or four times ; now, as to time, your exercise has been equal, but he may have passed over double the space in the same time. C. How does this apply to the explanation of the mechanical powers? F. You will find the application very easy : — without clear ideas of what is meant by time and space, it were in vain to expect you to comprehend the principles of mechanics. There are six mechanical powers. The lever ; the wheel and axle ; the pulley ; the inclined plane ; the wedge ; and the screw. E. Why are they called mechanical powers? F. Because, by their means, we are enabled me- chanically to raise weights, move heavy bodies, and overcome resistances, which, without their assistance, could not be done. C. But is there no limit to the assistance gained by these powers? for I remember reading of Archime- des, who said, that with a place for his fulcrum he would move the earth itself. F. Human power, with all the assistance which art can give, is very soon limited, and upon this prin- ciple, that what ice gain in power, we lose in time. That is, if by your own unassisted strength, you are able to raise fifty pounds to a certain distance in one minute, and if by the help of machinery, you wish to raise five hundred pounds to the same height, you will require ten minutes to perform it in ; thus you increase your power ten-fold, but it is at the expense of time. Or in other words, you are enabled to do that with one effort in ten minutes, which you could have done in ten separate efforts in the same time. E, The importance of mechanics, then, is not so very considerable as one, at first sight, would ima- MECHANICAL POWETIS. 47 gine ; since there is no real gain offeree acquired by the mechanical powers. . F. Thouoh there be not any actual mcrease ot force gamed°by these powers, yet the advantages which men derive from them are inestimable. It there are several small weights, manageable by human strength, to be raised to a certain height, it may be full as con- venient to elevate them one by one, as to take the ad- vantao-e of the mechanical powers, in raismg them all at once. Because, as we have shewn, the same time will be necessary in both cases. But suppose you have a large block of stone of a ton weight to carry away, or a weight still greater, what is to be done { E. I did not think of that, . F. Bodies of this kind cannot be separated mto parts proportionable to the human strength without immense labour, nor, perhaps, without rendering them unfit for those purposes for which they are to be ap- plied. Hence then you perceive the great importance of the mechanical powers, by the use of which, a man is able with ease to manage a weight many times p:reater than himself. , CI have, indeed, seen a few men, by means ot pulleys, and seemingly with no very great exertion, raise an enormous oak into a timber-carnage, in order to convey it to the dock-yard. , , , . . F. A very excellent instance; for if the tree had been cut into such pieces as could have been ma- naged by the natural strength of these men, it would not have been worth carrying to Deptford or Chat- ham for the purpose of ship-building. E. 1 acknowledge my error what is a fulcrum, ^^E. It is a fixed point, or prop, round which the other parts of a machine move. C. The pivot, upon which the hands of your watch move, is a fulcrum then ? i ^ E It is and you remember we called it also the centre of motion the rivet of these scissars is also a fulcrum, and also the centre of motion. 48 MECHANICS. E. Is tliat a fixed point, or prop ? F. Certainly it is a fixed point, as it regards the two parts of the scissars ; for that always remains m the same position, while the other parts move about it. Take the poker and stir the fire ; now that part of the bar on which the poker rests is a fulcrum, for the poker moves upon it as a centre. ' CONVERSATION XV. OF THE LEVER. F. We will now consider the Lever, which is generally called the first mechanical power. The lerer is any inflexible bar of wood, iron, &c. which serves to raise weights, while it is supported at a point by a prop or fulcrum, on which, as the centre of motion, all the other parts turn, tt 6 , will represent a lever, and the point c „ ^.-^1 the fulcrum or centre of motion. Now it is evident, if the lever turn on its \r'' centre of motion c, so that b comes ^ ^. into the position ti; a at the same time §'* must come into the position e. If both the arms of the lever be equal, that is, if ac is equal to he, there is no advantage gained by it, for they pass over equal spaces in the same time ; and, according to the fun- damental principle already laid down, (p. 46,) " as advantage or power is gained, time must be 'lost:" therefore, no time being lost by a lever of this kind, there can be no power gained. ^Vhy then is it called a mechanical power ? F. Strictly speaking, perhaps, it ought not to be numbered as one. But it is usually reckoned among them, having the fulcrum between the weight and the ])ow{?i , uliich is the distinguishing property of levers of tlie liisl kind. And when the fulcrum is exactly the middle point between the weight and power it is the common balance : to which, if scales be sus- MECHANICAL POWEllS. 49 ponded at a and b, it is fitted for weighing all sorts of (jommodities. E. You say it is a lever of the first kind j are there several sorts of levers 1 F, I'here are three sorts ; some persons reckon four ; the fourth, however, is but a bended one of thtj first kind. A lever of the fir bt kind has the fulcrum betv/een the weight and power. 6^ 4 ' "fl Fig. 18. ^^ig- The second kind of lever has the fulcrum at one end, the power at the other, and the weight between them. ^ Fig. 20. Fig. 21. In the third kind, the power is between the ful- crum and the weight. . . , /-n- m X Let us take the lever of the first kmd, (Fig. 18.) which if it be moved into the position cd, by turning on its fulcrum e, it is evident that while a has tra- velled over the short space ac, b has travelled over the greater space bd, which spaces are to one another exactly in proportion to the length of the arms ae and be. If, now, you apply your hand first to the point a, and afterwards to b, in order to move the lever into the position cd, using the same velocity in both cases, you will find, that the time spent in moving the lever when the hand is at b, will be as much greater, as that spent when the hand is at a, as the arm be is longer than the arm ae; but then the exertion re- quired will, in the same proportion, be less at b than at a. C. The arm be appears to be four times the length of ae. I) 50 MECHANICS. F. Then it is a lever which gains power in the proportion of four to one. That is, a single pound weight applied to the end of the arm be, as at p, will balance four pounds suspended at a, as u\ C. 1 have seen workmen move large pieces of tim- ber to very small distances, by means of a long bar of wood or iron ; is that a lever 1 F, It is ; they force one end of the bar under the timber, and then place a block of wood, stone, &c. beneath, and as near the same end of the lever as possible, for a fulcrum, applying their own strength to the other : and power is gained in proportion as the distance from the fulcrum to the part where the men apply their strength, is greater than the distance from the fulcrum to that end undei the timber. Hand- spikes are levers of this kind, and by these the hea- viest cannon are moved, as well as other heavy bodies. C. It must be very considerable, for IJiave seen two or three men move a tree in this way, of several tons' weight, I should think, F. That is not difficult ; for supposing a lever to gain the advantage of twenty to one, and a man by his natural strength is able to move but a hundred weight, he will find that by a lever of this sort he can move twenty hundred weight, or a ton ; but, for single exertions, a strong man can put forth a much greater power than that which is sufficient to remove a hundred weight ; and levers are also frequently used, the advantage gained by which is still more considerable than twenty to one. C. I think you said, the other day, that the com- mon steelyard made use of by the butcher is a lever] F. I did ; the short arm ac (Fig. 19.) is, by an increase in size, made to balance the longer one be, and from c, the centre of motion, the divisions must commence. Now if he be divided into as many parts as it will contain, each equal to ac, a single weight, as a pound, will serve for weighing any thing as heavy as itself, or as many times heavier as there are OF THE LEVER. 51 divisions in the arm c. If the weight p be placed at the division 1 in the arm he, it will balance one pound in the scale at «; if it be removed to 3, 5, or 7 it will balance 3, 6, or 7 pounds m the scale ; tor these divisions being respectively 3, 5, or 7 times tae distance from the centre of motion c, that a is, it be- comes a lever, which gains advantage, in those points, in the proportion of 3, 5, and 7. If, now, the mter- vals between the divisions on the longer arm be sub- divided into halves, quarters, &c. any weight may be accurately ascertained, to halves, quarters of pounds, &c. CONVEKSATION XVI. OF THE LEVER. E. What advantage has the steelyard, which you described in our last conversation, over a pair of scales 1 F. it may be much m.ore readily removed from place to place ; it requires no apparatus, and only a single weight for all the purposes to which it can be applied. — Sometimes the arms are not of equal weight. In that case the weight p must be moved along the arm be, till it exactly balance the other arm without a weight, and in that point a notch must be made, marking over it a cypher 0, from whence the divisions must commence. C. Is there not required great accuracy in the ma- nufacture of instruments of this kind 1 F. Yes ; of such importance is it to the public that there should be no error or fraud by means of false weights, or false balances, that it is the business of certain public officers to examine at stated seasons the weights, measures, &c. of every shopkeeper in the land. Yet it is to be feared that, after ail pre- cautions, much fraud is practised upon the unsus- pecting. E. I one day last summer bought, as I supposed, a pound of cherries at the door ; but Charles think- 52 MECHANICS. ing there was not a pound, we tried them in your scales, and found but twelve ounces, or three quar- ters, instead of a pound, and yet the scale went down as if the man had given me full weight. How was that managed ? F, It might be done many ways : by short weights ; — or by the scale in which the fruit was put being heavier than the other: — but fraud may be practised with good weights and even scales, by making the arm of the balance on which the weights hang shorter than the other, for then a pound weight v/ill be balanced by as much less fruit than a pound as that arm is shorter than the other j this was pro- bably the method by v/hich you were cheated. E. By what method could I have discovered this cheat? F. The scales when empty are exactly balanced, but when loaded, though still in equilibrio, the weights are unequal, and the deceit is instantly discovered by changing the weights to the contrary scales. I will give you a rule to find the true weight of any body by sueh a false balance ; the reason of the rule you will understand hereafter: "Jind the weights of the body by both scales, multiply them together, and then find the square root of the product, which is the true loeight" C. Let me see if I understand the rule : — suppose a body weigh 16 ounces in one scale, and in the other 12 ounces and a quarter, I multiply 16 by 12 and a quarter, and I get the product 196, the square root of which is 14; for 14 multiplied into itself gives 196 ; therefore the true weight of the body is 14 ounces. F. That is just what I meant. — To the lever of the first kind may be referred many common instruments, such as scissars, pincers, snuffers, &c. which are made of two levers, acting contrary to one another. F, The rivet is the fulcrum, or centre of motion, the hand the power used, and whatever is to be cut is the resistance to be overcome. OP THE LEVER. 53 C. A poker stirriog the fire is also a lever, for the bar is the fulcrum, the hand the power, and the coals the resistance to be overcome. F, W e nov^ proceed to levers of the second kind, in which the fulcrum c (Fig. 20.) is at one end, the power p applied at the other b, and the weight to be raised at iv, somewhere between the fulcrum and the power. C. And how is the advantage gained to be esti- mated in this lever? F, By looking at the figure, you will find that power or advantage is gained in proportion as the dis- tance of the power p is greater than the distance of the weight iv from the fulcrum. C. Then if the weight hang at one inch from the fulcrum, and the power acts at five inches from it, the power gained is five to one, or one pound at p will balance five at ivl. F. It will 3 for you perceive that the power passes over five tirhes as great a space as the weight, or while the point e in the lever moves over one inch, the point h will move over five inches. E, What things in common use are to be referred to the lever of the second kind ? F. The most common and useful of all things; every door, for instance, which turns on hinges is a lever of this sort. The hinges may be considered as the fulcrum, or centre of motion, the whole door is the weight to be moved, and the power is applied to that side on which the lock is usually fixed. E. Now I see the reason why there is considerable difficulty in pushing open a heavy door, if the hand is applied to the part next the hinges, although it may be opened with the greatest ease in the usual method, C. This sofa, with sister upon it, represents a lever of the second kind. F, Certainly ; if while she is sitting upon it, in the middle, you raise one end, while the other remains fixed as a prop or fulcrum. To this kind of lever may be also reduced nut-crackers j oars ; rudders of 51 MECHANICS. ships ; those cutting knives which have one end fixed in a block, such as are used for cutting chaff, drugs, wood for pattens, &c. E. I do not see how oars and rudders are levers of this sort. jp. The boat is the weight to be moved, the w^ater is the fulcrum, and the waterman at the handle the power. The masts of ships are also levers of the second kind, for the bottom of the vessel is the ful- crum, the ship the weight, and the wind acting against the sail is the moving power. The knowledge of this principle may be useful in many situations and circumstances of life : — if two men unequal in strength have a heavy burden to carry on a pole between them, the ability of each may be consulted, by placing the burden as much nearer to the stronger man, as his strength is greater than that of his partner. E. Which would you call the prop in this case 1 F, The stronger man, for the weight is nearest to liim ; and then the weaker must be considered as the power. Again, two horses may be so yoked to a carriage that each shall draw a part proportional to his strength, by dividing the beam in such a manner that the point of traction, or drawing, may be as much nearer to the stronger horse than to the weaker, as the strength of the former exceeds that of the latter. We will now describe the third kind of lever. In this the prop or fulcrum e (Fig. 21.) is at one end, the weight w at the other, and the power p is applied at b, somewhere between the prop and weight. C. In this case, the weight being farther from the centre of motion than the power, must pass through more space than it. F, And what is the consequence of that ? C. That the power must be greater than the weight, and as much greater as the distance of the weight from the prop exceeds the distance of the power from it, that is, to balance a weight of three pounds at a, OF THE WHEEL AND AXIS. 55 tlieie will require tlie exertion of a power p, acting at l^tn'ef tbent;er of this kind is a disad.an- taoVto the moving power, it is but seldom used, and oTy n cases of nec'^.ssity ; such as m tna of a ad- der! which being fixed a^ -d ^^I.:";;^ S tfot^W^enlicufarluUion . But the most IZortan application of this third kind of lever, is Tan" esHn the structure of the limbs of animals, par- Tuto V in those of man; to take the f^^^^^^X stoce when we lift a weight by the hand, it is effected by means of muscles coming from the shoulder-blade, and terminating about one-tenth as far below the elbow as the hand is: now the elbow being the centre of motion round which the lower mrt of the arm turns, according to the prmciple just laTd down, the muscles must exert a force ten times i treat^ the weight that is raised. At first view this may appear a disadvantage, but what is lost m powe" iJ gained in velocity, and thus the human ^gure is better adapted to the various functions it has to perform. CONVERSATION XVIL OF THE WHEEL AI^D AXIS. F Well, Emma, do you understand the principle of the lever, which we discussed so much at large ^'^E^'l^he' lever gains advantage in proportion to the space passed through by the acting power ; that is it the wetht to be raled be at the distance of one inch from he fulcrum, and the power is applied nme nches distant from it, then it is a lever which gams advantage as. nine to one, because the space passed through by the po^i-.r is nine times greater than that passed through by the weight ; and, therefore, what 50 MECHANICS. is lost in time, by passing through a greater space» is gamed m power. F. You recollect also what the different kinds of levers are, I hope ? E. I shall never see the fire stirred without think- mg of a simple lever of the first kind ; my scissars wiil frequently remind me of a com.bination of two - levers of the same sort. The opening and shuttino- of the door will prevent me from forgetting the na° ture of the lever of the second kind ; and I am sure that I shall never see a workman raise a ladder against a house without recollecting the third sort of lever. Besides, I believe a pair of tongs is a lever of this kind ? i^. You are right ; for the fulcrum is at the joint, and the power is applied between that and the parts used m taking up coals, &c.~Can you, Charles, tell us how the principle of momentum applies to the lever ? C. The momentum of a body is estimated by its weight multiplied into its velocity; and the velocity must be calculated by the space passed through in a given time. Now, if I examine the lever (iio-. 18. 20.) and consider it as an inflexible bar turning on a centre of motion, it is evident that the same time is used for the motion both of the weight and the power but the spaces passed over are very different ; that which the power passes through being as much greater than that passed by the weight, as the leno-th of the distance of the power from the prop is crrealer tnan the distance of the weight from the prop''; and the velocities being as the spaces passed in the same time, must be greater in the same proportion. Con- sequently, the velocity of p, the power, multiplied into its weight, wall be equal to the smaller velocity of 10, multiplied into its weight, and thus their mo- "'T'^^.n^.^'S- equal, they will balance one anotlier. ^. Ihis applies to the first and second kind of lever ; what do you say to the third ? C. In the third, the velocity of the power;) (Ih OF THE WHEEL AND AXIS. 57 21.) being less than that of the weight it is evi- dent, in order that their momenta may be equal, that the weight acting at p must be as much greater than that of lu as ae is less than be, and then they will be in equiUbrio. F. The second mechanical power is the Wheel and Axis, which gains power in proportion as the circum-. ference of the wheel is greater than that of the axis • this machine may be referred to the principle of the lever, ah is the wheel, cd its axis ; and if the circumference of the wheel be eight times as great as that of the axis, then a single pound, JO, will balance a weight, w, of eight pounds. C. Is it by an instrument of this kind that water is drawn from those deep wells so common in many parts of the country 1 F, It is ; but as in most cases of this kind only a single bucket is raised at once, there requires but little power in the operation, and therefore, instead of a large wheel, as ab, an iron handle fixed at c is made use of, which, you know, by its circular mo- tion, answers the purpose of a wheel. C. I once raised some water by a machine of this kind, and I found that as the bucket ascended nearer the top the difficulty increased. F. That must always be the case, where the wells are so deep as to cause, in the ascent, the rope to coil more than once the length of the axis, because the advantage gained is in proportion as the circumfer- ence of the wheel is greater than that of the axis ; so that if the circumference of the wheel be 12 times greater than that of the axis, one pound applied at the former will balance twelve hanging at the latter ; but by the coiling of the rope round the axis, the differ* ence between the circumference of the wheel and that of the axis continually diminishes ; consequently the advantage gained is lei-^s every time a new coil of D2 5B MECHANICS. rope is wound on the whole length of the axis : tliis explains why the difficulty of drawing the v/ater, or any other weight, increases as it ascends nearer the top. C. Then by diminishing the axis, or by increasing the length of the handle, advantage is gained 1 F. Y es, by either of these methods you may gain power ; but it is very evident that the axis cannot be diminished beyond a certain limit, without rendering it too weak to sustain the weight ; nor can the handle be managed, if it be constructed on a scale much larger than what is commonly used. C. We must, then, have recourse to the v;heel with spikes standing out of it, at certain distances from each other, to serve as levers. F, You may by this means increase your power according to your wish, but it must be at the expense of time, for you know that a simple handle may be turned several times, while you are pulling the wheel round once. — To the principle of the ivJieel and axis may be referred the capstan, windlass, and all those numerous kinds of cranes, which are to be seen at the different wharfs on the banks of the Thames. C, I have seen a crane, which consists of a wheel large enough for a man to vvalk in. F. In this the weight of the man, or men (for there are sometimes two or three), is the moving power ; for, as the man steps forwards, the part upon which he treads becomes the heaviest, and conse- quently descends till it be the lowest. On the same principle, you may see at the door of many bird-cage makers, a bird, by its weight, give a wicker cage a circular motion ; now, if there were a small weight suspended to the axis of the cage, the bird by its mo- tion would draw it up, for, as it hops from the bottom bar to the next, its momentum causes that to descend; and thus the operation is performed, both with regard to the cage, and to those large cranes which you i ave seen. E. Is there no danger if the man happens to slip 1 OF THE WHEEL AND AXIS. 59 F. Tf the weight be very great, a slip with the foot may be attended with very dangerous consequences. To prevent which, there is generally fixed at one end of the axis a little wheel,/, (Fig. 22.) called a racket-wheel, with a catch, e, to fall into its teeth ; this will, at any time, support the weight in case_ of an accident. Sometimes, instead of men walking within the great wheel, cogs are set round it on the outside, and a small trundle-wheel made to work in the cogs, and to be turned by a winch. C. Are there not other sorts of cranes, in which all danger is avoided ^. F. The crane is a machine of such importance to the commercial interests of this country, that new in- ventions of it are continually offered to the public : I will, when we go to the library, shew you in the 10th vol. of the Transactions of the Society for the Encouragement of Arts and Sciences, an engraving of a safe, and, I believe, truly excellent crane. It was invented by a friend of mine, Mr. James White, who possessed a most extraordinary genius for me chanics. C. You said that this mechanical power might be considered as a lever of the first kind. F, 1 did ; and if you conceive the wheel and axis to be cut through the middle in the di- rection ab, fgb will represent a section of it. ab is a lever, whose centre of mo- /'( lion is c; the weight w, sustained by the rope aw, is applied at the distance ca, the radius of the axis ; and the power p, acting in the direction bp, is applied at -p. 23 the distance cb, the radius of the wheel ; &• * therefore, according to the principle of the lever, the power will balance the v/eight when it is as much less than the weight as the distance cb is greater than the distance of the weight ac. 00 MECHANICS. CONVERSATION XVIII. OF THE PULLEY. -F. The third mechanical power, the Pulley, may be likewise explained on the principle of the lever. The Ime ab may be conceived to be a lever, whose arms uc and be are equal, '^^^'^^ and c the fulcrum, or centre of motion. H If now two equal weights, w and p, be aPS^ 'f) hung on the cord passing over the pulley, they will balance one another, and the fulcrum will sustain both. O'^'K) C. Does this pulley, then, like the com- Fig. 24 mon balance, give no advantage ? ' F. From the single fixed pulley no mechanical advantage is derived : it is, nevertheless, of great im- portance in changing the direction of a power, and is very much used in buildings for drawing up small weights, It being much easier for a man to raise such burthens by means of a single pulley, than to carry them up a long ladder. E, Why is it called a mechanical power ? F , Though a single fixed pulley gives no advan- tage, yet when it is not fixed, or v/hen two or more are combined into what is called a system of pulleys, they then possess all the properties of the other me- chanical powers. Thus in cdb c is the ful- ^ crum; therefore a power p acting at h, vVl will sustain a double weight w, acting at \f ^ a, for be is double the distance of ac from the fulcrum, | Again, it is evident, in the present case, that the whole weight is sustained by the cord edp, and whatever sustains half the 'O cord, sustains also half the weight ; but Fig. 25. one half is sustained by the fixed hook e, consequently the powei at p has only the other OF THE PULLEY. 01 half to sustain, or, iu other words, any given power at p will keep in equilibrio a double weight at w. C. Is the velocity of p double that of ly'? F Undoubtedly if you compare the space passed throuoh by the hand at p with that passed by w you will find that the former is just double of the latter, and therefore the momenta of the power and weight, as in the lever, are equal. C I think J see the reason of this; tor it the wekht be raised an inch, or a foot, both sides of the cord must also be raised an inch, or foot, but this cannot happen without that part of the cord at p passing through two inches or two feet oi space. F. You will now easily infer, from what has been already shewn of the single move- able pulley, that in a system of pulleys the power gained must be estimated by dou- bling the number of pulleys in the lower or moveable block. So that when the fixed block a contains two pulleys which only turn on their axes, and the lower block b contains also two pulleys, which not only turn on their axes, but also rise with the weight, the advantage is as four ; that is, a single pound at p will sustain four C. In the present instance, also, 1 per- lig. ^t), ceive, that by raising w an inch, there are four ropes shortened each an inch, and therefore the hand must have passed through four inches of space in raising the weight a single inch; which establishes the .maxim that what is gained in power is lost m space. But, papa, you have only talked ot the power balancing or sustaining the weight ; something more must, I suppose, be added to raise it. F. There must^; considerable allowance must like- wise' be made for the friction of the cords, and of the pivots, or axes, on which the pulleys turn. In the mechanical powers, in general, one-third ot power must be added for the loss sustained by tnction, and 02 MECHANICS. for the imperfect manner in which machines are com- monly constructed. Thus, if by theory you gain a power of 600, in practice you must reckon only upon 400. In those pulleys which we have been describ- ing, writers have taken notice of three things, which take much from the general advantage and conve- nience of pulleys as a mechanical power. The Jirst is, that the diameters of the axes bear a great propor- tion to their own diameters. The secojid is, that in working they are apt to rub against one another, or against the side of the block. And the third disad- vantage is the stiffness of the rope that goes over and under them. The two first objections have been, in a great degree, removed by the concentric pulley, invented by Mr. Jam.es White : 6 is a solid block of brass, in which grooves are cut, in the proportion of 1, 3, 5, 7, 9, &c. and a is another block of the same kind, whose grooves are in the proportion of 2, 4, 6, 8, 10, &c. and round these grooves a cord is passed, by which means they answer the purpose of so many distinct pulleys, every point of which moving with the velocity of the string in contact with it, the whole friction p. is removed to the tvv^o centres of motion of the blocks a and b ; besides, it is of no small advantage, that, the pulleys being all of one piece, there is no rubbing one against the other. E. Do you calculate the power gained by this pul- ley in the same method as with the common pulleys ? -F. Yes, for pulleys of every kind the rule is gene- ral ; the advantage gained is found by doubling the number of the pulleys in the lower block : in that before you there are six grooves, which answer to as many distinct pulleys, and consequently the power gained is twelve, or one pound at p will balance twelve pounds at w. OF THE INCLINED PLANE. 63 CONVERSATION XIX. OF THE INCLINED PLANE. F. We may now describe the inclined plane, which is the fourth mechanical power. C. You will not be able, I think, to reduce this also to the principle of the lever. F, No, it is a distinct principle, and some writers on these subjects reduce at once the six mechanical powers to two, viz. the lever and the inclined plane, E. How do you estimate the advantage gained by this mechanical power 1 F. The method is very easy, for just as much as the length of the plane exceeds its perpendicular height, so much is the advantage gained. Suppose ah is a plane stand- ing on the table, and cd another plane inclined to it ; if the length cd be three times greater than the ^ ^ perpendicular height, then the cy- pig. 28. Under e will be supported upon the plane cd by a weight equal to the third part of its own weight. E. Could I then dra w up a weight on such a plane with a third part of the strength that I must exert in lifting it up at the end'? F, Ceitaialy you might; allowance, however, must be made for overcoming the friction ; but then you perceive, as in the other mechanical powers, that you will have three times the space to pass over, or that as you gain power you will lose time. C. Now I understand the reason why sometimes there are two or three strong planks laid from the street to the ground-floor warehouses, making there- with an inclined plane, on which heavy packages are raised or lowered. E. The inclined plane is chiefly used for raising heavy weights io small heights, for in warehouses situ- Gl MECHANICS. ated in the upper part of buildings, cranes and pulleys are better adapted for the purpose. C. I have sometimes, papa, amused myself by ob- serving the difference of time which one marble has taken to roll down a smooth board, and another which has fallen by its own gravity without any sup- port. -F. And if it were a long plank, and you took care to let both marbles drop from the hand at the same instant, I dare say you found the difference very evident. C. I did, and now you have enabled me to account for it very satisfactorily, by shewing me that as much more time is spent in raising a body along an inclined plane, than in lifting it up at the end, as that plane is longer than its perpendicular height. For I take it for granted that the rule holds in the descent as well as in the ascent. F. If you have any doubt remaining, a few words will make every thing clear. Suppose your marbles placed on a plane, perfectly horizontal, as on tliis table, they will remain at rest wherever they are placed : now if you elevated the plane in such a manner that its height should be equal to half the length of the plane, it is evident from what has been shewn before, that the marbles would require a force equal to half their weight to sustain them in any par- ticular position : suppose then the plane perpendicu- lar to the table, the marbles will descend with their whole weight, for now the plane contributes in no respect to support them, consequently they would re- quire a power equal to their whole weight to keep tliem from descending. C. And the swiftness with which a body falls is to be estimated by the force with which it is acted upon ? F. Certainly ; for you are now sufficiently ac- quainted with philosophy to know that the efiect must be estimated from the cause. Suppose an inclined plane is thirty-two feet long, and its perpendicular height is sixteen feet, what time will a marble take in OF THE WEDGE. 65 falling down the plane, and also in descending from the to^p to the earth by the force of gravity '? C By the attraction of gravitation, a body talis sixteen feet in a second ; therefore the marble will be one second in falling perpendicularly to the ground ; and as the length of the plane is double its height, the marble must take two seconds to roll t^own it. F. I will try you with another example. If there be a' plane 64 feet perpendicular height, and 3 times 64, or 192 feet long, tell me what time a marble will take in fallino- to the earth by the attraction of gravity, and how long it will be in descending down the ^^^C.^ By the attraction of gravity it will fall in two seconds; because, by multiplying the sixteen feet which it falls in the first second, by the square of tvvo seconds (the time), or four, I get sixty-four, the heii^ht of the plane. But the plane being three times as fong as it is perpendicularly high, it must be three times as many seconds in rolling down the plane, as it was in descending freely by the force of gravity, that is, six seconds. E. Pray, papa, what common instruments are to be referred to this mechanical power, in the same way as scissars, pincers, &c. are referred to the lever? F. Chisels, hatchets, and whatever other sharp in- struments which are chamfered, or sloped down to an edge on one side only, may be referred to the princi- ple of the inclined plane. F. The next mechanical power is the wedge, which is made up of the two inclined planes ^ ffe/ and cef joined together at their bases is applied, and df and cf are the length of its sides , now there will be an equili- -^.^^ "29, brium between the power impelling the CONVERSATION XX. OF THE WEDGE. hrfg: dc is the whole thickness of the wedge at its back abed, where the power G(5 MECHANICS, wedge downward, and the resistance of the wood, or other^ substance acting against its sides, when the thickness do of the wedge is to the length of the two sides, or, which is the same thing, when half the thickness de o( the wedge at its back is to the length of df one of its sides, as the power is to the resistance. C. This is the principle of the inclined plane. F. It is, and notwithstanding ail the disputes which the methods of calculating the advantage gained by the wedge have occasioned, I see no reason to depart from the opinion of those who consider the wedge as a double inclined plane. E. I have seen people cleaving wood with wedges, but they seem to have no effect, unless great force and great velocity are also used. F. ]Mo, the power of the attraction of cohesion, by which the parts of v/ood stick together, is so great as to require a considerable momentum to separate them. Did you observe nothing else in the operation worthy of your attention ? C. Yes, I also took notice that the wood generally split a little below the place to which the wedge reached. ° F, This happens in cleaving most kinds of wood, and then the advantage gained by this mechanical power, must be in proportion as the length of the sides of the cleft in the wood is greater than the length of the whole back of the wedge. There are other varieties in the action of the wedge, but at present it is not necessary to refer to them. E. Since you said chat all instruments which sloped of! to an edge on one side only, were to be explained by the principle of the inclined plane ; so, I suppose, that those which decline to an edge on both 'sides, must be referred to the principle of the wedge. F. They must, which is the case with many chisels, and almost all sort? of axes, &c. C. Is the wedge much used as a mechanical power ] F. It is of great importance in a vast variety of cases, in which the other mechanical powers are of OF THE SCEEW. 07 110 avail ; and this arises from the momenturn of the blow, which is greater beyond comparison than the application of any dead weight or pressure, such as is employed in the other mechanical powers. Hence it is used in splitting wood, rocks, .^J^f , 7,^^^^^ largest ship may be raised to a small height by duving a wed^e below it. It is also used for raising up the . beam of a house when the floor gives way, by reascm of too great a burden bemg laid upon it. It is usual also in separating large mill-stones from the silicious Id-roclJin som^e par\s of Derbyshire to bore hor, cental holes under them m a circle and fi l ^^^^se w h peers or wedges made of dry wood, which gradually swell by the moisture of the earth, and m a day or two lift up the mill-stone without breakmg it. CONVERSATION XXI. OF THE SCREW. F Let us now examine the properties of the sixth and last mechanical power, the screw ; which, however, cannot be called a simple mechanical power, since it is never used with- out the assistance of a lever or winch ; by which it be- comes a compound engine, of great power in pressing bodies togetber, or in rais- in a- great weights, cib is the representation of one, together with the lever h. , • , F You said just now, papa, that all the mechanical powers were reducible either to the lever or inclined p]nne : how can the screw be referred to either F. The screw is composed of two parts, one ot which ab is called the screw, and consists of a spiral protuberance, called the thread, which may be sup- posed to be wrapt round a cylinder j the other part, g, 68 ME€HANIC8. called the nut, is perforated to the dimensions of the cylinder ; and in the internal cavity is also a spiral groove adapted to receive the thread. Now if you cut a slip of vi^riting-paper in the form of an inclined plane cde, and then wrap it round a cylinder of wood, you will find that it makes a spiral answering to the spiral part of the screw ; moreover, if you consider the ascent of the screws, it will be evident that it is precisely the ascent of an- inclined plane. C. By what means do you calculate the advantage gamed by the screw ? F. There are, at first sight, evidently two things to be taken into consideration ; the first is the distance between the threads of the screw ; — and the second is the length of the lever. C. Now I comprehend pretty clearly how it is an mclmed plane, and that its ascent is more or less easy as the threads of the spiral are nearer or farther distant from each other. F, Well, then, let me examine by a question whether your conceptions ))e accurate ; suppose two screws, the circumferences of whose cylinders are equal to one another ; but in one, the distance of the threads to be an inch apart ; and that of the threads of the other only one-third of an inch ; what will be the difference of the advantage gained by one of the screws over the other ? C. The one whose threads are three times nearer than those of the other, must, I should think, give three times the most advantage. F . Give me the reason for what you assert. C. Because, from the principle of the inclined plane, I learnt that if the height of two planes were the same, but the length of one twice, thrice, or four tmies greater than that of the other, the mechanical advantage gained by the longer plane would be two, three, or four times more than that gained by the' shorter. Now in the present case, the height gained m both screws is the same, one inch, but °the space passed in that, three of whose threads go to an inch, OP THB SCREW. 69 must be three times as great as the space passed in the other ; therefore, as space is passed, or time lost, just in proportion to the advantage gained, I infer that three times more advantage is gained by the screw, the threads of which are one-third of an inch apart, than by that whose threads are an inch apart. F, Your inference is just, and naturally follows from an accurate knowledge of the principle of the inclined plane. But we have said nothing about the lever. C. This seemed hardly necessary, it bemg so obvious to any one, who will think a moment, that power is gained by that as in levers of the first kind, according to the length h from the nut. F, Let us now calculate the advantage gamed by a screw, the threads of which are half an inch dis- tance from one another, and the lever 7 feet long. C. I think you once told me, that if the radms of a circle were given, in order to find the circumference I must multiply that radius by 6. F. I did ; for though that is not quite enough, yet it will answer all common purposes, till you are a little more expert in the use of decimals. C. Well, then, the circumference of the circle made by the revolution of the lever will be 7 feet, multiplied by 6, which is 42 feet, or 504 inches; but, during this revolution, the screw is raised only half an inch, therefore the space passed by the moving power will be 1008 times greater than that gone through by the weight, consequently the advantage gained is 1008, or one pound applied to the lever will balance 1008 pounds acting against the screw. F. You perceive that it follows as a corollary from what you have been saying, that there are two methods by which you may increase the mechanical advantage of the screw. C. I do ; it may be done either by takmg a longer lever, or by diminishing the distance of the threads of the screw. 70 MECHANICS. F. Tell me the result then, supposing the threads of the screw so fine as to stand at the distance of but one quarter of an inch asunder ; and that the length of the lever were 8 feet instead of 7. C. The circumference of the circle made by the lever will be 8 multiplied by 6, equal to 48 feet or 576 inches, or 2304 quarter inches, and as the eleva- tion of the screw is but one quarter of an inch, the space passed by the power will, therefore, be 2304 times greater than that passed by the weight, which is the advantage gained in this instance. F. A child, then, capable of moving the lever suf- ficiently to overcome the. friction, with the addition of a power equal to one pound, will be able to raise 2304 pounds, or something more than 20 hundred weight and a half. The strength of a powerful man would be able to do 20 or 30 times as much more. C. But I have seen at IMr. VVilmot's paper-mills, to which I once v/ent, six or eight men use all their strength in turning a screw, in ( rder to press out the water of the newly-made paper. The power applied in that case must have been very great indeed. F. It was ; but I dare say that you are aware that it cannot be estimated by multiplying the power of one man by the numxber of men employed. C. That is, because the men standing by the side of o«e another, the lever is shorter to every man the nearer he stands to the screw, consequently, though he may exert the same strength, yet it is not so effec- tual in moving the machine, as the exertion of him who stands nearer to the extremity of the lever. jp. The true method therefore of calculating the power of this machine, aided by the strength of these men, would be to estimate accurately the power of each man according to his position, and then to add all these separate advantages together for the total power gained. E. A machine of this kind is, I believe, used by bookbinders, to press the leaves of the books together before they are stitched ? OF THE PENDULUM. Tl F Yes it is found in every bookbinder's work- shop, and is particularly useful where persons are , desirous of having small books reduced to a stil smaller size for the pocket. It is also he prmcipal machine used for coining money ; for takmg off cop- per^plate prints; and for printmg m general. Mr. Boulton invented a magnificent apparatus for coinmg : the whole machinery is worked by an improved steam- endne, which rolls the copper for half-i)ence ; works the screw-presses for cutting out the circular pieces of copper, and coins both the faces and edges of the money at the same time : and since the circulation of the new half-pence, we are all acquainted with the superior excellence of the workmanship. By this loachinery four boys, of ten or twelve years old, are capable of striking 30,000 guineas m an hour and the machine itself keeps an unerrmg account ot the ^^^E!^And?have seen the cyder-press in Kent, which consists of the same kind of machine. F It would, my dear, be an almost endless task, were* we to attempt to enumerate all the purposes to which the screw is applied in the mechanical arts o life - it will, perhaps, be sufficient to tell you, that wheVever great pressure is required, there the power of the screw is uniformly employed. CONVERSATION XXII. ON THE PENDULUM, E. T have been so delighted with the Conversa- tions you have permitted us to have with you my dear papa, that I can scarcely thmk of any thmg but what you have been explaining to us ; and now when I see a machine of any kind, I begin to examme the various combinations of levers, wheels, and pulleys. F I am very glad to find that my explanations ot the mechanical powers have excited your curiosity so much ; and 1 shall always feel a pleasure m com- municating any thing I know. E. I am very much obliged to you, papa. When T2 MECHANICS. Charles and I were down stairs, we were examining the kitchen clock ; we saw wheels and axles, levers, screws, pulleys, &c. but neither of us knew what to call the pendulum. Is that a mechanical power ? It is not called a mechanical power, because it does not convey any mechanical advantage, but the theory of the pendulum depends on that of the in- clined plane. -E. What is meant by the term pendulum ? F. The. name is applied to any body so suspended that it may swing freely backwards and forwards, of which the great law is, that its oscillations are always performed in equal times ; and it is this remarkable property which makes it a time-keeper. A common pendulum consists of a ball, as a, suspended by a rod from a fixed point, at b, and made to swing backwards and forwards under this point. The ball being raised to c, and then set at liberty, falls back to o, with an accelerating motion, like a ball rolling down a slope (or inclined plane) ; and when arrived there. It has just acquired force enough to carry it to d, at an equal elevation on the other side; from this it falls back again, again to rise, and v»'ould so continue on for ever, if there was no impediment either from the air or friction. C. Are the laws which regulate these movements so simple that we can understand them ? F . I think they are ; the most important of them ore these : — 1. If a pendulum vibrates in very small circles, the times of vibration may be considered as equal, whatever be the proportion of the circles. — 2. Pendulums which are of the same length vibrate in the same manner, whatever be the proportion of the V eight of the bob.— 3. The velocity of the bob, or I all, in the lowest point, will be sr, the length of tiie chord of the arch which it describes in its descent.— OF THE PENDULUM. 4. The times of vibration of different pendulums in similar arches are proportional to the isquare roots of their different lengths. — 5. Hence the lengths of pen- dulums are as the squares of»the times of vibrations, — 6. In the latitude of London, a simple pendulum vv^ill depart 07ice in a second in a small arch, if its length be 39^ inches. C. Does the length of a pendulum influence the time of its vibrations ? F. Yes ; long pendulums depart more slowly than short ones ; because, in corresponding arcs, or paths, the bob or ball of the large pendulum has a greater jour- ney to perform, without having a steeper line of descent. C. If I understand you rightly, a pendulum which describes seconds is 39i inches in length, and the leftgths are as the square roots of the times ; therefore, the length of a half-second pendulum is 9| inches, and the length of a quarter-second pendulum will be 2|g inches. F. You are quite correct. E. But, if you wished the pendulum to beat longer than one second, how could that be done'? F. As a body falls four times as fast in two seconds as m one, a pendulum must be four times as long to beat once in two seconds as to beat every second. E. How is it made to denote the time on the clocks 1 F, A com.mon clock is merely a pendulum with wheel-work attached to it to record the number of vibrations. The wheels shew how many swings of the pendulum have taken place, because at every beat a tooth of the last wheel is allowed to pass ; now, if this wheel have 60 teeth, it will just turn round once for 60 beats of the pendulum, or seconds, and a hand fixed on the axis projecting through the dial-plate will be the second-hand of the clock. The other wheels are so arranged, and their teeth so pro- portioned, that one turns 60 times slower than the firsj, to fit its axis to carry a minute-hand, and another, by moving twelve times slower still, is fitted to carry an hour-hand. ASTRONOMY. CONVERSATION I. OP THE FIXED STARS. TUTOR CHARLES JAMES. Charles. The delay occasioned by our unusually long walk has afforded us one of the most brilliant views of the heavens that I ever saw. James. It is uncommonly clear, and the longer I keep my eyes fixed upwards the more stars seem tc appear : how is it possible to number these stars 1 and yet I have heard that they are numbered, and ev%n arranged in catalogues according to their apparent magnitudes. Pray, sir, explain to us how this busi- ness was performed. Tutor. This I will do, with great pleasure, some time hence ; but at present I must tell you, that in viewing the heavens with the naked eye, we are very much deceived as to the supposed number of stars that are at any time visible. It is generally admitted, and on good authority too, that there are never more than one thousand stars visible to the sight, unassisted by glasses, at any one time, and in one place. J. What! can I see no more than a thousand stars if I look all around the heavens ? I should sup- pose there were millions. T, This number is certainly the limit of what you can at present behold ; and that which leads you, and persons in general, to conjecture that the number is so much laiger, is owing to an optical deception. J. Are we frequently liable to be deceived by our senses ? T. We are, if we depend on them singly: but where we have an opportunity of calling in the assist- ance of one sense to the aid of another, we are seldom subject to this inconvenience. OF THE FIXED STARS. 75 C. Do you not know, that if you place a small marble in the palm of the left hand, and then cross the second finger of the right hand over the first, and in that position, with your eyes shut, move the marble with those parts of the two fingers at once which are not accustomed to come into contact with any object at the same time, that the one marble will appear to the touch as two 1 In this instance, without the as- sistance of our eyes, we should be deceived by the sense of feeling. T. This is to the point, and shews that the judg- ment formed by means of a single sense is not always to be depended upon. J, I recollect the experiment very well ; we had it from papa, a great while ago. But that has no- thing to do with the false judgment which we are said to form about the number of stars. T. You are right ; it does not immediately concern the subject before us, but it may be useful as af- fording a lesson of modesty, by instructing us that we ought not to close our minds against new evi- dence that may be offered upon any topic, notwith- standing the opinions we may have already formed. You say that you see millions of stars, whereas the ablest astronomers assert, that with the naked eye you cannot at one time see so many as a thousand. C. I should indeed have thought with my brother, had you not asserted the contrary : and I am anxious to know how the deception happens, for I am sure there must be a great deception somewhere, if I do not at this time behold very many thousands of stars in the heavens. T. You know that we see objects only by means of the rays of light which proceed from them in every direction. And you must, for the present, give me credit, when I tell you that the distance of the fixed stars from us is immensely great ; consequently the rays of light have to tiavel this distance, in the course of which, especially in their p?.ssage through our at- mosphere, they are subject to numberless rejiections 7G ASTRONOMY, and refractions. By means of these, other rays of light come to the eye, every one of which, perhaps, impresses upon the mind the idea of so many sepa- rate stars. Hence arises that optical fallacy by which we are led to believe the stars which we behold are innumerable. J. 1 should like to see an experiment to confirm this. r. I have no objection : — in every case you ought to require the best evidence that the subject will ad- mit of. I will shew you two experiments, which will go a good way to remove the difficulty. But, for this purpose, we must step into the house. Here are two common looking-glasses, which, phi- losophically speaking, are plane mirrors. I place them in such a manner on the table that they sup- port one another from falling by meeting at the tops. I now place this half-crown between them, on a book, to raise it a little above the table. 'J'ell me how many pieces of money you would suppose there were, if you did not know that I had used but one. /. There are several in the glasses. T. I will alter the position of the glasses a Ihtle, by making them almost parallel to one another ; now look into them, and say what you see. J. There are more half-crowns now than there were before. T. It is evident, then, that by reflection only, a single object, for I have made use of but one half- crown, will give you the idea of a vast number. C. If a little contrivance had been used to conceal the method of making the experiment, I sliould not have believed but that there had been several half- crowns instead of one. T. Bring me your multiplying glass ; look through it at the candle : how many do you see 1 or rather how many candles should you suppose there were, did you not know that there was but one on the table ? OF THE F1XET> STARS. 77 J. A great many \ and a pretty sight it is. C. Let me see ; — yes, there are : but 1 can easily count them ; there are sixteen. T. There will be just as many images of the candle, or any other object at which you look, as there are different surfaces on your glass. For by the principle of refraction, the image of the candle is seen in as many different places as the glass has sur- faces ; consequently, if instead of 16 there had been 60, or, if they could have been cut and polished so small, 600, then the single candle would have given you the idea of 60, or 600. What think you now about the stars ? J. Since I have seen that reflection and refraction will each, singly, afford such optical deceptions, I can no longer doubt but that, if both these causes are combined, as you say they are, with respect to the rays of light coming from the fixed stars, a thou- sand real luminaries may have the power of exciting in my mind the idea of millions. T, I will mention another experiment, for which you may be prepared against the next clear starliglit night. Get a long narrow tube, the longer and narrower the better, provided its weight does not render it unma- nageable : examine through it any one of the largest fixed stars, which are called stars of the first magni- tude, and you will find that, though the tube takes in as much sky as would contain many such stars, yet that the single one at which you are looking is scarcely visible, by the few rays which come directly from it : this is another proof that the brilliancy of the heavens is much more owing to reflected and re- fracted light, than to the direct rays flowing from the stars. CONVERSATION IL OF THE FIXED STARS. C. Another beautiful evening presents itself j shall 78 ASTRONOMY. we take the advantage which it offers of going on with our astronomical lectures ? T. I have no objection, for we do not always enjoy such opportunities as the brightness of the present evening affords. J. 1 wish very much to know how to distinguish the stars, and to be able to call them by their proper names. T, This you may very soon learn ; a few evenmgs, well improved, will enable you to distinguish all the stars of the first magnitude v/hich are visible, and all the relative positions of the different constellations. J. What are constellations, sir ? r. The ancients, that they might the better distin- guish and describe the stars, with regard to their situ- ation in the heavens, divided them into constellations, that is, systems of stars, each system consisting of such stars as were near to each other, giving them the names of such men or things as they fancied the space which they occupied in the heavens represented. C. Is it then perfectly arbitrary, that one collection is called the great hear, another the dragon, a third Hercules, and so on? T. It is ; and though there have been additions to the number of stars in each constellation, and various new constellations invented by modern astronomers, yet the original division of the stars into these collec- tions was one of those few arbitrary inventions which have descended without alteration, otherwise than by addition, from the days of Ptolemy down to the pre- sent time. Do you know how to find the four car- dinal points, as they are usually called, the Aorth, South, West, and East ? J. O yes, 1 know that if I look at the sun at twelve o'clock at noon, I am also looking to the South, where he then is ; my back is towards the North ; the West is on my right hand, and the East on my left. T, But you must learn to find these points without OF THE FIXED STARS. T9 the assistance of the sun, if you wish to be a young astronomer. t / * C I have often heard of the north pole star; that will perhaps answer the purpose of the sun, when he ^^T.'' You are right; do you see those seven stars which are in the constellation of the Great, Bear ? some people have supposed their position will aptly represent a plough; others say that they are more like a waggon and horses ; — the four stars representing the body of a wag- gon, and the other three the hoises, and hence they are called by some the plough, and by others they are ^ called Charles's wain or waggon. > ' u ^ . Here is a drawing of it ; abdg re- ^ ^ present the four stars, and e 2 b the " ether three. ' C. What is the star p ? T That represents the polar star to which you just now alluded; and you observe, that if a line were drawn through the stars h and a, and produced tar enough, it would nearly touch it. J. Let me look at the heavens for it by this guide. There it is, I suppose; it shines with a steady and rather dead kind of light, and it appears to me that it would be a little to the right of the Ime passmg throueh the stars b and a. T. It would, and these stars are generally known by the name of the pointers, because they point to p, the north pole, which is situated a little more than two degrees from the star p. C. Is that star always in the same part ot the heavens'? . • ^ • • T It may be considered as uniformly maintaining its position, while the other stars seem to move round it as a centre. We shall have occasion to refer to this star again; at present, I have directed your atten- tion to it, as a proper method of finding the cardinal points by starlight. m ASTRONOMY. J. Yes, I understand now, that if I look to the north, by standing with my face to that star, the south is at my back, on my right hand is the east, and the west on my left. T. This is one important step in our astronomical studies ; and we can make use of these stars as a kind of standard, in order to discover the names and positions of others in the heavens. C. In what way must we proceed in this business ? T. I will give you an example or two : conceive a line drawn from the star z, (Fig. 1,) leaving b a little to the left, and it will pass through that very brilliant star near the horizon towards the west. J. I see the star, but how am I to know its name ? T. Look on the celestial globe for the star z, and suppose the line drawn on the globe, as we conceived it done in the heavens, and you will find the star, and its name. C. Here it is ; — its name is A returns. r. Take the figure, (Fig. 1.) and place Arcturus at A, which is its relative position, in respect to the constellation of the Great }3ear. Now, if you con- ceive a line drawn through the stars g and b, and ex- tended a good way to the right, it will pass just above another very brilliant star. Examine the globe as before, and find its name. C. It is Cape 11/ 1, the gont. T, Now, vvhtiiiever you see any of these stars, you will know where to look for the others without hesitation. J. But do they never move from their places ? T. With respect to us, they seem to move together with the whole heavens. But they always remain in the same relative position, with respect to each other. Hence they are called /z'xeJ stars, in opposition to the planets, which, like our earth, are continually chano-- ing their places, both with regard to the fixed stars, and to tiiemselves also. C. I now understand pretty well the method of acquiring a knowledge of the names and places of the stars. OF THE ECLIPTIC. 81 T. And with this, we will put an end to our present conversation. CONVERSATION HI. OF THE FIXED STARS, AND ECLIPTIC. T. I dare say that you will have no difficulty in finding the north polar star as soon as we go into the open air. J. I shall at once know where to look for that and the other stars which you pointed out last night, if they have not changed their places. T. They always keep the same position, with respect to each other, though their situation, with regard to the heavens, will be different at different seasons of the year, and in different hours of the night. Let us go into the garden. C. The stars are all in the same place as we left them last evening. Now, Sir, if we conceive a straight line drawn through the two stars in the plough, which, in your figure, (Fig. 1.) are marked d and g, and to extend a good way down, it will pass or nearly pass through a very bright star, though not so bright as Arcturus, or Capella; what is that called 1 T. It is a star of the second magnitude, and if you refer to the celestial globe, in the same way as you were instructed last night, you will find it is called Regulus, or Cor heonis, the Lion's Heart. By this method you may quickly discover the names of all the principal stars, and afterwards, with a little patience, you will easily distinguish the others which are less conspicuous. C. But they have not all names ; how are they specified ? T. If you look on the globe, you will observe, that tliey are distinguished by the different letters of the Greek alphabet ; and in those constellations, in which there are stars of different apparent magnitudes^ E 2 ^2 ASTRONOMY, the largest is « alpha, the next in size /3 beta, the third y gamma, the fourth d delta, and so on. J. Is there any particular reason for this? T. The adoption of the characters of the Greek alphabet, rather than any other, was perfectly arbi- trary; it is, however, of great importance, that the same characters should be used in general by astro- nomers of all countries, for by this means the science is in possession of a sort of universal language. C. Will you explain how this is ? T, Suppose an astronomer in North America, Asia, or any other part of the earth, observe a comet in that part of the heavens where the constellation of the Plough h situated, and he wishes to describe it to his friend m Great Britain, in order that he may know, whether it was seen by the inhabitants of this island.' For this purpose, he has only to mention the time when he discovered it ; its position, as nearest to some one of the stars, calling it by the Greek letter by which it is designated ; and the course which it took from one star towards another. Thus he might say, that on such a time he saw a comet near d in the Great Bear, and that its course was directed from d to/3, or any other as it happens. C. Then, if his friend here had seen a comet at the same time, he would, by this means, know whether it w^as the same or a different comet ? T, Certainly; and hence you perceive of what im- portance it is that astronomers in different countries should agree to mark the same stars, and systems of stars, by the same characters. But to return to that star, to which you just called my attention, the Cor Lconis; it is not only a remai'kable star, but its position is also remarkable : it is situated in the Ecliptw. J. What is that, sir ? T. The ecliptic "is an imaginary' circle in the heavens, which the sun appears to describe in the course of a year. If you look on the celestial alobe, you will see it marked with a red line, perhaps an OF THE ECLIPTIC. 83 emblem of the fierce heat communicated to us by that body. J. But the sun seems to have a circular motion m the heavens every day ? r. It does; and this is called its apparent diurnal, or daily motion, which is very different from the path it appears to traverse in the course of a year. The former is observed by the most inattentive spectator, who cannot but know that the sun is seen every morning in the East, at noon in the South, and in the evening in the West ; but the knowledge of the latter must be the result of patient observation. ^ C. And what is the ^reen line which crosses it ? r. It is called the Equator; this is an imaginary circle belonging to the earth, which you must take for granted, a little longer, is of a globular form. If you can conceive the plane of the terrestrial equator to be produced to the sphere of the fixed stars, it would mark out a circle in the heavens, called the celestial equator, or equinoctial, which would cut the ecliptic in two parts ; one of which would make an angle with the other of about 23| degrees. J. Can we trace the circle of the ecliptic m the heavens ? T. It may be done with tolerable accuracy by two methods; Jirst, by observing several remarkable fixed starsj to which the moon in its course seems to approach. The second method is by observing the places of the planets. C. Is the moon then always in the ecliptic 1 T. Not exactly so ; but it is always either in the ecliptic, or within five degrees and a third of it on one side or the other. The planets also, by which I mean, Mercury, Venus, Mars, Jupiter, Saturn, and the Herschel, are never more than eight decrees dis- tant from the line of the ecliptic. J. How can we trace this line, by help of the fixed stars ? T. By comparing the stars in the heavens with their representatives on the artificial globe a practice 84 ASTRONOMY. which may be readily acquired, as you have seen. I will mention to you the names of those stars, and you may first find them on the globe, and then refer to as many of them as are now visible in the heavens. The first is in the Ram's horn, called a Arietis, about ten degrees to the north of the ecliptic ; the second is the star Aldeharan in the BuWs eye, six degrees south of the ecliptic. C. Then if at any time I see these two stars, I know that the ecliptic runs between them, and nearer to Aldebaran, than to that in the Ram's horn. T. Yes : now carry your eye eastward to a distance somewhat greater from Aldebaran, than that is east of a Arietis, and you will perceive two bright stars at a small distance from one another, called Castor and Pollux ; the lower one, and that which is least bril- liant is Pollux, seven degrees on the north side of the ecliptic. Following the same tract, you will come to Regains, or the Cor Leonis, which 1 have already ob- served is in the line of the ecliptic. Beyond this, and only two degrees south of that line, you will find the beautiful star in the virgin's hand called Spica Virginis, You then arrive at Antares, or the Scorpion's Heart, five degrees on the same side of the ecliptic. After- wards you will find cc AqailcF., which is situated nearly thirty degrees north of the ecliptic ; and farther on is the star Fomalhant in the fish's mouth, about as many degrees south of that line. The ninth and last of these stars is Pegasus, in the wing of the flying- horse, which is north of the ecliptic nearly twenty degrees. /. Upon what account are these nine stars par- ticularly noticed ? T. They are selected as the most conspicuous stars near the moon's orbit, and are considered as proper stations, from which the moon's distance is calculated for every three hours of time: and hence are con- structed those tables in the Nautical Alwanac, by means of which navigators in tlieir most distant voyages are enabled to estimate, on the trackless OF THE EPHEMERIS. 85 ocean, the particular part of the globe on which they are. C. What do you mean by the Nautical Almanac ? T. It is a kind of National Almanac, intended chiefly for the use of persons traversing the mighty ocean. It was begun in the year 1767, by Dr. Maskelyne, the Astronomer Koyal ; and is published by anticipation for several years beforehand, for the convenience of ships going out upon long voyages. This work has been found eminently important in the course of the late voyages round the world for making discoveries, and is highly useful to all engaged in navigation. CONVERSATION IV. OF THE ErHEMEMS. C. Your second method of tracing the ecliptic was by means of the position of the planets : will you ex- plain that now ? r. 1 will ; and to render you perfectly qualified for observing the stars, I will devote the present con- versation to the purpose of explaining the use of White's Ephemeris, a little "book which is published annually, and which is a necessary companion to every young astronomer. J. Must we understand all this to study the stars 1 T, You must ; or some other book of the same kind, if you would proceed on the best and most rational plan. Besides, when you know the use of this book, which you will completely with half an hour's attention, you have nothing more to do in order to find the position of the planets at any day of the year, than to turn to that day in the Ephemeris, and you will instantly be directed to those parts of the heavens, in which the different planets are situated. Turn to the second page. C. Here the astronomical characters are explained. T, The first twelve are the representatives of the 85 ASTRONOMY. signs into which the circle of the ecliptic is divided, called also the tw^elve signs of the Zodiac. Aries. ^ Leo. f Sagittarius. ^ Taurus. TT^ Virgo. Capricorn, n Gemini. =5= Libra. CCC Aquarius. 25 Cancer. ^1 Scorpio. X Pisces. In astronomical inquiries every circle is supposed to be divided into 360 parts, called degrees, and since that of the ecliptic is also divided into 12 signs, each sign must contain 30 degrees. Astronomers subdi- vide each degree into minutes and seconds ; thus, if I would express an angle of 25 degrees, 1 1 mi- nutes, and 45 seconds, I should write 25^. .1]/. .45". Or, if I would express the situation of the sun for the first of January, 1822, I look into the Ephemeris and find it in Capricorn, or 1^ 10^. .35'. .48". J. What do you mean by the Zodiac 1 T. It is a broad circle or belt surrounding the heavens, about sixteen degrees wide, along the mid- dle of which runs the ecliptic. The term Zodiac is derived from a Greek word signifying an animal, be- cause each of the twelve signs formerly represented some animal ; that which we now call Libra, beino- by the ancients reckoned a part of Scorpio. J. Why are the signs of the Zodiac called by the several names of Aries, Taurus, Leo, &c.? I see no likeness in the heavens to Rams, or Bulls, or Lions, which are the English words for those Latin ones. T. jVor do I ; nevertheless, the ancients saw, by the help of a strong imagination, a similarity between those animals, and the places which certain systems or stars took up in the heavens, and gave them the names which have continued to this day^ C. Perhaps these were originally invented in the same way as we sometimes figure to our imagination the appearances of men, beasts, ships, trees, &c. in the flying clouds or in the fire. T. They might possibly have no better authority OF THE EPHEMERIS. 87 for their origin. At any rate it will be useful for you to have the names of the twelve signs in your memory, as well as the order in which they stand : I will there- fore repeat some lines written by Dr. Watts, in which they are expressed in English, and will be easily re- membered : The Ram, the Bull, the heavenly Ttvins, And next the Crab the Lio7i shines, The Virgi7i and the Scales ; The Scorpion, Archer, and Sea-Go«if, The Man that holds tlie watering-pot. And Fish with glittering tails. C. We come now to the characters placed before the planets. T, These, like the former, are but a kind of short- hand characters, which it is esteemed easier to write than the names of the planets at length. They are as follow : — Venus. Mercury. The Moon. Ceres. ~\ Pallas, f new planets, Juno. Cot asteroids. Vesta. 3 With the other characters you have no need to trou- ble yourselves, till you come to calculate eclipses, and construct astronomical tables, a labour which may be deferred for some years to come. Turn to the eighth page of the Ephemeris. /. Have we no concern with the intermediate pages between the second and eighth 1 T, They do not contain any thing that requires ex- planation. In the eighth page after the common almanac for January, the two first columns point out the exact time of the sun's rising and setting at London : thus on the 10th day of January he rises at 58 minutes after 7 in the morning, and sets at 2 The Herschel, or ? Uranus, ^ b Saturn. 3) U Jupiter. $ Mars. $ e The Earth. 0 The Sua, s 88 ASTRONOMY. minutes past 4 in the afternoon. The third column gives the declination of the sun. J. What is that, sir ? T. The declination of the sun, or of any heavenly body, is its distance from the imaginary circle in the heavens, called the equinoctial. Thus you observe that the sun's declination on the first of January is 23^. .3' south ; or, it is so many degrees south of the imaginary equator. Turn to March 1822, and you will see that between the 20th and 21st days it is in the equator, for at 12 o'clock at noon on the 20th it is only 16' south, and at the same hour on the 21st it is 8' north of that line ; and when it is in the equator, then it has no declination. C. Do astronomers always reckon from 12 o'clock at noon ? T, They do ; and hence the astronomical day be-* gins 12 hours later than the day according to common reckoning ; and therefore the declination, longitude, latitude, &c. of the sun, moon, and planets, are always put down for 12 o'clock at noon of the day to which they are opposite. Thus the sun's declination for the 17th of January at 12 o'clock is 20«. .48' south. C. Is that because it is the commencement of the astronomical day, though in common life it be called 12 o'clock? T. It is. The three next columns contain the moon's declination, the time of her rising and setting, and the time of her southing, or when she comes to the meridian or south part of the heavens. C. Does she not come to the south at noon as well as the sun ] T. No ; the moon never comes to the meridian at the same time as the sun, but at the time of new moon. And this circumstance takes place at every new moon, as you may see by casting your eye down the several columns in the Ephemeris which relate to the moon's southing. J. What do you say of the column which is marked OF THE EPHEMERIS. 89 sometimes clock before the sun, at others clock after the sun ? , , r j T. A full explanation of that must be deterred till we come to speak of the equation of time ; at present it will be sufficient for you to know that if you are m possession of a very accurate and weli-regulated clock, and also of an excellent sun-dial, they will be together only four days in a year ; now this column m the Ephemeris points out how much the clock is be- fore the sun, or the sun before the clock, for every day in the year. On twelfth-day, 1822, for mstance the clock is faster than the sun by 6 minutes and 7 seconds ; but if you turn to May-day, you will find that the clock is 3' 2" slower than the sun. /. What are the four days in the year when the clock and dial are together 7 T. About the 15th of April, the 15th of June, the 1st of September, and Christmas-day. C. By this table then we may regulate our clocks and watches. J. In what manner ? ,11 C. Examine on any particular day the clock or watch, and dial at the same time, say 12 0 clock, and ob-^erve whether the difference between them answer to the difference set down in the table opposite to the day of observation. Thus on the 12th ol March, 1822, the clock did not shew true time unless it was 10' 3" before the dial, or when the dial is li o'clock it must be 10' . . 3" past 12 by the clock or watch. ^ , ^ ry^T ^ r Well let us proceed to the next page, l tie three first short columns, relating only to the duration of daylight and twilight, require no explanation ; he fourth we shall pass over for the present ; and the remaining five give the latitude of the planets. J. What do you mean by the latitude sir ? ^ r. The latitude of any heavenly body is its dis- tance from the ecliptic north or south, fhe atitude of Venus, on new-year's day 1822, was P . . 1 south. a Then the latitude of heavenly bodies has the 90 ASTRONOMY. same reference to the ecliptic that declination has to the equator ? T. It has. J. But I do not see any table of the sun's latitude. T, I dare say your brother can give you a reason for this. C. Since the latitude of a heavenly body is its dis- tance from the ecliptic, and since the sun is always in the ecliptic, therefore he can have no latitude. T, The longitude of the sun and planets is the only thing- in this page that remains to be explained. The longitude of a heavenly body is its distance from the first point of the sign Aries, and it is measured on the ecliptic. It is usual, however, as you observe in the Ephemeris, to express the longitude of a heavenly body by the degree of the sign in which it is. In this way the sun's longitude on the first of January, 1822, was m Capricorn 10° . . 35' . . 48" ; that of the moon in Aries, 17<* . . 44'. C. There are som.e short columns at the bottom of the former page that you have omitted. r. The use of these will be better understood when we come to converse respecting the planets.* CONVERSATION V. OF THE SOLAR SYSTEM. T. We wdll now proceed to the description of the Solar Sijstern. J. Of what does that consist, sir? T, It consists of the sun and planets, with their satellites, or moons. It is called the Solar Sijstem from Sol, the sun, because the sun is supposed to be fixed in the centre, while the planets, and our earth among them, revolve round him at different distances. * For the explanation of Heliocentric Longitude, see Conversation XX. OF THE SOLAR SYSTEM. 91 C. Bat are there not some people who beUeve that the sun goes round the earth '] T. Yes, it is an opinion embraced by the generahty of persons, not accustomed to reason on these subjects. It was adopted by Ptolemy, who supposed the earth perfectly at rest, and the sun, planets, and fixed stars, to revolve about it every twenty-four hours. J. And is not that the most natural supposition ? T. If the sun and stars were small bodies in com- parison of the earth, and were situated at no very great distance from it, then the system maintained by Ptolemy and his followers might appear the most probable. /. Are the sun and stars very large bodies, then I T, The sun is more than a miUion of times larger than the earth which we inhabit, and many of the fixed stars are probably much larger than he is. C. What is the reason, then, that they appear so small ? T. This appearance is caused by the immense dis- tance there is between us and these bodies. It is known with certainty that the sun is more than 95 millions of miles distant from the earth, and the near- est fixed star is probably more than two hundred thousand times further from us than even the sun himself.* C. But we can form no conception of such dis- tances. r. We talk of millions, with as much ease as of hundreds or tens, but it is not, perhaps, possible for the mind to form any adequate conceptions of such high numbers. Several methods have been adopted to assist the mind in comprehending the vastness of * The youn^ reader will, when he is able to manage the subject, see this clearly demonstrated by a series of propositions in the 5th book of Dr. Enfield's Institutes of Natural Philosophy. Second Edition. See p. 346, to the end of book V. ^2 ASTRONOMY, these distances. You have some idea of the swiftness with which a cannon-ball proceeds from the mouth of the gun ? 1 have heard at the rate of eight miles in a minute. T. And you know how many minutes there are in a year ? /. I can easily find that out, by multiplying 365 days by 24 for the number of hours, and that product by 60, and I shall have the number of minutes in a year, which number is 525,600. r. Now if you divide the distance of the sun from the earth by the number of minutes in a year, multi- plied by 8, because the cannon-bail travels at the rate of 8 miles in one minute, you will know how long any body issuing from the sun, with the velocity of a cannon-ball, would employ in reachiutr the earth. ^ C. If I divide 95,000,000 by 525,600, multiplied by 8, or 4,204,800, the answer will be more than 22 tlie^ number of years taken for the journey. ' T. Is It then probable that bodies so large, and at such distances from the earth, should revolve round it every day ? C. I do not think it is.— Will you, sir, go on with the description of the Solar Sqsteiii ? T, According to this system, the sun is in the centre, about which the planets revolve from wea to east, according to the order of the signs in the ecliptic • Uiat IS, if a planet is seen in Aries, it advances to' 1 aurus, then to Gemini, and so on. J. Hov/ many planets are there belongino- to the sun ? ° ° T, There are seven, besides some smaller bodies discovered during the present century. C is the sun' the nearest to which iUercurv revolves in the circle « ; next to him is the beautiful planet Venus, who pcrtorms her revolution in the circle h ; then comes tiie Earth m t; next to which b Mars in e; then OF THE SOLAR SYSTEM. 9S Jupiter in the circle f; afterwards Saturn in g ; and far beyond him the Herschet plar.et performs his revo- hiiion in the circle h. J, For what are the smaller circles which are attached to several of the larger ones intended ? T, They are intended to represent the orbits of the several satellites or moons belonging to some of the J. What do you mean by the word orbit I T. The path described by a planet in its course round the sun, or by a moon round its primary planet, is called its orbit. Look to the orbit of the earth in t, and you will see a little circle, which represents the orbit in which our moon performs its monthly journey. » C. Has neither Mercury nor Venus any moonl T, None have ever been discovered belonging either to Mercury, Venus, or Mars. J upiter, as you observe by the figure, has four moons : Saturn has seven : and the Herschel planet (which also goes by the name of Uranus) has six, which, for want of room, are not drawn in the plate. 94 ASTRONOMY. C. The Solar Si^slem, then, consists of the sun as a centre, round which revolve seven planets, and eighteen satellites or moons. Are there no other bodies be- longing to it ? T. Yes, four other planetary bodies have been very lately discovered as belonging to the solar sys- tem. These are very small, and called the Piazzi, Olbers, &c. from the gentlemen v^ho discovered them. They are likewise called Ceres Ferdincmdea, Pallas, Juno, and Vesta. There are comets also which make their appearance occasionally ; and it would be wrong positively to affirm that there can be no other planets belonging to the Solar System ; since, besides the four bodies just mentioned, it is only within these few years that the seventh, or the Herschel, has been known to exist as a planet connected vvith this system. C. Who first adopted the system of the world which you have been describing ? T. It was conceived and taught by Pythagoras to his disciples 600 years before the time of Christ. But it seems soon to have been disregarded, or perhaps totally rejected till about 300 years ago, when it was revived by Copernicus, and is at length generally adopted by men of science. CONVERSATION VI. OF THE FIGURE OF THE EARTH. T. Having, in our last conversation, given you a description of the Solar System in general, we will now proceed to consider each of its parts separately : and since we are most of all concerned with the earth, we will begin with that body. J . You promised to give us some reasons why this earth must be in the form of a globe, and not a m.ere extended plane, as it appears to common observation. T, Suppose you were standing by the sea-shore, on a level with the water, and at a very considerable distance, as far as the eye can reach, you observe a OF THE FIGURE OF THE EARTH. 95 sbip approaching, what ought to be the appearance, supposing the surface of the sea to be a flat plane ? C. We should, I think, see the whole ship at once, that is, the hull would be visible as soon as the top-mast. T. It certainly must, or indeed rather sooner, be- cause the body of the vessel being so much larger than a slender mast, it must necessarily be visible at a greater distance. J. Yes, I can see the steeple of a church at a much greater distance than I can discern the iron conductor which is upon it, and that I can perfectly see long before the little piece of gold wire, which is fixed at its extremity, is visible. T. Well, but the top-mast of a vessel at sea is always in view some little time before the hull of the vessel can be discerned. Now, if the surface of the sea be globular, this ought to be the appearance, be- cause the protuberance or swelling of the water be- tween the vessel and the eye of the spectator will hide the body of the ship some time after the pendant is seen above. C. In the same way as if a high building, a church for instance, were situated on one side of a hill, and I was walking up on the opposite side, the steeple would come first in sight, and as I advanced towards the summit, the other parts v/ould come successively in view. T. Your illustration is quite to the purpose : m the same way, two persons walking up a hill on the oppo- site sides, will perceive each other's heads first ; and as they advance to the top, the other parts of their bodies will become visible. With respect to the ship, the following figure will convey the idea very com- pletely. Suppose cha represent a small part of the curved surface of the sea ; if a spectator stand at a while a ship is at c, only a small part of the mast is visible to him, but, as it advances, more of the ship is seen, till it arrive at e, when the whole will be in sight. 96 ASTRONOMY. Fig. 3. C. When I stood by the sea side the water did not appear to me to be curved. i . Perliaps not ; but its convexity may be dis- covered upon any still water, as upon a river, which k extended a mile or two in length ; for you might see a very small boat at that distance while standing upright; if then you stoop down so as to bring your eye near the water, you will find the surface of it rising in such a manner as to cover the boat, and intercept its view completely. Another proof of the globular figure of the earth is, that it is necessary for those who are em- ployed in cutting canals, to make a certain allowance for the convexity ; since the true level is not a straight line, but a curve which falls below it eight inches in every mile. C. I have heard of people sailing round the world, which is another proof, I imagine, of the globular figure of the earth. T, It is a well-known fact that navigators have set out from a particular port, and by steering their course continually westward, have at length arrived at the same place from whence they first departed. Now had the earth been an extended plane, the longer they had travelled the farther must they have been from home. C. How is it known that they continued the same course 1 might they not have been driven round at open sea? T. By means of the mariner's compass, the history, properties, and uses of which 1 will explain very par- ticularly in a future j)art of our lectures, the method OF THE FIGUUE OF THE EARTH» 97 of sailing on the ocean by one certain tract, is as sure as travelling on the high London road from the me- tropolis to York. By this method, Ferdinand Magel- lan sailed, in the year 1519, from the western coast of Spain, and continued his voyage in avi^estward course till he arrived after 1124 days in the same port from whence he set out. The same, with respect to Great Britain, was done by our own countrymen Sir Francis Drake, Lord Anson, Captain Cook, and many others, C. Is then the common terrestrial globe a just representation of the earth 1 T. It is, with this small difference,* that the artifi- cial globe is a perfect sphere, whereas the earth is a spheroid, that is, in the shape of an orange, the diame- ter from pole to pole being about 37 miles shorter than that at the Equator, J. What are the poles, sir 1 T. In the artificial globe there is an axis ns about which it turns ; now the two extremities or ends of this axis n and s are called the poles. C. Is there any axis be- longing to the earth ? T. No ; but, as we shall to-morrow shew, the earth turns round once in every 24 hours ; so astronomers imagine an axis upon which it revolves as upon a centre, the extremities of which imaginary axis are the poles of the earth ; of these, n, the north pole, points at all times exactly to the north pole of the * What the earth loses of its sphericity, by mountains and valleys, is very inconsiderable ; the highest moun- tain bearing so little proportion to its bulk, as scarcely to be equivalent to the minutest protuberance on the surface of an orange. F 98 ASTRONOMY. heavens, whieh we have already described, and which is, as you recollect, within two degrees of the polar star. (See Fig. 1. p. 79.) /. And how do you define the equator ? T. The equator ab (Fig. 4.) is the circumference of an imaginary circle passing through the centre of the earth, perpendicular to the axis ns, and at equal distances from the poles. C. And I think you told us, that if we conceived this circle extended every way to the fixed stars it would form the celestial equator, T. I did ; it is also called the equinoctial, and you. must not forget, that in this case it would cut the circle of the ecliptic cd in two points. /. Why is the ecliptic marked on the terrestrial globe, since it is a circle peculiar to the heavens ? T. Though the ecliptic be peculiar to the heavens, and the equator to the earth, yet they are both drawn on the terrestrial and celestial globes, in order, among other things, to shew the position which these imaginary circles have to one another. I shall now conclude our present Conversation, with observing, that besides the proofs adduced for the globular form of the earth, there are others equally conclusive, which will be better understood a few days hence. CONVERSATION YII. OF THE DIURNAL MOTION OF THE EARTH. T. Well, gentlemen, are you satisfied that the earth on which you tread is a globular body, and not a mere extended plane ? C. Admitting the facts which you mentioned yester- day, viz. that the topmast of a ship at sea is always visible before the body of the vessel comes into sight ; that navigators have repeatedly, by keeping the same DIURNAL MOTION OF THE EARTH. 99 direction, sailed round the world ; and that persons employed in digging canals can only execute their work with effect by allowing for the supposed globu- lar shape of the earth, it is evident the earth cannot be a mere extended plane. /. But all these facts can be accounted for upon the supposition that the earth is a globe, and there- fore you conclude it is a globe : this was, 1 believe, the nature of the proof? r. It was : let us now advance one step farther, and shew you that this globe turns on an imaginary axis every twenty-four hours; and thereby causes the succession of day and night. J. I shall wonder if you are able to afford such satisfactory evidence of the daily motion of the earth, as of its globular form. T. I trust, nevertheless, that the arguments on this subject will be sufficiently convincing, and that be- fore we part you will admit, that the apparent mo- tion of the sun and stars is occasioned by the diurnal motion of the earth. C. 1 shall be glad to hear how this can be proved ; for if, in the morning, I look at the sun when rising, it appears in the east, at noon it has travelled to the south, and in the evening I see it set in the western part of the heavens. J. Yes, and we observed the same last night (March the 1st) with respect to Arctnrus ; for about eight o'clock it had just risen in the north-east part of the heavens, and when we went to bed, two hours after, it had ascended a good height in the heavens, evidently travelling towards the west. T. It cannot be denied that the heavenly bodies appear to rise in the east and set in the west ; but the appearance will be the same to us, whether those bodies revolve about the earth while that stands still, or they stand still while the earth turns on its axis the contrary way. C. Will you explain this, sir 1 100 ASTRONOMY. T. Suppose grch to represent the earth, t the centre on v/hich it turns from west to east, according to the order of the letters grcb. If a spec- tator on the surface of the earth at r, see a star at h, it v^ill appear to him to have just risen ; if now the earth be supposed to turn on its axis a fourth part of a revolution, the spectator will be carried from r to c, and the star will be just over his head ; when another fourth part of the revolution is completed the spectator will be at b, and to him the star at h will be setting, and will not be visible again till he arrive, by the rotation of the earth, at the station r. C. To the spectator, then, at r the appearance would be the same whether he turned with the earth into the situation h, or the star at h had described, in a contrary direction, the space hzo in the same time. T. It certainly would. J. But if the earth really turned on its axis, should we not perceive the motion 1 T. The earth in its diurnal rotation being subject to no impediments by resisting obstacles, its motion cannot affect the senses. In the same way ships on a smooth sea are frequently turned entirely round by the tide, without the knowledge of those persons who happen to be busy in the cabin, or between the decks. C. That is, because they pay no attention to any other object but the vessel in which they are, every part of which moves with themselves. J. But if while the ship is turning, without their knowledge, they happen to be looking at fixed distant objects, what will be the appearance T. To them, those objects which are at rest will appear to be turning round the contrary way. In tlie same manner we are deceived in the motion of the earth round its axis, for, if we attend to nothing but what is connected with the earth, we cannot per- ceive a motion of which we partake ourselves, and if DIURNAL MOTION OF THE EARTH. 101 we fix our eyes on the heavenly bodies, the motion of the earth being so easy, they will appear to be turn- ing in a direction contrary to the real motion of the earth. C. I have sometimes seen a skylark hovering and singing over a particular field for several minutes toge- ther ; now, if the eartii is continually in motion while the bird remains in the same part of the air, why do we not see the field, over which he first ascended, pass from under him 1 r. Because the atmosphere in which the lark is suspended is connected with the earth, partakes of its motion, and carries the lark along with it ; and therefore, independently of the motion given to the bird by the exertion of its wings, it has another in common with the earth, yourself, and all things on it, and being common to us all, we have no methods of ascertaining it by means of the senses. J. Though the motion of a ship cannot be observed vdthout objects at rest to compare with it, yet I can- not help thinking that if the earth moved we should be able to discover it by means of the stars, if they are fixed. r. Do you not remember once sailmg very swiftly on the river, when you told me that you thought all the trees, houses, &c. on its banks were in motion ? J. I now recollect it well ; and I had some diffi- culty in persuading myself that it was not so. C. This brings to my mind a still stronger decep- tion of this sort : when travelling with great speed in a post-chaise, I suddenly waked from a sleep in a smooth but narrow road, and I could scarcely help thinking, for several minutes, but th-at the trees and hedges were running away from us, and not we from them. r. I will mention another curious instance of this kind : if you ever happen to travel pretty swiftly in a carriage, by the side of a field ploughed into long nar- row ridges, and perpendicular to the road, you will think that all the ridges are turning round in a direction con- 10^ ASTRONOMY, trary to that of the carriage. These facts may satisfy you that the appearances will be precisely the same to us, whether the earth turn on its axis from -west to easts or the sun and stars move from east to west. J. They will ; but which is the more natural con- clusion ? T. This you shall determine for yourself. If the earth (Fig, 4.) turn on its axis in 24 hours, at what rate will any part of the equator ab move I C. To determine this we must find the measure of its circumference, and then dividing this by 24, we shall get the number of miles passed through in an hour. T, Just so : now call the semi-diameter of the earth 4000 miles, which is rather more than the true measure. /. Multiplying this by six * will give 24,000 miles for the circumference of the earth at the equator, and this divided by 24, gives 1000 miles for the space passed through in an hour, by an inhabitant of the equator. r. You are right. The sun, I have already told you, is 95 millions of miles distant from the earth ; tell me, therefore, Charles, at what rate that body must travel to go round the earth in 24 hours 1 C, I will : 95 millions multiplied by six will give 910 millions of miles for the length of his circuit ; this divided by 24 gives nearly 24 millions of miles for the space he must travel in an hour, to go round in a day. • If the reader would be accurate in his calculations, he must take the mean radius of the earth at 39G5 miles, and this, multiplied hy C2S,31S will give 24,912 miles for the circumference. Through the remainder of this work, the decimals in multiplication are omitted, in order that the mind may not be burdened with odd numbers. It seemed necessary, however, in this place to give the true serai-diameter of the earth, and the number ( accurate to five places of decimals) by which, if the radius of any circle be multiplied, the circumference is obtained. OF DAY AND NIGHT. 103 T Which now is the more probable conclusion, either that the earth should have a diurnal motion on its axis of 1000 miles in an hour, or that the sun, which is a million of times larger than the earth, should travel 24 millions of miles in the same time . J. It is certainly more rational to conclude that the earth turns on its axis, the effect of which you told us was the alternate succession of day and mght. T. I did ; and on this and some other topics we will enlarge to-morrow. CONVEKSATION VIII. OF DAY AND NIGHT. J. You are now, sir, to apply the rotation of the earth about its axis to the succession of day and night. r. I will ; and for this purpose suppose grcb {tig. 5.) to be the earth, revolving on its axis, accordmg to the order of the letters, that is, from gtor, r to c, &c. If the sun be fixed in the heavens at s, and a line ho be drawn through the centre of the earth it will represent that circle, which, when extended to the heavens, is called the rational horizon, ^ C In what does this differ from the sensible horizon ? T The sensible horizon is that circle m the heavens which bounds the spectator's view, arid which is greater or less according as he stands higher or lower, lor example; an eye placed at >e feet above the surface of the earth or sea, sees 2-J miles every way : but if it be at 20 feet high, that is, four times the height, it will see 5| miles, or twice the distance. C Then the senshle differs from the ratimial hori- zon in this, that the former is seen from the surface ot the earth, and the latter is supposed to be viewed from its centre. . . „ , T. You are right ; and the rising and setting ot the sun and stars are always referred to the rational horizon. . , . /. Why sol they appear to rise and set as soon as 104 ASTRONOMY. they get above, or sink below, that boundary which separates the visible from the invisible part of the heavens. T, They do not, however ; and the reason is this, that the distance of the sun and fixed stars is so great m comparison of 4000 miles (the difference between the surface and centre of the earth), that it can scarcely be taken into account. C. But 4000 miles seem to me an imir ense space. T, Considered separately they are so, but when compared with 95 millions of miles, the distance of the sun from the earth, they almost vanish as nothing. J. But do the rising and setting of the moon, which is at the distance of 240 thousand miles onlv respect also the rational horizon ? " ' Certainly; for 4000 compared with 240,000, bear only the proportion of 1 to 60. Now if two spaces were marked out on the earth in different directions, the one 60 and the other 61 yards, should you at once be able to distinguish the greater from the less \ C. 1 think not. T. Just in the same manner does the distance of the centre from the surface of the earth vanish in comparison of its distance from the moon. But this IS not the proper time to explain that peculiar differ- ence connected with what astronomers call parallax, /. We must not, however, forget the succession of day and night. T. Well then ; if the sun be supposed at 2, it will Illuminate by its rays all that part of the earth that is above the horizon ho; to the inhabitants at its %yestern boundary, it will appear just rising ; to "those situated at r, it will be noon ; and to those in the eastern part of the horizon, c, it will be setting. C. Isee clearly why it should be noon to those who live at r, because the sun is just over their heads, but it is not so evident why the sun must ap- pear rising and setting to those who are at and c. i . \ ou are satisfied that a spectator cannot, from OF DAY ANT) NIGHT. ^ 105 any place, observe more than a semi-circle of the heavens at any one time ; now what part of the hea- vens will the spectator at g observe 1 J. He will see the concave hemisphere zon, T. The boundary to his view will be s and n, will it not] C. Yes ; and consequently the sun at z will to him be just coming into sight. T. Then, by the rotation of the earth, the spectator at g will in a few hours come to r, when, to him, it will be noon ; and those who live at r will have de- scended to c ; now what part of the heavens will they see in this situation 1 J. The concave hemisphere nhz, and s bemg the boundary of their view one way, the sun will to them be setting. T. Just so. After which they will be turned away from the sun,, and consequently it will be night to them till they come again to g. Thus, by this sim- ple motion of the earth on its axis, every part of it is, by turns, enlightened and warmed by the cheering beams of the sun. C. Does this motion of the earth account also for the apparent motion of the fixed stars 1 r. It is owing to the revolution of the earth round its axis, that we imagine the whole starry firmament revolves about the earth in 24 hours. J. If the heavens appear to turn on an axis, must there not be two points, namely, the extremities of that imaginary axis, which always keep their position 1 T. Yes, we must be understood to except the two celestial poles which are opposite to the poles of the earth, consequently each fixed star appears to de- scribe a greater or a less circle round these, accord- ing as it° is more or less remote from those celestial poles. . , C. When we turn from that hemisphere in wnich the sun is placed, we immediately gain sight of the other in which the stars are situated. F 2 108 ASTRONOMY. T, Every part of the heavens is decorated with these glorious bodies. /. If every part of the heavens be thus adorned, v/hy do we not see the stars in the day as well as the night '? T. Because in the day-time the sun's rays are so powerful, as to render those coming from the fixed stars invisible. But if you ever happen to go down into any very deep mine, or coal-pit, where the rays of the sun cannot reach the eye, and it be a clear day, you may, by looking up to the heavens, see the stars at noon as well as in the night. C. If the earth always revolve on its axis in 24 hours, why does the length of the days and nights dif- fer in different seasons of the year 1 T. This depends on other causes connected with the earth's annual journey round the sun, upon which we will converse the next time we meet. CONVERSATION IX. OF THE ANNUAL MOTION OF THE EARTH. r. Besides the diurnal motion of the earth, by which the succession of day and night is produced, it has another, called its annual motion, which is the journey it performs round the sun in 365 days, 5 hours, 48 minutes, and 49 seconds. C. Are the different seasons to be accounted for by this motion of the earth 1 T. Yes, it is the cause of the different lengths of the days and nights, and consequently of the different seasons, viz. Spring, Summer, Autumn, and Winter, J. How is it known that the earth makes this an- nual journey round tlie sun 1 T. I told you yesterday, that through the shaft of a very deep mine, the stars are visible in the day a3 well as in the night ; they are also visible in the day- time, by means of a telescope properly fitted up for ANNUAL MOTION OF THE EARTH. 107 the purpose ; by this method, the sun and stars are visible at the same time. Now if the sun be seen in a line with a fixed star to-day at any particular hour, it will, in a few weeks, by the motion of the earth, be found considerably to the east of him : and, if the observations be continued through the year, we shall be able to trace him round the heavens to the same fixed star from which we set out ; consequently, the sun must have made a journey round the earth in that time, or the earth round him. C. And the sun being a million of times larger than the earth, you will say that it is more natural that the smaller body should go round the larger, than the reverse. T, That is a proper argument ; but it may be stated in a much stronger manner. The sun and earth mu- tually attract one another, and since they are in equilibrio by this attraction, you know, their momenta must be equal;* therefore the earth, being the smaller body, must make out by its motion what it wants in the quantity of its matter, and, of course, it must be that which performs the journey. J. But if you refer to the principle of the lever, to explain the mutual attraction of the sun and earth, it is evident, that both bodies must turn round some point as a common centre. T, They do ; and that is the common centre of gravity of the two bodies. Now this point between the earth and sun is within the surface of the latter body. C. I understand how this is ; because tbe centre of gravity between any two bodies must be as much nearer to the centre of the larger body than the smaller, as the former contains a greater quantity of matter than the latter. T, You are right : but you will not conclude that, because the sun is a million times larger than the earth, therefore it contains a quantity of matter a * See Mechanics, Conversation XIV. 108 ASTRONOMY. million of times greater than that contained in the earth. J. Is it then known, that the earth is composed of matter more dense than that which composes the body of the sun ? T, The earth is composed of matter four times denser than that of the sun ; and hence the quantity of matter in the sun is between two and three hun- dred thousand times greater than that which is con- tained in the earth. C. Then for the momenta of these two bodies to be equal, the velocity of the earth must be between two and three hundred thousand times greater than that of the sun. T. It must : and to effect this, the centre of gravity between the sun and earth must be as much nearer to the centre of the sun, than it is to the centre of the earth, as the former body contains a greater quantity of matter than the latter : and hence it is found to be several thousand miles within the surface of the sun. J. I now clearly perceive, that since one of these bodies revolves about the other in the space of a year, and that they both move round their common centre of gravity, that it must, of necessity, be the earth which revolves about the sun, and not the sun round the earth. T. Your inference is just. To suppose that the sun moves round the earth, is as absurd as to main- tain, that a mill-stone could be made to move round a pebble. CONVERSATION X. OF THE SEASONS. T. I will now shew you how the different seasons are produced by the annual motion of the earth. J. Upon what do they depend, sir ? T. The variety of the seasons depends (1) upon tlie length of the days and nights ; and, (2) upon the position of the earth with respect to the sun. OF THE SEASONS. 109 C. But, if the earth turn round its imaginary axis every 24 hours, ought it not to enjoy equal days and nights all the year 1 T, This would be the case if the axis of the earth ns were perpendicular to a line ce drawn through the centres of the sun and earth ; for then as the sun always enlightens one half of the earth by its rays, and as it is day at any given place on the globe so long as that place continues m the enlightened hemisphere, every part except the two poles must, during its rotation on its axis, be one half of its time in the light and the other half in darkness; or, in other words, the days and nights would be equal to all the inhabitants of the earth, excepting to those, if any, who live at the poles. J, Why do you except the people at the poles 7 r. Because the view of the spectator situated at the poles n and s, must be bounded by the line ce ; consequently to him the sun would never appear to rise or set, but would always be in the horizon. C. If the earth were thus situated, would the rays of the sun always fall vertically to the same part of it? r. They would : and that part would be' eq the equator ; and, as we shall presently shew, the heat excited by the sun being greater or less in proportion as its rays come more or less perpendicularly upon any body, the parts of the earth about the equator would be scorched up, while those between 40 and 50 degrees on each side of that line and the poles would be desolated by an unceasing winter. J. In what manner is this prevented 1 T, By the axis of the earth ns being inclined or bent about 23 degrees and a half out of the perpendicular. In this case you observe, that all the parallel circles, except the equator, are Iig- divided into two unequal parts, 110 ASTRONOMY. having a greater or less portion of their circumferences in the enlightened, than in the dark hemisphere, according to their situation with respect to n the north, or s the south pole. C. At what season of the year is the earth repre- sented in this figure 1 T. At our summer season : for you observe that the parallel circles in the northern hemisphere have their greater parts enlightened and their smaller parts in the dark. If dl represent that circle of latitude on the globe in which Great Britain is situated, it is evident that about two-thirds of it is in the light, and only one-third in darkness. You will remember that parallels of latitude are circles on the surface of the earth, or its representative the terrestrial globe, drawn parallel to the equator. J. Is that the reason why our days towards the middle of June are 16 hours long, and the nights but eight hours 1 T, It is : and if you look to the parallel next be- yond that marked d l, you will see a still greater dis- proportion between the day and night, and the parallel more north than this is entirely in the light. C. Is it then all day there 1 T. To the whole space between that and the pole it is continual day for some time, the duration of which is in proportion to its vicinity to the pole ; and at the pole there is a permanent day-light for six months together. J. And during that time it must, I suppose, be night to the people who live at the south pole 1 T, Yes, the figure shews thatthe south pole is in dark- ness ; and you may observe, that to the inhabitants liv- ing in equal parallels of latitude, the one north, and the other south, the length of the days to the one will be always equal to the length of the nights to the other. C. What then shall we say to those who live at the equator, and consequently have no latitude 1 T, To them the days and nights are alwaijs equal, and of course twelve hours each in length ; and this is OF THE SEASONS. HI also evident from the figure, for in every position of the globe one half of the equator is m the light and the other half in darkness. ^ J. If, then the length of the days is the cause ot the diflferent seasons, there can be no variety m this respect to those who live at the equator 1 ^ T. You seem to forget that the change m the seasons depends upon the position of the earth with respect to the sun, that is, upon the perpendicuLarity with which the rays of light fall upon any particular part of the earth ; as well as upon the length oi the ^^C.* Does this make any material difference with regard to the heat of the sun 1 T. It does : let a b represent a portion of the earth's surface, on which the sun's rays fall per- pendicularly ; let BC represent an equal portion on which they fall obliquely, or aslant. It is mani- Yw, 8. fest that BC, though it be equal to ° AB receives but half the light and heat that ab does. Moreover, by the sun's rays coming more perpen. dicularly, they come with greater force, as well as m greater numbers, on the same place. CONVEKSATION XI. OF THE SEASONS. T. If you now take a view of the earth in its an- nual course round the sun, considering its axis as inclined 23| degrees to a line perpendicular to its orbit, and keeping, through its whole journey, a di- rection parallel to itself, you will find, that accordmg as the earth is in different parts of its orbit, the rays ot the sun are presented perpendicularly to the equator, and to every point of the globe withm 23^ degrees of it both north and south. 112 ASTRONOMY. Fig. 9. This figure represents the earth in four different parts of its orbit, or as it is situated with respect to the sun in the months of March, June, September, and December. C. The earth's orbit is not made circular in the figure. T, No ; but the orbit itself is nearly circular : you are supposed to view it from the under side, and there- fore, though almost a circle, it appears to be a long ellipse. All circles appear elliptical in an oblique view, as is evident by looking obliquely at the rim of a bason, at some distance from you. For the true figure of a circle can only be seen when the eye is directly over its centre. You observe that the sun is not in the centre. J. I do ; and it appears nearer to the earth in the winter than in the summer. T. We are indeed more than three millions of miles nearer to the sun in December than we are in June. C. Is tliis possible, and yet our winter is so much colder than the summer ? T. Notwithstanding this, it is a well-known fact ; for it is ascertained that our summer, that is, the time that passes between the vernal and autumnal equi- noxes, is nearly eight days longer than our winter, or the time between the autumnal and vernal equinoxes. OF THE SEASONS. 113 Consequently the motion of the earth is slower in the former case than in the latter, and therefore, as we shall see, it must be at a greater distance from the sun. Again, the sun's apparent diameter is greater in our winter than in summer, but the apparent diameter of any object increases in proportion as our distance from the object is diminished, and therefore we con- clude, that we are nearer the sun in winter than in summer. The sun's apparent diameter in winter is 32'. .47" ; in summer, 31'. .40". J. But if the earth is farther from the sun in sum- mer than in winter, why are our winters so much colder than our summers ? r. Because first, in the summer, the sun rises to a much greater height above our horizon, and therefore its rays coming rnore perpendicularly, more of them, as we shewed you yesterday, must fail upon the sur- face of the earth, and come also with greater force, which is the principal cause of our greater summer's I heat. Secondly, in the summer, the days are very i long, and the nights short ; therefore the earth and i air are heated by the sun in the day more than they I are cooled in the night. I J. Why have we not, then, the greatest heat at the time when the days are longest 1 ' T. The hottest season of the year is certainly a month or two after this, which may be thus accounted for. A body once heated does not grow cold again i instantaneously, but gradually : now, as long as more heat comes from the sun in the day than is lost in the night, the heat of the earth and air will be daily increasing, and this must evidently be the case for some weeks after the longest day, both on account of the number of rays which fall on a given space, and also from the perpendicular direction of those rays. J, Will you now explain to us in what manner the seasons are produced 1 T, By referring to the last figure you will observe, ' that in the month of June the north pole of the earth ; inclines towards the sun, and consequently brings ail I i 114 ASTRONOMY. the northern parts of the globe more into light, than at any other time in the year. C Then to the people in those parts it is summer ? r. It is : but in December, when the earth is in the opposite part of its orbit, the north pole declines from the sun, which occasions the northern places to be more in the dark than in the light 3 and the re- verse at the southern places. J. Is it then summer to the inhabitants of the southern hemisphere ? T, Yes, it is ; and winter to us. In the months of March and September the axis of the earth does not incline to, nor decline from, the sun, but is perpen- dicular to a line drawn from its centre. And then the poles are in the boundary of light and darkness, and the sun being directly vertical to, or over, the equator, makes equal day and night at all places. jNow trace the annual motion of the earth in its orbit for yourself, as it is represented in the figure. C. I will, sir : about the 20th of March the earth is in Libra, and consequently to its inhabitants the sun will appear in Aries, and be vertical to the equator. T. Then the equator and all its parallels are equally divided between the light and dark. C. Consequently, the days and nights are equal all over the world. As the earth pursues its journey from March to June its northern hemisphere comes more into light, and on the 21st of that month tlie sun is vertical to the tropic of Cancer. T. You then observe, that all the circles parallel to the equator are unequally divided; those in the northern half have their greater parts in the light, and those in the southern half have their larger parts in darkness. C. Yes ; and, of course, it is summer to the in- habitants of the northern hemisphere, and winter to the southern. I now trace it to September, when I find the sun vertical again to the equator, and, of course, the days OF THE SEASONS. 115 and nights are again equal. And following the earth in its journey to December, or when it has arrived at Cancer, the sun appears in Capricorn, and is vertical to that part of the earth called the tropic of Capri- corn; and now the southern pole is enlightened, and all the circles on that hemisphere have their larger parts in light, and, of course, it is summer to those parts, and winter to us in the northern hemisphere. r. Can you, James, now tell me, why the days lengthen and shorten frorn the equator to the polar circles every year 1 J. I will try to explain myself on the subject. I Because the sun in March is vertical to the equator, ' and from that time to the 21st of June it becomes I vertical successively to all other parts of the earth be- j tween the equator and the tropic of Cancer ; and in I proportion as it becomes vertical to the more northern I parts of the earth, it declines from the southern, and, I consequently, to the former the days lengthen, and to ! the latter they shorten. From June to September the ! sun is again vertical successively to all the same parts I of the earth, but in a reverse order. C. Since it is summer to all those parts of the 1 earth where the sun is vertical, and we find that the j sun is vertical twice in the year to the equator, and every part of the globe between the equator and tropics, there must be also two summers in a year to all those places. r. There are ; and in those parts near the equator they have two harvests every year. — But let your brother finish his description. J. From September to December it is successively vertical to all the parts of the earth situated between the equator and the tropic of Capricorn, which is also the cause of the lengthening of the days in the southern hemisphere, and of their becoming shorter in the northern. I T. Can you, Charles, tell me why there is some- ' times no day or night for some little time together within the polar circles 1 \ 116 ASTRONOMY. C. The sun always shines upon the earth 90 de- grees every way, and when he is vertical to the tropic of Cancer, which is 23| degrees north of the equator, he must shine the same number of degrees beyond the pole, or to the polar circle, and while he thus shines there can be no night to the people within that polar circle, and, of course, to the inhabitants at the south- ern polar circle there can be no day at the same time ; for as the sun's rays reach but 90 degi'ees every way, they cannot shine far enough to reach them. T, Tell me, now, why there is but one day and night in the whole year at the poles 1 C. For the reason which I have just given, the sun must shine beyond the north pole all the time he is vertical to those parts of the earth situated between the equator and the tropic of Cancer, that is, from March the 21st to September the 20th, during which time there can be no night at the north pole, nor any day at the south pole. The reverse of this may be applied to the southern pole. J. I understand now, that the lengthening and shortening of the days, and different seasons, are pro- duced by the annual motion of the earth round the sun ; the axis of the earth, in all parts of its orbit, being kept parallel to itself. But, if thus parallel to itself, hov/ can it in all positions point to the pole- star in the heavens ? T, Because the diameter of the earth's orbit is nothing in comparison of the distance of the earth from the fixed stars. Suppose you draw two parallel lines at the distance of three or four yards from one another, will they not both point to the moon when she is in the horizon ? /. Three or four yards cannot be accounted as any thing in comparison of 240 thousand miles, the dis- tance of the moon from us. T. Perhaps three yards bear a greater proportion to 240 thousand miles, than 190 millions of miles bear to our distance from the polar star. OF THE EQUATION OF TIME. 117 CONVERSATION XII. OF THE EQUATION OF TIME. T. You are now, I presume, acquainted with the motions peculiar to this globe on which we live '? C. Yes : it has a rotation on its axis from west to east every 24 hours, by which day and night are pro- duced, and also the apparent, diurnal motion of the heavens from east to west. J. The other is its annual revolution m an orbit round the sun, likewise from west to east, at the dis- tance of about 95 millions of miles from the sun. T. You understand, also, in what manner this an- nual motion of the earth, combined with the inclina- tion of its axis, is the cause of the variety of seasons. We will therefore proceed to investigate another curious subject, viz. the equation of time, and to ex- plain to you the difterence between equal, or mean, and apparent time. C. Will you tell us what you mean by the words equal and apparent, as applied to time r. Equal or mean time is measured by a clock, that is supposed to go without any variation, and to measure exactly 24 hours from noon to noon. And apparent time is measured by the apparent motion ot the sun in the heavens, or by a good sun-dial. C. And what do you mean, sir, by the equation oj time? . T, It is the adjustment of the difference ot time, as shewn by a well-regulated clock and a true sun- dial. J. Upon what does this difference depend 1 T. It depends, first, upon the inclination of the earth's axis ; and, secondly, upon the elliptic form of the earth's orbit ; for, as we have already seen, the earth's orbit being an ellipse, its motion is quicker when it is in perihelion, or nearest to the sun ; and slower when it is in aphelion, or farthest from the sun. 118 ASTRONOMY. C. But. I do not yet comprehend what the rotation of the earth has to do with the going of a clock or watch. T. The rotation of the earth is the most equable and uniform motion in nature, and is completed in 23 hours, 56 minutes, and 4 seconds ; this space of time is called a sidereal day, because any meridian on the earth will revolve from a fixed star to that star again in this time. But a solar, or natural day, which our clocks are intended to measure, is the tinie which any meridian on the earth will take in revolving from the sun to the sun again, which is about 24 hours, sometimes a little more, but generally less. J. What occasions this difference between the so- lar and sidereal day ? T, The distance of the fixed stars is so great, that the diameter of the earth's orbit, though 190 millions of miles, when compared with it, is but a point, and therefore any meridian on the earth will revolve from a fixed star to that star again in exactly the same time as if the earth had only a diurnal motion, and remained always in the same part of its orbit. But with respect to the sun, as the earth advances almost a degree eastward in its orbit, in the same time that it turns eastward round its axis, it must make more than a complete rotation before it can come into the same position with the sun that it had the day before. In the same way as when both the hands of a watch or clock set off together at twelve o'clock, the minute- hand must travel m.ore than a whole circle before it will overtake the hour-hand, that is, before they will be in the same relative position again. Thus the sidereal days are shorter than the solar ones by about four minutes, as is evident from observation. C. Still I do not understand the reason why the clocks and dials do not agree. T. A good clock is intended to measure that equa- ble and uniform time which the rotation of the earth on its axis exhibits ; whereas the dial measures time by the apparent motion of the sun, which, as we have OF THE EQUATION OF TIME. 119 explained, is subject to variation. Or thus : though the earth's motion on its axis be perfectly uniform, and consequently the rotation of the equator, the plane of which is perpendicular to the axis, or of any other circle parallel to it, be likewise equable, yet we measure the length of the natural day by means of the sun, whose apparent annual motion is not in the equator, or any of its parallels, but in the ecliptic, which is oblique to it. /. Do you mean by this that the equator of the earth, in its annual journey, is not always directed towards the centre of the sun ? T. I do ; twice only in the year, a line drawn from the centre of the sun to that of the earth passes through those points where the equator and ecliptic cross one another ; at all other times, it passes through some other part of that oblique circle which is repre- sented on the globe by the ecliptic line. Now when it passes through the equator, or the tropics, which are circles parallel to the equator, the sun and clocks go together, as far as regards this cause, but at other times they differ, because equal portions of the eclip- tic pass over the meridian in unequal parts of time, on account of its obliquity. C. Can you explain this by a figure 1 T. It is easily shewn by the n globe, which this figure ^ n =?!b s may represent : cy' =^ will be the equator, 53 — the northern half of the eclip- t tic, and c\pVS ~ the southern half. Make chalk or pencil marks a, h, c, d, e,f, h, all round the equator and ecliptic, at equal distances (suppose 20 degrees) from each other, be- ginning at Aries. Now by turning the globe on its axis, you will perceive that all the marks in the first quadrant of the ecliptic, that is, from Aries to Cancer, come sooner to the brazen meridian than their corre- 120 ASTRONOMY. spending marks on the equator: — those from the be- ginning of Cancer to Libra come later: — those from Libra to Capricorn sooner : — and those from Capri- corn to Aries later. Now time as measured by the sun-dial is repre- sented by the marks on the ecliptic; that measured by a good clock, by those on the equator, C. Then while the sun is in the first and third quarters, or, what is the same thing, while the earth is travelling through the second and fourth quarters, that is, from Cancer to Libra, and from Capricorn to Aries, the sun is faster than the clocks, and while it is travelling the other two quarters it is slower. T. Just so : because while the earth is travelling through the second and fourth quadrants, equal por- tions of the ecliptic come sooner to the meridian than their corresponding parts of the equator ; and during its journey through the first and third quadrants, the equal parts of the ecliptic arrive later at the meridian than their corresponding parts of the equator. J. If I understand what you have been saying, the dial and clocks ought to agree at the equinoxes, that 'is, on the 20th of March and the 23d of September ; but if I refer to the Ephemeris, I find that on the for- mer day (1822) the clock is nearly eight minutes before the sun ; and on the latter day the clock is more than seven minutes behind the sun. T. If this difference between time measured by the dial and clock depended only on the inclination of the earth's axis to the plane of its orbit, the clocks and dial ought to be together at the equinoxes, and also on the 21st of June and the 21st of December, that is, at the summer and winter solstices ; because, on those days, the apparent revolution of the sun is parallel to the equator. But I told you there was another cause why this difference subsisted. C. You did ; and that was the elliptic form of the earth's orbit. T. If the earth's motion in its orbit were uniform, which it would be if the orbit were circular, then the OF LEAP YEAR. 121 whole difference between equal time as shewn by the clock, and apparent time as shewn by the sun, would arise from the inclination of the earth's axis. But this is not the case ; for the earth travels, when it is nearest the sun, that is, in the winter, more than a degree in 24 hours, and when it is farthest from the sun, that is, in summer, less than a degree in the same time ; consequently, from this cause, the natu- ral day would be of the greatest length when the earth was nearest the sun, for it must continue turn- ing the longest time after an entire rotation, in order to bring the meridian of any place to the sun again ; and the shortest day would be when the earth moves the slowest in her orbit. Now these inequalities, combined with those arising from the inclination of the earth's axis, make up that difference which is shewn by the equation table, found in the Epheme- ris, between good clocks and true sun-dials. CONVERSATION XIIE OF LEAP-YEAR. J. Before we quit the subject of time, will you give us some account of what is called in our alma- nacs Leap- Year ? T. I will. The length of our year is, as you know, measured by the time which the earth takes in per- forming her journey round the sun, in the same man- ner as the length of the day is measured by its rota- tion on its axis. Now, to compute the exact time taken by the earth in its annual journey, was a work of considerable difficulty. Julius Caesar was the first person who seems to have attained to any accuracy on this subject. C. Do 3^ou mean the first Roman Emperor, who landed also in Great Britain ? T. I do.» He was not less celebrated as a man of science, than he was renowned as a general, Julius Caesar, who was well acquainted with the learning of G 122 ASTRONOMY. the Egyptians, fixed the length of the year to be 365 days and six hours, which made it six hours longer than the Egyptian year. Now, in order to allow for the odd six hours in each year, he introduced an ad- ditional day every fourth year, which accordingly consists of 366 days, and is called Lea;^- Year, while the other three have only 365 days each. From him it was denominated the Julian year. /. It is also called Bissextile in the almanacs ; what does that mean 1 T. The Romans inserted the intercalary day be- tween the 23d and 24tli of February ; and because the 23d of February, in their calendar, was called sexto calendas Martii, the 6th of the calends of March, the intercalated day was called his sexto calendas Mar- tii, the second sixth of the calends of March, and hence the year of intercalation had the appellation of Bissextile. This day was chosen at Rome, on account of the expulsion of Tarquin from the throne, which happened on the 23d of February. We intro- duce, in Leap- Year, a new day in the same month namely, the 29th. C. Is there any rule for knowing what year is Leap- Year 1 T, It is known by dividing the date of the year by 4; if there be no remainder it is Leap- Year ; thus 1831 divided by 4 leaves a remainder of 3, shewing that it is the third year after Leap- Year. These two lines contain the rule : Divide by 4; what's left shall be For Leap-year 0 ; for past 1, 2, 3. J. The year, however, does not consist of 365 days and 6 hours, but of 365 days, 5 hours, 48 mi- nutes, and 49 seconds.* Will not this occasion some error 1 T. It will ; and, by subtracting the latter number from the former, you will find that the error amounts * See Conversation IX. OF LEAP YEAR. 123 to 11 minutes and 11 seconds every year, or to a whole day in about 130 years : notwithstanding this, the Julian year continued to be in general use till the year 1582, when Pope Gregory the 13th undertook to rectify the error, which at that time amounted to 10 days. He accordingly commanded the 10 days between the 4th and 15th of October in that year to be suppressed, so that the 5th day of that month was called the 15th. This alteration took place through I the greater part of Europe, and the year was after- ii wards called the Gregorian year, or New Style, In li this country, the method of reckoning according to the New Style was not admitted into our calendars { till the year 1752, when the error amounted to nearly j 11 days, which were taken from the month of Sep- tember, by calling the 3d of that month the 14th. C. By what means will this accuracy be main- tained? ; T, The error amounting to one whole day in about : 130 years, it is settled by an act of parliament, that I the year 1800 and the year 1900, which are, accord- ing to the rule just given, Leap-years, shall be com- i puted as common years, having only 365 days in ; each ; and that every four hundredth year afterwards ij shall be common years also. If this method be ad- I hered to, the present mode of reckoning will not vjiry a single day from true time in less than 5000 years. \ By the same act of parliament, the legal beginning !' of the year was changed from the 25th of March to the 1st of January. So that the succeeding months of January, February, and March, up to the 24th day, which would, by the Old Style, have been i reckoned part of the year 1752, were accounted as ' the three first months of the year 1753. Which is the reason you sometimes meet with such a date as this, March 15, 1774-5 ; that is, according to the Old Style it was 1774 — according to the New, 1775. Russia is the only country in Europe where the Old Style still prevails. 124 ASTRONOMY. CONVERSATION XIV OF THE MOON. T, You are now, gentlemen, acquainted with the reasons for the division of time into days and years. C. These divisions have their foundation in nature, former depending upon the rotation of the earth on its axis ; the lalter upon its revolution in an ellip- tic orbit about the sun as a centre of motion. J. Is there any natural reason for the division of years into weeks, or of days into hours, minutes, and seconds ] T. The first of these divisions was introduced by Divine Authority ; the second class was invented for the convenience of mankind. There is, however, another division of time marked out by nature. C. What is that, sir? T. The length of the month: not, indeed, that month which consists of four weeks, nor that by which the year is divided into 12 parts. These are both arbitrary. But by a month is meant the time which the moon takes' in performing her journey round the earth. J. How many days does t^e moon take for this purpose 1 T. If you refer to the time in which the m.oon re- volves from one point of the heavens to the same point again, it consists of 27 days, 7 hours, and 43 minutes ; this is called the periodical month : but if you refer to the time passed from new moon to new moon again, the month consists of 29 days, 12 hours, and 44 minutes ; this is called the siinociical month. C, Pray explain the reason of this difference. r. It is occasioned by the earth's annual motion in its orbit. Let us refer to our watch as an example. The two hands are together at 12 o'clock ; now, when the minute-hand has made a complete revolution, are they together again 1: OP THE MOON. 125 J. No ; for the hour-hand is advanced the twelfth part of its revolution, which, in order that the other may overtake, it must travel five minutes more than the hour. T. And something more, for the hour-hand does not wait at the figure 1, till the other comes up ; and therefore they will not be together till between five and six minutes after one. Now apply this to the earth and moon ) suppose Fig. 11. 3 to be the sun ; t the earth, in a part of its orbit ql ; and E to be the position of the moon : if the earth had no motion, the moon would miOve round its orbit EHC into the position e again in 27 days, 7 hours, 43 minutes ; but while the moon is describing her journey, the earth has passed through nearly a twelfth part of its orbit, which the moon must also describe before the two bodies come again into the same position that they before held with respect to the sun : this takes up so much more time as to make her synodical month equal to 29 days, 12 hours, and 44 minutes : hence the foundation of the division of time into months. We will now proceed to describe some other parti- culars relating to the moon, as a body depending, like the earth, on the sun for her light and heat. C. Does the moon shine with a borrowed lio-ht only ? 120 ASTRONOMY. 7'. This is certain j for if, like the sun, she were a luminous body, she would always shine with a full orb, as the sun does. Her diameter is nearly 2200 miles in length. J. And 1 remember she is at the distance of 240,000 miles from the earth. 7\ The sun s (Fig. 11.) always enlightens one half of the moon e, and its whole enlightened hemi- sphere, or a part of it, or none at all, is seen by us according to her different positions in the orbit with respect to the earth ; for only those parts of the en- lightened half of the moon are visible at t which are cut off by, and are within, the orbit. J, Then when the moon is at e, no part of its en- lightened side is visible to the earth. T. You are right : it is then new moon, or change, . for it is usual to call it new moon the first day it is visible to the earth, which is not till the second day after the change. And the moon being in a line be- tween the sun and earth, they are said to be in con- junction, C. And at a all the illuminated hemisphere is turned to the earth. T. This is called full moon; and the earth being between the sun and moon, they are said to be in opposition. The enlightened parts of the little figures on the outside of the orbit, represent the appearance of the moon as seen by a spectator on the earth. J. Is the little figure then opposite e wholly dark to shew that the moon is invisible at change ? 2\ It is : and when it is at f a small part of the illuminated hemisphere is icithin the moon's orbit, and therefore to a spectator at t it appears /lorned; at g one half of the enlightened hemisphere is visible, and it is said to be in quadrature : at h three-fourths of the enlightened part is visible to the earth, and it is then said to be gibbous : and at a the whole enlight- ened face of the moon is turned to the earth, and it is said to he fall. The same may be said of the rest. The horns of the moon, before conjunction or new OF THE MOON. 127 moon, are turned to the cast: after conjunction they are turned to the west. C. I see the figure is intended to shew that the moon's orbit is elliptical : does she also turn upon her axis 1 T. She does ; and she requires the same time for her diurnal rotation as she takes in completing her revolution about the earth : and consequently, though every part of the moon is successively presented to the sun, yet the same hemisphere is alv^^ays turned to the earth. This is knov^^n by observation with good telescopes. /. Then the length of a day and night in the moon is equal to more than twenty-nine days and a half of ours. r. It is so : and therefore, as the length of her year, which is measured by her journey round the sun, is equal to that of ours, she can have but about twelve days and one-third in a year. Another re- markable circumstance relating to the moon is, that the hemisphere next the earth is never in darkness ; for in the position e, when it is turned from the sun, it is illuminated by light reflected from the earth, in the same manner as we are enlightened by a full moon. But the other hemisphere of the moon has a fortnight's light and darkness by turns. C. Can the earth, then, be considered as a satellite to the moon 1 r. It would, perhaps, be maccurate to denominate the larger body a satellite to the smaller ; but with regard to affording reflected light, the earth is to the moon what the moon is to the earth, and subject to the same changes of horned, gibbous, full, &c. C. But it must appear much larger than the moon. r. The earth will appear to the inhabitants of the moon about 13 times as large as the moon appears to us. When it is new moon to us it is full earth to them, and the reverse. J. Is the moon then inhabited as well as the earth ? T. Thdugh we cannot demonstrate this fact, yet 128 ASTRONOMY. there are many reasons to induce us to believe it ; for the moon is a secondary planet of considerable size ; —its surface is diversified like that of the earth with mountains and valleys : — the former have been mea- sured by Dr. Herschel, and some of them found to be about a mile in height. The situation of the moon, with respect to the sun, is much like that of the earth, and by a rotation on her axis, and a small inclination of that axis to the plane of her orbit, she enjoys, though not a considerable, yet an agreeable va- riety of day and night and of seasons. To the moon, our globe appears a capital satellite, undergoing the same changes of illumination as the moon does to the earth. The sun and stars rise and set there as they do here, and heavy bodies will fall on the moon as they do on the earth. Hence we are led to conclude that, like the earth, the moon also is inhabited. Dr. Herschel discovered some years ago three volcanoes, ail burning, in the moon ; but no large seas or tracks of water have been observed there, nor is the exist- ence of a lunar atmosphere certain. Therefore, her inhabitants must materially differ from those who live upon the earth. CONVERSATION XV. OF ECLIPSES. C. Will you, sir, explain to us the nature and causes of eclipses ? T. I will, with great pleasure. You must observe, then, that eclipses depend upon this simple principle, that all opaque or dark bodies, when exposed to any light, and therefore to the light of the sun, cast a shadow behind them in an opposite direction. J. The earth being a body of this kind must cast a very large shadow on its side which is opposite to the sun. OF ECLIPSES. 129 T. It does : and an eclipse of the moon happens when the earth t passes between the sun s and the moon m, and it is occasioned by the earth's shadow being cast on the moon. C. When does this happen ? T, It is only when the moon is full, or in opposition, that it comes within the shadow of the earth. J. Eclipses of the moon, however, do not happen every time it is full 3 what is the reason of this ? T, Because the orbit of the moon does not coincide with the plane of the earth's orbit, but one half of it is ele- vated about five degrees and a third above it, and the other half is as much below it : and therefore, unless the full moon happen in or near one of the nodes, that is, in or near the points in which the two orbits intersect each other, she will pass above or below the shadov/ of the earth, in which case there can be no eclipse. C. What is the greatest distance from the node, at which an eclipse of the moon can happen ? T. There can be n-o eclipse if the moon, at the time when she is full, be more than 12 degrees from the node ; when she is within that distance, there will be a partial, or total eclipse, according as a part, or the whole disc or face of the moon falls within the earth's shadow. If the eclipse happen exactly when the moon is full in the node, it is called a central eclipse. J, I suppose the duration of the eclipse lasts all the tim.e that the moon is passing through the shadow. T. It does : and you observe that the shadow is considerably wider than the moon!s diameter, and therefore an eclipse of the moon lasts sometimes three I or four hours. The shadow also you perceive is of a II conical shape, and consequently, as the moon's orbit is an ellipse, and not a circle, the moon will, at differ- G 2 130 ASTRONOMY. ent times, be eclipsed when she is at different dis- tances from the earth. C. And according as the moon is nearer to, or farther from the earth, the eclipse will be of a greater or less duration ; for the shadow being conical be- comes less and less, as the distance from the body by which it is cast is greater. r. It is by knowing exactly at what distance the moon is from the earth, and of course the width of the earth's shadow at that distance, that all eclipses are calculated, with the greatest accuracy, for many years before they happen. Now, it is found that in all eclipses the shadow of the earth is conical, which is a demonstration, that the body by which it is pro- jected is of a spherical form, for no other sort of figure would, in all positions, cast a conical shadow. This is mentioned as another proof, that the earth is a spherical body, J. It seems to me to prove another thing, viz. that the sun must be a larger body than the earm. T. Your conclusion is just, for if the two bodies were equal to one another the shadow would be Fig. 13. Fig. 14. cylindrical ; and if the earth were the larger body, its shadow would be the figure of a cone, which had lost its vertex, and the farther it were extended the larger would it become. In either case the shadow would run out to an infinite space, and accordingly must sometimes involve in it the other planets, and eclipse them, which is contrary to fact. Therefore, since the earth is neither larger than, nor equal to, the sun, it must be the lesser body. — We will now proceed to the eclipses of the sun, C. How are these occasioned ? OF ECLIPSES, 13X T. An eclipse of the sun happens when the moon M, passing between the sun s and the earth T, intercepts the sun's light. J. The sun then can be eclipsed only at the new moon ? T, Certainly ; for it is only when the moon is in conjunction, that it can pass directly between the sun and earth. C. Is it only when the moon at her conjunction is near one of its nodes, that there can be an eclipse of the sun ? T. An eclipse of the sun depends upon this circum- stance : for unless the moon is in, or near, one of its nodes, she cannot appear in the same plane with the sun, or seem to pass over his disc. In every other part of the orbit she will appear above or below the sun. If the moon be in one of the nodes she will, in most cases, cover the whole disc of the sun, and pro- duce a total eclipse; if she be any where within about 16 degrees of a node, a partial eclipse will be produced. The sun's diameter is supposed to be divided into 12 equal parts called digits, and in every partial eclipse, as many of these parts of the sun's diameter as the moon covers, so many digits are said to be eclipsed. J. I have heard of annular eclipses; what are they, sir? T. When a ring of light appears round the edge of the moon during an eclipse of the sun, it is said to be annular, from the Latin word annulus, a ring ; these kind of eclipses are occasioned by the moon being at her greatest distance from the earth at the time of an eclipse, because, in that situation, the vertex or tip of the cone of the moon's shadow does not reach the surface of the earth. I C. How long can an eclipse of the sun last ? 132 ASTRONOMY. T. A total eclipse of tlie sun is a very curious and uncommon spectacle ; and total darkness cannot last more than three or four minutes. Of one that was observed in Portugal 180 years ago, it is said that the darkness was greater than that of the night ; — that stars of the first magnitude made their appearance ; — and that the birds were so terrified that they fell to the ground. J. Was this visible only at Portugal ? T. It must have been seen at other places, though we have no account of it. The moon being a body much smaller than the earth, and having also a coni- cal shadow, can with that shadow only cover a small part of the earth ; whereas an eclipse of the moon may be seen by all those that are on that hemisphere which is turned towards it. (Figs. 15. and 12.) You will also observe, that an eclipse of the sun may be total to the inhabitants near the middle of the earth's disc, and annular to those in places near the edges of the disc ; for in the former case the moon's shadow will reach the earth, and in the latter, on account of the earth's sphericity, it will not. C. Have not eclipses been esteemed as omens pre- saging some direful calamity ? r. Till the causes of these appearances were dis- covered, they were generally beheld with terror by the inhabitants of the world. CONVERSATION XVI. OF THE TIDES. T. We will proceed to the consideration of the Tides, or the flowing and ebbing of the ocean. J. Is this subject connected with astronomy ? r. It is, inasmuch as the tides are occasioned by the attraction of the sun and moon upon the waters, but more particularly by that of the latter. You will readily conceive that the tides are dependent upon some known and determinate laws, because, if you turn to the Ephemeris, or indeed to almost any almanac, OF THE TIDES. 133 you will see that the exact tune of high water at London-bridge on the morning and afternoon of every day in the year is set down. C. I have frequently wondered how this could be known with such a degree of accuracy : indeed there is not a waterman that plies at the stairs but can readily tell when it will be high water. r. The generality of the watermen are probably as ignorant as yourself of the cause by which the waters flow and ebb ; but by experience they know that the time of high water diflfers on each day about three quarters of an hour, or a little more or less, and there- fore, if it be high water to-day at six o'clock, they will, at a guess, tell you, that to-morrow the tide will not be up till a quarter before seven. /. Will you explain the causes ? r. I will endeavour to do this in an easy and con- cise manner, without fatiguing your memory with a great variety of particulars. You must bear in your mind, then, that the tides are occasioned by the attraction of the sun and moon upon the waters of the earth : perhaps a figure may be Fig. 16. of some assistance to you. Let a^ln be supposed the earth, c its centre ; let the dotted circle represent a mass of water covering the earth : let m be the moon in its orbit ; and s the sun. Since the force of gravity or attraction diminishes as the squares of the distances increase,* the waters * See Mechanics, Conversation VIL 134 ASTRONOMY. on the side a are more attracted by the moon m, than the central parts at c ; and the central parts are more attracted than the waters at / ; consequently the waters at d will recede from the centre ; there- fore, while the moon is in the situation m, the waters will rise towards h and d on the opposite sides of the earth. C. You mean that the waters will rise at d by the immediate attraction of the moon n?, and will rise at h, by the centre c receding and leaving them more elevated there, r. That is the explanation. It is evident that the quantity of water being the same, a rise cannot take place at b and d, without the parts at e and / being at the same time depressed. J. In this situation the water may be considered as partaking of an oval form. T. If the earth and moon were without motion, and the earth covered all over with water, the attrac- tion of the moon would raise it up in a heap in that part of the ocean to which the moon is vertical, and there it would always continue ; but, by the ro- tation of the earth on its axis, each part of its surface to which the moon is vertical is presented twice a day to the action of the moon, and thus are produced two floods and two ebbs. C. How twice a day 1 T. In the position of the earth and moon as it is in our figure, the waters are raised at d by the direct attraction of the moon, and a tide is accordingly produced : but when, by the earth's rotation, a comes, 12 hours afterwards, into the position /, another tide is occasioned by the receding of the waters there from the centre. J. You have told us that the tides are produced in those parts of the earth to which the moon is verti- cal ; but this effect is not confined to those parts ? T. It is not ; but there the attraction of the moon has the greatest eflfect ; in all other parts the force is weaker, because it acts in a more oblique direction. OF THE TIDES. 135 C. Are there two tides in every 24 hours ? T, If the moon were stationary this would be the case ; but because that body is also proceeding every day about 13 degrees from west to east in her orbit, the earth must make more than one revolution on its axis before the same meridian is in conjunction with the moon, and hence two tides take place in about 24 hours and 50 minutes. J. But I remember when we were at the sea, that the tides rose higher at some seasons than at others : how do you account for this 1 T. The moon goes round the earth in an elliptic orbit, and therefore she approaches nearer to the earth in some parts of her orbit than in others. When she is nearest, the attraction is the strongest, and cori- sequently it raises the tides most : and when she is farthest from the earth, her attraction is the least, and the tides the lowest. /. Do they rise to different heights in different places 1 T. They do : in the Black Sea and the Medi- terranean the tides are scarcely perceptible. At the mouth of the Indus the water rises and falls full 30 feet. The tides are remarkably high on the coasts of Malay, in the Straits of Punda, in the Ked Sea, along the coasts of China, Japan, &c. In general, the tides rise highest and strongest in those places that are narrowest. C. You said the sun's attraction occasioned tides as well as that of the moon. r. It does ; but, owing to the immense distance of the sun from the earth, it produces but a small effect in comparison of the moon's attraction. Sir Isaac Newton computed that the force of the moon raised the waters in the great ocean 10 feet, whereas that of the sun raised it only 2 feet. When both the attrac- tions of the sun and moon act in the same direction, that is at new and full moon, the combined forces of both raise the tide 12 feet. But when the moon is in her quarters, the attraction of one of these bodies 136 ASTRONOMY. raises the water, while that of the other depresses it ; and therefore the smaller force of the sun must be subtracted from that of the moon, consequently the tides will be no more than 8 feet. The highest tides are called spring tides, and the lowest are denominated neap tides. J. I understand that, in the former case, the height to which the tides are raised must be calculated by adding together the attractions of the sun and moon , and in the latter, it must be estimated by the differ- ence of these attractions. T, You are right. When the sun and moon are both vertical to the equator of the earth, and the moon at her least distance from the earth, then the tides are highest. C. Do the highest tides happen at the equinoxes 1 T, Strictly speaking, these tides do not happen till some little time after, because in this, as in other cases, the actions do not produce the greatest effect when they are at the strongest, but some time after- wards ; thus the hottest part of the day is not when the sun is on the meridian, but between two and four o'clock in the afternoon. — Another circumstance must be taken into consideration : the sun being nearer to the earth in winter than in summer, it is of course nearer to it in February and October, than in IMarch and September ; and therefore, all these things being *put together, it will be found that the greatest tides happen a little before the vernal, and some time after the autumnal, Equinoxes. As the tides are more affected by the attraction of the moon than by that of the sun, the magnitude of the tides varies with the distance of the moon from the earth ; the attraction is also greatest when she is in her perigee, or nearest the earth ; and it is least when she is in her apogee, or the point farthest from the earth. OF THE HARVEST MOON. 137 CONVERSATION XVII. OF THE HARVEST MOON. T. From what we said yesterday, you will easily understand the reason why the moon rises about three quarters of an hour later every day than on the one preceding. C. It is owing to the daily progress which the moon is making in her orbit, on which account any meridian on the earth must make more than one complete rota- tion on its axis, before it comes again into the same situation with respect to the moon that it had before. And you told us that this occasioned a difference of about 50 minutes. ' T. At the equator this is generally the difFerence of time between the rising of the moon on one day and the preceding. But in places of considerable latitude, as that in which we live, there is a remarkable ' dilference about the time of harvest, when at the sea- son of full moon she rises for several nights together only about 20 minutes later on the one day than on i that immediately preceding. By thus succeeding the 1} sun before the twilight is ended, the moon prolongs { the light, to the great benefit of those who are engaged in gathering in the fruits of the earth ; and heifte the full moon at this season is called the harvest moon. ! It is believed that this was observed by persons en- gaged in agriculture, at a much earlier period than it ; was noticed by astronomers ; the former ascribed it to the goodness of the Deity, not doubting but that I he had so ordered it on purpose for their advantage. J. But the people at the equator do not enjoy this benefit. i T. Nor is it necessary that they should, for in : those parts of the earth the seasons vary but little, and the weather changes but seldom, and at stated i times ; to them, then, moon-light is not wanting for i gathering the fruits of the earth. 138 ASTRONOMY. C. Can you explain how it happens, that the moon at this season of the year rises one day after another with so small a difference of time ? r. With the assistance of a globe I could at once clear the matter up. But I will endeavour to give you a general idea of the subject without that instru- ment. That the moon loses more time in her risings when she is in one part of her orbit, and less in another, is occasioned by the moon's orbit lying some times more oblique to the horizon than at others. J. But the moon's path is not marked on the globe. T. It is not ] you may, however, consider it, with- out much error, as coinciding with the ecliptic. And in the latitude of London, as much of the ecliptic rises about Pisces and Aries in two hours as the moon goes through in six days ; therefore, while the moon is in these signs she differs but two hours in rising for six days together ; that is, one day with another, about 20 minutes later every day than on the preceding. C. Is the moon in those signs at the time of harvest ? r. In August and September you know that the sun appears in Virgo and Libra, and, of course, when the moon is full, she must be in the opposite signs, viz. Pisces and Aries, C. Will you explain, sir, how it is that the people at the equator have no harvest moon ? T. At the equator, the north and south poles lie in the horizon, and therefore the ecliptic makes the same angle southward with the horizon when Aries rises, as it does northward when Libra rises ; but as the har- vest moon depends upon the different angles at which different parts of the ecliptic rises, it is evident tliere can be no harvest moon at the equator. The farther any place is from the equator, if it be not beyond the polar circles, the angle which the ecliptic makes with the horizon, when Pisces and Aries rise, gradually diminishes, and therefore when the moon is in these signs she rises with a nearly proportionable diliercnce later every day than on the OF THE HARVEST MOON. 139 former, and this is more remarkable about the time of full moon. J. Why have you excepted the space on the globe beyond the polar circles ? At the polar circles, when the sun touches the summer tropic he continues 24 hours above the hori^ zon, and 24 hours below it when he touches the winter tropic. For the same reason the full moon neither rises in the summer, when she is not wanted, nor sets in the winter, when her presence is so neces- sary. These are the only two full moons which hap- pen about the tropics ; for all the others rise and set. In summer the full moons are low, and their stay above the horizon short : in winter they are high, and stay long above the horizon. A wonderful display this of the divine wisdom and goodness, in apportion- ing the quantity of light suitable to the various neces- sities of the inhabitants of the earth, according to their different situations. C. At the poles, the matter is, I suppose, still dil- ferent. r. There one half of the ecliptic never sets and the other half never rises ; consequently the sun con- tinues one half year above the horizon, and the other half below it. The full moon being always opposite to the sun can never be seen to the inhabhants of the poles, while the sun is above the horizon. But all the time that the sun is below the horizon, the ful moons never set. Consequently to them the full moon is never visible in their summer ; and in their winter they have her always before and after the full, shining for 14 of our days and nights without inter- mission. And when the sun is depressed the lowest under the horizon, then the moon ascends with her highest altitude. J. This indeed exhibits in a high degree the atten- tion of Providence to all his creatures. But if I understand you, the inhabitants of the poles have ill their winter a fortnight's light and darkness by turns ? 140 ASTRONOMY. T. This would be the case for the whole six months that the sun is below the horizon, if there were no refraction,* and no substitute for the light of the moon. But by the atmosphere's refracting the sun's rays, he becomes visible a fortnight sooner, and continues a fortnight longer in sight, than he would otherwise do were there no such property belonging to the atmosphere. And in those parts of the winter, when it would be absolutely dark in the absence of the moon, the brilliancy of the Aurora Boreatis is probably so great as to afford a very comfortable de- gree of light. Mr. Hearne, in his travels near the polar circle, has this remark in his journal : " De- cember 24. The days were so short, that the sun only took a circuit of a few points of the compass above the horizon, and did not at its greatest altitude rise half way up the trees. The brilliancy of the Aurora Borealis, however, and of the stars, even without the assistance of the moon, made amends for this deficiency, for it was frequently so light all night, that I could see to read a small print." CONVERSATION XVIII. OF MERCURY. T. Having fully described the earth and the moon, the former a primary planet, and the latter its at- tendant satellite, or secondary planet, we shall next consider the other planets in their order, with which, however, we are less interested. Mercury, yon recollect, is the planet nearest the sun ; and Venus is the second in order. These are called inferior planets. C. Why are they thus denominated? T. Because they both revolve in orbits which are included ivithin that of the earth ; thus (Fig. 2. p. 93.) * The subject of refraction will be very particularly explained when we come to Optics. OF MERCURY. 141 Mercury makes bis annual journey round the sun in the orbit a ; Venus in l>, and the earth, farther from that luminary than either of them, makes its circuit in t. J. How is this known 1 T. By observation : for by attentively watching the progress of these bodies, it is found that they are continually changing their places among the fixed stars, and that they are never seen in opposition to the sun, that is, they are never seen in the western side of the heavens in the morning when he appears in the east ; nor in the eastern part of the heavens in the evening when the sun appears in the west. C. Then they may be considered as attendants upon the sun 7 T. They may : Mercury is never seen from the earth at a greater distance from the sun than about 28 degrees, or about as far as the moon appears to be from' the sun on the second day after its change ; hence it is that we so seldom see him ; and when we do, it is for so short a time, and always in twilight, that sufficient observations have not been made to ascertain whether he has a diurnal motion on his axis. J. Would you then conclude that he has such a motion 1 T. i think we ought ; because it is known to exist in all those planets upon which observations of suf- ficient extent have been made, and therefore we may surely infer, without much chance of error, that it be- longs also to Mercury, and the Hevschel planet ; the former from its vicinity to the sun, and the latter from its great distance from that body, having at present eluded the investigation of the most indefati- gable astronomers. C. At what distance is Mercury from, the sun ? T. He revolves round that body at about 37 mil- lions of miles distance, in 88 days nearly ; and there- fore you can now tell me how many miles he travels in an hour. J. I can ; for supposing his orbit circular, I must 142 ASTRONOMY. multiply the 37 millions by 6,* which will give 222 millions of miles for the length of his orbit ; this I shall divide by 88, the number of days he takes in per- forming his journey, and the quotient resulting from this must be divided by 24, for the number of hours in a day ; and by these operations I find, that Mercury travels at the rate of more than 105,000 miles in an hour. C. How large is Mercury ? T. He is the sm.allest of all the planets. His dia- meter is something more than 3200 miles in length. J, His situation being so much nearer to the sun than ours, he must enjoy a considerably greater share of its heat and light. T. So much so, as would indeed infallibly burn every thing belonging to the earth to atoms, were she similarly situated. The heat of the sun at Mercury, must be 7 times greater than our summer heat. C. And do you imagine that, thus circumstanced, this planet can be inhabited ? T. N ot by such beings as we are ; you and I could not long exist at the bottom of the sea ; yet the sea is the habitation of millions of living creatures; why then may there not be inhabitants in Mercury, fitted for the enjoyment of the situation which that planet is calculated to afford ? If there be not, we must be at a loss to know why such a body was formed ; certamly it could not be intended for our benefit, for it is rarely even seen by us. CONVERSATION XIX. OF VENUS. T. We now proceed to Venus, the second planet in the order of the solar system, but by far the most beautiful of them all. /. How far is Venus from the sun ? * Sec Conver. VU. Ae>troiiomy. OF VENUS. 143 T, That planet is 68 millions of miles from the sun, and she finishes her journey in 224| days, conse- quently she must travel at the rate of 75,000 miles m an hour. C. Venus is larger than Mercury, I dare say. T. Yes, she is nearly as large as the earth, which she resembles also in other respects, her diameter bemg about 7700 miles in length, and she has a rotation about her axis in 23 hours and 20 minutes. The quantity of light and heat which she enjoys from the sun, must be double that which is experienced by the inhabitants of this globe. J. Is there also a difference m her seasons, as there is here 1 T. Yes, in a much more considerable degree. The axis of Venus inclines about 75 degrees, but that of the earth inclines only 23 1 degrees, and as the variety of the seasons in every planet depends on the degree of the inclination of its axis, it is evident that the seasons must vary more with Venus than with us. C. Venus appears to us larger sometimes than at others. r. She does; and this, with other particulars, I will explain by means of a figure. Suppose s to be Fig. 17. the sun, t the earth in her orbit, and a, h, c, d, /, 14i ASTRONOMY. V enus in her's : now it, is evident, that when Venus is at a between the sun and earth, she wouhi, if visible, appear much larger than when she is at d, in oppo- sition. J. That is because she is so much nearer in the former case than in the latter, being in the situation a but 27 millions of miles from the earth t, but at d she is 163 millions of miles off. T, Now as Venus passes from a, through h, c, to d, she may be observed, by means of a good telescope, to have all the same phases as the moon has in pass- ing from new to full ; therefore, when she is at d she is full, and is seen among the fixed stars ; during her journey from d to e, she proceeds with a direct motion in her orbit, and at e she will appear to an inhabitant of the earth, for a few days, to be stationary, not seem- ing to change her place among the fixed stars, for she is coming toward the earth in a direct line : but in passing from e to/, though still with a direct motion, yet, to a spectator at t, her course will seem to be back again, or retrograde, for she will seem to have gone back from x to y ; her path will appear retro- grade till she gets to c, when she will again appear stationary, and afterwards, from c to d, and from d to e, it will be direct among the fixed stars. C. When is Venus an evening and when a morn- ing star 1 T, She is an evening star all the while she appears east of the sun, and a morning star while she is seta west of him. When she is at a she will be mvi^-ib'e, her dark side being towards us, unless she be exactly in the node, in which case she will pass over the sun's face like a little black spot. J. Is that called the transit of Venus 1 r. It is ; and it happens twice only in about 120 years. By this phenomenon astronomers have been enabled to ascertain with great accuracy the distance of the earth from the sun j and, having obtained this, the distances of the other planets are easily found. By the two transits which happened in 1761, and OF VENUS.' 145 1769, it was clearly demonstrated, that the mean dis- tance of the earth from the sun was between 95 and 96 millions of miles. The next transit of Venus will be in 1874. C. How do you find the distances of the other planets from the sun, by knowing that of the earth 1* T. 1 will endeavour to make this plain to you. Kepler, a great astronomer, discovered that all the planets are subject to one general law, which is, that the squares of their periodical times are proportional to the cubes of their distances from the sun, J. What do you mean by the periodical times ? T. I mean the times which the planets take m re- volving round the sun ; thus the periodical time of j the earth is 365| days ; that of Venus 224| days ; 1 that of Mercury 88 days. '> C. How then would you find the distance of Mer- i cury from the sun 1 i T. By the rule of three ; I would say, as the square of 365 days (the time which the earth takes in revolving about the sun) is to the square of 88 days i' (the time in which Mercury revolves about the sun), I so is the cube of 95 millions (the distance in miles of j the earth from the sun) to a fourth number. J, And is that fourth number the distance in miles of Mercury from the sun 1 T. No : you must extract the cube root of that number, and then you will have about 37 millions of miles for the answer, which is the true distance at which Mercury revolves about the sun. i * The remainder of this Conversation may be omitted t by those youn^ persons who are not ready in arithmetical operations. The author, however, knows from experi- i; ence, that children may, at a very early age, be brought i to understand these higher parts of arithmetic. H IIG ASTRONOMY. CONVERSATION XX. OF MARS. T. Next to Venus is the earth and her satellite the moon, but of these sufficient notice has already been taken, and therefore we shall pass on to the planet Mars, which is known in the heavens by a dusky red appearance. Mars, together with Jupiter, Saturn and the Herschel, are called superior planets, because the orbit of the earth is enclosed by their orbits. C. At what distance is Mars from the sun 1 T. About 144 millions of miles ; the length of his year is equal to 687 of our days, and therefore he travels at the rate of more than 53 thousand miles in an hour : his diurnal rotation on his- axis is performed in 24 hours and 39 minutes, which makes his figure tliat of an oblate spheroid. J. How is the diurnal motion of this planet dis- covered ? T. By means of a very large spot which is seen distinctly on his face, when he is in that part of his orbit which is opposite to the sun and earth. C. Is Mars as large as the earth 1 T. No : his diameter is only 4189 miles in length, which is but little more than half the length of the earth's diameter. And owing to his distance from the sun he will not enjoy one half of the light and heat which we enjoy. /. And yet, I believe, he has not the benefit of a moonl T. No moon has ever been discovered belonging either to Mercury, Venus, or Mars. C. Do the superior planets exhibit similar appear- ances of direct and retrograde motion to those of the inferior planets 1 T. They do : suppose s the sun ; a, h, d, /, ^, h OF MARS. Fig. 18. tlie earth in different parts of its orbit, and in Mars in his orbit. When the earth is at a, Mars will ap- pear among the fixed stars at :r.- when, by its annual motion, the earth has arrived at b, d, and /, respect- ively, the planet Mars will appear in the heavens at |: y, Zf and w : when the earth has advanced to g, I Mars will appear stationary at o : to the earth in its j; journey from g to h the planet will seem to go back- I wards or retrograde in the heavens from o to z, and j this retrograde motion will be apparent till the earth I has arrived at a, when the planet will again appear j stationary. ! /. I perceive that Mars is retrograde when in op- I position, and the same is I suppose applicable to the f other superior planets, but the retrograde motion of Mercury and Venus is when those planets are in conjunction. T, You are right ; and you see the reason, T dare I say, why the superior planets may be in the west in the morning when the sun rises in the east, and the i reverse. j C. For when the earth is at Mars may be at n, I in which case the earth is between the sun and the I planet: I observe also that the planet Mars, and 148 ASTRONOMY- consequently the otlier superior planets, are much nearer the earth at one time than at others. r. The difference with respect to Mars is no less than 190 millions of miles, the whole length of the orbit of the earth. This will be a proper time to ex- plain what is meant by the heliocentric longitude of the planets referred to in the Ephemeris. J. Yes, I remember you promised to explain this when you came to speak of the planets : I do not know the meaning of the word heliocentric. T. It is a term used to express the place of any heavenly body as seen from the sun ; whereas the geocentric place of a planet, is the position which it has when seen from the earth. C. Will you shew us by a figure in what this dif- ference consists 1 T, I will : let s repre- sent the place of the sun, b Venus in its orbit, a the earth in hers, and c Mars in his orbit, and the outermost circle will represent the sphere of fixed stars. Now to a spectator on the earth, a, Venus will appear among the fixed stars in the beginning of Scorpio, Fig. 19. but as viewed from the sun, she will be seen beyond the middle of Leo. Therefore the geocentric longitude of Venus will be in Scorpio, but her heliocentric longitude will be in Leo. Again, to a spectator at a, the planet Mars at c will appear among the fixed stars towards the end of the sign of Pisces ; but, as viewed from the sun, he will be seen at the beginning of the sign Aries : consequently, the geocentric longitude of Mars is in Pisces, but his heliocentric longitude is in Aries. OF JUPITER. 149 CONVERSATION XXI. OF JUPITER. T. We now come to Jupiter, the largest of all the planets, which is easily known by his peculiar mag- nitude and brilliancy c C. Is Jupiter larger than Venus 1 T. Though he does not appear so large, yet the magnitude of Venus bears but a very small proportion to that of Jupiter, whose diameter is 90,000 miles in length, consequently his bulk will exceed the bulk of Venus 1500 times: his distance from the sun is estimated at more than 490 millions of miles. J. Then he is Jive times farther from the sun than the earth ; and, consequently, as light and heat dimi- nish in the same proportion as the squares of the distances from the illuminating body increase, the in- habitants of Jupiter enjoy but a twenty-fifth part of the light and heat of the sun that we enjoy. T. Another thing remarkable in this planet is, that it revolves on its axis, which is perpendicular to its orbit, in 10 hours, and in consequence of this swift diurnal rotation, his equatorial diameter is 6000 miles greater than his polar diameter. C. Since, then, a variety in the seasons of a planet depends upon the inclination of the axis to its orbit, and since the axis of Jupiter has no inclination, there can be no difference in his seasons, nor any in the length of his days and nights. r. You are right ; his days and nights are always five hours each in length ; and at his equator, and its neighbourhood, there is a perpetual summer ; and an everlasting winter in the polar regions. J. What is the length of his years 1 T. It is equal to nearly 12 of ours, for he takes 11 years, 314 days, and 10 hours, to make a revolution round the sun, consequently he travels at the rate of more than 28,000 miles in an hour. 150 ASTRONOMY. This noble planet is accompanied with four satel- lites, which were first discovered by Galileo in 1610 ; they revolve about him at different distances, and in different periodical times ; the first in about 1 day and 18 hours : the second in 3 days 13 hours : the third in 7 days 3 hours : and the fourth in 16 days and 16 hours. C. And are these satellites, like our moon, subject to be eclipsed ? T, They are ; and these eclipses are of consider- able importance to astronomers, in ascertaining with accuracy the longitude of different places on the earth. By means of the eclipses of Jupiter's satellites, a method has been also obtained of demonstrating that the motion of light is progressive, and not instantane- ous, as was once supposed. Hence it is found, that the velocity of light is nearly 11,000 times greater than the velocity of the earth in its orbit, and more than a million of times greater that that of a ball issuing from a cannon. Kays of light come from the sun to the earth in 8 minutes, that is, at the rate of about 12 millions of miles in a minute. CONVERSATION XXII. OF SATURN. T. We are now arrived at Saturn in our descrip- tions, which, till within these fifty years, was es- teemed the most remote planet of the solar system. C. How is he distinguished in the heavens ? T. He shines with a pale dead light, very unlike the brilliant Jupiter, yet his magnitude seems to vie with that of Jupiter himself. The diameter of Saturn is nearly 80 thousand miles in length : his distance from the sun is more than 900 millions of miles, and lie performs his journey round that luminary in a little less than 30 of our years, consequently he must OF SATURN. 151 travel at a rate not much short of 21,000 miles an hour. J. His great distance from the sun must render an abode on Saturn extremely cold and dark too, in comparison of what we experience here. r. His distance from the sun being between nine and ten times greater than that of the earth, he must enjoy about 90 times less light and heat; it has nevertheless been calculated that the light of the sun at Saturn is 500 times greater than what we enjoy from OMYfiili moon. C. The daylight at Saturn, then, cannot be very contemptible : I should hardly have thought that the light of'the sun here was 500 times greater tlmn that experienced from a full moon. T. So much greater is our meridian light than this, that during the sun's absence behind a cloud, when the light is much less strong than when we behold him in all his glorious splendour, it is reckoned that our daylight is 90,000 times greater than the light of the moon at its full. J. But Saturn has several moons, I believe 1 T. He is attended by seven satellites, or moons, whose periodical times differ very much ; the one nearest to him performs a revolution round the pri- mary planet in 22 hours and a half ; and that which is most remote takes 79 days and 7 hours for his monthly journey : this last satellite is known to turn on its axis, and in its rotation is subject to the same law which our moon obeys, that is, it revolves on its axis in the same time in which it revolves about the planet. Besides the seven moons, Saturn is encompassed with two broad rings, which are probably of con- siderable importance in reflecting the light of the sun to that planet : the breadth of the inner ring is 20,000 miles, that of the outer ring 7200 miles,^and the va- cant space between the two rings is 2839 miles. These rings give Saturn a very different appearance 152 ASTRONOMY. to any of the other pla- nets. This figure is a representation of Sa- turn, as seen through a good telescope. C. Does Saturn turn on its axis 1 T. According to Dr. Herschel it has a rota- tion about its axis in 12 hours 13| minutes : this he computed from the equatorial diameter being longer than the polar diameter in the proportion of 11 to 10. Dr. Herschel has also discovered, that the ring, ju&t mentioned, revolves about the planet in 10 hours and a J. He also considers that the ring is no less solid than the body of the planet itself ; and has observed that it casts a shadow upon the planet, and that the light of the ring is brighter than that of the planet. CONVERSATION XXIII. OF THE HERSCHEL PLANET, T. We have but the planet Herschel now to describe. J. Was it discovered by Dr. Herschel ? T. It was, on the 13th of March, 1781 ; and therefore by many astronomers it is denominated the Herschel planet ; though by the Doctor himself it was named the Georgium Sidus, or Georgian Star, in honour of his late Majesty George the Third, who was for many years a liberal patron to this great and most indefatigable astronomer : but by foreign astro- nomers it is usually called Uranus. C. I do not think that I have ever seen this planet. T. Its apparent diameter is too small to be dis- cerned readily by tlie naked eye, but it may be easily discovered in a clear night, when it is above the hori- zon, by means of a good telescope, its situation being previously known from the Ephemeris. J. Is it owing to the smallness of this planet, or to its great distance from the sun, that we cannot see it with the naked eye 1 OF THE HERSCHEL PtANET. 153 T. Both these causes are combined : in compari- son of Jupiter and Saturn it is small, his diameter being less than 35 thousand miles in length ; and his distance from the sun is estimated at more than 1800 millions of miles from that luminary, around which, however, he performs his journey in 84 of our years, consequently he must travel at the rate of 16,000 miles an hour. C. But if this planet has been discovered only 52 years, how is it known that it will complete its revo- tion in 84 years? T. By a long series of observations, it was found to move with such a velocity as would carry it round the heavens in that period. And it has been ascer- tained, that all the computations of its places, con- ducted upon that supposition, are correct. J. How many moons has the Herschel ? T. He is attended by six satellites or moons, of which, the one nearest to the planet performs his revolution round the primary in 5 days and 25 hours, but that which is the most remote from him takes 107 days and 16 hours for his journey. C. Is there any idea formed as to the light and heat enjoyed by this planet 1 T. His distance from the sun is 19 times greater than that of the earth ; consequently, since the square of 19 is 361, the light and heat experienced by the inhabitants of that planet must be 361 times less than ive derive from the rays of the sun. The proportion of light enjoyed by the Herschel has been estimated at about equal to the effect of 248 of our full moons. The following Synopsis presents, at one view, the periods, distances, and magnitudes, of all the planets^ including the four small ones lately discovered : — 112 354 ASTRONOMY. Eartl 1 — 'eal greatei ba times •tenth and 3- o o CO r— 1 o [ETS, 1,448; ^ ^ ^ toD bb bo CO CO O) !-< ^ . „ _ 02 (:Ncococ^r-ic;, by some of the ablest philosophers that ever lived. And it has been found that water v/ill find its v^^ay through the pores of gold, rather than suffer itself to be compressed into a smaller space. C. How did that happen ? F, At Florence, a celebrated city in Italy, a globe made of gold wdiS filled with water, and then closed so accurately that none of it could escape. The globe was then put into a press, and a little flattened at the sides : the consequence of which was, that the w^ater came through the fine pores of the golden globe, and stood upon its surface like drops of dew. C. Would not the globe, then, contain so much after its sides were bent in as it did before 1 F. It would not ; and as the water forced its way through the gold, rather than suffer itself to be brought into a smaller space than it naturally occu- pied, it was concluded at that time that water was in- compressible. Later experiments have, however, shewn, that those fluids which v/ere esteemed incom- pressible are, in a very small degree, as, perhaps, one part in twenty thousand, capable of compression. E. Is it on this account you conclude that the par- ticles are very hard ? F. Undoubtedly ; for, if they were not so, you can easily conceive, that since there are vacuities between them, as we have shewn, and as are represented in Fig. 1. they must by very great pressure be brought closer together, and vi^ould evidently occupy a less space, which is contrary to fact. CONVERSATION IT. OF THE WEIGHT AND PRESSURE OF FLUIDS. F. In our last conversation we considered the nature of the component parts of fluids : I must now tell you, that these parts or particles act, with respect to their weight or pressure independently of each other. OF THE PRESSURE OF FLUIDS. 167 E. Will you explain what you mean by this ? F, You recollect, that by the attraction of cohesion* the parts of all solid substances are kept together, and press into one common mass. If I cut a part of this wooden ruler away, the rest will remain in precisely the same situation as it was before. But if 1 take [some water out of the middle of a vessel, the remain- der flows instantly into the place from whence that was taken, so as to bring the whole mass to a level. C. Have the particles of water no attraction for each other 1 » F. Yes, in a slight degree. The globules of dewf on cabbage plants prove, that the particles of water have a greater attraction to one another, than they have to the leaf on which they stand. Nevertheless, ithis attraction is very small, and you can easily con- jCeive, that if the particles are round, they will touch |each other in very few parts, and slide with the smallest pressure. Imagine that a few of the little {globules were taken out of the vessel, (Fig. 1.) and jit is evident that the surrounding ones would fall into •their place. It is upon this principle that the surface iof every fluid, when at rest, is horizontal or level. C. Is it upon this principle that water levels are i constructed 'i F. It is : the most simple kind of water level is a long wooden trough, which being filled to a certain height with water, its surface shews the level of the place on which it stands. J C. I did not allude to this kind of levels, but to those smaller ones contained in glass tubes. I F. These are, more properly speaking, air-levels. [They are thus constructed : d is a „ ^ glass tube fixed into l, a socket made C^^^^^S") generally of brass. The glass is filled with water, or some other fluid. Fig. 2. in which is enclosed a single bubble * See Mechanics, Conver. III. t See Mechanics, Conver. IV. 1€8 HYDROSTATICS, of air. When this bubble fixes itself at the mark a, made exactly in the middle of the tube, the place on which the mstrument stands is perfectly level. When It IS not level, the bubble will rise to the higher end. E, What is the use of these levels ? F. They are fixed to a variety of philosophical in- slruments, such as quadrants, and telescopes for sur- veymg the heavens, and theodolites for takino- the level of any part of the earth. They are also itseful in the more common occurrences of life. A sino^le mstance will shew their value : clocks will not ke'ep true time unless they stand very upright ; now by means of one of these levels you may easily ascer- tam whether the bracket, upon which the clock in the passage stands, is level. E, But I remember when Mr. F brouo-ht home your clock, he tried if the bracket was even "by means of one of Charles's marbles. How did he know by this ? F, The marble, being round, touched the board m a point only, consequently the line of direction* could not fall through that point, but the marble would roll, unless the bracket was very level ; there- fore, when the marble was placed in two or more different parts of the board, and did not move to one side or the other, he might safely conclude that it was level. C. Then the water-level and the rolling of the marble depend on the same principle. F. They do, upon the supposition that the particles of water are round. The water-level will, however^ be the most accurate, because we may imagine that the parts of which water is composed are perfectly round, and, therefore, as may be geometricallv proved, they will touch only in an infinitely small point ; whereas, marbles made by human contrivance touch in many such points. We now come to another very curious principle in * See Mechanics, Coaver. IX. OF THE UPWARD PRESSURE. 1G9 tills branch of science, viz. that fluids press equally in all directions. All bodies, both fluid and solid, press liownwards by the force of gravitation, but fluids of all kinds exert a pressure upwards and sideways equal to their pressure downwards. -E. Can you shew any experiments in proof of this ? F. a, b, c, is a bended glass fube : with a small glass funnel pour into the mouth a a quan- :ity of sand. You will find .hat, when the bottom part is illed, whatever is poured in ifterwards will stand in the side Fig. 3, Fig. 4. Df the tube a 6, and not rise m Lhe other side b c. C. The reason of this is, that by the attraction of g'ravitation all bodies have a tendency to the earth,* hat is, in this case, to the lowest part of the tube ; 3ut, if the sand ascended in the side b c, its motion jVould be directly the reverse of this principle. F. You mean to say that the pressure would, be ipwards, or from the centre of the earth. C. It certainly would. |i F. Well, we will pour away the sand, and put ijvater in its place : what do you say to this ? I E. The water is level in both sides of the tube, i F. This then proves, that with respect to fluid&y [here is a pressure upwards at the point b as well as lownwards. I will shew you another experiment. A B is a large tube or jar having a flat bot- « om : a 6 is a smaller tube open at both ends. Jk While I fill the jar with water, I take care 0 hold the small tube so close to the bottom )f the jar as to prevent any water from get- ing into the tube. I then raise it a little, ind you see it is instantly filled with water ^ Tomthejar. * See Mechanics, Conver, Y. 1 170 HYDROSTATICS. C. It is; and the water is level in the jar and tube. F. The tube you saw was filled by means of the pressure upwards, contrary to its natural gravity. Take out the tube ; now the water having escaped, it is filled with air. Stop the upper end a with a cork, and plunge it into the jar, the water will only rise as high as h, E. What is the reason of this, papa ? F. The air with which the tube was filled is a body, and unless the water were first to force it out from the tube, it cannot take its place. While this ink- stand remains here, you are not able to put any other thing in the same part of space. C. If air be a substance, and the tube is filled with it, how can any water make its way into the tube I F. This is a very proper question. Air, though a substance, and, as we have already observed, a fluid too, differs from water in this respect, that it is easily compressible, that is, the air, wiiich, by the natural pressure of the surrounding atmosphere, fills the tube, may, by the additional upward pressure of the water, be reduced into a smaller space, as a b. Another experiment will illustrate the difference between com- pressible and incompressible fluids. Fill the tube, which has still a cork in one end, with some coloured liquor, as spirits of wine ; over the other end place a piece of pasteboard, held close to the tube, to prevent any of the liquor from escap- ing : in this way introduce the tube into a vessel of water, keeping it perpendicular all the time : you may now take away the pasteboard, and force the tube to any depth, but the spirit of wine is not like the air, it cannot in this manner be reduced into a space smaller than it originally occupied. E. Why did not the spirits of wine run out of the tube into the water ? F, Because spirits are lighter than water, and it is a general principle that tiie lighter fluid always as- cends to the top. Take a thin piece of horn or pasteboard, and while OF THE PRESSURE OF FLUIDS. 17i you hold it by the edges, let your brother put a i pound weight upon it : what is the result ? E. It is almost bent out of my hand. i F. Introduce it now into a vessel of water at the depth of twelve, or fifteen inches, and bring it parallel to the surface. In this position, it sustains many pounds' weight of water. I C. Nevertheless, it is not bent in the least. F. Because the upward pressure against the lower jsurface of the horn is exactly equal to the pressure down vizard, or, which is the same thing, it is equal to the weight of the water which it sustains on the upper surface ; in other words, fluids press equally in all [directions." You may vary these experiments by yourselves till we meet again j when we shall resume the same Isubject. I CONVERSATION III. I OF THE WEIGHT AND PRESSURE OF FLUIDS. i C. When you were explaining the principle of the iWheel and Axis,*' I asked the reason why, as the ^bucket ascended near the top of the well, the difficulty iiin raising it increased? 1 have just now found another ipart of the subject beyond my comprehension. After ;the bucket is hlled with water, it sinks to the bottom {of the well, or as far as the rope will suffer it ; but in jdrawing it up through the water, it seems to have ■little or no weight till it has ascended to the surface iOf the water. How is this accounted for 1 i F. 1 do not wonder that you have noticed this cir- icumstance as singular. It was long believed by the I ancients that water did not gravitate, or had no weight 'in water : or, as they used to express it more generally, ithat fluids do not gravitate in propria loco. 1 E. I do not understand the meaning of these hard I words. i * See Mechanics, Conver. XVII. 172 HYDROSTATICS. F, Nor would I have made use of them, only that you can scarcely open a treatise on this subject with- out finding the phrase. I will explain their meaning without translating the words, because a mere trans- lation would give you a very inadequate idea of what the writers intended to express by them. No one ever doubted that water and other fluids had weight when considered by themselves ; but it was supposed that they had no weight when immersed in a fluid of the same kind. The fact which your brother has just mentioned respecting the bucket was the grand argument upon which they advanced and maintained this doctrine. E. Does it not weigh any thing, then, till it is drawn above the surface ? F. You must, my little girl, have patience, and you shall see how it is. Here is a glass bottle A, with a stop-cock b cemented to it, by means of which the air may be exhausted from the bottle, and prevented from returning into it again. The whole is made suf- ficiently heavy to sink in the vessel of water c d . The bottle must be weighed in air, that is, in the common method, and suppose it weighs 12 ounces ; let it now be put into the situation which is represented by the figure, when the weight of the bottle must be again taken by putting weights into the scale z. I then open the stop-cock while it is under water, and the water immediately rushes in and fills the bottle, which overpowers the weights in the scale. I now put other weights, say 8 ounces, into the scale, to restore the equilibrium between the bottle and scale It is evident, then, that 8 ounces is the weight of th water in the bottle, while weighed under water Fasten the cock and weigh the bottle in the usu way in the air. C. It weighs something more than 20 ounces. F. That is 12 ounces for the bottle, and 8 ounc OF THE PRESSURE OF FLUIDS. 173 lor the water, besides a small allowance to be made for the drops of water that adhere to the outside of the bottle. Does not this experiment prove that the water in the bottle weighed just as much in the jar of water, as it weighed in the air ? jE. I think it does. F. Then we are justified in concluding that the water in the bucket, which the bottle may represent, weighed as much while under water in the well, as it did after it was raised above the surface. C. This fact seems decisive, but the difficulty still remains in my mind, for the weight of the bucket is not felt till it is rising above the surface of the water. F. It may be thus accounted for : any substance of the sam.e specific gravity with water, may be plunged into it, and it will remain wherever it is placed, either near the bottom, in the middle, or towards the top, consequently it may be moved in any direction with the application of a very small force. E. What do you mean by the specific gravity of a body ? ! F. The specific gravity of any body is its weight compared with that of any other body. Hence it is also called the comparative gravity : thus, if a cubic inch of water be equal in weight to a cubic inch of I any particular kind of wood, the specific or compara- tive gravities of the water and that wood are equal. But, since a cubic inch of deal is lighter than a cubic inch of water, and that is lighter than the same bulk of lead or brass, we say the specific gravity of the lead, or brass, is greater than that of water, and the specific gravity of water greater than that of deal. C. The water in the bucket must be of the same specific gravity with that in the well, because it is a part of it. jP. And the wooden bucket differs very little in this respect from the water ; because, though the wood is lighter, yet the iron of which the hoops and handle are composed is specifically heavier than 174 HYDROSTATICS, water ; so that the bucket and water are nearly of the same specific gravity with the water in the well, and therefore it is moved very easily through it. Again, we have already proved that the upward pressure of fluids is equal to the pressure downwards, therefore the pressure at the bottom of the bucket upwards being precisely equal to the same force in a contrary direction, the application of a very small force, in addition to the upward pressure, will cause the bucket to ascend. E. Do you account for the easy ascent of the bucket upon the same principle by which you have shewn that horn or pasteboard will not be bent, when placed horizontally at any depth in water 1 F, Yes, I do : and I will shew you some other experiments to prove the effect of the upward pressure. Take a glass tube, open at both ends, the diameter of which is about the eighth of an inch, fill it with water, and close the top with your thumb ; you may now take it out of the water, but it will not empty itself so long as the top is kept closed. C. This is not the upward pressure of water, be- cause the tube was taken out of it. F. You are right : it is the upward pressure of the air, which, while the thum.b is kept on the top, is not counterbalanced by any downward pressure, and therefore it keeps the water suspended in the tube. Take this ale-glass and fill it with water, and cover it with a piece of writing-paper : then place your hand evenly over the paper, so as to hold it very tight about the edge of the glass, which you may now invert, and take away your hand without any danger of the water falling out. E. Is the water sustained by the upward pressure of the air 1 F. The upward pressure of the air against the paper sustains the weight of water and prevents it from falling You have seen the instrument used for tasting of beer or wine ? OF THE PRESSURE OE FLUIDS. 1T5 E. Yes : it is a tin tube, that holds about half a i^int^ into which very small tubes are inserted at the iitop and bottom. , , , i r F. The longer one is put into the hole made tor the vent-peg, and then the beer or wine is, by draw- incr out the air from it, forced into the large part of the tube, and by putting the thumb or finger on the upper part, the whole instrument may be taken out ot the cask, and removed any where, for the pressure of the air against the bottom surface of the lower tube keeps the liquor from running out, but the moment the thumb is taken from the top, the liquor descends i,by the downward pressure of the air. C. Is it for a similar reason that vent-holes are made in casks 1 F. It is : for when a cask is full, there is no down- : ward pressure, and therefore the air pressing against ' the mouth of the cock keeps the liquor from runnmg ilout ; a hole made at the top of the cask admits the ; external pressure of the air, by which the liquor is ' forced out. In large casks of ale or porter, where ; the demand is not very great, the vent-hole need i seldom be used, for a certain portion of the air con- tained in the liquor escapes, and being lighter than I the beer, ascends to the top, by which a pressure is ' created without the assistance of the external air. CONVERSATION IV. OF THE LATERAL PRESSURE OF FLUIDS. F. It is time now to advance another step in this science, and to shew you that the lateral or side pres- sure is equal to the perpendicular pressure. E. If the upward pressure is equal to the down- ward, and the side pressure is also equal to it, then the pressure is equal in all directions. F. You are right. Though the side direction may be varied in many ways, yet there are only the up- HYDROSTATICS, ward, downward, and lateral directions. The two lormer we have shewn are equal. That the side pres- sure is equal to the perpendicular pressure downwards IS demonstrated by a very easy experiment. AB IS a vessel filled with water, havino- a~ two equal orifices or holes, ah, bored with the same tool, one at the side, and the other in c the bottom ; if these holes are opened at the same instant, and the water suffered to run -MJ^ into two glasses, it will be found that, at the x^/ '* ^ end of a given time, they will have discharged ^' equal quantities of water ; which is a clea^r proof that w^rds^^^"" P'^^^^s side-wise as forcibly as it does down- *u ^^^"^ ^^^^ a general principle that fluids press in every possible direction ? F. This I think our experiments have proved . but you must not forget that it is only true upon the sup- position that the perpendicular heights are equal. For m the last experiment, if the hole h had been bored an inch or two higher in the side of the vessel, as at c, the quantity of water running out at a would have been greater than that at h, and much greater would It have been if the hole had been bored at four or hve inches above the bottom of the vessel This subject of pressure may be farther illustrated. A.t the bottom of this tube nv, open at both ends, 1 have tied a piece of bladder, and have poured in water till it stands at the mark r. Owing to the pressure of the water, the bladder is convex, that is, bent outwards • ' dip It mto thejar, (Fig, 5. p. 169.) theblad' . der is stdl convex : thrust it gently down, the surface of the water in the tube is now even with that in thejar. ^^E. It is ; and the bladder at the bottom is become F. The perpendicular depths being equal the pressure upwards is equal to that downwards, and the OF THE PRESSURE OF FLUIDS. m water in the tube is exactly balanced by the water in the jar. Let the tube be thrust deeper into tlie water. C. Now the bladder is bent upwards. F. The upward pressure is estimated by the per- pendicular depth of the water in the jar, measured from the surface to the bottom of the tube ; but the pressure downwards must be estimated by the per- pendicular height of the water in the tube, which being less than the former, the pressure upwards m the same proportion overcomes that downwards, and forces up the bladder into the position as you see it. This and the following experiment are some of the best that can be exhibited in proof of the upward pressure of fluids. Dip an open end of a tube, having a very narrow bore, into a vessel of quicksilver ; then stopping the upper orifice with the finger, lift up the tube out of the vessel, and you will see a sort of column of quick- silver hanging at the lower end, which, when dipped in water lower than fourteen times its own length, will, upon removing the finger, be pressed upwards into the tube. E. Why do you fix upon 14 times the depth ? F. Because quicksilver is 14 times heavier than water. Upon this principle of the upward pressure, lead or any other metal may be made to swim in water, ab is a vessel of water, and ab is a glass tube open throughout, d is string by "^j^j^' which a flat piece of lead x may be held ""^ — ^ fast to the bottom of the tube. To prevent the water from getting in between the lead and the glass, a piece of wet leather is first put over the lead. In this situation, let the tube be immersed in the vessel of water, and if it be plunged to the depth of about eleven times the thickness of the lead before the string be let go, the lead will not fall from the tube, but be kept adhering to it by the upward pressure below it. I 2 178 HYDROSTATICS. E. Is lead 11 times heavier than water? F. It is between 11 and 12 times heavier ; and therefore to make the experiment sure, the tube should be plunged somewhat deeper than 11 times the thick- ness of the lead. C. Is it not owing to the wet leather that the lead sticks to the tube, rather than to the upward pres- sure ? F. If that be the case, it will remain fixed if I draw up the tube an inch or two higher : — I will try it. E. It has fallen off. F. Because, when the tube was raised, the up- ward pressure was diminished so much as to become too small to balance the weight of the lead. But if the adhering together of the lead and tube had been caused by the leather, there would be no reason why it should not operate the same at six or nine times the depth of the lead's thickness as well as at 11 or 12 times that thickness. CONVERSATION V. OF THE IIYDROSTATICAL PARADOX. E. You are to explain a paradox to-day : I thought natural philosophy had excluded all paradoxes. F. Dr. Johnson has given this definition of a para- dox, an assertion contrary to appearances ;" now the assertion to which I am to refer you is, that any Qiuutiilii of water, however small, may be made to ba- UDice and support any quantity, hoicever lari^e. That a pound of water, for instance, should, without any mechanical advantage, be made to support ten pounds, or a hundred, or even a ton weight, seems at first in- credible ; certainly it is contrary to what one should expect, and on that account the experiment to shew this fact has usually been called the hydrostatical paradox. HYDROSTATIC PARADOX. 179 C. It does appear unaccountable : I hope the ex- periments may be very easy to be understood. F. Many have been invented for the purpose, but I know of none better than those described by Mr. Ferguson in his Lectures on Select Subjects. OEGH is a glass vessel, consisting of two tubes of very different sizes, joined together, and ^ ^ freely communicating with one another. ^ J // Let water be poured in at h, which will H ' pass through the joining of the tubes, and "7/ rise in the wide one to the same height ex- / ^' actly as it stands in the smaller ; which shews that the small column of water in -p-^ DG balances the large one in the other tube. °' This will be the case if the quantity of water in the small tube be a thousand or a million of times less than the quantity in the larger one. If the smaller tube be bent in any oblique situa- tion, as GF, the water v/ill stand at f, that is, on the same level as it stands at a. This would be the case, if instead of two tubes there were any given number of them connected together at b, and varied in all kinds of oblique directions, the water would be on a level in them all; that is, the iper-pendicidur height of the water would be the same. C. This does not quite satisfy me, because it ap- pears that a great part of the water in the large tube is supported by the parts b about the bottom, and therefore that the water in the smaller tube only sus- tains the pressure of a column of water, the diameter of which is equal to its own diameter. -F. This would be the case if the pressure of fluids was only downwards, but we have shewn that it acts in all directions. And therefore the pressure of the parts near the side of the tube acts against the column in the middle, which you suppose is the only part of the water sustained by that which is contained in the small tube; consequently the smaller quantity of v/ater in db sustains the larger one in ab. 180 HYDROSTATICS. Fi^. 11 Let us try another experiment. AB and AB are two ves- sels, having their bottoms ■Dd and i>d exactly equal, but the contents of one vessel is 20 times greater than the other ; that is, Fig. 11, when filled up to A, will hold but one pint of v/ater, whereas Fig. 12.. when filled to the same height, will hold 20 pints. Brass bottoms, cc, are fitted exactly to each vessel, and made water tight by pieces of wet leather. Each bottom is joined to its vessel by a hinge d, so that it opens downwards, like the lid of a box. By means of a little hook d, a pulley f, and a weight e, the bottom is kept close to the vessel, and will hold a certain quantity of water. E. That is, till the weight of the water overcome the weight e. F. I should rather say, till the pressure of the water overcome the weight e. Now hold the vessel (Fig. 12.) upright in your hands, while I gradually pour water into it with a funnel ; the pressure bears down the bottom, and, of course, raises the weight, and a small quantity of the water escapes. Let us mark the height h, at which the surface of the water stood in the vessel when the bottom began to give way. Try the other vessel (Fig. 11.) in the same manner, and we shall see that when the water rises to a, that is, to just the same height m this vessel as in the for- mer, the bottom will also give way, as it did in the other case. Thus equal weights are overcome in the one case by 20 pints of water, and in the other by a single pint. The same would hold good if tlie dif- ference was greater or less in any given proportion. E. What is the reason of this, papa ? F. It depends upon two principles with which you arc acquainted. The first is, that fluids press equally HYDROSTATIC PARADOX. 181 m all directions ; and the second is, that action and re-action are equal and contrary to each other.* The water, therefore, below the fixed part Bgf will press as much upward against the inner surface, by the ac- tion of the small column Ag, as it would by a column of the sinne height, and of any other diameter whatso- ever : and since action and re-action are equal and contrary, the action against the inner surface Bo-y will cause an equal re-action of the water in the cavity b/"cd against the bottom c ; consequently the pressure upon cc, Fig. 11., will be as great as it was upon the same part of Fig. 12. C. Can you prove by experiment that there is this upward pressure against the inner surface Bg f ? F. Very easily : suppose at f there were a little cork, to which a small string was fixed : I might place a tube over the cork, and then draw it out, the consequence of which would be, that the water in the vessel would force itself into the tube, and stand as high in it as it does in the vessel. Would not this experiment prove that there was this upward pres- sure against Bg/? C. It would : and I can easily conceive, that if other tubes were placed in the same manner, in dif- ferent parts of Bgf\ the same effect would be pro- duced. F, Then you must admit, that the action against Eg-/, or, which is the same thing, the re-action against c, that is, the pressure of the water against the bot- tom, is equally great as it would be if the vessel were as large in every part as it is at the bottom, and the water stood level to the height ^a. C. Yes, I do : because if tubes were placed in every part of b/, the same eflPect would be produced ia them all, as in the single one at / ,- but if the whole surface were covered with small tubes, there would then be little or no diflference between the two vessels. Figs. 11. and 12. * See Mechanics, Conver. XL 182 HYDROSTATICS. F. There would be no difference, provided you kept filling the large tube, so that the water should stand in them all at the same level Aa. Otherwise, the introduction of a single tube o/ would make a material difference. For though the water in ac vv'ould overcome the weight e, yet if with my hand I prevent any of the water from running out till I have taken out the cork, and suffered the water to force itself out of the vessel into the small tube, I may re- move my hand with safety ; for the water will not overcome the weight now, though there is certainly the same quantity of water in it as there was before the little tube a/ was inserted. E. I think I see the reason of this : the water stood as high as Aa before the little tube was introduced, but now it stands at the level xx, and you told us yester- day that the pressures were only equal, provided the perpendicular heights were also equal. F. I am glad to find you so attentive to what I say. In order that the pressure may overcome the weight E, you must put in more water till it rise to the level Aa, and now you see the weight rises, and the water flows out. I will now put another tube at ^, and the water rushing into that causes the level to descend again to XX, and I must put more water in to bring the level up to A, before it can overcome the weight e. What I have shewn in these two cases will hold true in all, supposing you fill the cover with tubes. C. I see, then, that it is the difference of the per- pendicular heights which causes the difference of pressure, and can now fully comprehend the reason why a pint of water may be made to balance or sup- port a hogshead : or, in the words with which you set out, that any quantity of water, however small, may he made to halance and support any other quantity, how- ever larcre. F. What has been proved respecting water holds with regard to wine, oil, or any other fluid : but not if diifcront fluids are used together, as water and oil. HYDROSTATIC BELLOWS. 183 CONVERSATION VI. OF THE HYDROSTATIC BELLOWS. F. I think we have made it sufficiently clear that the pressure of fluids of the same kind is always pro- portional to the area of the base multiplied into the perpendicular height at which the fluid stands, without any regard to the form of the vessel, or the quantity of fluid contained in it. E. 1 cannot help saying, that it still appears very mysterious to me that a pint of water (in Fig. 11.) should have an equal pressure with the 20 pints in the next vessel. You will not say that one pint weighs as much as the 20. F. Your objection is proper. The pressure of the water upon the bottom cc, does not in the least alter the weight of the vessel and water considered as one mass, for the action and re-action which cause the pressure, destroy one another with respect to the weight of the vessel, which is as much sustained by the action upwards as it is pressed by the re-action downwards. The pressure of water and other fluids differs from its gravity or weight in this respect : the weight is according to the quantity ; but the pressure is accord- ing to the perpendicular height. C. Suppose both vessels were filled with any solid substance, would the effect produced be very dif- ferent l F. If the water were changed into ice, for instance, the pressure upon the bottom of the smaller vessel would be much less than that upon the larger. Here is another instrument to shew you that a very few ounces of water will lift up and sustain a large weight. E, What is the instrument called? F. It is made like common bellows, only without valves, and writers have given it the name of the 184 HYDROS! A.TICS. hydrostatic bellows. This small tin pipe eo communicates with the inside of the bellows. At present the upper and lower board are kept close to one another with the weight w. The in- side of the boards are not very smooth, so that water may insinuate itself -p-^ between them: pour this half pint of ^' water into the tube. C. It has separated the boards and lifted up the weight. F. Thus you see that seven or eight ounces of water has raised and continues to sustain a weight of 561b. By diminishing the bore of the pipe, and in- creasing its length, the same or even a smaller quan- tity of water will raise a much larger weight. C. How do you find the weight that can be raised by this small quantity of water ? F. Fill the bellows with water, the boards of which, when distended, are three inches asunder : I will screw in the pipe. As there is no pressure upon the bellows, the water stands in the pipe at the same level with that in the bellows at z. Now place weights on the upper board till the water ascend exactly to the top of the pipe e : these weights express the weight of a pillar or column of water, the base of which is equal to the area of the lower board of the bellows, and the height equal to the distance of that board from the top of the pipe. E. Will you make the experiment ? F. Your brother shall first make the calculation. C. But I must look to you for assistance. F, You will require very httle of my help. "Mea- sure the diameter of the bellows, and the perpendi- cular height of the pipe from the bottom board. C. The bellows is circular and 12 inches in diame- ter ; the height of the pipe is 36 inches. F. Well ; you have to find the solid contents of a cylinder of these dimensions : that is, the area of the base multiplied by the height. HYDROSTATIC BELLOWS. 185 C. To find the area I multiply the square of 12 inches, that is 144, by the decimals .7854, and the product is 113, nearly the number of square inches m the area of the bottom board of the bellows. And 113 multiplied by 36 inches, the length of the pipe, gives 4068, the number of cubic inches in such a cy- linder ; this divided by 1728 (the number of cubic inches in a cubic foot) leaves a quotient of 2.3 cubic feet, the solid contents of the cylinder. Still I have not the weight of the water. F, The weight of pure water is equal in all parts of the known world, and a cubical foot of it weighs 1000 ounces. C. Then such a cylinder of water as we have been conversing about weighs 2300 ounces, or 144 pounds nearly. E. Let us now see if the experiment answers to Charles's calculation. F, Put the weights on carefully, or you will dash the water out at the top of the pipe, and I dare say that you will find the fact agrees with the theory. C. If instead of this pipe, one double the length was used, would the water sustain a double weight ] F. It would : and a pipe three or four times the length w^ould sustain three or four times greater weights. C, Are there then no limits to this kind of experi- ment except those which arise from the difficulty of acquiring length in the pipe ? F, The bursting of the bellows would soon deter- mine the limit of the experiment. Dr. Goldsmith says that he once saw a strong hogshead split by this means. A strong small tube made of tin, about 20 feet long, was cemented into the bung-hole, and then water was poured in to fill the cask ; when it was full and the water had risen to within about a foot of the top of the tube, the vessel burst with prodigious force. E. It is very difficult to conceive how this pressure acts with such power. F. The water at o is pressed with a force propor- iSo HYDROSTATICS, tional to the perpendicular altitude e o ; this pressure is communicated horizontally in the direction o and the pressure so communicated acts as you know equally in all directions : the pressure therefore down- wards upon the bottom of the bellows is just the same as it would heii p q n r were a cylinder of water. The experiment made on the bellows might, for want of such an instrument, be made by means of a bladder in a box with a moveable lid. E. TTas this property of hydrostatics been applied to any practical purposes ? F. The knowledge of it is of vast importance in the concerns of life. On this principle a press of im- mense power has been formed, (Conversation XX IT) which we shall describe after you are acquainted with the nature and structure of valves. It is used for the purpose of compressing soft substances, such as hay, cotton, wool, and other commodities, which it is neces- sary to transport on board of ships ; also soft manu- factured goods, as silks, cottons, woollen cloths, &c. CONVERSATION VII. OF THE PRESSURE OF FLUIDS AGAINST THE SIDES OF VESSELS. F. Do you recollect, Charles, the law by which you calculate the accelerated velocity of falling bodies ?* C. Yes ; the velocity increases in the same pro- portion as the odd numbers, 1, 3, 5, 7, 9, &c.; that is, if at the end of one second of time it has carried the body through 16 feet, then in the next second the body will descend three times 16, or 48 feet ; in the third it will descend five times 16 feet ; and in the next seven times 16 feet, and so on continually in- creasing in the same proportion. F. How many feet has it fallen altogether at the end of the third second ? * See Mechanics, Conver. VII. and VIII. OF THE PRESSURE OF FLUIDS. 187 E. I recollect this very well ; the whole space through which it will fall in three seconds is nine times 16, or 144 feet; because the rule is, that the whole spaces described by falling bodies are in pro- portion to the squares of the times, and the square of three is nine, therefore if it falls through 16 feet in the first second, it will in three seconds fall through nine times 16, and in five or eight seconds it will descend in the former case through 25 times 16 feet, and in the latter through 64 times 16 feet, for 25 is the square of five, and 64 is the square of eight. The example of the arrow which you gave me to work has fixed the rule in my mind. Well, then, what I am going to tell you will tend to impress the rule still stronger in your memory. The pressure of fluids against the sides of any vessel increases in the same proportion, and is governed by the same laws. Suppose a 6 c to be a cubical vessel filled with water or any other fluid, and one of the sides to be accurately divided into any number of equal parts by the lines 1, 7 ; 2, 8, &c. Now if the pressure of the v^^ater upon the part of the vessel a I b 1 he equal to an ounce or a pound, then the Fig. 14. pressure upon the part 1, 2, 7, 8, will be equal to three ounces or three pounds ; and the pres- sure upon the part 2, 3, 8, 9, will be equal to five ounces or pounds, and so on. C. Then I see the reason why the other part of the rule holds true, viz. that the pressure against the whole side must vary as the square of the depth of the vessel. F. Explain to us the reason. C. The pressure upon the^iVst part being 1, and that upon the second 3, and that upon the third 5, then the pressure upon the first and second taken together is by addition 4 : upon the first, second and third it 188 HYDROSTATICS. must be 9 ; and upon the first, second, third and fourtli, it will be 16; but 4, 9, 16 are the squares of 2, 3, 4. E. And the pressure upon the whole side abed must be 36 times greater than that upon the small part a\b 1 , C. And if there are three vessels, for instance, whose depths are as 1, 2, and 3, the pressure against the side of the second will be four times greater than that against the first ; and the pressure against the side of the third will be nine times greater than that against the first. F. That is right ; the beautiful simplicity of the rule, and its being the same by which the accelerating velocity of falling bodies is governed, will make it impossible that you should hereafter forget it. The use that I shall hereafter call you to make of the rule induces me to put a question to Emma. In two canals, one five feet deep, and the other 1 5, what difference of pressure will there be against the sides of these canals ? E. The pressure against the one will be as the square of 5, or 25 ; that against the other will be as the square of 15, or 225 ; now the latter number di- vided by the former gives 9 as a quotient, which shews that the pressure against the sides of the deep canal is nine times greater than that against the sides of the shallow one. Can this principle be proved by an experiment 1 F. By a very simple one : this is a vessel of the same size as the last ; the bottom and side h are wood mortised together ; the front and opposite side are glass carefully inserted in the wooden /-^^ parts, and made water tight. A thin rijv^ board c hangs by two hinges x y, and is ^\ v held close to the glass panes by means of the pulley and weight ly. The board -^^ is covered with cloth and made water tisfht. OF THE PRESSURE OF FLUIDS 189 Now observe the exact weight which is overcome when the water is poured in and rises to the line 1 ; then hang on four times that weight, and you will see that water may be poured into the vessel till it rise to the line 2, when the side c will give way and let part of it out. ^ But why does only a part run away ? F . Because when a small quantity of the water has escaped, the weight 20 is greater than the pres- sure of the water against c, and therefore the door c will be drawn close to the glass panes, and confine the rest within the vessel. You may now hang on a weight nine times greater than the first, and then the vessel will contain water till it rise up to the mark 3, when the side will give way by the pressure and part of the water escape. C. You have explained the manner of estimatino- the pressure of fluids against the sides of a vessel^; by what rule are we to find the pressure upon the bottom ? F. In such vessels as those which we have just described, that is where the sides are perpendicular to the bottom, and the bottom parallel to the horizon, the I pressure will he equal to the weight of the fluid. j E. If then the vessel y s hold a gallon of water, I which weighs about eight pounds, and if the bottom were made moveable like the side, would a weight of eight pounds keep the water in the vessel ? F, It would ; for then there would be an equilibri- um between the pressure of the water and the weight. And the pressure upon any one side is equal to half the pressure upon the bottom ; that is, provided the bottom and sides are equal to one another. C. Pray, sir, explain how this is made out. F. The pressure upon the bottom is, as we have shewn, equal to the weight of the fluid. But we have I also shewn that the pressure on the side grows less , and less continually, till at the surface it is nothing, i Since then the pressure upon the bottom is truly re- ; presented by the area of the base multiplied into the 1 190 HYDROSTATICS. altitude of the vessel ; the pressure upon the side will be represented by the base multiplied into half the altitude. E. Is the pressure upon the four sides equal to twice the pressure upon the bottom 1 F. It is : consequently, the pressure of any fluid upon the bottom and four sides of a cubical vessel is equal to three times the weight of the fluid. Can you, Charles, tell me the difference between the weight and the pressure of a conical vessel of water standing on its base 1 C. The iveight of a conical vessel of any fluid is found by multiplying the area of the base by one third part of its perpendicular height, and then by the specific gravity :* but the pressure is found by multi- plying the base by the whole perpendicular height ; therefore the pressure upon the base will be equal to three times the weight. CONVEKSATION VIII. OF THE MOTION OF FLUIDS. F. We will now consider the pressure of fluids with regard to the motion of them through spouting pipes, v/hich is subject to the same law. If the pipes at 1 and 4 (Fig. 15. p. 187.) be equal in size and length, the discharge of water by the pipe at 4, will be double that at I. Because the velocity with which water spouts out at a hole in the side or bottom of a vessel is as the square root of the distance of the hole below the surface of the water. E. What do you mean by the squnre root? F. The square root of any number is that which being multiplied into itself produces the said number. Thus the square root of 1 is 1 ; but of 4 it is 2 ; of 9 it is 3 ; and of 16 it is 4, and so on. C. Then if you had a tall vessel of water with a * See Bonny castle's Introduction to Mensuration. OF THE MOTION OP FLUIDS. 191 cock inserted within a foot of the top, and you v/ished to draw the liquid off three times faster than it could be done with that, what would you do ? F. I might take another cock of the same size, and insert it into the barrel at nine feet distance from the surface, and the thing required would be done. E. Is this the reason why the water runs so slowly out of the cistern when it is nearly empty, in compari- son of what it does when the cistern is just full? F. It is : because the more water there is in the cistern, the greater the pressure upon the part where the cock is inserted; and the greater the pressure, the greater the velocity, and consequently the quan- tity of water that is drawn off in the same time. In some large barrels there are two holes for cocks, the one about the middle of the cask, the other at the bottom ; now if when the vessel is full you draw the beer or wine from both cocks at once, you will find that the lower one gives out the liquor much the fastest. C. In v/hat proportion ? F . As the square root of 2 is greater than that of 1 ; that is, while you had a quart from the upper cock, three pints nearly would run from the lower one. E. Are we then to understand, that the pressure against the side of a vessel increases in proportion to the square of the depth ; but the velocity of a spout- ing pipe, which depends upon the pressure at the orifice itself, increases only as the square root of the depth 1 F. That 13 the proper distinction. C. Is not the velocity of water running out of a vessel that empties itself continually decreasing ? F . Certainly : because in proportion to the quan- tity drawn off, the surface descends, and consequently the perpendicular depths become less and less. The spaces described by the descending surface, in equal portions of time, are as the odd numbers 1, 3, 5, 7, 9, &c. taken backwards. If the height of a vessel filled with any fluid 192 HYDROSTATICS. be divided into 25 parts, and in a given space of time, as a minute, the surface descend through nine of those parts, will it in the next minute descend through seven of those parts, and the third minute five, in the fourth tltree, and in the fifth one 1 F. This is the law, and from it have been invented elepsydrtp^ or water-clocks. C. How are they constructed? F. Take a cylindrical vessel, and having ascer- tained the time it will require to empty itself, then divide, by lines, the surface into portions, which are to one another as the odd numbers 1, 3, 5, 7, &c. E. Suppose the vessel require six hours to empty itself, how must it be divided ? F, It must be first divided into 36 equal parts ; then, beginning from the surface, take eleven of those parts for the first hour, nine for the second, seven for the third, five for the fourth, three for the fifth, and one for the sixth ; and you will find that the surface of the water will descend regularly through each of these divisions in an hour. I believe both of you have seen the locks that are constructed on the river Lea ? C. Yes : and I have wondered why the flood-gates were made of such an enormous thickness. F. But after what you have heard respecting the pressure of fluids, you will see the necessity that there is for the great strength employed. C. 1 do : for sometimes the height of the water is 20 or 30 times greater on one side of the gates than it is on the other, therefore the pressure will be 400 or even 900 times greater against one side than it is against the other. E. How are the gates opened when such a weight presses against them ? F. There is hardly any power by which they could be moved when this weight of water is against them ; therefore there are sluices by the side, which being drawn up, the water gets away through them into the bason, till it becomes level on both sides : then the OP THE MOTION OF FLUIDS. 193 o-ates are opened with the greatest ease, because, the pressure being equal on both sides, a small force applied will be sufficient to overcome the friction of the hinges, or other trifling obstacles. C. Is it this great pressure that sometimes beats down the banks of rivers 1 F. It is ; for if the banks of a river or canal do not mcrease in' strength in the proportion of the square of the depth, they cannot stand. Sometimes the water m a river will insinuate itself through the bank near the bottom, and if the weight of the bank be not equal to that of the water it will assuredly be torn up, perhaps with great violence. I will make the matter clear by a figure. Suppose this figure be a section of a river, and c a crevice or drain made by time under '-^^ ^ the bank ^; by what we -p-^^ have shewn before the up- ° . ward pressure of the water in that drain is equal to the downward pressure of the water in the river ; therefore, if that part of the bank be not as heavy as a column of water the same height and width, it must be torn up by the force of the pressure. C. Is there no method of securing leaks that hap- pen in the embankments of rivers ? F. The only method is that called puddling. If n be the bank of a canal in which a leak is discovered, the water must be first drawn off below the leak, and a trench 18 or 20 inches wide dug length-wise along the side of the canal, and deeper than the bottom of the canal ; this is filled by a little at a time with clay or loam reduced into a half fluid state by mixing it with water : when the first layer, which is seldom above six or eight inches deep, is nearly dry, another is worked in the same manner till the whole be filled. By this means, if the operation be performed by skilful hands, and time be allowed for all the parts to dry and cohere, the bank becomes strong and impenetrable. K 19-4 HYDROSTATICS. CONVERSATION IX. OF THE MOTION OF FLUIDS. F. I will now shew you an experiment by which you will observe the uniformity of nature's operations in regard to spouting fluids. C. Do you refer to any other facts besides those which relate to the quantity of water issuing from pipes ? F\ Yes, I do. Let ab represent a tall vessel of wa- ter, which must be always kept full while the experi- ments are making. From the centre of this vessel, I have drawn a semi-circle, the diameter of which is the Fig. 17. height of the vessel a b. I have drawn three lines, d 2 from the centre of the vessel ; c 1, a 5, at equal distances from the centre, the one above and the other below it : all three are drawn perpendicular to the vessel. By taking out the plug from the centre you will see that the water spouts to M. Take your compasses, and you will find that the distance n m is exactly double the length of d 2. I will now stop this plug and open the next below. C. The water reaches to k, which is double the length of a 5. Try in the same manner the pipe c. C, It falls in the same spot k, as it did from the lower one. F. Because the lines c 1 and a 5 being equally distant from the centre of the semi-circle, they are equal to one another. E. Then n k is the double of c 1, as well as of a 5. F. It is : the general rule deduced from these ex- periments is, that the horizontal distance to which a fluid will spout from an horizontal pipe, in any part OF THE MOTION OF FLUIDS. 195 of the side of an upright vessel below the surface of the fluid, is equal to twice the length of a perpen- dicular to the side of the vessel, drawn from the moutli of the pipe to a semi-circle described upon the altitude of the vessel. Can you, Charles, tell me in what part the pipe should be placed, in order that the fluid should spout the farthest possible. C. In the centre : for the line d 2 seems to be the greatest of all the lines that can be drawn from the vessel to the curved line. F, Yes, it is demonstrable by geometry that this is the case : and also, that lines at equal distances from the centre above and below are also equal to each other. E, Then, in all cases, if pipes are placed equally distant from the centre they will spout to the same point. F. They will. Instead of horizontal pipes,' I will fix three others which shall point obliquely upwards at different angles ; one at 22<* 30', the second at 45^, and the third at 67^ 30', and you will see that when I open the cocks, the water will cut the curve line in those places to which the lines were drawn. C, That which spouts from the centre is thrown to the point m, as it was from the centre horizontal pipe. The two others fall on the point k, on which the upper and lower horizontal pipes ejected the stream. E. I thought the water from the upper cock did not reach so high as the mark. F, It did not. The reason is, that it had to pass through a larger body of air, and the resistance from that retarded the water, and prevented it from ascend- ing to the point to which it would have ascended if the air had been taken away. While we are on this subject, I will just mention, that as you see the water spouts the farthest when the pipe is elevated to an angle of 45*', so a gun, cannon, &c. will project a bullet the farthest, if it be elevated to an angle of 45'^. 196 HYDROSTATICS. C. Will a cannon or mortar carry a ball equally distant if it be elevated at angles equally distant from 45°, the one above and the other below 1 F. It will, in theory : but owing to the great resis- tance which very swift motions meet with from the air, there must be allowances made for some variation between theory and practice. A regard to this will explain the reason why water will not rise so high in a jet, as it does in a tube, E. I do not know what this means. F. You have seen a fountain 1 E. Yes, I have often been amused with that in the Temple. F, All fountains are called jets, or jets d'eau. Now if the water of that in the Temple ascended in a pipe, it would rise higher than it does in the open air. Turn to Fig. 10. p. 179 ; the water in the small tube rises to a level with that in the larger one ; now if the tube HG was broken off at t, the water would spout up like a fountain, but not so high as it stands in the tube, perhaps no higher than to d* C, Is that owing wholly to the resistance of the air ? F. It is to be ascribed to the resistance which the water meets with from the air, and to the force of gravity, which has a tendency to retard the motion of the stream. £. Why does the fountain in the Temple some- times play higher and sometimes lower 1 F. Near the Temple-hall there is a reservoir of water, from which a pipe communicates with the jet in the fountain : and according as the water in the reservoir is higher or lower, the height to which the fountain plays is regulated. C, By turning a cock near the pump the fountain is instantly lowered. F, That cock is likewise connected with the reser- voir, and therefore taking water from it must have the effect of lowering the stream at the fountain, as well as that in the reservoir. OF THE MOTION OF FLUIDS. 197 E. It soon recovers its force again. F. Because there is a constant supply of water to the reservoir, which, however, does not come in so quick as the cock lets it out, or the fountain would always play to the same height. From what you have already learnt on this subject you will be able to know how London and other places are supplied with water. C, London is, I believe, supplied from the New River, but I do not know in what manner. F. The New River is a stream of water that comes from Ware in Hertfordshire ; it runs into a reservoir situated on the high ground near Islington. From this reservoir pipes are laid into those parts of town that have their water from the New River, and through these pipes the water flows into cisterns be- longing to different houses. E. Then the reservoir at Islington must be higher than the cisterns in London, F, Certainly, because water will not rise above its level. On this account some of the higher parts of town have hitherto been supplied from ponds at Hampstead and Highgate ; and others were, till lately, supplied from the Thames by means of the water-works at London Bridge, C. Are pipes laid all the way from Hampstead to town 1 F, They are : but these supply the intermediate villages as well as London. And Hampstead stand- ing so high, the water is carried up into the first and second stories in some houses. Thus you see that water may be carried to any distance, and houses on different sides of a deep valley may be supplied by water from the same spring-head. You must remem- ber that if the valleys are very deep the pipes must be exceedingly strong near the bottom, because the pres- sure increases in the rapid proportion of the odd numbers 1, 3, 5, 7, &c. and therefore, unless the strength of the wood or iron be increased in the same proportion, the pipes will be continually burstmg. IDS HYDROSTATICS. E, You told me the other day, that the large mound of earth, for it appears nothing else, near the end of Tottenham Court lload was intended as a reservoir for the New River. -F. What appears to you, and others who pass by it, only a mound of earth, is an exceedingly large bason, capable of containing a great many thousand hogsheads of water. C. How will they get the water into it 1 F. At Islington near the New River Head is made a large reservoir upon some very high ground, into which, by means of a steam-engine, they will con- stantly throw water from the New River. This reser- voir being higher than that in Tottenham Court Road, nothing more is necessary than to lay pipes from Islington to that place in order to keep 'it con- stantly full of water. By this contrivance the New River Company will be able to extend their business to other parts of London, where their present head of water caiHiot reach. C. The weight of water in this place must be im- mensely great. F, It is ; and therefore you observe what a thick- ness the mound of earth against the wall is towards the bottom, and that it diminishes towards the top as the pressure becomes less and less. E, Would not the consequences be very serious if the water were to insinuate itself through the earth at the bottom 1 F, If such an accident were to happen when the reservoir was full of water, it would probably tear up the works and do incredible mischief. To prevent this, the vast bank of earth is sloped within, as well as without ; it will then be covered with a strong coat- ing of clay ; after this it will be built up with\ very thick brick wall, which will be carefully terraced over, so that the whole mass will be as firm and com- pact as a glass bottle. OF SPECIFIC GRAVITIES. 199 CONVERSATION X. OF THE SPECinC GRAVITIES OF BODIES. E What is the reason, papa, that some bodies, as lead or iron, if thrown into the water, sink, while others, as wood, will swim '? F. Those bodies that are heavier than water will sink in it, but those that are lighter will swim, E. I do not quite comprehend your meaning ; a pound of wood, another of water, and another of lead, are all equally heavy. For Charles played me a trick the other day : he suddenly asked which was heavier, a pound of lead, or a pound of feathers : 1 said the lead, and you all laughed at me, by which I was soon led to perceive, that a pound, or 16 ounces of any substance whatever, must be always equal to the same weight. i t i * i F You are not the first person that has been taken in by this question. It is a common trick. Although a pound of lead and another of water be equally heavy, yet they are not of equal magnitudes. Do you know how much water goes to a pound 1 C. Yes : about a pint. E. Do you think that if I were to fill the same pint measure with lead, that would weigh a pound only 1 C, Oh no : that would weigh a great deal more. I do not believe that the 14 pounds weight below stairs is much larger than a pint measure. E. Yes it is, by about a fourth part ; the same measure that contains one pound of water Nould, how- ever, contain about 11 pounds of lead ; but it would contain 14 pounds of quicksilver, which, you know, I could as readily pour into the vessel as if it were water. Here are two cups of equal size ; fill the one with water, and I will fill the other with quicksilver 1 E. Why did you not let Charles pour out the quicksilver 1 r i F. The loss of water is a matter of little conse- quence ; but if, by chance, he had thrown down the 200 HYDROSTATICS. quicksilver, the accident might have occasioned the loss of sixpence, or a shilling; and economy is right in all the affairs of life. Take the cups in your hand : which is the heavier ? C. The quicksilver by much. F. But the two cups are of equal size. E. Then there must be equal quantities of water and quicksilver. F, They are equal in bulk. C. But very unequal in weight : shall I try how much heavier the one is than the other 7 F, If you please. In what manner will you ascer- tain the matter? C. I will carefully weigh the two cups, and then, di- viding the larger weight by the smaller, I shall see how many times heavier the quicksilver is than the water. F. You will not come to the point accurately by that means , because the weight of the cups is proba- bly equal, but by this method they ought to differ in weight in the same proportion as the two substances. E. Then pour the quicksilver first into the scale and weigh it ; afterwards do the same with the water ; and divide the former by the latter : will not that give the result 1 F. Yes, it will : or you may make the experiment in this method. Here is a small phial, that weighs, now it is empty, an ounce ; fill it with pure rain water, and the weight of the whole is two ounces. C. Then it contains one ounce of water. F. Pour out the water, and let it be well dried both within and without : fill it now very accurately with quicksilver, and weigh it again. E. It weighs a little more than 15 ounces : but as the bottle weighs one ounce, the quicksilver weighs something more than 14 ounces. F. What do you infer from this, Charles'? C. That the quicksilver is more than 14 times heavier than water. jp. I will now pour away the quicksilver, and fill OF SPECIFIC GRAVITIES. 201 the phial with pure spirits of wine, or, as the chemists call it, with alcohol. E. It does not weigh two ounces now ; conse- quently the fluid does not weigh an ounce. The alcohol is, then, lighter than water. F. By these means, which you cannot fail of under- standing, we have obtained the comparative weights of three fluids : philosophers, as I have before told you, call these comparative weights the specific gravities of the fluids : they have agreed also to make pure rain water the standard to which they refer the comparative weights of all other bodies, whether solid or fluid. C. Is there any particular reason why they prefer water to every other substance 1 F. I told you a few days ago that rain water, if very pure, is of the same weight in all parts of the world : and, what is very remarkable, a cubical foot of water weighs exactly a thousand ounces avoirdu- pois : on these accounts it is admirably adapted for a standard, because you can at once tell the weight of a cubical foot of any other substance, if you know its specific gravity. E. Then a cubical foot of quicksilver weighs 14,000 ounces. F, You are right ; and if lead is 11 times heavier than water, a cubical foot of it will weigh 11,000 ounces. CONVERSATION XL OP THE SPECIFIC GRAVITIES OF BODIES. F, Before we enter upon the methods of obtaining the specific gravities of different bodies, it will be right to premise a few particulars, which it is neces- sary should be well understood. ^"ou now understand, that the specific gravity of dif- ferent bodies depends upon the different quantities of matter which equal bulks of these bodies contain. C. As the momenta* of different bodies are esti- * See Mc'clianic.;, Conver. VI, K 2 202 HYDROSTATICS. mated by the quantities of matter when the velocities are the same ; so the specific gravities of bodies are estimated by the quantities of matter when the bulks or magnitudes are the same. This, I believe, is what you mean. F, I do ; if you had a piece of wood, and another piece of lead, both exactly equal in size to a copper penny-piece, the former would be much lighter, and the latter considerably heavier than the copper. C. And I should say that the specific gravity of the wood is less than that of the copper, but of the lead it is greater. E. Is it then the density that constitutes the specific gravity 1 F. Undoubtedly it is ; and, as we observed yester- day, water is made use of as a medium to discover the different specific gravities of diflPerent bodies ; and also as a standard to which they may be all referred. Here are three pieces of different kinds of wood, which I will put into this vessel of water : one sinks to the bottom ; a second remains in any position of the water in which it is placed ; and the third swims on the water with more than half of the substance above its surface. C. The first, then, is heavier than the water, the second is of the same weight with an equal bulk of the fluid, and the third is lighter. F. Since fluids press in all directions, a solid that is immersed in water sustains a pressure on all sides, which is increased in proportion to the height of the fluid above the solid. E. That seems natural, but an experiment would fix it better in the mind, F, Tie a leathern bag to the end of a glass tube, and pour in some quicksilver. Dip the bag in water, and the upward pressure of the fluid will raise the quicksilver in the tube, '^J the ascent of which will be higher or lower in proportion to the height of the water abov« the bag. Fig. 18 OF SPECIFIC GRAVITIES. 203 E. I now understand that, the upper part of the tube being empty, or, at least, only filled with air, the upward pressure of the water against the bag must be greater than the downward pressure of the air : and that, as the pressure increases according to the depth, therefore the mercury must keep rising in the tube. What is the reason that a body heavier than water, as a stone, sinks to the bottom, if the pressure up- wards is always equal to that downwards 1 F, This is a very proper question. The stone en- deavours to descend by the force of gravity : but it cannot descend without moving away as much of the water as is equal to the bulk of the stone : therefore it is resisted, or pressed upwards, by a force equal to the weight of as much water as is equal in magnitude to the bulk of the stone : but the weight of the water is less than that of the stone, consequently the force pressing against it upwards is less than its tendency downwards, and therefore it will sink with the differ- ence of these two forces. You will now be at no loss to understand the rea- son why bodies lighter than water swim. C. The water being heavier, the force upwards is greater than the natural gravity of the body, and it will be buoyed up by the difference of the f6rces. F, Bodies of this kind, then, will sink in water till so much of them is below the surface, that a bulk of water equal to the bulk of the part of the body which is below the surface, is of a weight equal to the weight of the whole body. E, Will you explain this more particularly ? F, Suppose the body to be a piece of wood, part of which will be above and part below the surface of the water ; in this state conceive the wood to be frozen into the water. C. I understand you i if the wood be taken out of the ice, a vacuity will be left, and the quantity of water that is required to fill that vacuity will weigh as much as the whole substance of the wood. 204 HYDROSTATICS. F. That was what I meant to have said. There is one case remaining : — where equal bulks of the water and the wood are of the same weight, the force with which the wood endeavours to descend, and the force that opposes it, being equal to one another, and acting in contrary directions, the body will rest between them, so as neither to sink by its own weight, nor to ascend by the upward pressure of the water. E. What is the meaning of this glass jar with the images in it ? F. I placed it on the table in order to illustrate our subject to-day. You ob- serve, that, by pressing the bladder with my hand, the three images all sink. E. But not at the same moment. F. The images are made of glass, and about the same specific gravity with the water surrounding them, or perhaps Fig. 19. rather less than it, and consequently they all float near the surface. They are hollow, with little holes in the feet. When the air which lies be- tween the bladder and the surface of the water is pressed by my hand, there is a pressure on the water which is communicated through it, and that part of it which lies contiguous to the feet of the imas^es will be forced into the bodies, by which their weight is so much increased as to render them heavier than the water, and they descend. C, Why do they not all descend to the same depths 1 F, Because the hollow part of the image e is larger than the hollow part of d, and that is larger than that of c ; consequently, the same pressure will force more water into e than into d, and more into d than into c. E. Why do they begin to ascend now you have taken your hand away 1 F. I said the hollow parts of the images were empty, which was not quite correct ; they were full of OF SPECIFIC GRAVITIES. 205 air, which, as it could not escape, was compressed into a smaller space when the water was forced in by the pressure upon the bladder. But as soon as the pressure is removed, the air in the images expands, drives out the water, and they become as light as at first, and will therefore rise to the surface. CONVERSATION XII. OP THE METHODS OF FINDING THE SPECIFIC GRAVITY OF BODIES. E. What are you going to weigh with these scales ? This instrument is called the hydrostatical balance ; it dif- fers but little from the balance in common use. Some instru- ments of this kind are more com- plicated, but the most simple are best adapted to my purpose. To the beam two scale-pans are adjusted, and may be taken off at pleasure. There is also Fig. 20. another pan a, of equal weight with one of the others, furnished with shorter strings and a small hook, so that any body may be hung to it, and then immersed in the vessel of water n. C. Is it by means of this instrument that you find the specific gravity of diflferent bodies 1 F, It is : I will give you the rule, and then illus- trate it by experiments. The rule should be com- mitted to memory : ** Weigh the body first in air ; that is, in the com- mon way ; then weigh it in water, observe how much weight it loses by being weighed in water, and by dividing the former weight by the loss sustained, the result is its specific gravity compared with that of the water.'* I will give you an example. Here is a guinea: it weighs in the air 129 grains : I suspend it by a fine thread of horse-hair to the hook at the bottom of 200 HYDROSTATICS. the pan a, and you see that, by being immersed in water, It weig-hs only 121-| grains. E. Then in the water it has lost of its weight 7^ grains. F. Divide 129 by 7|, or, by turning the i into decimals, by 7.25. C. But 1 must add two ciphers to the 129 grains, because there must always be as many decimals in the dividend as there are in the divisor. And 129.00 divided by 7.25 gives for the quotient more than 17. F. The gold is therefore more than 17 times heavier than water. I do not understand the reason of this. F, In this scale is a bason filled accurately to the brim with water. I will put a piece of mahogany into It very gently; any thing else would answer the same purpose. £. The water runs over into the scale. 1^. So I expected it would : now every thing is at rest, and the bason is just as full as it was at first, only that the wood and water together fill the bason, whereas it was all water before. I will take away the bason, and put the mahogany by itself into the other scale. E. It balances the water that run out of the bason. C. The mahogany then displaced a quantity of water equal to itself in weight. F , And so did the guinea just now ; and if yon had taken the same precaution, you would have found that the quantity of water equal in bulk to the guinea weighed 7i grains, the weight which it lost by bein? weighed in the fluid. -E. Am I to understand that what any substance loses of its weight, by being immersed in water, is equal to the weight of a quantity of water of the same bulk as the substance itself ? F . This is true, if the body be wholly immersed in water; and with regard to all substances that are specifically heavier than water, you may take it as an axiom, that every body, when immersed in water, OF SPECIFIC GRAVITIES. 207 loses as much of its weight, as is equal to the weight of a bulk of water of the same magnitude." I will now place this empty box on the bason filled to the edge with water, and, as before, it drives over a quantity of fluid equal in weight to itself. Put in two penny-pieces, and you perceive the box sinks deeper into the water. C. And they drive more water over ; as much, I suppose, as is equal in weight to the copper coin. B\ Right : how long could you go on loading the box C. Till the weight of the copper and box, taken together, is something greater than the weight of as much water as is equal in bulk to the box, F» You understand, then, the reason why boats, barges, and other vessels, swim on water; and to what extent you may load them with safety. E, They will swim so long as the weight of the vessel and its lading together, is less than that of a quantity of water equal in bulk to the vessel. F, Can you, Charles, devise any method to make iron or lead swim, which are so much heavier than water 1 C. I think I can. If the metal be beat out very thin, and the edges turned up, I can easily conceive that a box or a boat of it may be made to swim. Of this kind is the copper ball which is contrived to turn off the water when the cistern is full. E. I have often wondered how that acts. F, If upon reflection you could not satisfy your- self about the mode of its acting, you should have asked ; it is better to get information from another than to remain ignorant. The ball, though made of copper, which is eight or nine times heavier than water, is beat out so thin, that its bulk is much lighter than an equal bulk of v/ater. By means of a handle it is fastened to the cock, through which the water flows, and as it sinks or rises, it opens or shuts the cock. If the cistern is empty, the ball hangs down, and 20S HYDROSTATICS. the cock is open, to admit the water freely. As the water rises in the cistern it reaches the ball, which, being lighter than the water, rises with it, and, by rising, gradually shuts the cock, and, if it be properly placed, it is contrived to shut the cock just at the moment the cistern is full. In the same way that these balls are made boats of iron are now constructed at the iron-works in Shropshire : they will last longer than wood, and cause less friction in passing through the water. Can you, Emma, find the specific gravity of this piece of silver] E, It weighs in air 318 grains: I now fasten it to the hook with the horse-hair, and it weighs in water 288 grains, which, taken from 318, leave 30, the weight it lost in water. By dividing 318 by 30 the quotient is about 10|; consequently, the specific gra- vity of the silver is ten and a half times greater than that of water. F. What is the specific gravity of this piece of flint-glass 1 It weighs 12 pennyweights in air. C, And in water it weighs only 8, and consequently loses 4 by immersion ; and 12 divided by 4 gives 3, therefore the specific gravity of flint-glass is three times greater than that of water. F. This is not the case with all flint-glass ; it varies from 2 to almost 4. Here is an ounce of quicksilver j let me know its specific gravity by the method now proposed. E, Hov/ will you manage that 1 you cannot hang it up on the balance. F» But you may suspend this glass bucket 0 on the hook at the bottom of a ; immerse it in T the water, and then balance it exactly with 4^ weights in the opposite scale. I will now put into the bucket the ounce, or 480 grains of quicksilver, and see how much it loses in water. A C. It weighs 445 grains, and consequently 0 it lost 35 gruins by immersion ; and 480 OF SPECIFIC GRAVITIES. 209 divided by 35 give almost 14, so that mercury is almost 14 times heavier than w^ater. F. In the same manner we obtam the specific gravity of all bodies that consist of small fragments. They must be put into the glass bucket and weighed ; and then, if from the weight of the bucket and body in the fluid, you subtract the weight of the bucket, there remains the weight of the body in the fluid. E. Why do you make use of horse-hair to suspend the substances with ; would not silk or thread do as well? F. Horse-hair is by much the best, for it is very nearly of the same specific gravity as water ; and its substance is of such a nature as not to imbibe moisture. CONVERSATION XIII. OF THE METHODS OF FINDING THE SPECIFIC GBAVITY OF BODIES. C. I have endeavoured to find out the specific gravity of this piece of beech-wood, but as it will not sink in the water I know not how to do it. F, It is true that we have hitherto only given rules for the finding of the specific gravity of bodies that are heavier than water ; a little consideration, how- ever, will shew you how to obtain the specific gravity of the beech. Canyon contrive means to sink the beech in the water? C. Yes ; if I join a piece of lead, or other metal, to the wood, it will sink. -F. The beech weighs 660 grains ; I will annex to It an ounce, or 480 grains of tin, which in water loses of Its weight 51 grains. In air the weight of the wood and metal taken together is 1140 grains; but in water they weigh but 138 grains: 138 taken from 1140 leave 1002, the difiference between the weights in air and in water. C. I now see the mode of finding what I want. 210 HYDROSTATICS. The whole mass loses 1002 grains by immersion, and the tin by itself lost in water 51 grains j therefore, the wood lost 951 grains of its weight by immersion : and 660 grains, the weight of the beech in air, di- vided by 951, which it may be said to lose by immer- sion, leaves in decimals for a quotient .694. F. Then making water the standard equal to 1, the beech is .694, or nearly seven-tenths of 1 : that is, a cubic foot of water is to a cubic foot of beech as 1000 to 694, for the one weighs 1000 ounces, and the other 694 ounces. E. It seems odd how a piece of wood that weighs but 660 grains in air, should lose of its weight 951 grains. F. You must, in this case, consider the weight necessary to make it sink in water, which must be added to the weight of the wood. I will now endeavour to make the subject easier by a different method. This small piece of elni I will place be- tween the tongs that are nicely balanced on the beam. The elm weighs 36 grains. To detain it under water, I must hang 24 grains to the end of the lever on which the tongs are fixed: then by the Rule of Three I say, as the specific gravity of the elm is to the specific gravity of water, so is 36, the weight Fig. 22. of the elm, to 60, the weight of the elm and the additional weight required to sink it in water.^ E. You have not obtained the specific gravity of the elm, but a proportion only. C. But three terms are given, because the water is always considered as unity or 1, therefore the specific gravity of the elm is J^-^^-l == . 6 E. I do not yet comprehend the reason of the pro- portion assumed. F, It is very simple. The elm is lighter than the water, but by hanging weights to the side of the balance, to which it is attached, in order to OF SPECIFIC GRAVITIES. 211 detain it just under water, I make the whole exactly equal to the specific gravity of the water; by this means it is evident, that the comparative gravity of the elm is to that of the water as 36 to 60. Try this piece of cork in the same manner. It weighs half an ounce, or 240 grains, in air ; and to detain the cork and tongs just under water, I am obliged to hang 2 ounces, or 960 grains, of lead on the lever : therefore, the specific gravity of the cork is to that of the water as 240 is to 1200 ; and 240 divided by 1200 give the decimal .2. Then the specific gravity of water is 5 times greater than that of cork. C. We have now obtained the specific gravities of water, beech, elm and cork, which are as 1, .7 nearly, .6 and .2. ^ F, You now understand the methods of obtaining the specific gravity of all solids, whether lighter or heavier than water. In making experiments upon light and porous woods, the operations must be per- formed as quickly as possible, to prevent the water from getting into the pores. C, And you have likewise shewn us a method of getting the specific gravity of fluids, by weighing cer- tain quantities of each. F,^ I have a still better method : the rule I will give in words : you shall illustrate it by examples : If the same body be weighed in diflerent fluids, the specific gravity of the fluids will be as the weights lost.'^ E, The body made use of must be heavier than the fluids. F. Certainly • this glass ball loses of its weight by immersion in water 803 grains; in milk it loses 831 grains ; therefore the specific gravity of the water is to that of milk as 803 to 831. Now a cubical foot of water weighs 1000 ounces ; what will be the weight of the same quantity of milk 1 E. As 803 : 831 : : 1000 : 12^.^1^ = 1035 803 ounces, nearly. HYDROSTATICS. F. Do you, Charles, tell me what is the specific gravity of vsome spirits of wine which I have here. C The glass loses in water 803 grains, in the spirit of wine it loses 699 grains, therefore the specific gravity of water is to the spirit as 803 is to 699 ; and to find the weight of a cubical foot of the spirit, I say, as 803 : 699 : : lOOO : ^^^^^no^^^ = ^^0 ounces. F. You may now deduce the method of comparing the specific gravities of solids one with another with- out making a common standard. Here is an ounce of lead and another of tin : I may weigh them in any fluid whatever ; in water the lead loses by immersion 42 grains, and the tin 63 grains. E. Is the specific gravity of the lead to that of the tm as 42 to 631 F. No : " the specific gravities of bodies are to one another inversely as the losses of weight sustained :" therefore, the specific gravity of the lead is to that of the tin as 63 to 42 ; or if a block of lead weighs 63 pounds, the same sized block of tin will weigh 42 pounds only. C. I think I see the reason of this : the heavier the body, the less it loses of its weight by immersion ; therefore, of two bodies whose absolute weights are tlie same, that is, each weighing an ounce, pound, &c., the one which loses least of its weight will be specifically the heaviest. F, You are right ; for the specific gravity of bodies is as their density, and their densities are inversely as the weights they lose by immersion, that is, the body that is most dense will lose the least in water, because it displaces the least quantity of water ; a pound of copper occupying seven or eight times less space than a pound of wood, would therefore remove seven or eight times less water. HIERO'S CROWN. 213 CONVERSATION XIV. OF THE METHODS OF OBTAINING THE SPECIFIC GRA- VITY OF BODIES. F. As I have shewn you the methods of finding the specific gravity of almost all kinds of bodies, it will be proper in this and one or two lessons, to shew you the practical utility of this part of science. E. To whom are we indebted for the discovery of the mode of performing these operations 1 F. To that most celebrated mathematician of anti- quity, Archimedes. C. Was he not slain by a common soldier at the siege of Syracuse ? F. He was, to the great grief of Marcellus, the Roman commander, who had ordered that his house and person should be respected : but the philosopher was too deeply engaged in solving some geometrical inquiries to think of seeking that protection which even the enemy intended for him. E. Had he at that time so high a reputation as to induce the general of a besieging army to give parti- cular orders for his preservation 1 F, His celebrity was so great among the literati of Rome, that his tragical end caused more real sorrow than the capture of the whole island of Sicily did joy- We are informed by history, that it was by the wisdom of Archimedes that the fate of Syracuse was Jong suspended : by his inventions multitudes of the Roman army were killed and their ships destroyed : and that he made use of burning glasses, which, at the distance of some hundreds of yards, set the Ro- man vessels on fire.* * We shall consider this subject at large in our con- versations on Optics. 214 HYDROSTATICS. C. I wonder then that he was not defended by his fellow-citizens. F, Alas! my child, I am sorry to say that m other countries as well as Sicily, there have been in- stances in which persons who have benefited then- country as much as Archimedes have experienced no more gratitude than he did. It is a fortunate circumstance when the efforts of philosophy are directed under able judgment to the defence of one's country. The Romans had no more right to plunder Sicily than the highwayman has to rifle your pockets or mine. In the eye of reason and justice all offensive war is the most deliberate and cruel system of robbery and murder. But to return to our subject. To Archimedes the world is indebted for the discovery ** That every body heavier than its bulk of water, loses so much of its weight, by being suspended in water, as is equaHo the weight of a quantity of water equal to its bulk.'* E. How did he make the discovery 1 F. Hiero, king of Syracuse, had given to a jew- eller a certain quantity of pure gold to make a crown for him. The monarch, when he saw the crown, suspected the artist of having kept back part of the gold. E. Why did he not weigh itl F. He did : and found the weight right : but he suspected, perhaps from the colour of the crown, that some baser metal had been mixed with the gold, and therefore though he had his weight, yet only a part of it was gold, the rest silver or copper. He applied to Archimedes to investigate the fraud. C, Did he melt the crown, and endeavour to sepa rate the metals ? F. That would not have answered Hiero's inten- tions ; his object was to detect the roguery, if any, without destroying the workmanship. While the philosopher was intent upon the problem, he went, according to his custom, into tlie bath, and he ob- served tliat a quantity of water flowed over, which he OF SPECIFIC GRAVITY. 215 thought must be equal to the bulk of his own body. He instantly saw the solution of Hiero's problem. In raptures at tfie discovery, he is said to have leaped from tiie water and run naked through the streets of the city, shouting aloud 'Ev^tjxa ! 'EvQijza I L have found it out ! I have found it out ! '* When the excess of his joy was abated, he got two masses, one of gold and the other of silver, each equal in weight to the crown ; and having filled a vessel very accurately with water, into which he first dipped the silver mass, and observed the quantity of water that flowed over, he then did the same with the gold, and found that a less quantity of water had flowed over than before, C. And was he, from these trials, led to conclude that the bulk of the silver was greater than that of the gold 1 F, He was ; and also, that the bulk of water dis- placed was, in each experiment, equal to the bulk of the metal. He then made the same trial with the crown, and found, that though of the same weight with the masses of silver and gold, yet it displaced more water than the gold, and less than the silver. E, Accordingly he concluded, I imagine, that it was neither pure gold nor pure silver. C. But how could he discover the proportions of each metal 1 F, I believe we have no other facts to carry us farther into the history of this interesting experiment. But to-morrow I will endeavour to explain and illus- trate the matter. CONVERSATION XV. OF THE METHODS OF OBTAINING THE SPECIFIC GRA- VITY OF BODIES. E. You are to describe, to-day, the method of de- tectino- the proportion of each metal if two are mixed together in one mass. 21G HYDROSTATICS. F. Suppose I take in change a guinea, which I suspect to be bad : upon trying it I find it weighs 129 grains, which is the standard weight of a guinea. I then weigh it in water, and it loses of its weight 8^ grains, by which I divide the 129^ and the quotient is 15.6, the specific gravity of the guinea. But you know the specific gravity of the gold, made at the Mint, is more than 17, and therefore I conclude the guinea is base metal, a mixture of silver, or copper, with stan- dard gold. C. But how will you get the proportions of the two metals ? F, Suppose, for example, that the mass be a com- pound of silver and gold. — " Compute what the loss of a mass of standard gold would be j and likewise the loss which a mass of silver equal in weight to the guinea would sustain. Subtract the loss of the gold from that of the compound, the remainder is the ratio or proportion (not the quantity) of the silver : then subtract the loss of the compound from that of the silver, the remainder is the proportion of the gold." I will propose you an example. What are the proportions of silver and gold in a guinea weighing 129 grains, whose specific gravity is found to be only 13.09 ; supposing the loss of stand- ard gold 7.25, and that of a piece of silver, equal in weight to a guinea, 12.45, and the loss of the com- pound 9 851 C. I first subtract the loss of standard gold 7.25 from the loss of the compound 9.85, the remainder is 2.6 : I now take the loss of the compound 9.85 from that sustained by the silver 12.45, and the remainder is also 2.6. jP. Then the proportions of silver and gold are equal to one another, consequently the false guinea is half standard gold and half silver. Here is another counterfeit guinea, which is full weight, but I know it is composed of standard gold, adulterated with copper, and its loss in water is, as you see, 8.64 : now tell me the proportions of the OF SPECIFIC GRAVITY. 217 two metals ; but you should be informed, that a piece of copper of the weight of a guinea would lose in water 14.65 grains. E. I deduct 7.25, the loss of a guinea standard gold, from 8.64, the remainder is 1.39 : I now take the loss of the compound 8.64 from 14.65, the loss sustained by a piece of copper equal in weight to a guinea, and the remainder is 6.01. Is not the pro- portion of gold to copper as 1.39 to 6.01 1 F. You are quite right. Now by the rule of three tell me the quantity of each metal. E. To find the weight of the copper, I add 6.01 and 1.39 together, which are the proportional weights of the two metals ; and say, as 7.40, the sum, is to 1.39, the proportional weight of copper, so is the weight of the guinea, 129 grains, to the real weight of copper contained in the counterfeit guinea: but 1.39 X jj£---24.1 ; therefore there is a little more 7.40 than 24 grains of copper in the compound. F. You have found then that there are 24 grains of copper in this counterfeit guinea. How will you find the weight of the gold 1 E. Very easily, for if the composition be copper and gold, and there are found to be 24 grains of cop- per, there must be 105 of gold. C. I have a question to propose. If by chance you take a bad guinea (I have heard you say that you never attempt to pass it upon others), how should you be able to ascertain the value it would fetch at the goldsmith's ? F. It is certainly very wrong knowingly to pass bad money upon the public : no man has a right to commit an injury because he has received one ; if therefore I have taken counterfeit money, I ought to abide by the loss, rather than run the risk of injuring my neighbour : besides, in the course of circulation, a bad guinea or a sovereign, or even coins of much less value, may fall into the hands of a poor and in- 218 HYDROSTATICS. dustrious family, which they perhaps lay by to answer the extraordinary demands of sickness ; and at that period of distress not being able to say from whom they received the counterfeit coin, they may possibly be reduced to serious and pitiable difficulties ; and therefore it is better for me to put up with the loss than run the hazard of injuring the poor. Now to answer your question. A piece of cop- per of equal weight with a guinea loses of its weight in water 14.65 grains, 7.4 more than is lost by a standard guinea. The value of a standard guinea is 252 pence : divide therefore 252 by 7.4, and you get 34, the number of pence that is deducted from the value of a guinea, for every grain it loses more than it would lose if it were sterling gold. E, In the guinea that lost 8.64 hov*' much must be deducted from the real value of a guinea standard gold? C. I can tell that: subtract 7.25 from 8,64 the re- mainder is 1.39, and this multiplied by 34 pence gives 47.26 pence, or very nearly 4 shillings, conse- quently that guinea is worth only 17 shillings. F, Suppose the compound were silver and gold, how would you proceed in making an estimate of its value 1 C. A piece of silver of tlie weight of a guinea would lose 12.45 grains, from which I deduct 7.25, and with the remainder 5.2 I divide the value of a guinea, or 252 pence, and the quotient is 48.4 pence, or rather more than 4 shillings is to be deducted from the value of a guinea adulterated with silver, for every grain it loses by immersion more than standard gold". E, How is that papa 1 silver is much dearer than copper, and yet you allow 4 shillings a grain when the guinea is alloyed with silver, and but 2s. lOd. when the mixture is made with copper 1 F, Because the specific gravity of silver is much nearer to that of gold than that of copper ; conse- OF SPECIFIC GllAVITIES. 219 quently, if equal quantities of silver and copper were mixed with gold, the silver would cause a much less loss by immersion in water than the copper. As it seldom happens that the adulteration of metal in guineas is made with all copper, or with all silver, but generally with a mixture of both, three shillings is upon the average allowed for every grain that the base m^etal loses by immersion in water more than sterling gold. E, There is a silver cream -jug in the parlour ; I have heard mamma say, she did not think it was real silver ; how could she fmd out whether she had been imposed on ] F, Go and fetch it. We will nov*' weigh it. E, It weighs 5| ounces, but I must weigh it in water, and it has lost in the water 10| dwts ; and di- viding 5| ounces, or 110 pennyweights, by lOJ, I get for answer 10.7, the specific gravity of the jug. F, Then there is no cause for complaint, for the specific gravity of good wrought silver is seldom more than this. TABLE OF SPECIFIC GRAVITIES. Mercury . 13.568 Iron (bar) . 7.788 Zinc .... . 7.191 Flint-glass . 3.290 Marble . . • . . 2.700 . 1.250 Ash .... .800 220 HYDROSTATICS. Maple 755 Elm 600 Fir 550 Cork -240 CONVERSATION XVI. OF THE HYDROMETER. -F. Before I describe the construction and uses of the hydrometer, I will shew you an experiment or two which will afford you entertainment, after the dry calculations in some of our former conversations. C. The arithmetical operations are rather tedious, to be sure, but they serve to bring to mind what we have already learnt, and at the same time shew to what uses arithmetic may be applied- F. You know that wine is specifically lighter than water, and the lighter body will always be uppermost ; upon these principles, I will exhibit two or three experiments : I have fdled the bulb b with port wine to the top of the narrow stem .r. 1 now fill a with water. -p-^ 23 E. The wine is gradually ascending like ®" a fine red thread through the water to its surface. -F. And so it will continue till the water and wine have changed places. C. 1 wonder the two liquids do not mix, as wine and water do in a common drinking glass. jP. It is the narrowness of the stem x which prevents the admixture : in time, however, this would be eflrected, because v/ater and wine have what the chemists call an attraction for each other. Here is a small bottle b with a neck three inches long, and about one-sixth of an inch wide ; it is full of red wine ; I will now place it at the bottom of a jar of water, a few inches deeper than the bottle is high. The wine you observe is ascend- Fig. 24. ing through the water. E. This is a very pretty experiment : the wine rises OP THE HYtpROMETER. 221 in a small column to the surface of the water, spread- ing itself over it like a cloud. F. Now reverse the experiment : fill the bottle with water, and plunge its neck quickly into a glass of wine, with its mouth downwards j the wine is taking- place of the water. C. Could you decanter a bottle of wine in this way without turning it up ? F. I could, if the neck of the decanter were suf- ficiently small. The negroes in the West Indies are said to be well acquainted with this part of hydrosta- tics, and to plunder their masters of rum by filling a common bottle with water, and plunging the neck of it into the bung-hole of the hogshead. E. Poor creatures, they ought to have something to console them for the miseries they endure. F, Indeed the cruelties that are in general exer- cised upon the slaves very much extenuate the crime of pilfering, of which they are said to be guilty. Upon the principle of lighter fluids keeping the uppermost parts of a vessel, several fluids may be. placed upon one another in the same vessel without mixing : thus in a long upright jar, three or four inches in diameter, I can place water first, then port wine, then oil, brandy, oil of turpentine, and alcohol. C. How would you pour them in one upon another without mixing ] -F. This will require a little dexterity : when the water is in, I lay a piece of very thin pasteboard upon its surface, and then pour in the wine ; after which I take away the pasteboard, and proceed in the same manner with the rest. Take a common goblet, or drinking glass, pour water in, and then lay a thin piece of toasted bread upon the water, and you may pour your wine upon the bread, and the two fluids will remain for some time separate. E. Is the toast placed merely to receive the shock of the wine when poured in ? F. That is the reason. Now I will proceed to ex- plain the principle of the hydrometer, an instrument 222 HYDROSTATICS. contrived to ascertain with accuracy and expedition the specific gravities of different fluids. A B is a hollow cylindrical tube of glass, ivory, copper, &c. five or six inches long, annexed to a hollow sphere of copper d : to the bottom of this is united a smaller sphere e, containing a little quicksilver, or a few leaden shot, suflficient to poise the machine, and make it sink vertically in the fluid. C. What are the marks on the tube 1 F. They are degrees, exhibiting the mag- nitudes of the part below the surface, con- sequently the specific gravity of the fluid in which it descends. If the hydrometer, when placed in water, sinks to the figure 10, and in spirits of wine to 11.1, then the specific gravity of the water is to that of the spirit as 11.1 to 10. For if the same body float upon different fluids, the specific gravity of these fluids will be to each other inversely as the parts of the body immersed. E. By inversely, do you mean that the fluid in which the hydrometer sinks the deepest is of the least specific gravity? jP. Yes 1 do : here is a piece of dry oak, which if I put into spirits of wine is entirely immersed ; in water the greatest part of it sinks below the surface ; but in mercury it scarcely sinks at all. Hence it is evident that the hydrometer will sink deepest in the fluid that is of the least specific gravity. To render this instrument of more service, a small stem is fixed at the end of the tube, upon which weights like that at g may be placed. Suppose then the weight of the instrument is 10 dwts. and by being placed in any kind of spirit it sinks to a certain point L, it will require an additional weight, suppose 1.6 dwt. to sink it to the same depth in water: in this case the specific gravity of the water to the spirit will be as 11.6 to 10. By the addition of different weights the specific gravity of any kind of liquor is OF THE HYDROMETER. 223 easily found. The point l should be so placed as to mark the exact depth to which the instrument will sink in the liquor that has the least specific gravity. C. But you always make the specific gravity of water 1, for the sake of a standard. F, Kight : and to find the specific gravity of the spirit compared with water at 1, I say as 11 6 : 1 : : 10: .862 nearly, so that I should put the specific gravity of this spirit down at .862 in a table wherp water was marked 1 : and as a cubic foot of water weighs 1000 ounces, a cubic foot of this spirit would weigh 862 ounces, which is generally the standard of pure rectified spirit, E. Is this what is usually called spirits of wine F, JVo ; it is the alcohol of the chemists, one pint of which added to a pint of water make a quart nearly of common spirits of wine. C. You said .862 was generally the specific gravity of alcohol : what causes the difference at other times ? F. It is not always manufactured of equal strength ; and the same fluids vary in respect to their specific gravity by the difterent degrees of heat and cold in the atmosphere. The cold of winter con- denses the fluid and increases the specific gravity ; the heat of summer causes an expansion of the fluid, and a diminution of its specific gravity. F. You said just now that a pint of water added to a pint of alcohol made nearly a quart of spirits of wine; surely two pints make a/u// quart? F, Irideed they will not. A pint of water added to a pint of water will make a quart : and a pint of spirit added to a pint of spirit will make a quart : but mix a pint of spirit with a pint of water, and there is a certain chemical union or penetration between the particles of the two fluids, so that they will not make a quart. This subject we will resume in our Chemical Conversations.* * See Dialogues on Chemistry. 22-1 HYDROSTATICS. CONVERSATION XVII. ■ OF THE HYDROMETER, AND SWIMMING. C. To what purposes is the hydrometer applied ? F. It is used in breweries and distilleries to ascer- tain the strength of their different liquors : and by this instrument the excise officers gauge the spirits, and thereby determine the duties to be paid to the revenue. I think, from the time we have spent in considering the specific gravity of different bodies, you will be at no loss to account for a variety of circumstances that will present themselves to your attention in the com- jnon concerns of life. Can you, Emma, explain the theory of floating vessels ? E, All bodies whatever that float on the surface of the water displace as much fluid as is equal in weight to the weight of the bodies ; therefore, in order that a vessel may keep above water, it is only necessary to take care that the vessel and its cargo, passengers, &c. should be of less weight than the weight of a quantity of water equal in bulk to that part of the vessel which it will be safe to immerge in the water. F. Salt water, that is, the water in the sea, is specifically heavier than fresh or river water. C. Then the vessel will not sink so deep at sea as It does in the Thames. F. That is true; if a ship is laden at Sunderland, or any other sea-port, with as much coals or corn as it can carry, it will come very safely till it reach the fresh water in the Thames, and there it will infallibly go to the bottom unless some of the cargo be taken out. E. How much heavier is sea water than the fresh ? F. About one-thirtieth part, which would be a guide to the master of a vessel, who was bent upon freighting it as deeply as possible. C. In bathing, I have often tried to swim, but have OF SWIMMING. 225 not yet been able to accomplish the task : is my body specifically heavier than the water ? F. I hope you will learn to swim, and well too ; it may be the means of saving your own life, and rescu- ing others who are in danger of drowning. J5y some very accurate experiments made by Mr. Robertson, a late librarian of the Royal Society, upon ten different persons, the mean specific gravity of the human body was found to be about one-ninth less than that of common river water. C. Why then do I sink to the bottom? I ought to swim like wood on the surface. F, Though you are specifically lighter than water, yet it will require some skill to throw yourself into such a position as to cause you to float like wood. C. What is that position *! F. Dr. Franklin recommends a person to throw himself in a slanting position on his back, but his whole body, except the face, should be kept under water. Unskilful persons in the act of attempting this are apt to plunge about and struggle : by this means they take water in at their mouths and nostrils, which of itself would soon render them as heavy, or heavier, than the water. Moreover the coldness of the stream tends to contract the body ; perhaps fear has the same tendency; all these things put together will easily account fpr a person sinking in the water. E. But if a dog or cat be thrown into the pond they seem as terrified as I should be in a like situa- tion, yet they never fail in making their way out by swimming. F. Of all land animals man is probably the most helpless in this element. The brute creation swim naturally: the human race must acquire the art by practice. In other animals the trunk of the body is large, and their extremities small : in man it is the reverse, the arms and legs are small in proportion to the bulk of the body, but the specific gravity of the extremities is greater than that of the trunk, conse- L 2 226 HYDROSTATICS. quently it will be more difficult for man to keep above water than for four-footed animals : besides, the act of swimming seems more natural to them than to us, as it corresponds more nearly to their mode of walking and running than to ours. C. I will try the next time I bathe to throw myself on my back according to Dr. Franklin's directions. F. Do not forget to make your experiments in water that is not so deep as you are high by at least a foot, unless you have an experienced person with you ; because an unsuccessful experiment in this element, where it is but a little out of your depth, may be the last you will m.ake. And neither your sister nor I can spare you yet. C. I once jumped into a part of the New Kiver, which I thought did not appear deeper than you say, and I found it was over my head ; but there were several persons there who soon put me in shallower water. F. It is not so generally known as it ought to be, that the depth of a clear stream of water is always one-fourth part greater than it appears to be.* C. If the river appear to be only three feet deep, may I reckon upon its being full four feet? F. Yes ; you must estimate it in this manner. Remember also, that if a person sink slowly in water ever so deep, a small effort will bring him up again, and if he be then able to throw himself on his back, keeping only his face above water, all will be well : but if instead of this he is alarmed, and by struggling throw himself so high above the water that his body does not displace so much of it as is equal to his weight, he will sink with an accelerated motion : a still stronger effort, which the sense of danger will inspire, may bring him up again, but in two or three efforts of this kind his strength fails, and he sinks to rise no more alive. * The reason of this deception will be explained in our conversations on Optics : Conversation IV. OF THE SYPHON. 227 E. Is It the upward pressure which brings up a person that is at a considerable depth in the v/ater ? F. It is : this upward pressure balances the weight of water which he sustains, or he would be crushed to pieces by it. ■ , • i. Cork an empty bottle ever so well, and with weights plunge it down a hundred yards into the sea, and the p'ressure of the water wiU force the cork into the bottle. .4 ll CONVERSATION XVIII. OF THE SYPHON. F, This bended tube is called a sy- /^v phon, and it is used to draw off water, dp wine, or other fluids, from vessels which ^ ' - it would be inconvenient to move from the place in which they stand. ^ C. I do not see how it can draw liquor jf out of any vessel :— why is one leg longer Yicr, 26. than the other 1 F. I will first shew you how the operation is per- formed, and then endeavour to explain the principle. I fill the tube edc with water, and then placing a finger on e, and another on c, I invert the tube, and immerse the shorter leg into a jar of water ; and having taken my fingers away you see the water runs over in a stream. E. Will it continue to flow over1 • F. It will till the water in the vessel comes as low as E, the edge of the syphon. C. Is this accounted for by pressure? F, To the pressure or weight of the atmosphere we are indebted for the action of the syphon, pumps, &c. At present you must take it for granted that the air which we breathe, thdugh invisible, has weight, and that the pressure occasioned by it is equal to about 14 228 HYDROSTATieS. or 15 pounds upon every square inch.* The surface of this table is equal to about six square feet, or 864 square inches, and the pressure of the atmosphere upon it is equal to at least 12,000 pounds. E. How does the pressure of the air cause the water to run through the syphon 1 F, The principle of the syphon is this : the two legs are of unequal length, consequently the weight of water in the longer leg is greater than that in the shorter, and therefore will, by its own gravity, run out at c, leaving a vacuum from d to e, did not the pressure of the atmosphere on the surface of the water in the jar force it up the leg de, and thus con- tinually supply the place of the water in dc. C. But since the pressure of fluids acts in all direc- tions, is not the upward pressure of the atmosphere against c, the mouth of the tube, equal to the down- ward pressure on the surface of the water ? jP, The pressure of the atmosphere may be con- sidered as equal in both cases. But these equal pressures are counteracted by the pressures of the two unequal columns of water, de and dc. And since the atmospheric pressure is more than sufficient to balance both these columns of fluid, that which acts with the lesser force, that is, the column de, vAW be more pressed against dc, than dc is against de at the vertex d ; consequently the column de will yield to the greater pressure, and flow off through the orifice c. E. Would the same thing happen if the outer leg DC were shorter than the other ] F. If DC were broken oflT at b, even with the sur- face of the water, no water would run over : or if it were broken olf any where lower than b, it would only run away till the surface of the fluid descended * If any of my young readers are unwilling- to admit this assertion without proof, they mnst be referred to the 4th, 5th, and Cth Conversations on Pneumatics of these Dialogues, for a complete demonstration of the fact. OF THE SYPHON, 229 to a level vi^ith the length of the outer tube, because then the column de will be no more pressed against DC, than DC is against de, and consequently the s}'- phon will empty itself, the water in the outer leg will run out at the lower orifice, and that in the inner will fall back into the jar. C. In decanting a bottle of wine, are you obliged first to fill the syphon with liquor, and then invert it? -F. No : a small pipe is fixed to the outer leg of the syphon, by which the air is drawn out of it by the mouth, and the short leg being immersed in the wine, the fluid will follow the air, and run out till the bottle is empty. The syphon is sometimes disguised for the sake of amusing young people. Tantalus's cup is of this kind. The longer leg of the sy- phon passes through and is cemented into the bottom of the cup : if water be poured into the cup, so as not to stand so high as the bend of the tube, the water will remain as in any common vessel ; but if it be raised over the bended part of the syphon, Fig. 27. it will run over, and continue to run til] the vessel is emptied. Sometimes a little figure of a man, representing Tantalus, conceals the syphon, so that Tarftalus, as in the fable, stands up to his chin in water, but is never able to quench his thirst, for just as it comes to a level with his chin, it runs out through the concealed syphon. This is another kind of Tantalus's cup, but the syphon is concealed in the handle, and when the water in '2" the cup, which communicates with the shorter leg at c, is raised above the bend of the handle, it runs out through the longer leg at p, and so continues till the cup is empty. This cup is often made to deceive the unwary, who, by taking it up to drink, cause the water, which was, while at rest, below the bend of 230 HYDROSTATICS. tlie syphon, to run over, and then there is no means of stopping the stream till the vessel is empty. C. I have frequently seen at the doors of public houses large hogsheads of spirits in carts or waggons, and persons drawing off the contents by means of an instrument like a syphon. F. That is called a distiller's crane or syphon, b represents one of these barrels with the crane at work from the bun^- hole n. The longer leg mr is about three feet long, with a stop-cock near the middle, which must be shut, and then the shorter leg is immersed in the Hquor. E. Is the air in the short leg forced into the other by the upward pressure of the fluid. F. It is ; and the cock being shut it cannot escape, but will be very much condensed. If then the cock be suddenly opened, the condensed air will rush out, and the .pressure of the air on the liquor in the vessel will force it over the bend of the syphon, and cause it to flow off in a stream, as the figure represents. If, however, the barrel be not full, or nearly so, then it is necessary to draw the air out of the syphon by means of a small tube, ah, fixed to it. By the principle of the syphon we are enabled to explain the nature of intermitting springs. E. What are these, papa ? F. They are springs, or rather streams, that flow pe- riodically. A figure will give a clearer idea of the subject than many words without. GFc represent a cavity in the bowels of a hill, or mountain, from the bottom of which, c, proceeds the irregular cavity ced, forni- Fior. OF THE DIVER'S BELL. 231 ing a sort of natural syphon. Now, as this cavity fills, by means of rain or snow draining through the pores of the ground, the water will gradually rise in the leg CE, till it has attained the horizontal level hh, when it will begin to flow through the leg ed, and continue to increase in the quantity discharged as the water rises higher, till a full stream is sent forth, and then, by the principle of the syphon, it must continue to flow till the water sink to the level ii, when the air will rush into the syphon and stop its motion. C. And being once brought so low, it cannot run over again till the cavity is full of water, or, at least, up to the level hh, which, as it is only supplied by the draining of the water through the ground, must take a considerable length of time. Is that the reason why they are called intermitting springs 1 F, It is ; Mr. Clare, in his treatise "Oh the Mo- tion of. Fluids," illustrates the subject by referring to a pond at Gravesend, out of which the water ebbs all the time the tide is coming into the adjacent river, and runs in while the tide is going out. Another in- stance mentioned by the same author is a spring in Derbyshire, called the Wedding-well, which, at cer- tain seasons, issues forth a strong stream, with a sing- ing noise, for about three minutes, and then stops again. At Lambourn, in Berkshire, there is a brook which in summer carries down a stream of water suf- ficient to turn a mill ; but during the winter there is scarcely any current at all. CONVERSATION XIX. OF THE diver's BELL. Take this ale-glass, and thrust it with the mouth downwards into a glass jar of water, and you will per- ceive that but very little water will enter into it. C. The water does not rise in it more than about a quarter of an inch : if I properly understand the sub- ject, the air which filled the glass before it was put in 232 HYDROSTATICS. water is now compressed into the smaller space ; and it is this body of air that prevents more water getting into the glass. F. That is the reason : for if you tilt the glass a little on one side, a part of the air will escape in the form of a bubble, and then the water will rise higher in the glass. Upon this simple principle machines have been in- vented, by which people have been able to walk about at the bottom of the sea, with as much safety as upon the surface of the earth. The original ma- chine of this kind was much improved by Dr. Halley, more than a century ago ; it was called the Diver's C. Was it made in the shape of a bell? F, It was ; and as great strength was required to resist the pressure of the water, he caused it to be made of copper ; this is a representation of it. The diameter of the bottom was five feet, that of the top three feet, and it was eight feet high : to make the vessel sink vertically in water, the bottom was loaded with a quantity of leaden balls. E. It was as large as a good sized closet; but how did he contrive to get light ? F. Light was let into the bell by means of strong spherical glasses fixed in the top of the machine. Bell. Fig. 31. OF THE DIVER'S BELL. 2'3S C. How were the divers supplied with air ? F. Barrels, filled with fresh air, were made suffi- ciently heavy, and sent down, such as that repre- sented by c ; from which a leathern pipe communi- cated with the inside of the bell, and a stop-cock at the upper part of the bell, let out the foul air. E. The little men seem to sit very contentedly under the bell, yet I do not think I should like a journey with them. F. Perhaps not ; but the principal mconvenience which divers experience arises from the condensation of the air in the bell, which though in the ale-glass was very trifling, yet at considerable depths in the sea is very great, and produces a disagreeable pres- sure upon all parts of the body, but more particu- larly in their ears, as if quills were thrust into them. This sensation does not last long, for the air pressing through the pores of the skin, soon becomes as dense within their bodies as without, when the sense of pres- sure ceases. E. They might stop their ears with cotton. F. One of them once thought himself as cun- ning as you, and for the want of cotton he chewed some paper, and stuflfed it into his ears : as the bell descended, the paper was forcibly pressed into the cavities, and it was with great difficulty and some danger that it was extracted by a surgeon. C. Are divers able to remain long under water ? F. Yes : when all things are properly arranged, if business require it they will stay several hours without the smallest difficulty. £. But how do they get up again ? F. They are generally let down from on board ship, and taking a rope with them, to which is fixed a bell in the vessel, they have only to pull the string, and the people in the ship draw them up. C. What does the figure E represent? F. A man detached from the bell, with a kind of inverted basket made of lead, in which is fixed another flexible leathern pipe, to give him fresh air from the 234 HYDROSTATICS. bell as often as he may find it necessary. By this method a man may walk to the distance of 80 or 100 yards from the machine. E. It is to be hoped his comrades will not forget to supply him with air. F. If his head is a little above that part of the bell to which the pipe communicates, he can by means of a stop-cock assist himself as often as he requires a new supply : and that man is always best helped who can help himself. C. I dare say that is a right principle ; in the pre- sent case, I am sure, it would be exceedingly wrong to depend on another for that which might be done by one's self. CONVERSATION XX. OF THE diver's BELL. F. You see how, by this contrivance, the parts of wrecked vessels and their cargoes are saved from the devouring ocean ; and by what means people are enabled to pursue the business of pearl and coral fishing. E, Have there been no accidents attending this business ] F. There are very few professions, hovrever simple, the exercise of which, either through carelessness or inattention, is not attended with danger. The diving bell proved fatal to Mr. Spalding, and an assistant, who went down to view the wreck of the Imperial East Indiaman, near Ireland. They had been down twice, but on descending the third time they remained about an nour under water, and had two barrels of air sent down to them, but on signals from below not being again repeated, after a certain time, they were drawn up by their assistants, and both found dead in tlie bell. This accident happened by the twisting of some ropes, which prevented the unfortunate suffer- ers from announcing their wants to their companions Fig. 32. OF THE DIVER'S BELL. 235 in the ship.— Mr. Day also perished at Plymouth in a diving bell of his own construction, in which he was to have continued, for a wager, twelve hours, one hundred feet deep in water. C. Did these accidents put an end to the experi- ments ? F. No ; but they have led to im- provements in the structure and use of the machine. Mr. Smeaton very suc- cessfully made use of a square cast- iron chest, the weight of which, 50 cwt., was heavy enough to sink itself. It was 4J feet in height, the same number of feet in length, and three feet wide,, and of course afforded suf- ficient room for two men to work under it at a time. £. What are those round things at the top ] F. They are four strong pieces of glass to admit the light. The great advantage which this had above Dr Haliey's bell was, that the divers were supplied with a constant influx of air, without any attention of their ovvn, by means of ^a forcing air-pump, worked in a boat upon the water's surface. G. That is not represented in the plate. F. Look to the next figure, which is a diving machine of a different construction, invented by the very ino-enious and truly respectable lec- turer, Mr. Adam Walker,* by whose politeness 1 am enabled to copy the figure. This machine is of the shape of a conical tub, but little more than one- third as large as Mr. Smeaton*s. ' The balls at the bottom are lead, sufficiently heavy to make it sink of itself; a bended metal tube, 6c, is * See Walker's System of Natural Philosophy, 2 vols. 4to. 236 HYDROSTATICS. attached to the outside of the machine, with a stop- cock, and a flexible leathern tube to the other end c ; this tube is connected with a forcing air-pump d, which abundantly supplies the diver with fresh air. jE. Can he move about with the machine 1 F. IMost readily; for the pressure of the water being equal on all sides, he meets with very little re- sistance ; and the ropes and leathern tube being fiexible, he can, with the machine over his head, walk about several yards, in a perpendicular posture ; and thus having a more ready access to pieces of the wreck than in a cumbrous bell, he can easily fasten ropes to them, an#perform any sort of business nearly as well as on dry land. Mr. Walker says, that the greatest part of the wreck saved fxom the rich ship -Belgioso was taken up by means of this .bell. The following anecdote, given by this gentleman, will en- tertain my young readers. " As the" diver had plenty of air to spare, he thought a candle might be supported in the bell, and he could descend by night. He made the experiment, and presently found himself surrounded by fish, some very large, and many such as he had never seen before. They sported about the bell, and smelt at his legs as they hung in the water : this rather alarmed him, for he was not sure but some of the larger might taie a fancy to him ; he therefore rang his bell to be taken up, and the fish accompanied him with much good nature to the surface." OF PUMPS. 237 CONVERSATION XXI OF PUMPS. F. Here is a glass model of a com mon pump which acts by the pressure of the atmosphere on the surface of the water in which it is placed. E. Is this like the pump below stairs? F. The principle is exactly the same : j i-^ a represents a rmg of wood, or metal, f^^J^^ with pliable leather fastened round it to — fit the cylinder A. Over the whole is a valve of metal covered with leather, Fig. 34. of which a part serves as a hinge for the valve to open and shut by. C, What is a valve, sir ? F. It may be described as a kind of lid or trap- door, th^t opens one way into a tube, but which the more forcibly it is pressed the other way, the closer the aperture is shut : so that it either admits the en- trance of a fluid into the tube, and prevents its return ; or permits it to escape, and prevents its re-entrance. Attend now to the figure : the handle and rod r end in a fork, which passes through the piston, and is screwed fast to it on the under side. Below this, and over a tube of a smaller bore, as s, is another valve V opening upward, which admits the water to flow up, but not to run down. E. That valve is open now, by which we see the size of the lower tube, but I do not perceive the upper valve. F. It is supposed to be shut, and in this situation the piston a is drawn up, and being air-tight, the column of air on its top is removed, and consequently leaves a vacuum in the part of the cylinder between the piston and the lower valve. C. I now see the reason of lifting up the handle to 238 HYDROSTATICS, pamp: because the piston then goes down to the lower valve, and by its ascent afterwards the vacuum is produced. F. And the closer the piston is to the lower valve the more perfect will be the vacuum. You know there is a pressure of the air on all bodies on or near the surface of the earth, equal to about 14 or 15 pounds on every square inch : this pressure upon the water in the well, into which the lower end of the pump is fixed, forces the water into the tube z above its level as high as /. C. What becomes of the air that was in that part of the tube 1 F. You shall see the operation : I will put the model into a dish of water which now stands at a level in the tube z, with the water in the.dish. I draw up the piston a, which causes a vacuum in the cylinder a. E. But the valve v opens, and now the water has risen as high as I. F. Because, when the air was taken out of the cylinder a, there was no pressure upon the valve v to balance that beneath it, consequently the air in the tube z opens its valve v, and part of it rushed into a. But as soon as part of the air had left the tube s, the pressure of the atmosphere upon the water in the dish was greater than that of the air in the tube, and therefore by the excess of pressure the water is driven into it as high as /. _ C. The valve v is again shut. F, That is because the air is diffused equally be- tween the level of the water at / and the piston a, and therefore the pressures over and under the valve are equal. And the reason that the water rises no higher than / is, that the air in that space is not only equally diffused, but is of the same density as the air without. Push down the piston a again. E. I saw the valve in the piston open. E. For the air between the piston and valve v could not escape by any other means than by lifting up the valve in a, I will draw up the piston. OF PUMPS. 239 C. The water has risen now above the valve v as high as ?«. .F. I dare say you can tell the cause of this ? C. Is it this : by lifting up the piston, the air that was between / and the valve v rushed into a, and the external pressure of the atmosphere forced the water after it ? F. And now that portion of air remains between the surface of the water m, and the piston. The next time the piston is forced down all the air must escape, the water will get above the valve in the piston, and in raising it up again it will be thrown out of the spout. E. Will the act of throwing that open the lower valve again, and bring in a fresh supply ? F, Yes : every time the piston is elevated, the ower valve rises, and the upper valve falls ; but every time the piston is depressed, the lower valve falls, and the upper one rises. E. This method of raising water is so simple and easy, that I wonder people should take the trouble of drawing water up from deep wells, when it might be obtained so much easier by a pump. F. I was going to tell you, my child, that the action of pumps, so beautiful and simple as it is, is very limited in its operation. If the water in the well be more than 32 or 33 feet from the valve v, you may pump for ever, but without any effect. C. That seems strange ; but why 33 feet in par- ticular ? F. I have already told you that it is the weight of the atmosphere which forces the water into the vacu- um of the pump : now if this weight were unlimited, the action of the pump would, be so likewise ; but the weight of the atmosphere is only about 14 or 15 pounds on every square inch ; and a column of water, of about 33 feet in height, and whose surface is one square inch, weighs also 14 or 15 pounds. I C. Then the weight of the atmosphere . would balance or keep in equilibrio only a column of water 240 HYDROSTATICS, of 33 feet high, and consequently could not support a greater column of water, much less have power to raise it up. F. The operation is entirely effected by the atmos- phere pressing on the surface of the water, by which it is forced into the place which the air previously occupied. E, A pump, then, would be of no use in the deep wells which we saw near the coast in Kent. F, None at all : the piston of a pump should never be set to work more than 28 feet above the water, because at some periods the pressure of the atmos- phere is so much less than at others, that a column of water something more than 28 feet, will be equal to the weight of the air. CONVERSATION XXII. OF TPIE FORCING-PUMP, FIRE-ENGINE, ROPE-PUMP, CHAIN-PUMP, AND WATER-PRESS. C. Why is this called the forcing- pump ? F. Because it not only raises the water into the barrel like the common pump, but afterwards forces it up into the reservoir k k. E. How is that operation performed, papa ] F. The pipe and barrel are the same as in the other pump, but the piston has no valve ; it is solid and heavy, and made air-tight, so that no water can get above it. C. Does the water come up through the valve a, as it did in the last? F, By raising up the piston, or, as it is generally called, the plunger g, a vacuum is made in the lower part of the barrel, into which, by the pressure of the air, the. water rushes from the well, as you shall see. £. And the valve is shut down. OF THE FORCING PUMP. 241 F. The water not being able to go back again, and being a fluid that is nearly incompressible, when the plunger is forced down it escapes along the pipe m and through the valve b into the vessel k. C. Though the water stands no higher than h, yet it flows through the pipe f to some height. F. The pipe f i is fixed into the top of the vessel, and is made air-tight, so that no air can escape out of it after the water is higher than i, the edge of the pipe. E. Then the whole quantity of air which occupied the space f 6 is compressed into the smaller space h F. F. You are right, and therefore the extra pressure on the water in the vessel forces it through the pipe, as you see. C. And the greater the condensation, that is, the more water you force into the vessel k, the higher the stream will mount. jP. Certainly : for the forcing pump difl^ers from the last in this respect, that there is no limit to the altitude to which water may be thrown, since the air may be condensed to almost any degree. The water-works at London-bridge, alluded to p. 197, exhibited a most curious engine, constructed on the principle of the forcing-pump : the wheel- work was so contrived as to move either way as the water ran? by these works 140,000 hogsheads of water were raised every day. E. Is there any rule to calculate the height to which an engine will throw water ? E. If the air's condensation be double that of the atmosphere, its pressure will raise water 33 feet ; if the condensation be increased three-fold, the water will reach 66 feet ; and so on, allowing the addition of 33 feet in height foi every increase of one to the number that expressed the air's condensation. C. Are fire-engines made in this manner ? E. They are all constructed on the same principle, M 242 HYDROSTATICS. but there are two barrels by which the water is alternately driven into the air-vessels : by this means the condensation is much greater; the water rushes out in a continued stream, and with such velocity, that a raging fire is rather dashed out than extin- guished by it. Garden-engines are also constructed on a principle similar to that we have been describing. This figure is the representation of a method of raising water from wells of con- a siderable depth. z&^^bz E. Is it a more convenient method than the wheel and axis ? f i F. The wheel and axis are adapted ^Jt^ merely to draw up water by buckets ; li~7j whereas the rope-pump is intended to L^^l throw water into a reservoir to almost any height. It consists of three hair-ropes Fig. 35. passing over the pulleys a and b, which have three grooves in each. The lower pulley b is immersed in the water, in which it is kept suspended by a weight ^r. The pulleys are turned round with great velocity by multiplying wheels, and the cords in their ascent carry up a considerable quantity of water, which they discharge into the box or reservoir Zy from whence by pipes it may be conveyed else- where. The ropes must not be more than about an mch apart. E. What is the reason of that, papa 1 F. Because, in that case, a sort of column of water will ascend between the ropes, to which it adheres by the pressure of the atmosphere. C. Ought not tliis column, in its ascent, to fall back by its own gravity ? F. Yes ; and so it would did not the great velocity of the ropes occasion a considerable rarefaction of the air near them, consequently the adjacent parts of the atmosphere pressing towards the vacuity, tend to sup- port the water. E. Can any considerable quantity of water be raised in this way 1 OF THE WATER-PRESS. 213 F, At Windsor a pump of this kind will raise, by the efforts of one man, about 9 gallons of water in a minute from a well 95 feet deep. In the beginning of motion, the column of water adhering to the rope is always less than when it has been worked for some time, and the quantity continues to increase till the surrounding air partakes of its motion. There is also another of these pumps at the same place, which raises water from the well in the round tower 178 feet in depth. C. What is a chain-pump 1 P. It consists of two square barrels, through which a chain passes, having several flat pistons, or valves, fixed to it, at certain distances. The chain passes round wheel-work, which is fixed at one end of the machine. A row of the pistons, which are free of the sides of the barrel, is always rising when the pump is at work ; and as it is usually worked with great velo- city, they bring up a full bore of water in the pump. C. What are the chief purposes for which the chain- pump is used 1 F. It has been used in the Navy, to prevent a repetition of the fatal accidents which have sometimes occurred on shipboard by the choking of pumps witli valves. It is adapted to raise water in all situations where it is mixed with sand, or other substances, which destroy common pumps, as in alum-works, mines, quarries, &c. It is now much improved — is simple and durable — and may be made of metal or wood. C. You told us some time ago that when we had seen the nature and understood the construction of valves, you would explain the action of the water-press. F, This is a good time for the purpose, and with it I shall conclude our Hydrostatical Couversations. a is a strong cast-iron cylinder, ground very accurately within, that the piston e may fit exceedingly close and well. I need scarcely tell you that the little figure repre- sents a forcing-pump, with a solid plunger c, and a valve n that opens Fig. 36. 244 HYDROSTATICS, upwards, through which the water is brought into the pipe n o. By bringing down the plunger r, the water in n o is forced through the valve x into the bottom of the cylinder, and thereby drives up the plunger e. C. What does m represent ? F. A bundle of hay, or bag of cotton, or any other substance that it may be desirable to bring into a compass twenty or thirty times less than it generally occupies. E, I see now the whole operation : the more water there is forced into o, the higher the plunger is lifted up, by which the substance m is brought into a smaller space. jP. Every time the handle s is lifted up the water rushes in from the well or cistern, and when it is brought down the water must be forced into the cylinder. The power of this engine is only limited by the strength of the materials of which it is made, and by the force applied to it. Mr. Walker says, a single man, working at s, can, by a machine of this kind, bring hay, cotton &c. into twenty times less compass than it was before ; con- sequently a vessel carrying light goods may be made to contain twenty times more package by means of the water-press than it could without its assistance. PNEUMATICS. CONVERSATION I. OF THE NATURE OF AIR. FATHER CHARLES EMMA. Father, That branch of natural philosophy which is called Pneumatics treats of the nature, weight, pressure, and spring of the air which we breathe, and of the several effects dependent upon these properties. Charles. You told us a few days ago, that the air, though to us invisible, is a fluid ; but it surely differs very much from those fluids which you conversed upon when treating of Hydrostatics. F. It does so ; but recollect the terms by which we defined a fluid. C. You distinguished a fluid as a body, the parts of which yield to the least pressure. F. The air in which we live and move will answer to this definition ; since we are continually immersed in it, as fish are in the water, if the parts did riot yield to the least force, we should be constantly reminded of its presence by the resistance made to our bodies ; whereas persons unaccustomed to think on these sub- jects are not even aware that they are surrounded with a fluid, the weight and pressure of which, if not counterbalanced by ^ome other power, would in- stantly crush the human frame. E. In a still calm day, such as the present is, when one can scarcely discern a single leaf in mo- tion, it is difficult to conceive of the existence of such a fluid ; but when Down at once Precipitant, descends a mingled mass Of roaring winds, and flames, and rushing- floods, ( Thomson's Summer) 246 PNEUMATICS. no doubi can remain as to the existence of some mighty unseen power. C. By this quotation, Emma, you take it for granted that the air and the winds are the same. F, This is really the fact, as we shall prove On a future day. C. But I am not quite satisfied that the air is such a body as you have described. F. I do not wish to proceed a single step till 1 have made your mind easy upon this head. — You see how easily those gold and silver fish move in the water : can you explain the reason of it 1 C, Is it not by the exertion of their fins 1 F, A fish swims by the help of his fins and tail ; and fish in general are nearly of the same specific gravity with water. Take away the water from the vessel, and the fish would still have the use of their fins and tail, at least for a short period. E. And they would flounder about at the bottom. F. Now consider the case of birds, how they fly ; the swallow, for instance, glides as smoothly along in the air as fish do in the water. But if 1 were to put a bird, or even a butterfly, under a glass receiver, however large, and take away the air, they would have no more use of their wings, than fish have of their fins when out of water. You shall see the ex- periment in a day or two. E. And would they die in this situation as fish die when taken from their natural element, the water ? jP. The cases are precisely similar : some fish, as the carp, the eel, and almost all kinds of shell-fish, will live a considerable time out of water ; so some creatures which depend upon air for existence will live a long time in an fexhausted receiver ; a butterfly, for instance, v^ll fall to the bottom apparently lifeless, but admit the air into the receiver, and it will revive ; whereas experiments have been made on mice, rats, birds, rabbits, &c. and it is found that they will live without air but a very few minutes. E. These are very cruel experiments. OF THE USES OF AIR. 247 F. And ought by no means to be indulged in ; they can be only justified upon the presumption, that io the hands, and under the direction, of able philoso- phers, they may lead to discoveries of importance to the health and happiness of the human race. C. Can fish live in water from which the air is wholly excluded ? , • • F. The air is, in fact, as necessary to their exis- tence, as it is to ours. Besides their fins, fish have the use of an air-vessel, which gives them full com- mand of their various motions in all depths of water, which their fins without it would not be equal to. E. What do you mean by an air-vessel 1 F, It is a small bladder of air, so disposed within them, that, by the assistance of their muscles, they are able to contract or dilate it at pleasure. By contraction they become specifically heavier than the water, and sink ; by dilatation they are lighter, and rise to the surface more readily. C. Are these opeiations effected by the external air 1 , . F. Very much so ; for if you take away the air from the water in which a lish is swimrning, it will no longer have the power of contracting the air- vessel within, which will then become so expanded as to keep it necessarily on the surface of the water, evidently to its great inconvenience and pam, and if the air-bladder be pricked or broken, the fish pre- sently sinks to the bottom, unable either to support or raise itself up again. Flat fish, as soles, plaise, tur- bot, &c. have no ajr-bladder. CONVERSATION II. OF THE AIR-PUMP. E. You told US, papa, of taking away the air from vessels ; will you shew us how that is performed 1 F. I will ; and I believe it will be the most con- 248 PNEUMATICS. vincing method of proving to you that the air is such a body as 1 have described. Fig. I- This instrument is called an air-pump, and its use is to exhaust the air from any vessel, as the glass receiver lk. C. Does it act like the common pump ? F. So much so, that if you comprehend the nature and structure of the one, you will find but little diffi- culty in understanding the other. I will, however, describe the different parts, a a are two strong brass barrels, within each of which, at the bottom, is fixed a valve, opening upwards ; these valves communicate with a concealed pipe that leads to k. The barrels include also moveable pistons, with valves opening upwards.* E. How are they moved ? F. To the upper parts of the pistons are attached rack-work, part of which you see at cc : these racks are moved up and down by means of a little cog- wheel, turned round by the handle r. C. You turn the handle but half way round. F. And by so doing, you perceive that one of the racks rises and the other descends. * The reader is supposed to have attended to the struc- ture of the common pump, described in Conversation XXI. of Hydrostatics. OF THE AIR-PUMP. E. What is the use of the screw v 1 F. It serves to re-admit air into the receiver when it is in a state of exhaustion, for without such a con- trivance, the receiver could never be moved out of its place, after the air was once taken from beneath it. But you shall try for yourselves. I first place a slip of wet leather under the edge of the receiver, be- cause the brass plate is liable to be scratched, and the smallest unevenness between the receiver and plate would prevent the success of our experiment. —I have turned the handle but a few times : try to take away the receiver. C. I cannot move it. F. I dare say not ; for now the greater part of the air is taken from under the receiver, and consequently it is pressed down with the weight of the atmosphere on the outside. E. Pray explain how the air was taken away. F. By turning the winch r half way round I raise one of the pistons, and thereby leave a vacuum in the lower part of the barrel, and a portion of the air in the receiver rushes through the pipe into the empty barrel. I then turned the winch the other way, which raised the other piston, and a vacuum would be left in that barrel, did not another portion of air rush from the receiver into it. . . C. When the first piston descended, did the air in the barrel open the little valve, and escape by the ^^V.^It did ; and by the alternate working of the pistons, so much of the air is taken away, that the quantity left has not force enough to raise the valve. C. Cannot you take all the air from the receiver ? F, Not by means of the air-pump. E. What is the reason that a mist comes on the inside of the glass receiver while the air is exhausting 1 E. It is explained by the sudden expansion of the air that is left in the receiver, which we shall notice more particularly in our Conversations on Chemistry. M 2 250 PNEUMATICS. C. You have not told us the use of the smaller receiver w, v^^ith the bottle of quicksilver within it. F. By means of the concealed pipe there is a com- munication between this and the large receiver, and the whole is intended to shew to what degree the air in the large receiver is exhausted. It is called the small barometer-guage, the meaning of which you will better understand when the structure of the barometer is explained. — I will now shew you an experiment or two, by which the resistance of the air is clearly demonstrated. £. Are these mills for the pur- pose ? F. Yes, they are ; the machine consists of two sets of vanes, a and b, made equally heavy, and to move on their axes with the same free- dom. C. But the vanes of a are placed edgeways, and those of b are breadthways. F, They are so placed to exhibit in a striking manner the resistance of the atmosphere ; for as the little mill a turns, it is resisted only in a small de- gree, and will go round a much longer time than the other, which, in its revolutions, meets the air with its whole surface. By means of the spring c resting against the slider d in each mill, the vanes are kept fixed. E. Shall I push down the sliders ? F. Do so : you see that both set off with equal velocities. C. The mill b is evidently declining in swiftness, while the other goes on as quick as ever. F, Not quite so ; for in a few minutes you will find them both at rest. Now we will place them under the receiver of the air-pump, and by a little contrivance we shall be able to set the mills going after the air is exhausted 1^1 Fig . 2. RESISTANCE OF THE AIR. 251 from the receiver, and then, as there is no sensible resistance against them, they will both move round a considerable time longer than they did in the open air, and the instant that one stops the other will stop also. E. This experiment clearly shews the resisting power of the air. F, It shews also that its resistance is in proportion to the surface opposed to it ; for the vane which met and divided the air by the edge only, continued to move the longest while they were both exposed to it ; but when that is removed, they both stop together, because there is nothing now to retard their motion but the friction on the pivots, which is the same in both cases. Take this guinea and a feather j let them both drop from your hand at the same instant. C. The guinea is soon at rest at my feet, but the feather continues floating about. Is the feather spe- cifically lighter than air ] F. No ; for if it were, it would ascend till it found the air no heavier than itself; whereas in a minute or two, you will see the feather on the floor as well as the guinea : it is however so light, and presents so large a surface, in comparison to its weight, to the air, that it is considerably longer in falling to the ground than heavier bodies, such as a guinea. Take away the resisting medium, and they will both reach the bottom at once. E. How will you do that ? y| F, Upon this brass flap I place the guinea cS^^ and the feather, and having turned up the flap and shut it into a small notch, I fix the whole on a tall receiver, with a piece of wet leather between the receiver and brass. I will now exhaust the air from under the receiver by placing it over the air-pump, and if I turn the wire/ a little, the flap will slip down, and the guinea and feather will fall with equal velocities. 252 PNEUMATICS. C. They are both at the bottom, but I did not see tliem fall. F. While I repeat the experiment, you must look stedfastly to the bottom, because the distance is too small for you to trace their motion ; but by keeping your eye at the bottom you will see the feather and the guinea arrive at the same instant. In this glass tube is some water, but the air is taken away, and the glass completely closed. Turn it up quick, so that the water may fall on the other end. E. It makes a noise like the stroke of a hammer. F, And for that reason it is usually called the philosophical hammer. The noise is ^ occasioned through the want of air to break j-- ^ the fall ; for if I take another glass in all respects like it, but having air inclosed in it as well as water, you may turn it as often as you please with hardly any noise. CONVERSATION III. OF THE TORRICELLIAN EXPERIMENT. C. If by means of the air-pump you cannot per- fectly exhaust the air from any vessel, by what means is it done 1 F. This glass tube is about 36 inches long, and open at one end only. I fill it very accurately with quicksilver, and placing my thumb over the open end, I invert the tube, and plunge it into a vessel of the same metal, taking care not to remove my thumb till the end of the tube is completely immersed in the quicksilver. — You observe the mercury is suspended in the tube to a certain height, and above it there is a perfect vacuum ; that is, in the six or seven inches at the upper part of the tube the air is perfectly ex- cluded. TORRICELLIAN EXPERIMENT. 253 E. Could not the air get in when you took away your thumb ? F. You saw that I did not remove my thumb till the open end of the tube was wholly under the quick- silver, therefore no air could get into the tube without first descending through the quicksilver : now you know that a lighter fluid will not descend through one that is heavier, and consequently it is impossible that any air should be in the upper part of the tube. C. What makes the quicksilver stand at that par- ticular height I F. Before I answer this, tell me what is the reason that water cannot be raised by means of a common pump higher than about 32 or 33 feet ? C. Because the pressure of the atmosphere is equal to the pressure of a column of water so many feet in height.* F. And the pressure of a column of quicksilver 29 inches long, a little more or less according to the variation of the air, is equal to the pressure of a column of water 32 or 33 feet high, and consequently equal to the pressure of the whole height of the atr/io- sphere. E. Is then the mercury in the tube kept suspended bv the weight of the air pressing on that in the cup 1 ■'F. It is. E. If you could take away the air from the cup, would the quicksilver descend in the tube ? F. If I had a receiver long enough to enclose the cup and tube, and were to place them on the air- pump, you would see the effect that a single turn of the handle would have on the mercury ; and, after a few turns, the quicksilver in the tube would be nearly on a level with that in the cup. I can shew you by means of this syringe, that the suspension of the quicksilver in the tube is owing to nothing but the pressure of the air, * See Hydrostatics, CDnversation XXI. 254 PNEUMATICS. C. What Is the structure of the syringe ? jP. If you understand in what manner a common water-squirt acts, you will be at no loss about the syringe, which is made like it. C. By dipping the small end of a squirt in water, and lifting up the handle, a vacuum is made, and then the pressure of the air on the surface of the water forces it into the squirt. F. That is the proper explanation. — This vessel d, containing some quick- silver, and the small tube gf, 33 inches long, open at both ends, immersed in it, are placed under a large receiver a b ; the brass plate c, put upon it with a piece of wet leather, admits the small tube to pass through it at h. I will now screw the syringe h on the tube g f, and by lifting up the handle i, a partial vacuum is made in the tube ; consequently the pressure of the air in the receiver upon the mercury in the cup d forces it up into Fig. 5. the little tube as high as a\ just in the same manner as water follows the piston in a common pump." E. But is not this rise of the quicksilver in the tube owing to the suction of the syringe 1 F. To prove to you that it is not, I place the whole apparatus over the air-pump, and exhaust the air out of the receiver a b. This operation, you must be sensi- ble, has not the smallest effect on the air in the syringe and little tube ; but you nevertheless observe, that the mercury has again fallen into the cup d : and the syringe might now be worked for ever without raising the mercury in the tube ; but admit the air into the receiver, and its action upon the surface of the quicksilver in the cup will force it instantly into the tube. This is called the Torricellian experiment, in honour of Tonicelli, a learned Italian, and disciple of Ga- OF THE PRESSURE OF THE AIR. 255 lileo, who invented it ; who was the first person that discovered the pressure and weight of the air, and the father of all modern discoveries respecting the proper- ties of the atmospheric air. CONVERSATION IV. OF THE PRESSURE OF THE AIR. C. It seems very surprising that the air, which is invisible, should produce such effects as you have described. F, If you are not satisfied with the evidence which your eyes are capable of affording, you would, perhaps, have no objection to the information which your feelings may convey to your mind. Place this little glass a b, open at both ends, over the hole of the pump plate, and lay your hand close upon the top b, while 1 turn the handle of the pump a few times. C. It hurts me very much : I cannot take my hand away. F. By letting in the air, I have released you. The pain was occasioned by the pressure of the air on the outside of your hand, that being taken away from under it which served to counterbalance its weight. This is a larger glass of the same kind ; over the large end I tie a piece of wet bladder very tight, and will place it on the pump, and take the air from under it. E. Is it the weight of the air that bends the bladder so much 1 Fig. 7. F. Certainly : and if I turn the handle a few more times it will burst. C. It has made a report as loud as a gun. F. A piece of thin flat glass may be broken in the 256 PNEUMATICS. Fio-. 8. same manner. — Here is a glass bubble a with a long neck, which I put into a cup of water b, and place them under a re- ceiver on the plate of the air-pump, and by turning- the handle the air is not only taken from the receiver, but that in the hollow glass ball will make its way through the water and escape. E. Is it the air which occasions the bubbles at the surface of the water ? F. It is. And now the bubbling is stopped, and therefore I know that as much of the air is taken away as can be got out by means of the pump. The hollow ball is still empty : and by turning the cock v of the pump (Fig. 1.) the air rushes into the receiver and presses upon the water, thereby filling the ball with the fluid. C. It is not quite full. F. That is because the air could not be perfectly exhausted, and the little bubble of air at the top is what, in its expanded state, filled the whole glass ball, and now by the pressure of the external air is re- duced into the size you see it. Another very simple experiment will convince you that suction has nothing to do with these experiments. On the leather of the air-pump at a little distance from the hole, I place lightly this small receiver .r, and pour a spoonful or two of water round the edge of it. I now cover it with a larger receiver a b, and exhaust the air. E, I see by the bubbles round the edge of the small receiver that the air is making its way from under it. F. I have pretty well exhausted all the air. Can you move the large receiver 1 C. No ; but by shaking the pump, I see the little one is loose. TW. 9. OF THE PRESSURE OF THE AIR. 257 F. The large one is rendered immoveable by the pressure of the external air. But the air being taken from the inside of both glasses, there is nothmg to fasten down the smaller receiver. E. But, if suction had any thing to do with this business, the little receiver would be fast as well as the other. F, Turn the screw v of the air-pump (Fig. 1.) quickly. You hear the air rushing in with violence. C. And the large receiver is loosened again. F. Take away the smaller one, Emma. E. I cannot move it with all my strength. F, Nor could you lift it up if you were a hundred times stronger than you are. For by admitting the air very speedily into the large receiver it pressed down the little one before any air could get under- neath it. C. Besides, I imagine you put the water round the edge of the glass to prevent the air from rushing be- tween it and the leather. F. You are right ; for air, being the lighter fluid, could not descend through the layer of water in order to ascend into the receiver. — Could suction produce the effect in this experiment ? C. I think not : because the little receiver was not fixed till after what might be thought suction had ceased to act. F, Kight : and to impress this fact strongly on your mind, I will repeat the experiment ; you observe that the air being taken from under both receivers, the large one must be fixed by the pressure of the atmosphere, and the smaller one must be loose, be- cause there is no pressure on its outside to fasten it. But by admitting the air, the inner one becomes fixed by the very means that the outer one is loosened. £. How will you get the small one away 1. F. As I cannot raise it, I must slide it over the hole in the brass plate ; and now the air gets under it, there is not the smallest difficulty ; otherwise, it could scarcely be lifted by the strength of any one. 258 PNEUMATICS. CONVERSATION V. OF THE PRESSURE OF THE AIR. C. Although suction has nothing to do in the ex- periments which you made yesterday, yet I think I can shew you an instance in which it has. This experiment, if such it may be called, I have made a hundred times. 1 fasten a string in the centre of a round piece of leather, and having thoroughly soaked it in water, I press it on a flat stone, and by pulling at the string the leather draws up the stone, although it be not more than two or three inches in diameter, and the stone weighs several pounds. Surely this is suction. J^. I should say so too if I could not account for it by the pressure of the atmosphere. By pressing the wet leather on the stone you displace the air, then by pulling the string a vacuum is left at the centre, and the pressure of the air about the edges of the leather is so great, that it requires a greater power than the gravity of the stone to separate them. I have seen you drink water from a spring by means of a hollow straw. E. Yes, that is another instance of what we have been accustomed to call suction. F. But now you know, that in this operation you make a syringe with the straw and your lips, and by drawing in your breath you cause a vacuum in the hollow straw tube, and the pressure of the air on the water in the spring forces it up through the straw into the mouth. C. I cannot, however, help thinking that this looks like suction, for the moment I cease the drawing in my breath, the Water ceases to rise in my mouth. F, That is, when there is no longer a vacuum in the straw, the pressure within is just equal to that without, and consequently the water will rest at its natural level. OF THE TRANSFERRER. 259 I will shew you anothei' striking instance of the efFects of the air's pressure. This instrument is called the transferrer. The screw gm c fits on to the plate of the «^ air-pump, and by means of 'J^\ the stop-cocks g and h, I ^ can take away the air from both, or either, of the re- Fig. 10. ceivers, i k, at pleasure. E. Is there a channel then running from c through DAB, and thence passing to the receivers? , F. There is. I will screw the whole on the air- pump, and turn the cock g, so that there is now no com- munication from C to the internal part of the receiver I. At present you observe that both the receivers are perfectly free. By turning the handle of the pump a few times the air is taken away from the receiver k, and to prevent its re-entrance I turn the stop-cock d. Try if you can move it. C. I cannot ; but the other is loose. F. The pressure of the atmosphere is evidently the same on the two receivers ; but with regard to i, the pressure within is equal to that without, and the glass is free ; in the other, the pressure from withm is taken away, and the glass is fixed. In this stage of the experiment you are satisfied that there is a vacuum m the receiver k. By turning the cock g, I open a communication between the two receivers, and you hear the air that was in i rush through the channel A B into K. Now try to move the glasses. E. They are both fixed : how is this 1. F. The air that was inclosed in the glass i is equally diflfused between the two, consequently the internal pressure of neither is equal to the external, and therefore they are both fixed by the excess of the external pressure over the internal. In this case it could not be suction that fixed the glass i, for it was free long after what might have been thought suction had ceased to act. 260 PNEUMATICS. C. What are these brass cups? /J^X F, They are called the hemispherical cups ; I will bring the two, b a, together, with a wet leather between them, and then screw them by D to the plate of the air- pump : and having exhaust- ed the air from the inside, I turn the stop-cock e, take them from the pump, and screw on the handle f. See if you two can separate them. E, We cannot stir them. F . If the diameter of these cups were four inches, the pressure to be overcome would be equal to 1801b! I will now hang them up in the receiver (Fig. 12.) and exhaust the air out of it, and you see they separate without the application of any force. C. Now there is no pressure on the outside, and therefore the lower cup falls off by its own gravity. -F. With this steel-yard you may try very accurately to what weight the pressure of the atmos- phere against the cups is equal.* E, For when the weight w is carried far enough to overcome the pressure of the cups, it lifts up the top one. F. I have exhausted the air of this receiver h, consequently it is fixed down to the brass plate i ; to the plate is joined a small tube with a stop-cock x ; by placing the lower end of the tube in a bason of water, and turning the cock, the pressure of the atmos- phere on the water in the bason forces it through the tube in the form of a fountain. This is called the fountain in vacuo. Fio-. 14. Fi^. 15. OF THE WEIGHT OF AIR. To this little square bottle a h is cemented a screw valve, by vi^hich I can fix it on the plate of the air-pump, and exhaust its air : and you will see that when there is no power within to support the pressure of the atmosphere from without, it will be broken into a thousand pieces. C. Why did you not use a round phial 1 F. Because one of that shape would have sustained the pressure like an arch. E. Is that the reason why the glass receivers are able to bear such a weight without breaking 1 F. It is : if mercury be poured into a wooden cup c, made of willow, and the air taken from under it, the mercury will, by the weight of the external air, be forced through the pores of the wood, and descend like a shower of rain. This experiment proves, satisfactorily, the great pressure of the atmosphere. Fig. 16, CONVERSATION VI. OF THE WEIGHT OF AIR. E. We have seen the surprising effects of the air's pressure ; are there any means of obtaining the exact weight of air ? F. If you do not require any very great nicety, the method is very simple. * The principle of the steel-yard (referred to p. 260) is explained in Conversation XV. of Mechanics. 2G2 PNEUMATICS This Florence flask is fitted up with a screw, and a fine oiled silk valve at d. I will now screw the flask on the plate of the air-pump, and exhaust the air. You see in its present ex- / „ i hausted state it weighs 3 ounces and 5 grains. Fig. 17. C. Cannot the air get through the silk? F, The silk, being varnished with a kind of oily substance, is impenetrable to air ; and, being ex- hausted, the pressure upon the outside effectually prevents the entrance of the air by the edges of the silk ; but if I lift it up by means of this sewing needle, you will hear the air rush in. E, Is that hissing noise occasioned by the re- entrance of the air 1 F, It is ; and when that ceases you may be sure the air within the bottle is of the same density as that without. C. If I weigh it again, the difference between the weight now, and when you tried it before, is the weight of the quantity of air contained in the bottle : — it weighs very accurately 3 ounces 19- grains, con- sequently the air weighs 14| grains. F. And the flask holds a quart, wine measure. E, Does a quart of air always weigh 14 j grains 1 F. The weight of the air is perpetually changing ; therefore though a quart of it weighs to-day 14~ grains, the same quantity may, in a few hours, weigh 14§ grains, or perhaps only 14 grains, or more or less. The air is much heavier this morning than it was at the same time yesterday. C. How do you know that ; did you weigh some yesterday 1 F, No ; but the rising and falling of the quick- silver in the barometer, an instrument which 1 shall hereafter very particularly describe, are sure guides by OF THE WEIGHT OF AIR. 203 which the real weight of ihe air is estimated ; and it stands full three-tenths of an inch higher now than it did yesterday. E. Will you explain how we may judge ot the different weights of the air by the barometer 1 F. This subject might, perhaps, be better discussed when we come to treat explicitly on that instrument ; but I will now answer your inquiry, although I should be in some danger of a repetition on a future day. I The mercury in a well-made barometer will always 1 subside till the weight of the column be exactly equivalent to the weight of the external air upon the : surface of the mercury in the bason, consequently ! the height of the mercury is a sure criterion by f which that weight is to be estimated.— Suppose, for example, the barometer stands at 29J inches, or, as it is usually expressed, at 29.5, and I find a quart of air at that time weighs 14| grains. Here then is a , standard by which I may ever after compare the gravity of the atmosphere. If to-morrow 1 find the ! quicksilver has fallen to 29.3, I shall know the air is not so heavy as it was ; because, in this case a column of quicksilver, 29.3 inches, balances the whole ' weight; whereas it required before a column equal ! to 29.5. If, on the contrary, when I look again, the ii mercury has risen to 30.7, as it really stands at this ! hour, I am sure the atmosphere is considerably I heavier than it was before, and that a quart of it will \ weigh more than 14| grains. i C. You intimated, that in weighing air the flask j could not be depended upon if great nicety were re- I quired ; what is the reason of that 1 ' F. I told you when explaining the operations of the air-pump, that it was impossible to obtain by I means of that instrument a perfect vacuum. The i want of accuracy in the flask experiment depends on 1 the small quantity of air that is left in the vessel after • the exhaustion is carried as far as it will go : this, however, if the pump be good, will, after 12 turns of 204 PNEUMATICS. the handle, be less than the 4000th part of the whole quantity. E. How do you know this ? F. You seem unwilling to take any thing upon my word ; and in subjects of this kind you do right never to rest satisfied without a reason for what is asserted. I suppose, then, each of the oarrels of the air-pump is equal in capacity to the flask ; that is, each will contain a quart ; then it is evident, that, by turning the handle of the pump, I exhaust all the air of one barrel and the air in the flask becomes at the same time equally diffused between the barrel and flask ; that is, the quart is now divided into two equal parts, one of which is in the flask and the other in the bar- rel. By the same reason, at the next turn of the handle, the pint in the flask will be reduced to half a pint ; and so it will go on decreasing, by taking away, at every turn, one half of the quantity that was left in by the last turn. C. Do you mean, then, that after the first turn of the handle, the air in the bottle is twice as rare as it was at first ; and after the second, third, and fourth turns, it is four times, eight times, and sixteen times as rare as it was when you beganl F, That is what I meant ; carry on your multipli- cation, and you will find that after the twelfth turn it is 4096 times more rare than it was at first. E. I now understand, that though absolute exact- ness be not attainable, yet in weighing this quart of air, the error is only equal to the 4096th part of the whole, which quantity may, in reasoning on the sub- ject, be overlooked. F. I will now exhaust the flask again of its air, and putting the neck of it under water, I will lift up the silk valve, and fill it with water. Now dry the out- side very thoroughly, and weigh it. C. It weighs 27 ounces. F. Subtract the weight of the flask, and reduce the remainder into grains, and divide by 14^, and you OF THE ELASTIGM Y OF AIR. 2fK) will obtain the specixic gravity of water compared with that of air. C. I have done it, and the water is something more than 800 times heavier than air. -F. Since, then, the specific gravity of water is always put at 1, that of air must be as 1 eight- hundredth, af least according to this calculation ; but following the more accurate experiments of Mr. Cavendish and others, whose authority may be safely appealed to, the specific gravity of air at the surface of the earth is 800 times less than that of water, when the barometer stands as high as 30 inches. Now tell me what the air in this room weighs, which room is 25 feet long, lO^ high, and 12| wide. E. By multiplying these three numbers together, the answer is 3281.25; so that the room contains rather more than 3281 cubic feet : the weight of a cubic foot of water is 1000 ounces, therefore the roomful of water would weigh 3,281,000 ounces ; but as air is 800 times lighter than water, the air in the room will weigh 3,281,000 800 = 4101oz. =r 2561bs. 5oz. It appears surprising that the invisible air should weigh so much, but as the computation is m.ade on careful experiments, the fact cannot be doubted. CONVERSATION VII. OF THE ELASTICITY OF AIR. F. I have told you that air is an elastic fluid. Now it is the nature of all elastic bodies to yield to pressure and to endeavour to regain their former figure as soon as the pressure is taken oflf. In pro- jecting an arrow from your bow, you exert your strength to bring the two ends nearer together, but the moment you let go the string, it recovers its for- mer shape ; the power by which this is effected is called elasticity. E, Is it not by this power that India rubber, after N 26G PNEUxMATIC3. it has been stretched, recovers its usual size aruj form 1 F, It is : and almost every thing that you make use of possesses this property in a greater or less de- gree : balls, marbles, the chords of musical instru- ments, are all elastic. C, I understand how all these things are elastic ; but do not see in what manner you can prove the elasticity of the air.* F, Here is a bladder, which we will fill v/ith air, and tie up its mouth, to prevent its escaping again. If you now press upon it with your hand, its figure will be changed ; but the moment the pressure is re- moved, it recovers its round shape. E. And if I throw it on the ground, or against an^' other obstacle, it rebounds, like balls or marbles. F. You are satisfied also, 1 presume, that it is the air that is the cause of it, and not the bladder that contains it. Let us have recourse to the air-pump, to exhibit some of the more striking eflfects of the air's elasticity. I will let a part of the air out of the bladder, and tie up its mouth again. The pressure of the external air renders it flaccid, and you may make what impression you please upon it, without its endeavouring to re- assume its former figure. E. What proof is there that this is owing to the external pressure of the air ? F. Such as will satisfy you both, I am sure. Place it under the receiver of the air-pump, exhaust the air, and see the consequences. C. It begins to swell out ; — and now it is as large as when it was blown out full of air. E. The outward pressure being in part removed, the particles of air, by their elasticity, distend, and fill up the bladder ; and if it were much larger, and the exhaustion were carried farther, the same small quan- tity of air would fill it completely. I will now let the air in again. * See Conver. XIII. Mechanics. OF THE ELASTICITY OF AIR. 267 E. This exhibits a very striking proof of the power and pressure of the external air, for the bladder is as flaccid as it was before. F. I put the same bladder into this square box without any alteration, and lay upon it a moveable lid, upon which I place this weight. By bringing the whole under a receiver, and exhausting the external air, the elasticity of that in the bladder will lift up the lid and weight together. C. If you pump much more the weight will fall against the side of the glass. F. I do not mean to risk that : — it is enough that you see a few grains, not half a dozen, of air will, by their elasticity, raise and sustain a weight of several pounds. Take this glass bubble (see Fig. 8.) ; the bore of the tube is too small for the water to run out ; but if I place it under the receiver of the air-pump, and take away the external air, the little quantity of air which is at the top of the glass will, by its elastic force, expand itself, and drive out all the water. E. This experiment shews, that a very small quan- tity of air is capable of filling a large space, pro« vided the external pressure is taken off. F. Certainly : I will take off the bladder from this glass. (See Hydrostatics, Fig. 19.) The little images all swim at the top, the air contained in them rendering them rather lighter than the water. Tie little leaden v/eights to their feet — these pull them down to the bottom of the vessel : I now place the glass under the receiver of the air-pump, and by ex- hausting the air from the vessel, that which is within the images, by its elasticity, expands itself, forces out more water, and you see they are ascending to the top, dragging the weights after them. I will let in the air, and the pressure forces the water into the images again, and they descend. Here is an apple very much shrivelled, which, Vv^hen placed under the receiver, and the external air taken 268 PNEUMATICS. away, will appear as plump as if it was newly gathered from the tree. E. Indeed it now looks so inviting, that I am ready to wish it was my own. F. Before, however, you can get it, all its beauty will fade. I will admit the air again. C. It is as shrivelled as ever. Do apples contain air] F, Yes, a great deal, and so in fact do almost all bodies that are specifically lighter than water, as well as many that are not so. It was the elastic power of the air within the apple that forced out all the shrivelled parts when the external pressure was taken away. Here is a small glass of warm ale, from which I am going to take away the air. E. It seems to boil now you exhaust the air from the receiver. F. The bubbling is caused by the air endeavouring to escape from the liquor. Let the air in again, and then taste the beer. C. It is flat and dead. F. You see of what importance air is to give to all our liquors their pleasant and brisk flavour, for the same will happen to wine and all other fermented fluids. E. How is it that the air, when it was re-admitted^ did not penetrate the ale again ? F. It could not insinuate itself into the pores of the beer, because it is the lighter body, and therefore will not descend through the heavier. Besides, it does not follow that it is the same sort of air which I admitted into the receiver, that was taken from the ale. E. Are there more kinds of air than one 1 F, Yes, very many ; as we shall shew you in our Conversations on Chemistry. That which I took from the beer, and which gives it the brisk and lively taste, is called fixed air, of which there is in general but a very small quantity in the atmosphere. OF THE ELASTICITY OF AIR. 269 The elasticity or spring of air contained in our flesh was clearly shewn by the experiment when I pumped the air from under your hand. C. Was that the cause of its swelling downward ? F, It was : and it will account for the pain you felt, which was greater, and of a very different kind, than what you would have experienced by a dead weight being laid on the back of the hand, equal to the pressure of the air. Cupping is an operation performed on this princi- ple ; the operator tells you he draws up the flesh, but if he were to speak correctly, he would say he took away the external air from ofl* the part of the body, and then the elastic force of the air within extends and swells out the flesh ready for his lancets. E, When I saw you cupped, he did not use an air-pump, but little glasses, to raise the flesh. F. Glasses closed at top are now generally made use of, in which the operator holds the flame of a lamp, by the heat of which the elasticity of the air in the glass is increased, and thereby a great part of it driven out. In this state the glass is put on the part to be cupped, and as the inward air cools, it con- tracts, and the glass adheres to the flesh by the difler- ence of the pressures of the internal and external air. By some persons, however, the syringe is con- sidered as the most eflectual method of performing the operation, because by flame the air cannot be rarefied more than one half, whereas by the syringe a few strokes will nearly exhaust it. Here is another little square bottle like that be- fore mentioned, (see Fig. 15.) only that it is full of air, and the mouth sealed so closely that none of it can escape. I enclose it within the wire cage b, and in this state bring them under the receiver, and ex- haust the external air. C. With what a loud report it has burst ! F, You can easily conceive now in what manner this invisible fluid endeavours, continually by its elastic force, to dilate itself. 270 PNEUMATICS. E. Why did you place the wire cage over the bottle 1 F. To prevent the pieces of the bottle froin break- ing the receiver, an accident that would be liable to happen without this precaution. Take a new-laid egg and make a small hole in the little end of it ; then, with that end downwards, place it in an ale-glass under the receiver and exhaust the air ; the whole contents of the egg will be forced out into the glass, by the elastic spring of the small bubble of air which is always to be found in the large end of a new-laid egg. CONVEUSATION VIII. OF THE COMPRESSION OF THE AIR. F. 1 have already alluded to the compressibility of air, which it is proper to describe here, it being a cori- sequence of its elasticity ; for whatever is elastic, is capable of being forced into a smaller space. In this respect, air differs very materially from other fluids. C. You told us, that water was compressible in a very small degree. F. I did so : but the compression which can be effected with the greatest power is so very small, that without the greatest attention and nicety in conductmg the experiments, it would never have been discovered. Air, however, is capable of being compressed into a very small space compared with what it naturally possesses. E. The experiment you made, by plungmg an ale- glass with its mouth downwards, clearly proved that the air which it contained was capable of being reduced into a smaller space. OF THE COMPRESSION OF AIR. 271 F. This bended tube a b c is closed at A and open at c. It is in the common state, full of air. I first pour into it a little quicksilver, just sufficient to cover the bottom a b ; now the air in each leg is of the same density, and, as that con- tained in A B cannot escape, because the lighter fluid vv^ill be always uppermost, when I pour more quicksilver in at c, Fig. 18. its vi^eight will condense the air in the leg A B, for the air which filled the whole length of the leg is, by the weight of the quicksilver in c b, pressed into the smaller space a x, which space will be diminished as the vv^eight is increased : so that by increasing the length of the column of mercury in c b, the air in the other leg will be more and more con- densed. Hence we learn that the elastic spring of air is always, and under all circumstances, equal to the force which compresses it. C. How is that proved ^ F. If the spring with v/hich the an- endeavours to expand itself, when it is compressed, were less than the compressing force, it must yield still farther to that force ; that is, if the spring of the air in a x were less than equal to the weight of the mercury in the other leg, it would be forced into a yet smaller space ; but if the spring were greater than the weight pressing upon it, it would not have yielded so much ; for you are well aware that action and re-action are equal, and act in opposite directions. You can nov/ easily understand why the lower regions of the atmosphere are more dense than those which are higher. £. Because they are pressed upon by all the air that is above them, and therefore condensed into a smaller space. F, Consequently, the air grows gradually thinner, till at a considerable height it may be conceived to degenerate to nothing. The different densities of the air may be illustrated by conceiving twenty or thirty 272 PNEUMATICS. equal paclcs of wool placed one upon another; the lowest will be forced into a less space, that is, its parts will be brought nearer together, and it will be more dense, than the next; and that will be more dense than the thu-d from the bottom, and so on till you come to the uppermost, which sustains no other pres- sure than that occasioned by the weight of the incumbent air. Let us now see the effects of condensed air by means of an artificial fountain. This vessel is made of strong copper, and about half full of water. With a syringe that screws to the pipe b a I force a con- siderable quantity of air into the vessel, so that it is very much condensed. By turning the stop-cock b while I take off the syringe no water can escape : and instead of the syringe I put on a jet, or very small tube, after which the stop- cock is turned, and the pressure of the condensed air forces the water through the tube to a very great height. C. Do you know how high it ascends ? -F. Not exactly : but as the natural pressure of the air will raise water 33 feet, so if by condensation its pressure be tripled, it will rise 66 feet. E. Why tripled 1 Ought it not to rise to this height by a double pressure ? F. You forget that there is the common pressure always acting against, and preventing the ascent of the water; therefore, besides a force within to balance that without, there must be a double pressure. C. You described a syringe to be like a common water squirt ; how are you able, by an instrument of this kind, to force in so great a quantity of air? will it not return by the same way it is forced in 1 F. 'V\\Q only difference between a condensing syringe and a squirt is, that, in the former, there is a valve that opens downwards, by which air may be forced through it, but the instant the downward pres- Fig. 19. OF THE COMPRESSION OF AIR. 273 sure ceases, the valve, by means of a strong spriQg, shuts of itself, so that none can return. E. Will not air escape back, during the time you are forcing in more of the external air ? F. That would be the case if the syringe pipe went no low^er than that part of the vessel which contams the air, but it reaches to a considerable depth in the water, and as it cannot find its way back up the pipe, it must ascend through the water, and cause that pressure upon it which has been described. C. To what extent can air be compressed 1 F. If the apparatus be strong enough, and a suf- ficient power applied, it may be condensed several thousand times ; that is, a vessel which will contain a gallon of air in its natural state may be made to contain several thousand gallons. By means of a fountain of this kind, young people, like yourselves, may receive much entertainment with only a few additional jets, which are made to screw on and olf. One kind is so formed that it will throw up and sustain on the stream a little cork ball, scat- tering the water all around. Another is made in the form of a globe, pierced with a great number of holes, all tending to the centre, exhibiting a very pleasing sphere of water. One is contrived to shew, in a neat manner, the composition and resolution of forces ex- plained in our Conversations on Mechanics.* Some will form cascades ; and by others you may, when the sun shines at a certain height in the heavens, exhibit artificial rainbows.f We will now force in a fresh supply of air, and try some of these jets. E. I observed in the upright jets that the height to which the water was thrown was continually di- minishing. F. The reason is this : that in proportion as the * See Conver. XIIT. of Meehanics. t This phenomenon we shall describe aiid explain when we treat of Optics. (Conver. XVIII. ") N 2 -74 PNEUMATICS, quantity of water in the fountain is lessened, the air has more room to expand, the compression is di- minished, and consequently the pressure becomes less, till at length it is no greater within than it is without, and then the fountain ceases altogether. CONVERSATION IX. MISCELLANEOUS EXPERIMENTS ON THE AIR-PUMP. F. I shall, to-day, exhibit a few experiments, without any regard to the particular subjects under which they might be arranged. In this jar of water I plunge some pieces of iron, zinc, stone, &c., and you will see that when I exhaust the external air, by bringing the jar under the receiver of the air-pump, the elastic spring of air contained in the pores of these solid substances will force them out in a multitude of globules, and exhibit a very pleasing spectacle, like the pearly dew-drops on the blades of grass ; but when I admit the air, they suddenly dis- appear. E. This proves what you told us a day or two ago, that substances in general contain a great deal of air. F. Instead of bodies of this kind, I will plunge in some vegetable substances, a piece or two of the stem of beet-root, angelica, &c. and now observe, when I have exhausted the receiver, what a quantity of air is forced out of the little vessels of these plants by means of its elasticity. C. From this experiment we may conclude that air makes no small part of all vegetable substances. F, To this piece of cork, which of itself would swim on the surface of water, 1 have tied some lead, MISCELLANEOUS EXPERIMENTS. 2?5 just enough to make it sink. But by taking off the external pressure, the cork will bring the lead up to the surface. E. Is that because, when the pressure is taken off, the substance of the cork expands, and becomes spe- cifically lighter than it was before? F, It is : this experiment is varied by using a blad- der, in which is tied up a very small quantity of air, and sunk in water : for when the external pressure is removed, the spring of air within the bladder will expand it, make it specifically lighter than water, and bring it to the surface. The next experiment shews that the ascent of smoke and vapours depends on the air. I will blow out this candle, and put it under the receiver ; the smoke now rises to the top, but as soon as the air is exhausted to a certain degree, the smoke descends like all other heavy bodies. C. Do smoke and vapours rise because they are lighter than the surrounding air ? F, That is the reason : sometimes you see smoke from a chimney rise very perpendicularly in a long column ; the air then is very heavy ; at other times you may see it descend, which is a proof that the density of the atmosphere is very much diminished, and is, in fact, less than that of the smoke. And at all times the smoke can ascend no higher than where it meets with air of a density equal to itself, and there it will spread about like a cloud. This figure is usually called the lungs- glass : a bladder is tied close about the little pipe ff, which is screwed into the bottle a. I introduce it under the receiver b, and begin to exhaust the air of the receiver, and that in the bladder communicating with it, will also be withdrawn ; the elastic force of the air in the bottle a will now press the bladder to the shrivelled state represented m the figure ; I will admit the air, which expands '^'^^ the 276 PNEUMATICS. bladder ; and thus by alternately exhausting and re- admitting the air, I shew the action of the lungs in breathing. But perhaps the following ex- periment will give a better idea of the sub- ject. A represents the lungs, b the wind- pipe leading to them, which is closely fixed in the neck of the bottle, from which the air cannot escape : d is a bladder tied to the bottom, and in its distended state will, with the internal cavity of the bottle, re- present that cavity of the body which sur- Fig. 21. rounds the lungs at the moment you have taken in breath : I force up d (as in this ii [] figure), and now the bladder is shrivelled by the pressure of the external air in the [ /' \ ] bottle, and represents the lungs just at the j p] moment of expiration. [ jy^/. | E. Does Fig. 21. shew the state of the '-^^^ lungs after I have drawn in my breath, and Fig. 22. when I have thrown it out Fig. 22. forcibly ? F. That is what the figures are intended to repre- sent, and they are well adapted to shew the eleva- tion and compression of the lungs, although I do not mean to assert, that the action of the lungs in breath- ing depends upon air in the same manner as that in the bladder does upon the air which is contained in the cavity of the bottle. I have exactly balanced on this scale-beam a piece of lead and a piece of cork : in this state I will intro- duce them under the receiver, and exhaust the air. C. The cork now seems to be heavier than the lead. F. In air each body lost a weight proportional to its bulk, but when the air is taken away, the weight lost will be restored ; but as the lead lost least, it will now retrieve the least, consequently the cork will preponderate with the difference of the weights re- stored by taking away the air. Thus you see that, in vacuo, a pound of feathers OF THE AIR-GUN. 277 would be heavier than a pound of lead ; because, as the air is a fluid substance, it tends to raise a body immersed in it, and its effect is proportional to the bulk of the body. CONVERSATION X. OF THE AIR-GUN, AND SOUND. F. The air-gun is an instrument, the effects of which depend on the elasticity and compression of air. E. Is it used for the same purposes as common guns 1 F. Air-guns will answer all the purposes of a musket or fowling-piece : bullets discharged from them will kill animals at the distance of 50 or 60 yards. They make no report, and on account of the great mischief they are capable of doing, without much chance of discovery, they are deemed illegal, and are, or ought to be, found no where but among the apparatus of the experimental philosopher. C. Can you shew us the construction of an air- gun ? E. It was formerly a very complex machine, but now the construction of air-guns is very simple ; this is one of the most approved- Fig. 23. E. In appearance it is very much like a common musket, with the addition of a round ball c. F, That ball is hollow, and co^ntainsthe condensed 2T8 PNEUMATICS. air, into which it is forced by means of a syringe, and then screwed to the barrel of the gun. C, Is there fixed to the bail a valve opening in- w^ards ? J^. There is : and when the leaden bullet is ram- med down, the trigger is pulled back, which forces down the hook b upon the pin connected with the valve, and liberates a portion of the condensed air, which rushing through a hole in the lock into the barrel, will impel the bullet to a great distance. E. Does net all the air escape at once 1 F. No : if the gun be well made, the copper ball will contain enough for 15 or 20 separate charges : so that one of these is capable of doing much more execution in a given time than a common fowling- piece. C, Does not the strength of the charges diminish each time ? -F. Certainly ; because the condensation becomes less upon the loss of every portion of air ; so that after a few discharges the ball will be projected only a short distance. To remedy this inconvenience, you might carry a spare ball or two ready filled with con- densed air in your pocket, to screw on when the other was exhausted. Formerly this kind of instrument was attached to gentlemen's walking-sticks. C. 1 should like to have one of them. F. I dare say you would : but you must not be trusted with instruments capable of doing much mis- chief, till it is quite certain that your reason will restrain you from actions that might annoy other persons. A still more formidable instrument is called the magazine wind-gun. In this there is a magazine of bullets as well as another of air, and when it is pro- perly charged the bullets may be projected one after another as fast as the gun can be cocked and the pan opened. In these the syringe is fixed to the butt of the gun, by which it is easily charged, and may be ke})t in that state for a great while. AHl THE MEDIUM OF SOUND. 1^79 E. Does air never lose its elastic power ? F. It would be too much to assert that it never will : but experiments have been tried upon different portions of it, which have been found as elastic as ever after the lapse of many months, and even years. C. What is this bell for 1 F. I took it out to shew you that air is the medium by which, in general, sound is communicated. I will place it under the receiver of the air-pump, and ex- haust the air. Now observe the clapper of the bell while I shake the apparatus. E. I see clearly that the clapper strikes the side of the bell, but 1 do not hear the least noise. F, Turn the cock and admit the air ; now you hear the sound plain enough and if I use the sy- ringe and a different kind of glass, so as to condense the" air, the sound will be very much increased. Dr. Desaguliers says, that in air that is twice as dense as comnion air, he could hear the sound of a bell at double the distance. C. Is it on account of the different densities of the atmosphere, that we hear St. Paul's clock so much plainer at one time than another? F. Undoubtedly the different degrees of density m the atmosphere will occasion some difference, but the principal cause depends on the quarter from which the wind blows, for as the direction of that is towards or opposite to our house, we hear the clock better or worse. E. Does it not require great strength to condense air? F. That depends much on the size of the pistx)n belonging to the syringe ; for the force required in- creases in proportion to the square of the diameter of the piston. Suppose the area of the piston is one inch, and you have already forced so much air into the vessel that its density is double that of common air, the resis- tance opposed to you will be equal to 15 pounds ; 280 PNEUMATICS. but if you would have it 10 times as dense, the resis- tance will be equal to 150 pounds. C That would be more than I could manage. F. Well, then, you must take a syringe, the area of whose piston is only half an inch ; and then the resistance would be equal to only the fourth part of 150 pounds, because the square of | is equal to j. E. You said that the air was generally the medium by which sound is conveyed to our ears is it not always so 1 F! Air is always a good conductor of sound, but water is a still better. Two stones being struck to- gether under water, the sound may be heard at a greater distance by an ear placed under water in the same river than it can through the air. In calm weather a whisper may be heard across the Thames. The slightest scratch of a pin at one end of a long piece of timber, may be heard by an ear applied near the other end, though it could not be heard at half the distance fhrough the air. The earth is not a bad conductor of sound : it is said, that by applying the ear to the ground, the trampling of horses may be heard much sooner than it could be through the medium of the air. Kecourse has sometimes been had to this mode of learning the approach of a hostile army. Take a long stiip of flannel, and in the middle tie a common poker, which answers as well as anything, leaving the ends at liberty : these ends must be rolled round the end of the first finger of each hand, and then stopping the ears with the ends of these fingers, strike the poker, thus suspended, against any body, as the edge of a steel fender : the depth of the tone which the stroke will return is amazing ; that made by the largest church bell is not to be compared with it. — Thus it appears that flannel is an excellent con- ductor of sound. OF SOUND. 281. CONVERSATION XI. OF SOUND. F. We shall devote this conversation to the con- sideration of some curious circumstances relating to sound ; which, as depending upon the air, will come very properly under Pneumatics. C. You shewed us yesterday that the stroke made by the clapper of a bell was not audible, when it was under an exhausted receiver; is air the cause of sound 1 F. Certainly in many cases it is : of this kind is thunder, the most awful sound in nature. E. Is thunder produced by the air 1 F. Thunder is generally supposed to be produced by the concussion or striking together of two bodies of air ; for lightning, darting through the air, causes, by its great velocity, a vacuum, and the separated bodies of air rushing together produce the noise we call thunder. The same effect, only in miniature, is produced by the inflammation of gunpowder. C. Can the report of a large cannon be called a miniature imitation 1 I remember being once^ in a room at the distance of but a few paces of the Tower guns, when they were fired, and the noise was infi- nitely worse than any thunder that 1 ever heard. F. This was because you were near to them : gun- powder, so tremendous as it is in air, when inflamed in a vacuum makes no more sound than the bell in like circumstances. Mr. Cotes mentions a very curious experiment, which was contrived to shew that sound cannot pene- trate through a vacuum. A strong receiver filled with common atmospheric air, in which a bell was suspended, was screwed down to a brass plate so tight that no air could escape, and this was included in a much larger receiver. When the air between the two receivers was exhausted, the sound of the bell could not be heard. 282 PNEUMATICS. E. Could it be heard before the air was taken away ? F. Yes ; and also the moment it was re-admitted. C. What is the reason that some bodies sound so much better than others ? Bell-metal is more musi- cal than copper or brass, and these sound much bet- ter than many other substances. F, All sonorous bodies are elastic, the parts of which by percussion are made to vibrate ; and as long as the vibrations continue, corresponding vibra- tions are communicated to the air, and these produce sound. Musical chords and bells are instances that will illustrate this. E. The vibrations of the bell are not visible ; and musical chords will vibrato after their sound has vanished. F. If light particles of dust be on the outside of a bell when it is struck, you will, by their motion, have no doubt but that the particles of the metal move too, though not sufficiently to be visible to the naked eye. And though the motion of a musical string continues after the sound ceases to be heard, yet it does not follow that sound is not still produced, but only that it is not sufficiently strong to produce a sensation in the ear. You see in a dark night the flash of a gun, but being at a considerable distance from it, you hear no report. If, however, you knew that the light was occasioned by the inflammation of gunpowder in a musket or pistol, you would conclude that it was attended with sound, though it was not sufficiently strong to reach the place where you are. C. Is it knov/n how far sound can be heard? F, We are assured upon good authority, that the unassisted human voice has been heard at the dis- tance of 10 or 12 miles, namely, from New to Old Gibraltar. And in the famous sea-fight between the English and Dutch, in 1672, the sound of cannon was heard at the distance of 200 miles from the place of action. — In both these cases the sound passed over water ; and it is well known that sound may be OF SOUND. 283 always conveyed much farther along a smooth than an uneven surface. Experiments have been instituted to ascertain how much water, as a conductor of sound, was better than land ; and a person was heard to read very distinctly at the distance of 140 feet on the Thames, and on land he could not be heard further than 76 feet. E. Might there not be interruptions in the latter case ? F, No noise w^hatever intervened by land, but on the Thames there was some occasioned by the flowing of the water. C. As we were walking last summer towards Hamp- stead, we saw a party of soldiers firing at a mark near Chalk Farm, and you desired Emma and me to take notice, as we approached the spot, how much sooner the report was heard after v/e saw the smoke, than it was when we first got into the fields. F. My intention was, that you should know from actual experiment that sound is not conveyed instan- taneously, but takes a certain time to travel over a given space. When you stood close to the place, did you not observe the smoke and hear the report at the same instant ? £. Yes, we did. F. Then you are satisfied that the light of the flash and the report are always produced together. The former comes to the eye with the velocity of light, the latter reaches the ear with the velocity with which sound travels : if then light travels faster than sound, you will, at any considerable distance from a gun that is fired, see the flash before you hear tlie report. Do you know with what velocity light travels ? C. At the rate of 12 millions of miles in a minute.* F. With regard then to several hundred yards, or even a few miles, the motion of light may be consi- dered as instantaneous, that is, there would be no assignable difference of time to two observers, one of whom should stand at the breech of the gun, and the * See Conver. XXVI. of Astronomy. 284 PNEUMATICS. other at the distance of six, or eight, or ten miles from it. -E. This I understand, because ten miles is as nothing when compared with 12 millions. F. Now sound travels only at the rate of about 13 miles in a minute, therefore as time is easily divisible into seconds, the progressive motion of sound is rea- dily marked by means of a stop-watch ; consequently, if persons situated, some close to a gun when it is discharged, others at a quarter of a mile from it, and others at half a mile, and so on, they v/ill all see the flash or smoke at the same instant, but the report will reach them at different times. C. Is it certain that sounds of all kinds travel at this rate 1 F. A great variety of experiments have been made on the subject, and it seems now generally agreed that sound travels with a velocity that is equal to 1142 feet in a second of time. E. Then with a stop-watch you could have told how far we were from the firing when we first saw it. F, Most easily, for I should have counted the number of seconds that elapsed between the flash and the report, and then have multiplied 1142 by the number, and I should have had the exact distance m feet between us and the gun. C. Has this knowledge been applied to any prac- tical purpose ? F. It has frequently been used at sea, by night, to know the distance of a ship that has fired her watch-guns. Suppose you were in a vessel, and saw the flash of a gun, and between that and a report, 24 seconds elapsed, what would be the distance of one vessel from another? E. I should multiply 1142 by 24, and then bring the product into miles, which in this instance is equal to somethmg more than five miles. -F. The mischief occasioned by lightning is sup- posed to depend much on the distance at which the storm is from the spot from whence it is seen. OF THE SPEAKING TRUMPET. 285 By counting the number of seconds elapsed be- tween the flash of lightning and the clap of thunder, j'ou may ascertain how far distant you are from the storm. C. I should like to have a stop- watch to be able to calculate this for myself. F. As it will, probably, be some time before you become possessed of this expensive instrument, I will tell you of something which you have always about you, and which will answer the purpose. E. What is that, papal F. The pulse at your wrist, which in healthy peo- ple generally beats about 75 times in a minute in the same space of time sound flies 13 miles : therefore, in one pulsation, sound passes over 13 miles divided by 75, that is about 915 feet, or the sixth part of a mile, consequently in six pulsations it will pass over a mile. E. If I see a flash of lightning, and between that and the thunder I count at my wrist 36 or 60 pulsa- tions, I say the distance in one case is equal to 6 miles, in the other to 10, because, if sound travel one- sixth of a mile in the interval between two pulsations 36 it will travel — = 6 miles, during 36 pulsations ; and ^ = 10 miles, during 60 pulsations. CONVERSATION XII. OF THE SPEAKING TRUMPET. C. I have been thinking about the nature of sound, and am ready to ask what it is ; I can conceive of particles of light issuing from the sun, or other lumi- nous bodies, but I know not what sound is. F. It would be but of little use to give you a defini- tion of sound, but I will endeavour to illustrate the subject. Sound is not a body like light, but it de- pends on the concussion or striking together of other * The pulse is quicker in children. '286 PNEUMATICS, bodies that are elastic, which being put into a tremu- lous motion excite a wave in the surrounding air. E. Is it such a wave as we see in the pond when it is ruffled by the wind? F. Rather such a one as is produced by throwing a pebble into still water. C. I have often observed this : the surface ot the water forms itself into circular waves. _ ^ F. It is probable that the tremulous motion of the parts of a sonorous body communicate undulations in the air in a similar manner. Two obvious circum- stances must strike every observer with regard to the undulations in water. (1.) The waves, the farther they proceed from the striking body, become less and less, till, if the water be of a sufficient magnitude, they become invisible, and die away. The same thing takes place with regard to sound, the farther a person is from the sounding body, the less plain it is heard, till at length the distance is too great for it to be audible : and (2.) the waves on the water are not propagated instantaneously, but are formed one after another in a given space of time. This, from what we have already shewn, appears to be the manner in which sound is propagated. E. Is sound the effect which is produced on the ear by the undulations of the air ? F. It is : and according as these waves are stronger or weaker, the impression, and consequently the sen- sation, is greater or less. If sound be impeded in its proo-ress by a body that has a hole in it, the waves pass through the hole, and then diverge on the other side as from a centre. Upon this principle the speak- 2 wo- trnmipet is constructed. C. What is that, sir? E. It is a long tube, used for the purpose ot making the voice heard at a considerable distance :-— the lenoth of the tube is from 6 to 12 or 15 feet, it is straio-ht'' throughout, having at one end a large aper- ture,° and the other terminates in a proper shape and s ze to receive the lips of the speaker. OF THF. SPEAKING TRUMPET. 237 E. Are these instruments much m use 1 F. It is believed that they were more used formerly than now; they are certainly of great antiquity; Alexander the Great made use of such a contrivance to communicate his orders to the army ; by means of which it is asserted he could make himself perfectly understood at the distance of 10 or 12 miles. Stentor is celebrated by Homer as one who could call louder than fifty men: and from Stentor the speaking- trumpet has been called the stentorophonic tube. C. Perhaps Stentor was employed in the army for the purpose of communicating the orders of the general, and he might make use of a trumpet for the purpose, and that is what is meant by brazen lungs. F, This is not an improbable conjecture. Well, besides speaking trumpets, there are others contrived for assisting the hearing of deaf persons, which differ but little from the speaking trumpet. Fig. 24. If A and B represent two trumpets, placed in an exact line at the distance of 40 feet or more from one another, the smallest whisper at a would be heard distinctly at h ; so that by a contrivance to conceal the trumpets, many of those speaking figures are constructed which are frequently exhibited in the metropolis and other large towns. E. I see how it may be done ; there must be two sets of trumpets, the one connected with the ear of the image into which the spectator whispers, and which conveys the sound to a person in another room, who by tubes connected with the mouth of the image returns the answer. C. How are the lips set in motion ? F. Very easily, by means of a string or wire pass- ing under the floor up the body of the image. 288 PNEUMATICS- CONVERSATION XIII. OF THE ECHO. F. Let US turn our attention to another curious subject relating to sound, and which depends on the air ; I mean the echo. E. I have often been delighted to hear my own words repeated, and I once asked Charles how it happened, that if I stood in a particular spot in the garden, and shouted loud, my words were distinctly repeated ; whereas if I moved a few yards nearer to the wall I had no answer. He told me that he knew nothing more than this, that in a part of Ovid's Me- tamorphoses, Echo is represented as having been a nymph of the woods, but that, pining away in love, her voice was all that was left of her. C. I did ; and you shall hear a translation of the whole passage : So wondrous are the effects of restless pain, That nothing but her voice and bones remain, " Nay, e'en the very bones at last are g'one, And metamorphosed to a thoughtless stone. Yet still the voice does in the woods survive ; The form's departed, but the sound's alive. E. But these lines say nothing of Echo being a nymph . C. Well, then, here are others applied immediately to Echo: A nymph she was, though only now a sound, Yet of her tongue no other use was found Than now she has; which never could be more Than to repeat what she had heard before. F. I doubt this will give your sister but little satisfaction respecting the cause of the echo which she has often heard, and which she may still hear, in ihe garden. E, No, I cannot conceive why a nymph of the OF THE ECHO. 289 woods should take up her residence in our garden • and the more so as I never saw her. ' F. If she is a mere sound, you cannot see her : I will endeavour to explain the subject. — When you throw a pebble into a small pool of water, what hap- pens to the waves when they reach the margin 1 C. They are thrown back again. F. The same happens with regard to the undula- tions in the air, which are the cause of sound. They strike _ against any surface fitted for the purpose, as the side of a house, a brick wall, a hill, or even against trees, and are reflected or beat back again ; this is the cause of an echo. E. I wonder then that we do not hear echoes more frequently. F . There must be several concurring circumstances before an echo can be produced. For an echo to be heard the ear must be in the line of re/lection. C. I do not knov/ what you mean by the line of reflection. F. I cannot always avoid using terms that have net been previously explained. This is an instance. I will, however, explain what is meant by the line of incidence, and the line of reflection. When you come to Optics these subjects will be made very familiar to you. You can play at marbles 1 C, Yes, and so can Emma. F. It is not a very common amusement for girls , however, as it happens, I shall find my advantage in it, as she will the more readily enter into my expla- nation. Suppose you were to shoot a marble agamst the wainscot; what would happen ? C. That depends on the direction in which I shoot it : if I stand directly opposite to the wainscot, the marble will, if I shoot it strong enough, return to my hand. F. The line which the marble describes in going to the wall is called the line of incidence, and that which it makes in returning, is the line of reflection. 290 PNEUMATICS. E. But they are both the same. F. In tliis particular instance they are so : but suppose you shoot obliquely or sideways against the board, will the marble return to the hand 1 C. Oh no : it will fly off sideways in a contrary direction. F. There the line it describes before the stroke, or the line of incidence, is different from that of reflec- tion, which it makes after the stroke, I will give you another instance : if you stand before the looking- glass you see yourself, because the rays of light flow from you, and are reflected back again in the same line. But let Emma stand on one side of the room, and you on the other : — you both see the glass at the upper end of the room. E. Yes, and I see Charles in it too. C. I see Emma, but I do not see myself. F. This happens just like the marble which you shot sideways. The rays flow from Emma obliquely on the glass, upon which they strike, and fly off in a contrary direction, and by them you see her. I will apply this to sound. If a bell a be struck, and the undulations of the air /' V - strike the wall c in a ^ .,/ perpendicular direction, they will be reflected '-^^ back in the Scime line ; and if a person were pro- perly situated between a and r, as at x, he would Fig. 25. hear the sound of the bell by means of the undulations as they went to the wall, and he would hear it again as they came back, which would be the echo of the first sound. E. I now understand the distinction between the direct sound and the echo. E. If the undulations strike the wall obliquely, they will, like the marble against the wainscot, or the rays of light against glass, fly off again obliquely on OF THE ECHO. 291 the other side, in a reflected line, as c m : now if there be a hill or other obstacle between the bell and the place m where a person happens to be standing, he will not hear the direct sound of the bell, but only the echo of it, and to him the sound will come along the line c m, C. I have heard of places where the sound is heard repeated several times. F, This happens where there are a number of walls, rocks, &c. which reflect the sound from one to the other ; and where a person happens to stand in such a situation as to intercept all the lines of reflec- tion. These are called tautological or babbling echoes. There can be no echo unless the direct and reflected sounds follow one another at a sufficient in- terval of time ; for if the latter arrive at the ear before the impression of the direct sound ceases, the sound will not be doubled, but only rendered more intense. E, Is there any rule by which the time may be ascertained ] F, Yes, there is : I will begin with the most simple case. If a person stand at x, (Fig. 25.) in order that the echo may be distinct, the difference between the space ax and ac, added to cx^ must be at least 127 feet. C. The space through which the direct sound travels to a person is a x, and the whole direct line to the wall is a c, besides which it has to come back through cx to reach the person again. AH this I comprehend : but why do you say 127 feet in par- ticular 1 F. It is founded on this principle. By experience it is known that about nine syllables can be articulately and distinctly pronounced in a second of time. But sound travels with the velocity of 1142 feet in a second, therefore in the ninth part of a second it passes over ^ , or 127 feet nearly, and conse- quently the reflected sound, which is the echo, to be 292 PNEUMATICS. distinct, must travel over at least 127 feet more than the direct. E. Ifcd in the figure represent the garden wall, how far must I be from it to hear distinctly any word I utter ? will 63 or 64 feet be sufficient, so that the whole space which the sound has to travel be equal in this case also to 127 feet ? F. It must be something more than this, because the first sound rests a certain time on the ear, which should vanish before the echo return, or it will appear a continuation of the former, and not a distinct sound : it is generally supposed the distance must not be less than 70 or 72 feet ; and this will give the distinct echo of one syllable only. C. Must the distance be increased in proportion to the number of syllables that are to be repeated ^ F, Certainly : and at the distance of about 1000 or 1200 feet, 8 or 10 syllables, properly pronounced, will be distinctly repeated by the echo. But I will finish this subject to-morrow. CONVERSATION XIV. OF THE ECHO. F, The following are among the most celebrated echoes. At Rossneath, near Glasgow, there is an echo that repeats a tune played with a trumpet three times completely and distinctly. In Gloucestershire, at Thornbury Castle, an echo repeats 10 or 11 times dis- tinctly. Near Rome there was one that repeated what a person said five times. At Brussels there is an echo that answers 15 times. Between Coblentz and Bingen an echo is celebrated as difl^erent from most others. In common echoes, the repetition is not heard till some time after hearing the vvords spoken or notes sung ; in this the person who speaks or sings is scarcely heard, but the repetition very clearly, and in surprising varieties: the echo in some cases OF THE VVHISPEHING GALLERY. 293 nppearsto be approaching, in others receding: some- times it is heard distinctly, at others scarcely at all : one person hears only one voice, while another hears several. And to mention but one more instance, in Italy, near Milan, the sound of a pistol is returned 56 times. E. This is indeed To fetch shrill echoes from their hollow earth. F. The ingenious Mr. Derham applied the echo to measuring inaccessible distances. C. How did he do this 1 F, Standing on the banks of the Thames, opposite Woolwich, he observed that the echo of a single sound was reflected from the houses in three seconds, consequently in that time it had travelled 3426 feet, the half of which, or 1713 feet, was the breadth of the river in that particular place. Did you ever hear of the Whispering-Gallery in the dome of St. Paul's Church ? E. Yes : and you promised to take us to see it some time. E. And I will perform my promise. In the mean time it may be proper to inform you, that the circum- stance that attracts every person's attention is, that the smallest whisper made against the wall on one side of the gallery is distinctly heard on the other side. C. Is this effect produced on the principle of the echo E. No : the undulations made in the air by the voice are reflected both ways round the wall, which is made very smooth, so that none may be lost, and meet at the opposite side ; consequently to the hearer the sensation is the same as if his ear were close to the mouth of the speaker. E. Would the effect be the same if the two persons were not opposite to one another ? E. In that case the woids spoken would be heard 294 PNEUMATICS. double, because one arc of the circle being less than the other, the sound will arrive at the ear sooner round the shorter arc than round the longer one. C. You said the wall is very smooth : is there a material difference, in the conveyance of sound, whether the medium be rough or smooth ? F . The difference is very great : still water is, perhaps, the best conductor of sound : the echo which I mentioned in the neighbourhood of Milan, depends much on the water over which the villa stands. Dr. Hutton, in his Mathematical Dictionary, gives the following instance as a proof that moisture has a considerable effect upon sound. A house in Lambeth-marsh is very damp during winter, when it yields an echo, which abates when it becomes dry in summer. To increase the sound in a theatre, at Home, a canal of water was carried under the floor, which caused a great difference. After water, stone is reckoned a good conductor of sound, though the tone is rough and disagreeable : a well-made brick wall has been known to convey a whisper to the distance of 200 feet nearly. Wood is sonorous, and produces the most agreeable tone, and is therefore the most proper substance for musical in- struments : of these we shall say a word or two before we quit the subject of sound. -E. All wind instruments, as flutes, trumpets, &c. must depend on the air : but do stringed instru- ments F . They all depend on the vibrations which they make in the surrounding air. I will illustrate what 1 have to say by means of the Eolian harp. If a cord eight or ten yards long be stretched very tight between two points, and then struck with a stick, the whole string will not vibrate, but there will be several still places in it, between which the cord will move. Now the air acts upon the strings of the harp in the same manner as the stroke of the stick upon the long cord just mentioned. OF THE EOLIAN HARP. 205 C Do not tlie different notes upon a violin depend upon the .Uffercnt length of the strings, which is varied by the lingers of tlie musician I F tLy do : and the current of air acts upon each string, and divides it into parts, as so m^^'jy ^^S'-;"/ bridges. Hence every string m an -Lolian harp though all are in unison, become capable of severa sounds, from which arises the wild and wonderful harmony of that instrument. , , ■ 'fhe undulations of the air, caused by the quick vibrations of a string, are well illustrated by a sort of mechanical sympathy that exists among accordant Tounds If two strings on different instruments are uned n unison, and one be struclc, the o her w,l! reply, Ihough they be several feet distant from one another. E. How is this accounted tor ! . , . n ,i ^ F. The waves made by the first string being of the same kind as would be made by the second if st.uck tCse waves give a mechanical stroke to the second ctrino- and produce its sound. C°'lf all the string-son the Eolian harp are set to the same note, will they all vibrate by striking only ""f They will : but the fact is well illustrated in this "method: bend little bits of F"- f string, and then strike one sufficiently to shake oft its pa;.er, and you will see the others will be shaken 'T'wm Sis happen if the strings are not in "xrv for yourself, alter the notes of all the strin'c^s but two, and place the papers on again : vibrate that string which is in umson "'''l;^^""*^/- , E. The papers on those are shaken off; but the wet finger pressed round the edge of a thin drinking-glass will produce its key ; if the glass be st"uck so\sto prodVe its pitch, and an uiuson to that pitch be strongly excited oa a violoncello, the 2^^'^ PNEUMATICS. glass will be set in motion, and if near the edge of the table will be liable to be shaken off. CONVERSATION XV. OF THE WINDS. F. Yoa know, my dear children, what the wind C. You told us, a few days ago, that vou should prove It was only the air in motion. F. I can shew you in miniature, that air in mo- tion will produce effects similar to those produced by a violent wind. I place this little mill under the receiver of the air-pump m such a manner, that the air when re- entermg may catch the vanes. I will exhaust the air;— now observe what happens when the stop- cock is opened. ^ F, The vanes turn round with an incredible velo- city ; much swifter than ever I saw the vanes of a i-eal wind-mill. But what puts the air in motion so as to cause the actual wind ? F. There are, probably, many conspiring causes to produce the effect. The principal one seems to be neat communicated by the sun. C. Does heat produce wind ? F. Pleat, you know, expands all bodies, conse- quently It rarefies the air, and makes it lio-hter. But you have seen that the lighter fluids Ascend, and thereby leave a partial vacuum, towards which the surrounding heavier air presses, with a greater or less motion, according to the degree of rarefaction or of heat which produces it. The air of this room, by means of the fire, is much warmer than that in the passage. E. Has that in the passage a tendency into the parlour? *^ F. Take thii lighted wax taper, and hold it at tlie bottom oi the door. OP THE WINDS. 29T E. The wind blows the flame violently into the room . F, Hold it now at the top of the door. C. The flame rushes outwards there. F. This simple experiment deserves your attention. The heat of the room rarefies the air, and the lighter particles ascending, a partial vacuum is made at the lower part of the room ; to supply the deficiency, the dense outward air rushes in, vv'hile the lighter particles, as they ascend, produce a current at the top of the door out of the room. If you hold the taper about the middle space between the bottom and top, you will find a part in which the flame is perfectly still, having no tendency either inwards or outwards. The smoke-jack^ so common in the chimneys of large kitchens, consists of a set of vanes, something like those of a wind-mill, or ventilator, fixed to wheel- work, which are put in motion by the current of air up the chimney, produced by the heat of the fire ; and of course the force of the jack depends on the strength of the fire, and not upon the quantity of smoke, as the name of the machine would lead you to suppose. E. Would you define the wind as a current of air ? F. That is a very proper definition : and its direc- tion is denominated from that quarter from which it blows. C. When the wind blows from the north or south, do you say it is in the former case a north wind, and in the latter a south wind ] F. We do. The winds are generally considered as of three kinds, independently of the names which they take from the points of the compass from which they blow. These are the constant, or those which always blow in the same direction : the periodical, or those which blow six months in one direction, and six in a contrary direction : and the variahie, which appear to be subject to no general rules. 2m PNEUMATICS. E. Is there any place where the wind always blows in one direction only ? F, This happens to a very large part of the earth ; to all that extensive tract that lies between 28 or 30 degrees north and south of the equator. C. What is the cause of this 1 F. If you examine the globe you will see * that the apparent course of the sun is from east to west, and that it is always vertical to some part of this tract of our globe ; and since the wind follows the sun, it must, of necessity, blow in one direction constantly E. Arid is that due east ? F. It is only so at the equator : for on the north of this line the wind declines a little to the north point of the compass, and this the more so as the place is situated farther towards the north ; on the south side the wind will be southerly. C. The greater part of this tract of the globe is water; and I nave heard you say, that transparent mediunis do not receive heat from the sun. F. The greater part is certainly water : but the proportion of land is not small : almost the whole contment of Africa, a great part of Arabia, Persia the East Indies, and China, besides the whole nearly of New Holland, and numerous islands in the Indian and Pacific oceans : and in the western hemisphere by far the greatest part of South America, ]^ew Spain, and the West India islands, come within the limits of 30 degrees north and south of the equator. 1 hese amazingly large tracts of land imbibe the heat, by which the surrounding air is rarefied, and thus the wind becomes constant ^ or blows in one direction. You will also remember, that neither the sea' nor the atmosphere are so perfectly transparent as to transmit all the rays of the solar light ; many are stopped m their passage, by which both the sea and * It is supjiosed tlie reader is acquainted M'ith tlie Conversations on Astronomy, OF THE WINDS. . 290 air are warmed to a considerable degree. These constant or general winds are usually called trade^ winds. E. In what part of the globe do the periodical winds prevail 1 F, They prevail in several parts of the eastern and southern oceans, and evidently depend on the sun ; for when the apparent motion of that body is north of the equator, that is, from the end of March to the same period in September, the wind sets m from the south-west : and the remainder of the year, while the sun is south of the equator, the wind blows from the north-east. These are called the monsoons, or shifting trade-winds, and are of considerable im- portance to those who make voyages to the East Indies. C. Do these changes take place suddenly 1 F, No; some days before and after the change there are calms, variable winds, and frequently the most violent storms. On the greater part of the coasts situated between the tropics, the wind blows towards the shore in the day-time, and towards the sea by night. These winds are called sea and land breezes ; they are affected by mountains, the course of rivers, tides, &c. E. Is it the heat of the sun by day that rarefies the air over the land, and thus causes the wind ? F. It is : the following easy experiment will illus- trate the subject. In the middle of a large dish of cold water put a water-plate filled with hot water ; the former repre- sents the ocean, the latter the land rarefying the air over it. Hold a lighted candle over the cold water, and blow it out ; the smoke, you see, moves towards the plate. Reverse the experiment by filling the outer vessel with warm water, and the plate with cold, the smoke will move from the plate to the dish._ C. In this country there is no regularity m the direction of the winds ; sometimes the easterly winds prevail for several days together, at other times 1 300 Pi^EUMATICS. have noticed the wind blowing from all quarters of the compass tvyo or three times in the same day, F. The variableness of the wind in this island de- pends probably on a variety of causes ; for whatever destroys the equilibrium in the atmosphere, produces a greater or less current of v/ind towards the place where the rarefaction exists. It is generally believed that the electric fluid, which abounds in the air, is the principal cause of the variableness of the wind here. You may often see one tier of clouds moving in a certain direction, and another m a contrary one ; that is, the higher clouds will be moving perhaps north or east, while the weather-cock stands directly south or west. In cases of this kind a sudden rarefaction must have taken place m the regions of one set of these clouds, and consequently the equilibrium destroyed. This phe- nomenon is frequently found to precede a thunder- storm; from v^hicb it has been supposed that the electric fluid is, in this and such like instances, the principal cause in producing the wind : and if in the more remarkable appearances we are able to trace hie operating cause, we may naturally infer that those which are less so, but of the same nature, depend on a like principle. E. Violent storms must be occasioned by sudden and tremendous concussions in nature. I remember to have seen once last year some very large trees torn up by the wind. It is difficult to conceive how so thin and light a body can produce such dire effects. jP. The inconceivable rapidity of lightning will account for the suddenness of any storm; and°when you are acquainted with what velocity a wind wiij sometimes move, you will not be surprised at the effects which it is capable of producing. C. Is there any method of ascertaining the velocity of the wind ? ^ F. Yes ; several machines have been mvented for tiic purpose. But Dr. Derham, by means of the VELOCITY OF THE WIND. ^01 flight of small downy feathers, contrived to measure the velocity of the great storm which happened in the year 1705, and he found the wind moved 33 feet in half a second, that is, at the rate of 45 miles per hour : and it has been proved, that the force of such a wind is equal to the perpendicular force of 10 pounds avoirdupois Vv^eight on every square foot. Now if you consider the surface which a large tree, with all its branches and leaves, presents to the wind, you will not be surprised, that, in great storms, some of them should be torn up by the roots. E. Is the velocity of 45 miles an hour supposed to be the greatest velocity of the wind ] F, Dr. Derham thought the greatest velocity to be about 60 miles per hour. But we have tables calculated to shew the force of the wind at all velo- cities from 1 to 100 miles per hour. C. Does the force bear any general proportion to the velocity 1 F. Yes, it does : the force increases as the square of the velocity. £. Do you mean, that if on a piece of board, ex- posed to a given wind, there is a pressure equal to 1 pound, and the same board be exposed to another wind of double velocity, the pressure will be in this case 4 times greater than it was before 1 F. That is the rule. The following short table, selected from a larger one out of Dr. Hutton's Dictionary, will fix the rule and facts in your me^ mory PNEUMATICS. TABLE. Velocity of the iiid, ill miles, per hour. 5 10 20 40 Perpendicular force on one square foot in pounds avoir- pois. .123 .492 1.968 7.872 31.488 Common appellations of the wind. Gentle, pleasant wind. Brisk gale. Very brisk. Very high wind. A hurricane. CONVERSATION XVI. OF THE STEAM-ENGINE. F. If you understand the principle of the forcing- pump you will easily comprehend in what manner the steam-engine, the most important of all hydro- statical machines, acts. C. Why do you call it the most important of all machines ; is it not a common one ? F, Steam-engines can be used only with advantage in those cases where great power is required. They are adapted to the raising of water from ponds and wells ; to the draining of mines; and perhaps with- out their assistance we should not at this moment have the benefit of coal fires. E. Then there cannot be two opinions entertained respecting their utility. I do not know what we should do without them in winter, or even in summer, since coal is the fuel chiefly used in dressing our food. ° F. Our ancestors had, a century ago, excavated all tFie mines of coal as deep as they could be worked without the assistance of these sort" of engines. For when the miners have dug a certain depth below the burface of the earthy the vvalcr pours in upon them from OF THE STEAM-ENGINE. 303 all skies : consequently they have no means of going on with their work without the assistance of a steam- engine, which is erected by the side of the pit, and being kept constantly at work, will keep it dry enough for all practical purposes. , . , . . The steam-engine was invented during the reign ot Charles II., though it was not brought to a degree of perfection sufficient for the draining of mmes till nearly half a century after that period. C. To whom is the world indebted for the dis- F^^It is difficult, if not impossible, to ascertain who was the inventor. The Marquis of Worcester ^de- scribed the principle in a small work entitled A Century of Inventions," which was first published in the year 1663. E. Did the Marquis construct one of these en- gines 7 , , F. No ; the invention seems to have been neg- lected for several years, when Captain Thomas ba- very, after a variety of experiments, brought it o some degree of perfection, by which he was able to raise water in small quantities, to a moderate height. C. Did he take the invention from the Marquis ot Worcester's book ? F Dr. Desao-uliers, who, in the middle ot the last century, entered at large into the discussion mam- tains that Captain Savery was wholly mdebted to the Marquis, and to conceal the piracy, he charges him with having purchased all the books which contained the discovery, and burned them. Captain Savery, however, declared, that he was led to the discovery bvthe following accident Having drank a flask of Florence wine at a tavern, and thrown the flask on the fire, he perceived that the few drops left in it were converted into steam, which induced liim to snatch it from the fire, and plunge its neck into a bason ot water, which, by the atmospheric pressure, was driven quickly into the bottle." . , , • . E. This was something like an experiment which mi P?nEUMATICS. I have often seen at the tea-table. Jf 1 pour half a cup of V, ater into the saucer, and then hold a piece ot lighted paper in the cup a few seconds, and when the cup is pretty warm plunge it with the mouth down- wards into the saucer, the water almost instantly dis- appears. F, In both cases, the principle is exactly the same : the heat of the burning paper converts the water that hung about tlie cup into steam, but steam being much lighter than air, expels the air from the cup; which being plunged into the water the steam is quickly condensed, and a partial vacuum is made in the cup ; consequently, the pressure of the atmos- phere upon the water in the saucer forces it into the cup, just in the same manner as the water follows the vacuum made in the pump. C. Is steam, then, used for the purpose of making a vacuum instead of a piston ? F. Just so : and Dr. Darwin ascribes to Captain Savery the honour of being the first person who ap- plied it to the purpose of raising water. E, Will you describe the engine, that we may see how it works. F , I shall endeavour to give you a general and correct explanation of the principle and mode of acting of one of Mr. Watt's engines, v/ithout entering into all the minutis of the several parts. OF THE STEAM-ENGINE. S?05 A is a section of the boiler, standing over a fire, about half full of water : b is the steam-pipe which conveys the steam from the boiler to the cylmder c, in which the piston d, made air-tight, works up and down, a and c are the steam valves through winch the steam enters into the cylinder ; it is admitted through a when it is to force the piston downwards and through c when it presses it upwards, b and d are the eduction valves, through which the steam passes from the cylinder into the condenser e, which is a separate vessel placed in a cistern of cold water, and which has a jet of cold water continually playing up in the inside of it. / is the air-pump, which ex- tracts the air and water from the condenser. It is worked by the great beam or lever rs, and the water taken from the condenser, and thrown into the hot well g, is pumped up again by means of the pump i/, and carried back into the boiler by the pipe z i. _ k is another pump, likewise worked by the engine itselj, which supplies the cistern, in which the condenser is fixed, with water. C. Are all three pumps, as well as the piston, worked by the action of the great beam 1 F, They are ; and vou see the piston-rod is fas- tened to the beam by^inflexible bars ; but, ttiat the stroke mio-ht be perpendicular, Mr. Watt invented the machinery called the parallel joint, the construc- tion of which will be easily understood trom the figure. 1 I, 4. -J E How are the valves opened and shut I F Lono- levers o and p are attached to them, which are moved°up and down by the piston-rod of the air- pump EF. In order to communicate a rotatory motion to any machinery by the motion of the beam, Mr. W att makes use of a large fly wheel x, on the axis of which is a small concentric toothed wheel n ; a similar toothed wheel i is fastened to a rod t coming from the end of the beam, so that it cannot turn on its axis, but must rise and fall with the motion of the great beam. A bar of iron connects the centres ot the two smaU 306 PNEUMATICS. toothed wheels : when therefore the beam raises llie Wheel I, it must move round the circumference of the wheel n, and with it turn the fly-wheel x ; which will make two revolutions while the wheel i goes round it once. These are called the Sun and Planet wheels ; H, like the sun, turns only on its axis, while i revolves about it as the planets revolve about the sun. If to the centre of the fly-wheel any machinery were fixed, the motion of the great beam r s would keep it in constant work. C. Will you describe the operation of the engine ? F. Suppose the piston at the top of the cylinder, as it is represented in the plate, and the lower part of the cylinder filled with steam. By means of the pump- rod E F, the steam valve a and the eduction valve d will be opened together, the branches from them being connected at o. There being now a communi- cation at d between the cylinder and condenser, the steam is forced from the former into the latter, leaving the lower part of the cylinder empty, while the steam from the boiler entering by the valve a presses upon the piston, and forces it down. , a As soon as the piston has arrived at the bottom, the steam valve c and the eduction fS|j valve b are opened, while those at a and d ny\ are shut ; the steam, therefore, immediately IS rushes through the eduction valve h into the condenser, while the piston is forced up ^ again by the steam, which is now admitted by the valve c. Fig. 27, CONVERSATION XVII. OF THE STEAM-ENGINE. C. I do not understand how the two sets of valves act, which you described yesterday, as the steam and eduction valves. -F. If you look to Fig. 27, there is a different view of this part of the machine, unconnected with the rest : s is part of the pipe which brings the steam from the OF THE STl^AM-ENGINE, 307 boiler, a represents the valve, which, being opened, admits the steam into the upper part of the cyhnder, forcing down the piston. E. Is not the valve d opened at the same time ' F It is : and then the steam which was under the piston is forced through into the condenser e. When the piston arrives at the bottom, the other pan- ot valves are opened, viz. c and b, through c the steam rushes to raise the piston, and through b the steam which pressed the piston down before, is driven out into the pipe r, leading to the condenser ; m this there is a jet of cold water constantly playing up and thereby the steam is instantly reduced into the shape of hot water. . -i, i r n C. Then the condenser e (Fig. 26) will soon be lull of water. -,1,1 F It would, if it were not connected by the pipe s with the pump/. And every time the great beam r s is brought down, the plunger at the bottom of the pis- ton rod E F descends to the bottom of the pump. £. Is there a valve in the plunger? F, Yes, which opens upwards, consequently all the hot water which runs out of the condenser into the pump will escape through the valve, and be at the top of the plunger, and the valve not admittmg it to return, it will, by the ascent of the piston-rod into the situation as is shewn in the plate, be driven through n into g, the cistern of hot water, from which, owing to a valve, it cannot return. C. And I see the same motion of the great beam puts 'the pump y in action, and brings over the hot- water from the cistern g, through the pipe i i into the little cistern v, which supplies the boiler. E. If the pump h brings in, by the same motion, the water from the well w, do not the hot and cold water intermix? ^ E. No: if you look carefullym the figure, you will • observe a strong partition v, which separates the one from the other. Besides, you may perceive that the hot water does not stand at so high a level as the cold, 308 PNEUMATICS, which is a sufficient proof that they do not communi- cate. Indeed, the operation of the enoine woukl be greatly nijured, if not wholly stopped, if the hot water commumcated with the cold ; as, in that case, the water, being at a medium heat, would be too warm to condense the steam in e, and too cold to be admitted into the boiler ^vithout checking the production of the steam. C. There are some parts of the apparatus belongin^^ to the boiler which you have not yet explained. What is the reason that the pipe q, which conveys the wat^r rom the cistern v to the boiler, is turned up at the lower end 1 F. If it were not bent in that manner the steam that IS generated at the bottom of the boiler would rise into the pipe, and in a great measure prevent the de- scent of the water through it. E, In this position I see clearly no steam can enter the pipe, because steam, being much lighter than \yater, must rise to the surface, and cannot possibly sink through the bended part of the tube. What does m represent ? F. It represents a stone suspended on a wire, which is shewn by the single line ; this stone is nicely ba- lanced by means of a lever, to the other end of which IS another wire, connected with the valve at the top of the pipe q, that goes down from the cistern. C. Is the stone so balanced as to keep the valve sufficiently open to admit a proper quantity of water ? ^ F. It is represented by the figure in that situation. By a principle in hydrostatics,* with which you are acquainted, the stone is partly supported by the water ; if then by increasiog the fire, too great an evaporation take place, and the water in the boiler sink below its proper level, the stone also must sink, which will cause the valve to open wider, and let that from the cistern come in faster. If, on the other hand, the eva- poration be less than^ it ought to be, the water will have a tendency to rise in the boiler, and with that * See Conrer. XI. ^ii Hydrostatics. OF THE STEAM-ENGINE. 309 the stone must rise, and the valve will, consequently, let the water in with less velocity. By this neat con- trivance the water in the boiler is always kept at one level. E. What are the pipes t and it for ? F, They are seldom used, but are intended to shew the exact height of the water in the boiler. ^The one at t reaches very nearly to the surface of the water when it is at the proper height : that at u enters a little below the surface. If then the Vv^ater be at its proper height, and the cocks t and u be opened, steam will issue from the former, and loater from the latter. But if the water be too high it will rush out at t instead of steam : if toa low, steam will issue out of u instead of water. i • i i C. Suppose things to be as represented m the plate, why will the water rush out of the cock u if it be opened '? It will not rise above its level. F. True : but you forget that there is a constant pressure of the steam on the surface of the water in the boiler which tends to raise the water in the pipe n. This pressure would force the water through the pipe, as in an artificial fountain. Conver. VIII. CONVERSATION XVIII. OF THE STEAM-ENGINE, AND PAPIn's DIGESTER. C. We have seen the structure of the steam-engine and its mode of operation ; but you have not told us the uses to which it is applied. F. The application of this power was at first wholly devoted to the raising of water, either from the mines, which could not be worked without such aid, or to the throwing it to some immense reservoir, for the purpose of supplying with this useful article places which are higher than the natural level of the stream. But it is novv applied to various other purposes, such as the working of mills, the threshing of corn, and coining. In making the copper money now in use, the ingeni- ous Mr. Boulton contrived, by a single operation of 310 PNEUMATICS, the steam-engine, to roll the copper out to a proper thickness, to cut it into circular pieces, and to make the faces and the edge. C. How is the power of these engines estimated? I\ The power varies according to the size. That at Messrs. Whitbread's brewhouse, to which I' have had access, has a cylmder 24 inches in diameter, and wdl perform the work of 24 horses, working night and day. E, But the horses cannot work incessantly. F, They will work only eight hours, at the average, out of the 24; therefore, since the engine is kept con- tinually at work, it will perform the business of 72 horses. The coals consumed by this engine are about seven chaldrons per week, or one chaldron in 24 hours. By the application of different machinery to this engine, it raises the malt into the upper warehouses, and grinds it; it pumps the wort from the under- backs into the copper ; raises the wort into the coolers ; it fills the barrels when the beer is made ; and when the barrels are full and properly bunged, they are, by the steam engine, driven into the storehouses in the next street, a distance of more than a hundred yards, and let down into the cellar. E, I have met with the term explosive steam : why is it so called ? F . From a great variety of accidents, that have happened through careless people, it appears that the expansive force of steam suddenly raised is much stronger than even that of gunpowder. At the cannon foundry in IVIoorfields, some years ago, hot metal was poured into a mould that accidentally contained a small quantity of water, which was instantly converted into steam, and caused an explosion that blew the foundry to pieces. A similar accident happened at a foundry in Newcastle, which occurred from a little water having insinuated itself into a hollow brass ball that was thrown into the melting pot. C. These facts bring to my mind a circumstance OF PAPIN'S DIGESTER. 311 that I have often heard you relate, as coming within your own knowledge. F, You do well to remind me of it. The fact is worth recording. A gentleman who was carrying on a long series of experiments, wished to ascertain the strength of a copper vessel, and gave orders to his workmen for the purpose. The vessel, however, burst unexpectedly, and, in the explosion, it beat down the brick wall of the building in which it was placed, and was, by the force of the steam, carried 15 or 20 yards from it ; several of the bricks were thrown 70 yards from the spot ; a leaden pipe, suspended from an adjoining building, was bent into a right angle ; and several of the men were so dreadfully scalded or bruised, that for many weeks they were unable to stir from their beds. A very intelligent person, who con- ducted the experiment, assured me, that he had not the smallest recollection how the accident happened, or by what means he got to his bed-room after the explo- sion. E. Is it by the force of steam that bones are dis- solved in Papin's Digester, which you promised to de- scribe '?* jP. No ; that operation is performed by the great heat produced in the digester. This is a representation of one of these machines. It is a strong metal pot, at least an inch thick in every part ; the top is screwed down, so that no steam can escape but through the valve v. Fig. 28. C. What kind of a valve is it? F. It is a conical piece of brass, made to fit very accurately, but easily moveable by the steam of the water v^'hen it boils : consequently, in its simple state, * See Conyer. IIL of Mecliaiiirs. S12 PNEUMATICS. the heat of the water will never be much greater than that of boiling water in an open vessel. A steel-yard is therefore fitted to it, and by moving the weight w backwards ot forwards, the steam will have a lesser or greater pressure to overcome. E. Is the heat increased by confining the steam ? F. You have seen that, in an exhausted receiver, water not near so hot as the boiling point will have every appearance of ebullition. It is the pressure of the atmosphere that causes the heat of boiling water to be greater in an open vessel, than in one from which the air is exhausted. In a vessel exposed to condensed air, the heat required to make the water boil would be still greater. Now, by confining the steam, the pressure may be increased to any given de- gree. If, for . instance, a force equal to 14 or 15 pounds be put on the valve^ the pressure upon the water will be double that produced by the atmos- phere, and of course the heat of the water will be greatly increased. C. Is there no danger to be apprehended from the bursting of the vessel ? F. If great care be taken not to load this valve too much, the danger is not very great. But in experi- ments made to ascertain the strength of any particu- lar vessel, too great precautions cannot be taken. Under the direclion of Mr. Papin, the original in- ventor, the bottom of a digester was torn off with a wonderful explosion : the blast of the expanded water blew all the coals out of the fire-place, the remainder of the vessel was hurled across the room, and striking the leaf of an oaken table an inch thick, broke it in pieces. The least sign of water could not be dis- cerned, and every coal was extinguished in a moment. OF THE BAROMETER. ^13 CONVERSATION XIX. OF THE BAROMETER. F. As these Conversations are intended to make you familiar with all those philosophical instruments that are in common use, as well as to explain the use and structure of those devoted to the teaching of science, I shall proceed with an account of the ba- rometer, which, with the thermometer, is to be found m almost every house. I will shew you how the barometer is made, without any regard to the frame to which it is attached. A B is a glass tube, about 33 or 34 inches long, closed at top, that is, in philo- sophical language, hermetically sealed; D is a cup, bason, or wooden trough, partly filled with quicksilver. I fill the ^ , tube with the quicksilver, and then put A k my finger upon the mouth, so as to pre- vent any of it from running out; I now invert the tube, and plunge it in the cup Fio-. 29. B. You see the mercury subsides three ^ or four inches ; and when the tube is fixed to a gra- duated frame it is called a barometer, or weather- glass, and you know it is consulted by those who study and attend to the changes of the weather. E. Why does not all the quicksilver run out of the tube ? ..7^ * ^- ^'!^ ^^^"^^^ ^^^^"g" another question : I What is the reason that water will stand in an ex- hausted tube, provided the mouth of it be plunwd into a vessel of the same fluid 1 C. In that case, the water is kept in the tube by the pressure of the atmosphere on the surface of the ; water into which it is plunged. If you resort to the : same principle, in the present instance, why does the ; water stand 33 or 34 feet, but the mercury only 29 or 130 inches? P PNEUMATICS, F. Do you not recollect that mercury is 14 timer, heavier than water? therefore, if the pressure of the atmosphere will balance 34 feet of water, it ought, on the same principle, to balance only a 14th part of that height of mercury : now divide 34 feet, or 408 mches, ^'^E^.'^The quotient is little more than 29 inches. F* By this method Torricelli was led to construct the barometer. It had been accidentally discovered that water could not be raised more than about 34 feet in the pump. Torricelli, on this, suspected that the pressure of the atmosphere was the cause of the ascent of water in the vacuum made in pumps, and that a column of water 34 feet high was an exact counter- poise to a column of air which extended to the top oi the atmosphere. Experiments soon confirmed the truth of his conjectures. He then thought, tnat it .34 feet of water were a counterpoise to the pressure ot the atmosphere, a column of mercury ^ as much shorter than 34 feet as mercury is heavier tnan water, would likewise sustain the pressure of the atmosphere : he obtained a glass tube for the purpose, and found his reasoning just. C. Did he apply it to the purpose of a weather- ^^^F No : it was not till some time after this that the pressure of the air was known to vary, at different times, in the same place. As soon as that was dis- covered, the application of the Torricellian tube o predicting the changes of the weather immediately succeeded. . . ^ ^ j r^,. C. A barometer, then, is an instrument used toi measuring the weight or pressure of the atmosphere. F ThSt is the principal use of the barometer : li the air be dense, the mercury rises in the tube, and indicates fair weather: if it grow the mercury falls, and presages ram, snow, &c. tUp The height of the mercury in the tube is called the * See the rules in the last Conversation on Pneumatics. OF THE BAROMETER. 315 standard altitude, which in this country fluctuates be- tween 28 and 31 inches, and the difference between the greatest and least altitudes is called the scale oj variation. E. Is the fluctuation of the mercury different m other parts of the world 1 F. Within and near the tropics, there is little or no variation in the height of the mercury in the barometer in all weathers ; this is the case at St. Helena. At Jamaica the variation very rarely exceeds three-tenths of an inch : at Naples it is about one inch : whereas in England it is nearly three inches, and at Peters- burgh it is as much as 3| inches. C. The scale of variation is the silvered plate, which is divided into inches and . tenths of an inch : but what do you call the moveable index? F. It IS called a vernier, from the inventor's name^ and the use of it is to shew the fluctuation of the mer- cury to the hundredth part of an inch. The scale of inches is placed on the right side of the barometer tube,, the beginning of the scale being the surface of the mercury in the bason : the vernier plate^and index are moveable, so that the index may, at any time, be set to the upper surface of the column of mercury. E. I have often seen you move the index, but I am still at a loss to conceive how you divide the inch into hundredth parts by it. F. The barometer-plate is divided into tenths ; the length of the vernier is eleven tenths, but divided into ten equal parts. C. Then each of the ten parts is equal to a tenth of an inch, and a tenth part of a tenth. F, True : but the tenth part of a tenth is equal ta a hundredth part, for you remember that to divide a fraction by any number, is to multiply the denomina- tor of the fraction by the number, thus — divided by Suppose the index of the vernier to comcide exactly 316 PNEUMATICS. with one of the divisions of the scale of variation, as 29.3. E. Then there is no difficulty ; the height of the barometer is said to be 29 inches and 3 tenths. F, Perhaps, in the course of a few hours, you ob- serve that the mercury has risen a very little ; what will you do ? E. 1 will raise the vernier even with the mercury. F. And you find the index so much higher than the division 3 on the scale, as to bring the figure 1 on the vernier even with the second tenth on the scale. E. Then the whole height is 29 inches, 2 tenths, and one of the divisions on the vernier, which is equal to a tenth and a hundredth ; that is, the height of the mercury is 29 inches, 3 tenths, and 1 hundredth, or 29.31. F, If figure 2 on the vernier stand even with a division ou the scale, how should you call the height of the mercury? E. Besides the number of tenths, I must add 2 hundredths, because each division of the vernier con- tains a tenth and a hundredth ; therefore I say the barometer stands at 29.32, that is, 29 inches, 3 tenths, and 2 hundredths. F. Here is a representation a b of the upper part of a barometer tube ; the quicksilver stands at c : from 2 to a: is part of the scale of variation ; 1 to 10 is the ver- nier, equal in length to eleven- tenths of an inch, but divided into 10 equal parts. In the pre- sent position of the mercury the figure 1 on the vernier coincides exactly with 29.5 on the scale ; and finding the index stand be- tween the 6th and 7th divisions on the scale, I therefore read the height 29.61 ; that is, 29 inches, 6 tenths, and 1 hundredth. Fig. 30. OF THE BAROMETER. 317 C. I luidei^tand the principle of the barometer, but I want a guide to teach me how to predict the changes of the weather, which the rising and faUing of the mercury presage. F. I will give you rules for this purpose in a few days.* CONVERSATION XX. OF THE BAROMETER, AND ITS APPLICATION TO THE MEASURING OF ALTITUDES. C. Is the height of the atmosphere known ? F. If the fluid air were similar to water, that is, every where of the same density, nothing would be easier than to calculate its height.— When the ba- rometer stands at 30 inches, the specific gravity of the atmosphere is 800 times less than that of water ;t but mercury is about 14 times heavier than water, consequently the specific gravity of mercury is to that of air as 800 multiplied by 14 is to 1 ; or mercury is 11,200 times heavier than air. In the case before us, a column of mercury, 30 inches long, balances the whole weight of the atmosphere ; therefore, if the air was eauallv dense at all heights to the top, its height must be 11,200 times 30 inches ; that is, the column of air must be as much longer than that of the mer- cury, as the former is lighter than the latter. Do you understand me 1 C. I think I do : 11,200 multiplied by 30 gives 336,000 inches, which are equal to 5| miles nearly. F. That would be the height of the atmosphere if it were equally dense in all parts : but it is found that the air, by its elastic quality, expands and contracts, and that at 3i miles above the surface of the earth it is twice as rare as it is at the surface ; that at 7 miles it is 4 times rarer ; at 10| miles it is 8 times rarer ; * See Conversation XXIV. + See Conversation VI. 31Q PNEUMATICS. at 14 miles it is 16 times rarer ; and so on, according to the following TABLE. o r 31 1 7 sur- th, . 2 1 4 altitude A lOi 1-4 17| ove the the ear e air is 8 . 16 . 32 03 21' -3 o-s . . 64 24| , miles . 128 . 256 Now if you were disposed to carry on the addition on one side, and the multiplication on the other, you would find that, at 500 miles above the surface of the earth, a single cubical inch of such air as we breathe, would be so much rarefied as to fill a hollow sphere, equal in diameter to the vast orbit of the planet Saturn. jE. Is it inferred from this that the atmosphere does not reach to any very great height 1 F. Certainly ; for you have seen that a quart of air at the earth's surface weighs but about 14 or 15 grains ; and by carrying on the above table a few steps, you would perceive, that the same quantity, only 49 miles high, would weigh less than the 16 thousandth part of 14 grains, consequently, at that height, its density must be next to nothmg. irom experiment and calculation it is generally admitted, that the atmosphere at the height of more than 45 or 50 miles above the surface of the earth, is not suf- ficiently dense to refract the rays of light ; conse- quently, that is generally denominated the height ot the atmosphere. C. By comparing the state of the atmosphere at the bottom and at the top of a mountain, should you perceive a sensible difference ? F, We must not trust to our feelings on such occasions. The barometer will be a sure guide. I OF THE BAROMETER. 3i9 will not trouble you with calculations, but mention two or three facts, with the conclusions to be drawn from them. In ascending the Puy de Dome, a very hio-h mountain in France, the quicksilver fell 3i inches ; and the height of the mountain was found, by measurement, to be 3204 feet. By a similar ex- periment upon Snowden, in Wales, the quicksilver was found to have fallen 3 inches 8-tenths, at the heio-ht of 3720 feet above the surface of the earth. ^ From these and many other observations it is in- ferred, that in ascending any lofty eminence, the mercury in the barometer will fall one-tenth of an inch for every 100 feet of perpendicular ascent. This number is not rigidly exact, but sufficiently so for common purposes, and it will be easily remem- bered. The three following observations were taken by Dr. Nettleton near the town of Halifax : Perpendicii- Lowest station Highest sta- . hi altifade of the tion of the Diftereiice. in feet. Barometer. Barometer. 102 29.78 29.66 0.12 236 29.50 29.23 0.27 507 30.00 29.45 0.55 E. If I ascend a high hill, and, taking a barometer with me, find the mercury has fallen 1| inch, may 1 conclude that the hill is 1500 feet perpendicular height? 1 . F. That number will be rather too large, but the height would be between 14 and 1500 feet. Are you aware how great a pressure you are continually sus- taining? . , T T f 1 E. No ; it never came into my head, i teel no burden from it, therefore it cannot be very great. E. You sustain every moment a weight equal to many tons, which if it were not balanced by the elas- tic force of the air within the body, would crush you to pieces. C. We might indeed have inferred that it was con- 320 PNEUMATICS. siderable from the sensations that we felt when the air j was taken from under our hands. But how, sir, do 1 you make out the assertion 1 F. When the barometer stands at 29.5 the pressure of the air upon every square inch is more than equal to 14 pounds ; call it 14 pounds for the sake of even numbers, and the surface of a middle-sized man is 14| feet : tell me now the weight he sustains. C. I must multiply 14 by the number of square inches in 14| feet: now there are 144 inches in a square foot, consequently in 14| feet there are 2088 square inches; therefore, 14 pounds multiplied by 2088 will give 29,232, the number of pounds weight which such a person has to bear up. F. That is equal to about 13 tons; now if Emma reckon herself only half the size of a grown person, she will sustain 65 tons. E. What must the whole earth sustain 1 F. This you may calculate at your leisure ; I will furnish you with the rule : — Find the diameter of the earth,* from which you will easily get the superficial measure in square inches, and this you must multiply by 14, and you get the answer to the question in pounds avoirdupois." CONVERSATION XXI. OF THE THERMOMETER. F. As the barometer is intended to measure the different degrees of density of the atmosphere, so the thermometer is designed to mark the changes in its temperature, with regard to heat and cold. E. Is there any difference between the ther- mometer that is attached to the barometer, and that which hangs out of doors ? F. No : they are both made by the same person, and are intended to shew the same effects. But for the purposes of accurate observation it is usual to * See Conversation VII. of Astronomy, p. 102. OF THE THERMOMETER. 321 have two instruments, one attached to, or near, the barometer, and the other out of doors, to which neither the direct nor reflected rays of the sun should ever come. Though my thermometers are both of the same construction, and such as are principally used in this country, yet there are others made of different materials and upon different principles. C. Does not this thermometer consist of mercury inclosed in a glass tube that is fixed to a graduated frame 1 F. That is the construction of Fahrenheit's ther- mometer : but when these instruments were first in- vented, about 200 years ago, air, water, spirits of wine, and then oil, were made use of, but these have given way to quicksilver, which is considered as the best of all the fluids, being highly susceptible of expansion and contraction, and capable of exhibiting a more extensive scale of heat. Fahrenheit's thermometer is chiefly used in Great Britain, and Reaumur's on the continent. E. Is not this the principle of the thermometer, that the quicksilver expands by heat, and contracts by cold 1 F. It is : place your thumb on the bulb of the thermometer. E. The quicksilver gradually rises. F. And it will continue to rise till the mercury and your thumb are of equal heat. Now you have taken away your hand, you perceive the mercury is falling as fast as it rose. C. Will it come down to the same point at which it stood before Emma touched it ? F. It will, unless, in this short space of time, there has been any change in the surrounding air. Thus, the thermometer indicates the temperature of the air, or, in fact, of any body with which it is in contact. Just now it was in contact with your thumb, and it rose in the space of a minute or two from bQ^ to 62^ ; had you held it longer on it, the mercury would have risen still higher. It is now falling. Plunge it into 322 PNEUMATICS. boiling water,* and you will find that the mercury rises to 212^. Afterwards you may place it in ice in its melting state, and it will fall to 32''. E. Why are these particular numbers pitched on 1 F. You will not perhaps be satisfied if I tell you, that the only reason why 212 was fixed on to mark the heat of boiling water, and 32 that to shew the freezing point, was, because it so pleased M. Fahren- heit : this however was the case. C. I can easily conceive that at the same degree of cold water will always begin to freeze ; but surely there are different degrees of heat in boiling water, and therefore it should seem strange to have only one number for it. F, In an open vessel, boiling water is always of the same heat, that is, provided the density of the at- mosphere be the same : and though you increase your fire in a tenfold proportion, yet the water will never be a single degree hotter ; for the superabundant heat, communicated to the water, flies off in the form of steam or vapour. E. But suppose you confine the steam. F. Before I should attempt this, I must be provided with a strong vessel, or, as you have seen under the article of the steam-engine, it would certainly burst. But in a vessel proper for the purpose, water has been made so hot as to melt solid lead. C. Will you explain the construction of the thermometer 1 F. A B represents a glass tube, the end A is blown into a bulb, and this, with a part of the tube, is filled with mercury. In good thermometers, the upper part of the tube approaches to a perfect vacu- um, and of course the end b is hermeti- Fig. 31. * This should be done very gradually, by holding it some time in the steam, to prevent its breaking by the sudden heat. OF THE THERMOMETER. S23 caliy sealed. If the tube be now placed in pounded ice, the mercury will sink to a certain point x, which must be marked on the tube, and on the scale oppo- site to this pomt 32 must be placed, which is called the freezing point. Then let it be immersed m boiling water, the mercury will rise, and after a few minutes will become stationary; against that point make another mark, and write on the scale 212 for the heat of boiling water. Between these points let the scale be divided into 180 equal parts. E. Why 130 parts? F. Because you begin from 32, and if you sub- tract that number from 212, the remainder will be 180. Also, below 32, and above 212, set off more divisions on the scale, equal to the others. The scale is finished when you have written against 0 extreme cold; digmnst 32 freezi7ig point; against 55 temperate heat; against 76 summer- heat; against 98 blood heat; against 112 fever heat; against 176 spirits boil, and against 212 water boils. E. You said the scale was to be divided higher than boiling water, but without mentioning the ex- tent. F. The utmost extent of the mercurial thermome- ter, both ways, are the points at which quicksilver boils and freezes ; beyond these it can be no guide : now the degree of heat at which mercury boils is 600, and it freezes when it is brought down as low as 39^ or 40« below 0 ; consequently, the whole extent oi the mercurial thermometer is 640 degrees. And though the cold is never so intense in this country as to sink the mercury 40« below the freezing point, yet it is in some parts of Lapland and Siberia ; and arti- Jicial cold may be produced here equal to this. 324 PNEUMATICS. CONVERSATION XXII. OF THE THERMOMETER. C. Is quicksilver, when frozen, a solid metal, like iron and other metals ? F. It is thus far similar to them, that it is mallea- ble, or will bear hammering. And when quicksilver boils, it goes off in v3.^om like boiling water, only much slower. Hence it has been inferred, that all bodies in nature are capable of existing either m a solid, fluid, or aeriform state, according to the degree of heat to which they are exposed. E. I understand that water may be either solid, as ice, or in its fluid natural state, or in a state of vapour or steam. , n • i c F, I do not wonder that you call the fluid state ot water its natural state, because we are accustomed, in general, to see it so, and when it is frozen into ice, there appears to us in this country a violence commit- ted upon nature. But if a person from the West or East Indies, who had never seen the effects of frost, were to arrive in Great Britain during a severe and long continued one, such as formerly congealed the surface of the Thames, unless he were told to the contrary, he would conclude that ice was some mine- ral, and naturally solid. E. Does it never freeze in the East or West Indies? F. It seldom freezes, unless in very elevated situa- tions, within 35 degrees of the equator north and south : it scarcely ever hails in latitudes higher than 60O. In our own climate, and indeed in all others between 35^ and 60^ it rarely freezes till the sun's meridian altitude is less than 40 degrees. The cold- est part of the 24 hours is generally about an hour before sun-rise, and the warmest part of the day is usually between two and four o'clock in the afternoon OF THE THERMOMETER. 325 C. Are there no degrees of heat higher than that of boiling mercury ? F. Yes, a great many : brass will not melt till it is heated more than six times hotter than boiling mer- cury ; and to melt cast-iron requires a heat more than six times greater than this. E. By what kind of thermometer are these degrees of heat measured 1 F, The ingenious Mr. Wedgewood has invented a thermometer for measuring the degrees of heat up to 32,277^ of Fahrenheit's scale. C. Can you explain the structure of his thermo- meter 1 F, All argillaceous bodies, or bodies made of clay, are diminished in bulk by the application of great heat. The diminution commences in a dull red heat, and proceeds regularly as the heat increases, till the clay is vitrified, or is transformed into a glassy sub- stance. This is the principle of Mr. Wedgewood's thermometer. E. Is vitrification the limit of this thermometer ? F. Certainly : the construction and application of this instrument is extremely simple, and it marks all the different degrees of ignition from the red heat, visible only in the dark, to the heat of an air furnace. It consists of two rulers fixed on a plane, a little far- ther asunder at one end than at the other, leaving a space between them. Small pieces of alum and clay, mixed together, are made just large enough to enter at the wide end : they are then heated in the fire with the body whose heat is to be ascertained. The fire, according to its heat, contracts the earthy body, so that, being applied to the wide end of the gauge, it will slide on towards the narrow end, less or more, according to the degree of heat to which it has been exposed.* * We have in tlie former parts of this work observed, that all bodies are expanded by heat. The diminution of the argillaceous substances made use of by Mr. 32G PNEUMATICS, Each degree of IMr. Wedgewood's thermometer answers to 130 degrees of Fahrenheit, and he begins his scale from red heat fully visible in daylight, which he finds to be equal to 1077<' of Fahrenheit's scale if it could be carried so high. Here is a small scale of heat, as it is applicable to a few bodies : — SCALE OF HEAT. Fahrenheit. Extremity of Wedgewood's « "\ scale ..... 240oL - i32>770 Cast iron melts . . at IGO 13fr2lS77 Fine gold melts . . • ^2 < g S 5237 Fine silver melts . . .28 J | ( ^''^^ Brass melts . • • * f I 1 ^^^^ Red heat visible by day . 0 V, J 1077 Mercury boils at GOO Lead melts* ^^^> Bismuth melts* 460 Tin melts* 408 Milk boils . . • • • • -213 Water boils 212 Heat of the human body . . . 92 to 97 Water freezes -^2 Milk freezes 30 A mixture of snow and salt sinks the ther- mometer to ^ Mercury freezes 40^ C You said that Reaumur's thermometer was chiefly used abroad ; what is the difference between that and Fahrenheit's ? , . . l\ Reaumur places the freezmg pomt at 0, or Wedgewood rti)i7e«r5 to be an exception: but as the con tractfon of these does not commence till they are ex- posed to a red heat, it may probably be accounted for from the expulsion of the fluid particles, rather than from any real contraction in the solids. * If these three metals be mixed together by fusion in the proportion of 5, 8, and 3, the mixture will melt in a heat below that of boiling water. OF THE PYROMETER. 327 zero, and each degree of his thermometer is equal to 2i, or I degrees of Fahrenheit's. E. What does he make the heat of boiling water ? F. Having fixed his freezing point at 0, and making one of his degrees equal to 2\ of Fahrenheit, the heat of boiling water must be SO'*. C. Let me see. The number of degrees between the freezing and boiling points on Fahrenheit's ther- mometer is 180, which, divided by 2|, or 2.25, gives 80 exactly. F. You have then a rule by which you may always convert the degrees of Fahrenheit into those of Reaumur: subtract 32 from the given number, and multiply by the fraction Tell me, Emma, what degree on Reaumur's scale answers to 167« of Fahrenheit. E, Taking 32 from 167 there remains 135, which, multiplied by 4, gives 540, and this divided by 9, gives 60. So that 60« of Reaumur answers to 167« of Fahrenheit. C. How shall I reverse the operation, and find a number on Fahrenheit's scale that answers to a given one on Reaumur's 1 F. IMultiply the given number by the improper fraction |, and add 32 to the product." Tell me what number on Fahrenheit's scale answers to 40 on Reaumur's. C. If I multiply 40 by 9, and divide the product by 4, I get 90 ; to which if 32 be added, the result is 122 ; which answers to 40 on Reaumur's scale. CONVERSATION XXIII. OF THE PYROMETEPw AND HYGROMETER, F. To make our description of philosophical instru- ments more perfect, I shall to-day shew you the con struction and uses of the pyrometer and hygrometer ; and conclude to-morrow with an account of the ram- gauge and some directions for judging of the weather 328 PNEUMATICS. E. What do you mean by a pyrometer 1 F. It is a Greek word, and signifies a fire-measurer. The pyrometer is a machine for measuring the expan- sion of solid substances, particularly metals, by heat. This instrument will render the smallest expansions sensible to the naked eye. C. Is all this apparatus necessary for the purpose ? F. This as far as I know is one of the most simple pyrometers, and, admitting of an easy explanation, 1 have chosen it in preference to a more complicated instrument, which might be susceptible of greater nicety. To a flat piece of mahogany a a are fixed three studs, B, c, and d, and at b there is an adjusting screw p. H F is an index, turning very easy on the pivot f, and L s is another, turning on l, and pointing to the scale MN. R is part of a watch-spring, fixed at y, and pressing gently upon the index l s. Here is a bar of iron, at the common temperature of the sur- rounding air ; I lay it in the studs c and d, and ad- just the screw p so that the index ls may point to 0 on the scale. C. The bar cannot expand without movmg the mdex FH, the crooked part of which pressing upon' L s, that also will be moved if the bar lengthens. F. Try the experiment : friction, you know, pro- duces heat ; take the bar out of the nuts, rub it briskly, and then replace it. E. The index l s has moved to that part of the OF THE PYROMETER. 329 scale which is marked 2 : it is now going back. How do you calculate the length of the expansion '? F. The bar pressed against the index fh at f, and that again presses against ls at l, and hence they both act as levers. C. And they are levers of the third kind, for in one case the fulcrum is at x, the power at f, and the point z to be moved may be considered as the weight : —in the other, l is the fulcrum, the power is applied at r, and the point s is to be moved.* F. The distance between the moving point f and H is 20 times greater than that between x and f ; the same proportion holds between ls and Lr ; from this you will get the spaces passed through by the differ- ent points. E. Then as much as the iron bar expands, so much will it move the point f, and of course the point z will move 20 times as much ; so that if the bar lengthens one-tenth of an inch, the point z would move twenty-tenths, or two inches. By the same rule the point s will move through a space 20 times as great as the point r. F. There are two levers, then, each of which gam power, or move over spaces, in the proportion of 20 to 1 ; consequently, when united, as in the present case, into a compound lever, we multiply 20 into 20, which make 400 ; and therefore if the bar lengthen one- tenth of an inch, the point s must move oyer 400 times that space, or 40 inches. But suppose it only expands one four-hundredth part of an inch, how much will s move 1 C. One inch. F, But every inch may be divided into tenths, and consequently, if the bar lengthen only one four-thou- sandth part of an inch, the point s will move through the tenth of an inch, which is very perceptible.— In the present case the point s has moved two inches, * For an account of the different levers, see Conver. XV. and XVI. of Mechanics. 330 PNEUMATICS. therefore the expansion is equal to two foar-hundredths, or one two-hundredth part of an inch. — An iron bar, three feet Jong, is about one 70th part of an inch longer in summer than in winter. C. I see that, by increasing the number of levers, you might carry the experiment to a much greater degree of nicety. F. Well, let us now proceed to the hygrometer, which is an instrument contrived for measuring the different degrees of moisture in the atmosphere. E, I have a weather-house that I bought at the fair, which tells me this ; for if the air is very moist, and thereby denotes wet weather, the man comes out; and in fair weather, when the atmosphere is dry, the woman makes her appearance. C. How is the weather-house constructed 1 F. The two images are placed on a kind of lever, which is sustained by catgut ; and catgut is very sen- sible to moisture, twisting and shortening by mois- ture, and untwisting and lengthening as it ^ becomes dry. On the same principle is fj'^ constructed another hygrometer. Anisa \i' catgut string, suspended at a with a little |[ weight B, that carries an index c round a Ij j-, circular scale de on a horizontal board or ^^-^A.lx^i - ^, «0-: V-J table : for as the catgut becomes moist, it ^^^ir^.-^^ twists itself, and untwists when it ap- p-,,. ^3 proaches to a dry state. E. Then the degrees of moisture are shewn by the index, which moves backwards and forwards by the twisting and untwisting of the catgut. Does all string twist with moisture 1 F, Yes. Take a piece of common packthread, and on it suspend a pound weight in a vessel of water, and you will see how soon the two strings are twisted round one another. C. I recollect that the last time the lines for dry- ing the linen were hung out in the garden, that they appeared to be much looser in tiie evening than they were next morning, so that I thought some person 0¥ THE HYGROMETER. 331 had been altering them. A sudden shower of rain has produced the same effect in a striking manner. E. Sometimes, when sudden damp weather has set in, the string of the harp has snapped when no per- son has been near it. F. These are the effects produced by the moisture of the air ; the damp of night always shortens hair and hempen lines; and owing to the changes to which the atmosphere in our climate is liable, the harp, violin, &c. that are set to tune one day, will need some alteration before they can be used the next. Here is a sensible and very ^ ^™_======a simple hygrometer : it consists of whipcord, or catgut, fastened ZZI^l at A, and stretched over several IZD ^ pulleys, B, c, D, E, F ; at the end is a little weight w, to which is an index pointing to a gradu- ated scale. i'jg- 34. C. Then according to the de- gree of moisture in the air, the string shortens or lengthens, and of course the index points higher or lower. F. Another kind of hygrometer consists of a piece of sponge e, prepared and nicely balanced on the beam xy ; and the fulcrum z lengthened out into an index pointing to a scale ac. ^ yIo- 35. E. Does the sponge imbibe °* moisture sufficiently to become a good hygrometer? F. Sponge of itself will answer the purpose, but it is made much more sensible in the following man- ner : — After the sponge is well washed from all impurities and dried again, it should be dipped into water or vinegar in which sal-ammoniac, salt of tartar, or almost any other salt has been dissolved, and then 332 PNEUMATICS. suffered to dry, when it should be accurately ba- lanced. C. Do the saline particles in damp weather im- bibe the moisture, and cause the sponge to prepon- derate ? F. They do. Instead of sponge a scale may be hung at E, in which must be put some kind of salt that has an attraction to the watery particles floating in the air. Sulphuric acid may be substituted in the place of salt, but this is not fit for your experiments, because a little spilt over will destroy your clothes, otherwise it makes a very sensible hygrometer. E. I have heard the cook complain of the damp weather when the salt becomes wet by it. F. Right : the salt-box in the kitchen is not a bad hygrometer ; and others may be easily constructed, as you extend your acquaintance with natural sub- stances. CONVERSATION XXV. OF THE RAIN-GAUGE. C. Does the rain-gauge measure the quantity of rain that falls 1 F, It shews the height to which the rain would rise on the place where it is fixed, if there were no evaporation, and if none of it were im- bibed by the earth. One which is made and sold by Mr. Jones, of Holborn, consists of a funnel a communicating with a cylindric tube B. The diameter of the funnel is ex- actly 12 inches, and that of the tube is 4 inches. Tell me, Emma, what propor- tion the area of the former has to that of the latter. E. I remember that all plane surfaces bear the same proportion to one another that the squares of their diameters have. Now the square of 12 is 144, and OF THE RAIN-GAUGE. 333 the square of 4 is 16, therefore the proportion of the area of the funnel is to that of the tube as 144 to 16. F. But 144 may be divided by 16 without leaving a remainder. C. Yes, 9 times 16 is 144, consequently the pro- portion is as 9 to one ; that is, the area of the fun- nel is 9 times greater than that of the tube. jP. If then the water in the tube be raised 9 inches, the depth of rain fallen will, in the area of the funnel, which is the true gauge, be only one inch. E. Does the little graduated rule mark the rise? F. Yes, it does. It is a floating index divided into inches. E. If then the float be raised 1 inch, is the depth of water reckoned only one-ninth of an inch 1 F, You are right : and each 9 inches in length being divided into 100 equal parts, the fall of rain can be readily estimated to the nine-hundredth part of an inch. Rain-gauges should be varnished or well paint- ed, and as much water should be first poured in as will raise the float to such a height, that 0 or zero on the ruler may coincide with the edge of the funnel. C. This is not like your rain-gauge. F. That which I use, though somewhat more diffi- cult of explanation, is a much cheaper instrument ; it may without the bottle be made for a single shilling. It consists of a tin funnel ; the area of the top is ex- actly 10 square inches, and the tube, about 5 or 7 inches long, passes through a cork that is fixed in a quart bottle. E. Is there any particular proportion between the area of the funnel and that of the bottle 1 F, No, it is not necessary ; for in this the quantity of the rain is calculated by its weight compared with the area of the funnel, which is known. For every ounce of water I allow .174 parts of an inch for the depth of the rain fallen. Thus the last time that I ex- amined the bottle, I found that the water weighed exactly 6 ounces, and 6 multiplied by .174 gives 1.0443 that is, the rain fallen in the preceding month 334 PNEUMATICS, was equal to rather more than 1 inch in depth. In the month of June (1801) the rain collected in the gauge weighed 11 J ounces, which is nearly equal to 2 inches in depth. C. Pray explain the reason of multiplying the number of ounces by the decimals .174. F. Every imperial gallon of pure rain water con- tains 277.3 cubic inches, and weighs 81b. or 160 ounces avoirdupois, consequently every ounce of water is equal to 1.74 cubic inches ; but the area of the funnel is 10 square inches, and 10 multiplied by .174 (the depth of rain fallen) is equal to 1.74. , You have now a pretty full account of all the in- struments necessary for judging of the state of the weather, and for comparing, at different seasons, the various changes as they happen. E. Yes ; the barometer informs us how dense the at- mosphere is ; the thermometer its heat ; the hygro- meter what degree of moisture it contains ; and by the rain-gauge how much rain falls in a given time. F. The rain-gauge must be fixed at some distance from all buildings which might in any way shelter it from particular driving winds ; and the height at which the surface of the funnel is from the ground must be ascertained. C. Does it make any difference in the quantity of rain collected whether the gauge stands on the ground, or some feet above it ? F. Very considerable ; as that which I have described is a cheap instrument, one may be placed on the top of the house, and the other on the garden wall, and you will find the difference much greater than you would imagine.— I will now give you some rules for judging of, and predicting, the state of the weather, which are taken from writers who have paid the most attention to these subjects, and which my own observations have verified. 1. The rising of the mercury presages, in general, fair weather ; and its falling foul weather, as rain, snow, high winds, and storms. When the surface of OF JIJDCING OF THE WEATHER. 335 the mercury is conves:, or stands higher in the middle than at the sides, it is a sign the mercury is then in a rising state ; but if the surface be concave, or hollow in the middle, it is then sinking. 2. In very hot vv^eather, the falling of the mercury indicates thunder. 3. In winter, the rising presages frost : and in frosty weather, if the mercury falls three or four divi- sions, there will be a thaw. But in a continued fi'ost, if the meicury rises, it will certainly snow. 4. When wet weather happens soon after the de- pression of the mercury, expect but little of it; on the contrary, expect but little fair weather when it proves fair shortly after the mercury has risen. 5. In wet weather, when the mercury rises much and high, and so continues for two or three days be- fore the bad weather is entirely over, then a con- tinuance of fair weather may be expected. 6. In fair weather, when the mercury falls much and low, and thus continues for two or three days be- fore the rain comes, then a deal of wet may be ex- pected,^ and probably high winds. 7. The unsettled motion of the mercury denotes unsettled weather. 8. The words engraved on the scale are not so much to be attended to as the rising and falling of the mercury : for if it stand at much rain, and then rises to changeable, it denotes fair weather, though not to continue so long as if the mercury had risen higher. If the mercury stands at fair, and falls to changeable, bad weather may be expected. 9. In winter, spring, and autumn, the sudden fall- ing of the mercury, and that for a large space, denotes high winds and storms ; but in summer it presages heavy showers, and often thunder. It always sinks lowest of all for great winds, though not accompanied with rain : but it falls more for wind and rain to- gether than for either of them alone. 10. If, after rain, the wind change into any part 336 PNEUMATICS. of the north, with a clear and dry sky, and the mer- cury rise, it is a certain sign of fair weather. 11. After very great storms of wind, when the mercury has been low, it commonly rises again very fast. In settled fair weather, except the barometer sink much, expect but little rain. In a v/et season, the smallest depressions must be attended to j for when the air is much inclined to showers, a little sinking in the barometer denotes more rain. And in such a season, if it rise suddenly fast and high, fair weather cannot be expected to last more than a day or two. 12. The greatest heights of the mercury are found upon easterly and north-easterly winds ; and it may often rain or snow, the wind being in these points, while the barometer is in a rising state, the effects of the wind counteracting. But the mercury sinks for wind as well as rain in all other points of the com- pass. By noticing these, and other rules which you will learn from experience, you will become as well ac- quainted with the weather as any persons can be in our variable climate. OPTICS. CONVERSATION L INTRODUCTION. OF LIGHT— THE SMALLNESS OF ITS PARTICLES THEIR VELOCITi — -THEY MOVE ONLY IN STRAIGHT LINES. TUTOR CHARLES JAMES. Charles. When we were on the sea, you told us that you would explain the reason why the oar, which was straight when it lay in the boat, appeared crooked as soon as it was put into the water. Tutor, I did ; but it requires some previous know- ledge before you can comprehend the subject. It would afford you but little satisfaction to bo told that this deception was caused by the different degrees of refraction which take place in water and in air. James, We do not know what you mean by the word refraction. T. It will therefore be right to proceed with caution ; refraction is a term frequently used in the science of optics, and this science depends wholly on light. J. What is light 1 T. It would, perhaps, be difficult to give a direct answer to your question, because we know nothing of the nature of light,- but by the effects which it pro- duces. In reasoning, however, on this subject, it is generally admitted that light consists of inconceivably small particles, which are projected, or thrown off, from a luminous body with great velocity in all directions. C. But how is it known that light is composed of small particles 1 T. There is no proof indeed that light is material, Q 3GS OPTICS. or composed of particles of matter, and therefore I said It was generally, not universally, admitted to be so ; but if it is allowed that light is matter, then the particles must be small beyond all computation, or in falling on the eye they would infallibly blind us. /. Does not the light come from the sun, in some such manner as it does from a candle ? T. This comparison will answer our purpose ; but there appears to be a great difference between the two bodies ; a candle, whether of wax or tallow, is soon exhausted ; but philosophers have never been able to observe that the body of the sun is diminished by the light which it incessantly pours forth. J. You say incessantly j but v/e see only during the hours of day. C. That is because the part of the earth which we inhabit is turned away from the sun during the night : but our midnight is mid-day to some other parts of the earth. T. Right : besides, yon know the sun is not in- tended merely for the benefit of this globe, but it is the source of light and heat to six other planets, and eighteen moons belonging to them. C. And you have not reckoned the four newly dis- covered little planets, which Dr. Herschel denomi- nates Asteroidsy but which are known by the name of Ceres Ferdinandea, Pallas, Juno, and Vesta. T. Well, then, the sun to these is the perpetual source of light, heat, and motion ; and to more distant worlds it is a fixed star, and will appear to some as large as Arcturus, to others no larger than a star of the sixth magnitude, and to others it must be invisible unless the inhabitants have the assistance of glasses, or are endowed with better eyes than ourselves. /. Pray, sir, how swift do you reckon that the particles of light move ? T. This you will easily calculate, when you know, that they are only about eight minutes in coming from the sun. THE SUN, THE SOURCE OF LIGHT. 339 C. And if you reckon that at the distance of ninety-five millions of miles from the earth, light pro- ceeds at the rate nearly of twelve millions of miles in a minute, or at 260,000 miles in a second of time. But how do you know that it travels so fast? T, It was discovered by M. Koemer, who observed that the eclipses of Jupiter's satellites took place about sixteen minutes later if the earth was in that part of its orbit which is farthest from Jupiter, than if it was in the opposite point of the heavens. C. I understand this : the earth may sometimes be in a line between the sun and Jupiter ; and at other times the sun is between the earth and Jupiter ; and therefore, in the latter case, the distance of Jupiter from the earth is greater than in the former, by the whole length of the diameter of its orbit. T, In this situation the eclipse of any of the satel- lites is, by calculation, sixteen minutes later than it would be if the earth were between Jupiter and the sun : that is, the light flowing from Jupiter's satellites is about sixteen minutes in travelling the diameter of the earth's orbit, or 190 millions of miles- J. It would be curious to calculate how much faster light travels than a cannon ball. T. Suppose a cannon ball to travel at the rate of twelve miles a minute, and light to move a million of times faster than that ; yet Dr. Akenside conjec- tures that there may be stars so distant from us that the light proceeding from them has not yet reached the earth : but Huygens, an eminent astronomer, threw out the idea before Akenside was born. /. And you say the particles of light move in all directions. T. Here is a sheet of thick brown paper — I make only a small pin-hole in it, and then, through that hole, I can see the same objects, such as the sky, trees, houses, &c. as I could if the paper were not there. C. Do we only see objects by means of the rays of light which flow from them ? 340 OPTICS. r. In no other way : and therefore the liglit that comes from the landscape which I view by looking through the small hole in the paper, must come in all directions at the same time. — Take another instance : if a candle be placed on an eminence in a dark night, it may be seen all round for the space of half a mile : in other words, there is no place within a sphere of a mile in diameter where the candle cannot be seen, that is, where some of the rays from the small flame will not be found. J. Why do you limit the distance to half a mile 1 T, The distance of course will be greater or less according to the size of the candle : but the degree of light, like heat, diminishes in proportion as you go farther from the luminous body. C. Does it follow the same law as gravity ?* T. It does : the intensity or degree of light de- creases as the square of the distance from the luminous body increases. J, Do you mean, that at the distance of two yards from a candle we shall have four times less light, than we should have if we were only one yard from it? T. I do : at three yards' distance nine times less light ; and at four yards' distance you will have six- teen times less light than you would were you within a yard of the object. — I have one more thing to tell you : light always moves in straight lines. /. How is that known ? T, Look through a straight tube at any object, and the rays of light will flow readily from it to the eye ; but let the tube be bent, and the object cannot be seen through it, which proves that light will flow only in a straight line. This is plain also from the shadows which opaque bodies cast ; for if the light did not describe straight lines, there would be no shadow. Hold any object * See Conver. VH. of Mechanics. OF RAYS OF LIGHT. 311 "in the light of the sun, or a candle, as a square board or book, and the shadow caused by it proves that light moves only in right or straight lines; for the space immediately behind the object is in shade. CONVERSATION II. OF RAYS OF LIGHT OF REFLECTION AND REFRACTION. C. You talked, the last time vi^e met, of the rays of light flowing or moving; what do you mean by a ray of light ? T. Light, you know, is supposed to be made up of indefinitely small particles ; now one or more of these I particles, in motion from any body, is called a ray of j light. — If the supposition be true, that light does consist of particles flowing from a luminous body, as the sun, and that these particles are about eight minutes in coming from the sun to us ; then, if the sun were blotted from the heavens, we should actually ! have the same appearance for eight minutes after the destruction of that body as we now have. /. I do not understand how we could see a thing that would not exist. r. The sun is perpetually throwing off particles of light, which travel at the rate of twelve millions of j miles in a minute, and it is by these that the image of I the body is impressed on our eye. The sun being i blotted from the firmament would not affect the ' course of the particles that had the instant before I been thrown from his body ; they would travel on as if nothing had happened, and till the last particles had reached the eye we should think we saw the sun I as much as we do now. C. Do we not actually see the body itself? i T. The sense of sight may, perhaps, not be un- aptly compared to that of smell : a grain of musk will I throw off its odoriferous particles all round, to a con- i siderable distance ; now if you or I happen to be near j it, the particles which fall upon certain nerves in the S42 OPTICS, nose, will excite in us those sensations by which we say we have the smell of musk. In the same way particles of light are flowing in every direction from the grain of musk, some of which fall on the eye, and these excite different sensations, from which we say we see a piece of musk. C. But the musk will in time be completely dis- sipated, by the act of throwing off the fine particles ; whereas a chair or a table may throw off its rays so as to be visible, without ever diminishing in size. T. True : because whatever is distinguished by the sense of smell, is known only by the particles of the odoriferous body itself flowing from it : ^ whereas a body distinguished by the sense of sight is known by the rays of light, which first fall on the body, and are then reflected from it. /. What do you mean by being reflected ? T. If I throw this marble smartly against the wainscot, will it remain where it was throwii 1 J. No: it will rebound or come back again, r. What you call rebounding, writers on optics denominate reflection. When a body of any kind, whether it be a marble with which you play, or a particle of light, strikes against a surface, and is sent back again, it is said to be reflected. If you shoot a marble straight against a board, or other obstacle, it comes back in the same line, or nearly so ; but sup- pose you throw it sideways, does it return to the hand] . , C. Let me see : I will shoot this marble against the middle of one side of the room, from the corner of the opposite side. J. You see, instead of coming back to your hand, it goes off to the other corner, directly opposite to the place from which you sent it. r. This will lead us to the explanation of one of the principal definitions in optics, viz. that the angle of reflection is always equal to the angle of incidence. You know what an angle is 1* * See Convcr. I. of ]\Ieclianics. INCIDENT AND REFLECTED RAYS. 343 C. We do : but not what an angle of incidence is. T. I said, a ray of light was a particle of light in motion: now there are incident x^y Sy^nA reflected x^ys. The incident rays are those which fall on the sur- face ; and the reflected rays are those which are sent off from it. C. Does the marble going to the wamscot represent the incident ray, and in going from it does it represent the reflected ray ? r. It does : and the wainscot may be called the re- flecting surface. J. Then what are the angles of mcidence and re- flection ] 1 . , 1 T. Suppose you draw the Imes on which the niarble travelled, both to the wainscot, and from it again. C. I will do it with a piece of chalk. T. Now draw a perpendicular* from the point where the marble struck the surface, that is, where your two lines meet. C. I see there are two angles, and they seem to be ^^^.^'We cannot expect mathematical precision in such trials as these ; but if the experiment were accu- rately made, the two angles would be perfectly equal : the angle contained between the incident ray, and the perpendicular, is called the angle of incidence, and that contained between the perpendicular and reflected ray is called the angle of reflection. J. Are these in all cases equal, shoot the marble as you will 1 T. They are : and the truth holds equally with rays of light : — both of you stand in front of the looking- glass. You see yourselves, and one another also ; for the rays of light flow from you to the glass, and are reflected back again in the same lines. Now both of you stand on one side of the room. ¥/hat do you see? * If tlie point be exactly in the middle of one side of the room, a perpendicular is readily drawn by finding the middle of the opposite side, and joining- the two points. 344 OFriCS. C. Not ourselves j but the furniture on the opposite side. T, The reason of this is, that the rays of light flowing from you to the glass are reflected to the other side of the room. C. Then if I go to that part, I shall see the rays of light flowing from my brother : — and I do see him in the glass. J. And I see you. T. Now the rays of light flow from each of you to the glass, and are reflected to one another : but neither of you sees himself. C. No : I will move in front of the glass, now I see myself, but not my brother ; and I think I under- stand the subject very well. T. Then explain it to me by a figure, which you may draw on the slate. C. Let a b represent the looking- glass : if I stand at o, the rays flow from me to the glass, and are reflected back in the same line, because now there is no angle of incidence, and of course no angle of reflection ; but Fig. 1 . if I stand at .t, then the rays flow from me to the glass, but they make the angle .r c o, and therefore they must be reflected in the line c y, so as to make the angle y c o, which is the angle of reflection, equal to the angle x c o. And if James stand at 3/, he will see me at a-, and I standing at x shall see him at y. CONVErxSATION III. OF THE REFRACTION OF LIGHT. C. If glass stop the rays of Hght, and reflect them, why cannot I see myself in the window ? T. It is the silvering on the glass which causes the reflection, otherwise the rays would pass through it without being stopped, aijjl jf they were not stopped REFRACTION OF LIGHT. 343 they could not be reflected. No glass however is so transparent, but it reflects some rays : put your hand to within three or four inches of the window, and you see clearly the image of it. J. So I do, and the nearer the hand is to the glass the more evident is the image ; but it is formed on the other side of the glass, and beyond it too. T, It is; this happens also in looking-glasses: you do not see yourself on the surface, but apparently as far behind the glass, as you stand from it in the front. Whatever suff'ers the rays of light to pass through it is called a medium. Glass, which is transparent, is a medium; so also is air; water, and indeed all fluids that are transparent are called media, ^ and the more transparent the body, the more perfect is the medium, C. Do the rays of light pass through these in a straight line 1 I T. They do : but not in precisely the same direc- tlon in which they were moving before they entered it. : They are be7it out of their former course, and this is i called refraction, J. Can you explain this term more clearly 1 J T. Suppose d / to be a piece of glass two or three ! inches thick; and a ray of light c a to fall upon it at a ; j it will not pass through it in j the direction c s, but when it i comes to a it will be bent to- wards the perpendicular a b, Fig. 2. and go through the glass in the course a x, and when it comes into the air it will pass on in the direction x z, which is parallel to c s. C. Does this happen if the ray fall perpendicularly ! on the glass, as p a f I T. In that case there is no refraction, but the ray proceeds in its passage through the glass, precisely in the same direction as it did before it entered it, namely, in the direction p b. Q 2 346 OPTICS. J. Refraction then takes place only when the rays fall obliquely or slantwise on the medium ? T. Just so : rays of light may pass out of a rarer into a denser medium, as from air into water or glass: or they may pass from a denser medium into a rarer, as from water into air. C. Are the effects the same in both cases ? r. They are not ; and I wish you to remember the difference. When light passes out of a rarer into a denser medium, it is drawn to the perpendicular ; thus if c a pass from air into glass, it moves, in its passage through it, in the line a x, which is nearer to the per- pendicular a b than the line a s which was its first direction. But when a ray passes from a denser medium into a rarer, it moves in a direction farther from the perpendicular ; thus if the ray x a pass through glass or water into air, it will not when it comes to a move in the direction a m, but in the line a c, which is farther than a m from the perpendicular a p, J. Can you shew us any experiment in proof of this? T. Yes, I can: here is a common earthen pan, on the bottom of which I will lay a shilUng, and will fasten it with a piece of soft wax, so that it shall not move from its place, while I pour in some water. Stand back till you j ust lose sight of the shilling. J. The side of the pan now completely hides the sight of the money from me. T. I will pour in a pitcher of clear water. J. I now see the shilling : how is this to be ex- plained 1 T. Look to the last figure, and conceive your eye to be at r, a b the side of the pan, and the piece of money to be at x : now when the pan is empty, the rays of light flow from x in the direction .t a m, but your eye is at c, of course you cannot see any thing by the ray proceeding along x am. As soon as I put the water into the vessel, the rays of light proceed from X to tJ, but there they enter from a denser to a rarer REFRACTION OF LIGHT. 347 medium; and, therefore, instead of moving in a m, as they did when there was no water, they will be bent from the perpendicular, and will come to your eye at c, as if the shilling were situate at n, J, And it does appear to me to be at T, Remember what I am going to tell you, for it is a sort of axiom in optics : "We see every thing in the direction of that line in which the rays approach us last." Which may be thus illustrated : I place a candle before the looking-glass, and if you stand also before the glass the image of the candle appears behind it ; but if another looking-glass be so placed as to re- ceive the reflected rays of the candle, and you stand before this second glass, the candle will appear behind that; because the mind transfers every object seen along the line in which the rays came to the eye last. C. If the shilling were not moved by the pouring in of the water, I do not understand how we could see it afterwards. T. But you do see it now at the point r?, or rather at the little dot just above it, which is an inch or two from the place where it was fastened to the bottom, and from which you may convince yourself it has not moved. /. 1 should like to be convinced of this : will you make the experiment again that I may be satisfied of it? T. You may make it as often as you please, and the effect will always be the satne : but you must not imagine that the shilling only will appear to move, the bottom of the vessel seems also to change its place. J. It appears to me to be raised higher as the water is poured in. T, I trust you are satisfied by this experiment : but I can shew you another equally convincing ; but in this we stand in need of the sun.^ Take an empty bason or pan a into a dark room, having only a very small hole in the a^/^ window shutter : so place the bason that a p/j ray of light s s shall fall upon the bottom of |/j it at a ; here I make a small mark, and then ^ fill the bason with water. Now where does y^^. ^ the ray fain 348 OPTICS. J, IVIuch nearer to the side at h. T. I did not move ihe bason, and therefore could Iiave no power in altering the course of the light. C. It is very clear that the ray was refracted by the water at s : and I see that the effect of refraction in this instance has been to draw the ray nearer to a per- pendicular, which may be conceived to be the side of the vessel. T. The same thing may be shewn wjth a candle in a room otherwise dark ; let it stand in such manner as that the shadow of the side of a pan or box may fall somewhere at the bottom of it; mark the place, and pour in water, and the shadow will not then fall so far from the side. CONVERSATION IV. OF THE REFLECTION AND REFRACTION OF LIGHT. T. We will proceed to some farther illustrations of the laws of reflection and refraction. We shut out all the light except the ray that comes in at the small hole in the shutter : at the bottom of this bason, where the ray of light falls, I lay this piece of looking-glass; and if the water be rendered in a small degree opaque by mixing with it a few drops of milk, and the room be filled with dust by sweeping a carpet, or any other means, then you will see the refraction which the ray from the shutter undergoes in passing into the water, the reflection of it at the surface of the looking-glass, and the refraction which takes place when the ray leaves the water, and passes again into the air. _ J. Does this refraction take place in all kinds of glass ? T. It does ; but where the glass is very thin, as in window glass, the deviation is so small as to be gene- rally overlooked. You may now understand why the oar in the vvalei appears bent, though it be really OPTICAL DECEPTIONS. 349 straight; for suppose a b be water, and max the oar, the image of the part a co in the water will lie above the object, so that the oar will appear in the shape ^ «i « /Zjinstead of m a a:. On this account also a fish in the water appears nearer the surface than it actually is, and a marksman shooting at it must aim below the place which it seems to occupy. C, Does the image of the object seen in the water always appear higher than the object really is 1 T. It appears one fourth nearer the surface than the object is. Hence a pond or river is a third part deeper than it appears to be, which is of importance to re- member, for many a school-boy has lost his life by imagining the water into which he plunged was within his depth. J. You say the bottom appears one fourth nearer the surface than it is ; and then that the water is a third deeper than it seems to be : I do not understand this. T. Suppose the river to be six feet deep, which is sufficient to drown you or me, if we cannot swim : I say the bottom will appear to be only four feet and a half from the surface, in which case you could stand and have the greater part of your head above it : of course it appears to be a foot and a half shallower than it is, but a foot and a half is just the third part of four feet and a half. C. Can this be shewn by experiment 1 T, It may : I take this large empty pan, and with a piece of soft wax stick a piece of money at the bot- tom, but so that you can just see it as you stand ; keep your position, and I will pour in a quantity of water gradually, and tell me the appearance. C. The shilling rises exactly in the same proportion as you pour in the water. T. Recollect, then, in future, that we cannot judge of distances so well in water as in air. 350 OPTICS. J. And I am sure we cannot of magnitudes : for in looking through the sides of a globular glass at some gold and silver fish, I thought them very large, but if 1 looked down upon them from the top they appeared much smaller indeed. r. Here the convex or round shape of the glass becomes a magnifier, the reason of vi^hich will be ex- plained hereafter. A fish will, however, look larger in water than it really is. — I will shew you another ex- periment which depends on refraction : here is a glass goblet two-thirds full of water ; I throw into it a shil- ling, and place a plate on the top of it, and turn it quickly over, that the water may not escape. What do you see ? C. There is certainly a half-crown lying on the plate, and a shilling seems swimming above it in the water. r. So it appears indeed ; but it is a deception, which arises from your seeing the piece of money in two directions at once, viz. through the conical surface of the water at the side of the glass, and through the flat surface at the top of the water. The conical surface, as was the case with the globular one in which the fish were swimming, magnifies the money; but by the flat surface the rays are only refracted, on which ac- count the money is seen higher up in the glass, and of its natural size, or nearly so. T. If 1 look side-ways at thje money I only see the large piece ; and if only at top, I see it in its natural size and state. C. Look again at the fish in the glass, and you will see through the round part two very large fish, and seeing them from the upper part they appear of their natural size; the deception is the same as with the shilling in the goblet. I'. The principle of refraction is productive of some very important effects. By this the sun every clear morning is seen several minutes before he comes to the horizon, and as long after he sinks beneath it in the evening. REFRACTION OF THE ATMOSPHERE. 351 C. Then the days are longer than they would be if there was no such thing as refraction. Will you ex- plain how this happens? T. I will : you know we are surrounded with an at- mosphere which extends all round the earth, and above it to about the height of forty-five miles ; now the dot- ted part of this figure represents that atmosphere : suppose a spectator no <> « stand at s, and the sun to be at a 3 if \ i there were no refraction the person / f at s would not see the rays from the ^""^,,53^^ sun till he were situate with regard to the sun in a line s x a, because Fio", 5. when it was below the horizon at 6, the rays would pass by the earth in the direction bxz; but owing to the atmosphere, and its refracting power, when the rays from b reach x, they are bent towards the perpendicular, and carried to the spectator at s. J. Will he really see the image of the sun while it is below the horizon? T, He will ; for it is easy to calculate the moment when the sun should rise and set, and if that be com- pared with exact observation, it will be found that the image of the sun is seen sooner and later than this by several minutes every clear day, C. Are we subject to the same kind of deception when the sun is actually above the horizon ? T, We are always subject to it in these latitudes, and the sun is never in that place in the heavens where he appears to be. Why in these latitudes particularly? T. Because with us the sun is never in the zenith, or directly over our heads; and in that situation alone his true place in the heavens is the same as his appa- rent place. C. Is that because there is no refraction when the rays fall perpendicularly on the atmosphere ? T. It is : but when the sun is at m, his rays will not proceed in a direct line m 0 r, but will be bent out of their course at 0, and go in the direction o s, and the 352 OPTICS. spectator will imagine he sees the sun in the line s 0 n. C. What makes the moon look so much larger when it is just above the horizon, than when it is higher up? r. The thickness of the atmosphere when the moon is near the horizon, renders it less bright than when it is higher up, which leads us to suppose it is farther off in the former case than in the latter ; and, because we imagine it to be farther from us, we take it to be a larger object than when it is higher up. It is owing to the atmosphere that the heavens ap- pear bright in the day-time. Without any atmosphere only that part of the heavens would appear luminous in which the sun is placed ; in that case, if we could live without air, and should stand with our backs to the sun, the whole heavens would appear as dark as night. CONVERSATION V. DEFINITIONS OF THE DIFFERENT KINDS OF LENSFS OF MR. Parker's burning lens, and the effects PRODUCED EY IT. T. I must claim your attention to a few other defi- nitions ; the knowledge of which will be wanted as we proceed, A pencil of rays is any number that proceed from a point. Parallel rays are such as move always at the same distance from each other. C. That is something like the definition of paralUd lines.* But, when you admitted the rays of light through the small hole in the shutter, they did not seem to flow from that point in parallel lines, but to recede from each other in proportion to their distance from that point. T, They did ; and when they do thus recede from * Parallel lines are tliosc which being' infinitely ex- tended never meet. OF THE DIFFERENT LENSES. 353 FiP . 6. each other, as in this figure from c to cd, then they are said to diverge. But if they continu- ally approach towards each other as in moving from cdto c, they are said to converge. J. What does the dark part of this figure represent ? T. It represents a glass lens, of which there are several kmds. C. How do you describe a lens ? T. A le7is is a glass ground into such a form as to collect or disperse the rays of light which pass through It. Lenses are of different shapes from which they take their names. They are represented here in one Fig. 7. view, a is such a one as that in the last figure, and It is called a plano-convex, because one side is flat, and the other convex ; 6 is a plano-concave, one side being flat, and the other is coyicave ; c is a double convex-lens, because both sides are convex; d is a double concave, because both sides are concave ; and e IS called a meniscus, being convex on one side', and concave on the other; of this kind are all watch glasses. /. I can easily conceive of diverging rays, or rays proceeding from a point ; but what is to make them converge, or come to a point 1 T. Look again to Fig. 6 ; now a, b, m, &c. re- present parallel rays, falling upon a convex surface, of glass for instance, all of which, except the middle one, fall upon it obliquely, and, according to what we saw yesterday, will be refracted towards the per- pendicular. 354 OPTluS. C. And I suppose they will all meet in a ccitain point in that middle line. T, That point c is called the/ocits : the dark part of this figure only represents the glass, at cdn. C. Have you drawn the circle to shew the exact curve of the different lenses 1 2\ Yes : and you see that parallel rays falling upon a plano-convex lens meet at a point behind it, the distance of which from the middle of the glass is exactly equal to the diameter of the sphere of which the lens is a portion. J. And in the case of a double ■ convex, is the distance of the focus of parallel rays, equal only to the radius of the sphere? T. It is : and you see the reason of it immediately, for two concave surfaces have dou- Fig. 8, ble the effect in refracting rays to what a single one has : the latter bringing them to a focus at the distance of the diameter, the former at half that distance, or of the radius. C. Sometimes, perhaps, the two sides of the same lens may have different curves : what is to be done then? T. If you know the radius of both the curves, the following rule will give you the answer : — As the sum of the radii of both curves or con- vexities is to the radius of either, so is double the radius of the other to the distance of the focus from the middle point." J, Then if one radius be four inches, and the other 24 3 three inches, I say, as 4 x 3:4::6:— = 3-^ » or to nearly three inches and a half. — I saw a gentle- man lighting his pipe yesterday by means of the sun's rays and a glass ; was that a double convex lens 1 T. I dare say it was : and you now sec the reason of what then you could not comprehend : all the rays of the sun that fall on the surface of the OF THE FOCAL DISTANCE. 355 glass (see Fig. 8.) are collected in the point/, which, in this case, may represent the tobacco in the pipe. C, How do j^ou calculate the heat which is col- lected in the focus 1 T. The force of the heat collected in the focus is in proportion to the common heat of the sun, as the area of the glass is to the area of the focus : of course it may be a hundred or even a thousand times greater in the one case than in the other. J, Have I not heard you say that Mr. Parker, of Fleet Street, made once a very large lens, which he used as a burning-glass ? T. He formed one three feet in diameter, and when fixed in its frame it exposed a clear surface of more than two feet eight inches in diameter, and its focus, by means of another lens, was reduced to a diameter of half an inch. The heat produced by this was so great that iron plates were melted in a few seconds : — tiles and slates became red hot in a moment, and were vitrified, or changed into glass : — sulphur, pitch, and other resinous bodies were melted under water : — wood-ashes, and those of other vege- table substances, were turned in a moment into tran- sparent glass. C. Would the heat produced by it melt all the metals 1 T, It would : even gold was rendered fluid in a few seconds ; notwithstanding, however, this intense heat at the focus, the finger might without the small- est injury be placed in the cone of rays within an inch of the focus. J. There was, however, I should suppose, some risk in this experiment, for fear of bringing the finger too near the focus. T, Mr. Parker's curiosity led him to try what the sensation would be at the focus, and he describes it like that produced by a sharp lancet, and not at all similar to the pain produced by the heat of fire or a candle. Substances of a white colour were difficult to be acted upon. 350 OPTICS. C. I suppose he could make water boil in a very short time with the lens. r. If the water be very clear, and contained in a clear glass decanter, it will not be warmed by the most powerful lens. But a piece of wood may be burned to a coal, when it is contained in a decanter of water. J. Will not the heat break the glass ? r. It will scarcely warm it : iff however, a piece of metal be put in the water, and the point of rays be thrown on that, it will communicate heat to the water, and sometimes make it boil. The same effect will be produced if there be some ink thrown into the water. If a cavity be made in a piece of charcoal, and the substance to be acted on be put in it, the effect pro- duced by the lens will be much increased. Any metal thus enclosed melts in a moment, the fire sparkling like that of a forge to which the blast of a bellows is applied. CONA^EKSATION VI. OF PARALLEL RAYS OF DIVERGING AND CON- VERGING RAYS or THE EOCUS AND EOCAL DIS- TANCES. C. I have been looking at the figures 6 and 8, and see that the rays falling upon the lenses are parallel to one another : are the sun's rays parallel 1 T, They are considered so : but you must not suppose that all the rays wliich come from the surface of an object, as the sun, or any other body, to the eye, are parallel to each other, but it must be understood of those rays only which proceed from a single pomt. Suppose s to be the sun, the rays which proceed from a ^^ y^^^^^^-f^tr;:^:^^^^^ single point a, do in reality '"c ' Ji form a cone, the base of Fig. 9. OF PARALLEL RAYS. 31)7 which is the pupil of the eye, and its height is the distance from us to the sun. / . B\it the breadth of the eye is nothing when com- pared to a line ninety-five millions of miles long. T. And for that reason the various rays that pro- ceed from a single point in the sun are considered as parallel, because their inclination to each other is in- sensible. The same may be said of any other point, as c. Now all the rays that we can admit by means of a small aperture or hole, must proceed from an indefinitely small point of the sun, and therefore they are justly considered as parallel. If now we take a ray from the point a, and another from c, on opposite points of the sun's disk, they will form a sensible angle at the eye ; and it is from this angle a eg that we judge of the apparent size of the sun, which is about half a degree in diameter. C. Will the size of the pupil of the eye make any difference with regard to the appearance of the ob- ject? T. The larger the pupil the brighter will the object appear, because the larger the pupil is the greater num- ber of rays it will receive from any single point of the object. — And I wish you to remember what I have told you before, that whenever the appearance of a given object is rendered larger and brighter, we always imagine that the object is nearer to us than it really is, or than it appears at other times. J. If there be nothing to receive the rays (Fig. 8.) at fy would they cross one another and diverge ? T. Certainly, in the same manner as they con- verged in coming to it ; and if another glass, fg, of the same convexity as d e, be placed in the rays at the same distance from the focus, it will so refract them, that, after going out of it, they will be parallel, and so proceed on in the same manner as they came to the first glass. C. There is, however, this difference — all the rays except the middle one have changed sides. T. You are right j the ray b, which entered at 358 OPTICS. bottom, goes out at the top h; and a, which entered at the top, goes out at the bottom c, and so of the rest. If a candle be placed at/, the focus of the convex glass, the divergmg rays in the space f/ g will be so refracted by the glass, that after going out of it, they will become parallel again. J. What will be the effect if the candle be nearer to the glass than the point/? r. In that case, as if the candle be at g, the rays will diverge after they have passed through the glass, and the divergency will be greater or less in proportion as the candle is more or^less distant from the focus. C. If the candle^be placed far- ther from the lens tten the focus/, will the rays meet in a point after they have passed, through iti T. They will : thus, if the candle be placed at g, the rays, , after passing the lens, will meet in X ; and this point x will be moj^e or less distant from the glass, as the candle is nearer to, or far- ther from, its focus. — Where the rays meet they form an inverted image of the flame of the candle. /. Why so? T. Because that is the point where the rays, if they are not stopped, cross each other : to satisfy you on this head I will hold in that point a sheet of paper, and you now see that the flame of the candle is in- verted. This may be explained in the following manner : Let a be represent an arrow placed beyond the focus g of a double convex lens, def; some rays will flow from every part of the arrow, and fall on the lens ; but we shall consider only those which flow from the points a, h, and c. The rays which come from a, as Fi.cr. 11. OF THE INVERTED IMAGE. 359 Fig. 12. ad, ae, and a/, will be refracted by the lens, and meet in a. Those which come from h, as hd^ be, and h f, will unite in b, and those which come from c will unite m c. ' C. I see clearly how the rays from h are refracted, and unite in b ; but it is not so evident with regard to those from the extremities a and c. T. I admit it, but yau must remember the difficulty consists in this, the rays fall mote; obliquely on the glass from those points than from the middle, and therefore the refraction is' very different. The ray i-n the centre suffers no refraction, hdis refracted into B : and if another ray went from i as id, it would be refracted to i somewhere between b and a, and the rays from a must for the same reason be refracted to A. J. If the object a 6c is brought nearer to the glass, will the picture be removed to a greater distance 1 r. It will : for then the rays will fall more diver- ging upon the glass, and cannot be so soon collected into the corresponding points behind it. C From what you have said, I see that if the ob- ject a ft c be placed in g, the rays, after refraction, will go out parallel to one another ; and if brought nearer to the glass than g, then they will diverge from one another, so that in neither case an image will be formed behind the lens. J. To get an image, must the object be beyond the focus g ? T, It must : and the picture will be bigger or less than the object, as its distance from the glass is 360 OPTICS greater or less than the distance of the object ; if abc (Fig. 12.) be the object, c b a will be the picture ; and if c B A be the object, abc will be the picture. C. Is there any rule to find the distance of the picture from the glass 1 T, If you know the focal distance of the glass, and the distance of the object from the glass, the rule is this : " Multiply the distance of the focus by the distance of the object, and divide the product by their differ- ence, the quotient is the distance of the picture." J. If the focal distance of the glass be seven inches, and the object be nine inches from the lens, I say, Z-^ = ^=:31| inches; of course the pic- ture will be very much larger than the object. For, as you have said, the picture is as much bigger or less than the object, as its distance from the glass is greater or less than the distance of the object, r. If the focus be seven inches, and the object at the distance of seventeen inches, then the distance of the picture will be found thus ^ = =12 inches nearly. CONVERSATION VII. IMAGES OF OBJECTS INVERTED OF THE SCIOPTRIC BALL OF LENSES AND THEIR FOCI. J. Will the image of a candle when received through a convex lens be inverted 1 T. It will, as you shall see : here is no light in this room but from the candle, the rays of which pass through a convex lens, and by holding a sheet of paper in a proper place, you will see a complete in- verted image of the candle on it. An object seen through a very small aperture appears also inverted, but it is very imperfect com- OF INVERTED IMAGES. 3g| pared to an image formed with the lens ; it is faint for want of light, and it is confused because the rays in- terfere with one another. ^ C. What is the reason of its being inverted J . Because the rays from the extreme parts of the object must cross at the hole. If you look through a very sjiall hole at any object, the object appears magnified Make a pm-hole in a she^t of brown paper, and look through it at the small print of this J. It is, indeed, very much magnified. imLA^ f^^""^- ^PP^^aches a convex lens, its mage departs from it ; and as the object recedes, its image advances. Make the experimtnt with a can aie and a ens, properly mounted, in a long room: theTmfo^ "-^^ throw image on the opposite wall, the image is lar^e and the ima|e is small and the distance between the candle and glass is ver^ much increased. ^ ^ f^^J''''' f^^:^. you an instrument called a Scioptr^c Ball, which is fastened into a window shutter in a room from which all light is banished excep what comes m through this glass. ^ C. Of what does this instrument consist ? , . ^ • V? ^ F^n^e A B and a ball of wood c rf^ m which IS a glass lens; and the ball cJ ^ moves eesily m the frame in all directions, H ^ so that the view of any surrounding objects H O may be received through it. shu'tte??' ^'^'^ ^""'^^ ^'^'^^ '"^^^ them l^X^tt '"^ \ 'h^^ P"^P««^ ^ inere are little brass screws belonging to it, such as hat marked 5. When it is fixed in its place a screen must be placed at a proper distance from ihe doors Th-r-' T '"'^'^ objects out of ficial eye ^"^^^^^^^"^ sometimes called an arti- R 362 OPTICS. C. In what respects is it like the eye 1 T, The frame has been compared to the socket in which tlie eye moves, and the wooden ball to the whole globe of the eye ; the hole in the ball repre- sents the pupil, the convex lens corresponds to the crystalline humour,* and the screen to the retina. J. The ball by turning in all directions is very like the eye, for without moving my head I can look on all sides, and upwards and downwards. T. Well, we will now place the screen properly, and turn the ball to the garden : — Here you see all the objects perfectly expressed. /. But they are all inverted. T, That is the great defect belonging to this in- strument ; but 1 will tell you how it may be re- medied : — take a looking glass and hold it before you with its face towards the picture on the screen, and inclining a little downwards, and the images will appear erect in the glass, and even brighter than they were on the screen. C. You have shewn us in what manner the rays of light are refracted by convex lenses, when those rays are parallel : will there not be a difference if the rays converge or diverge before they enter the lens 1 T. Certainly : if rays converge before they enter a convex lens, they will be collected at a point nearer to the lens than the focus of parallel rays. But if they diverge before they enter the lens, they will then be collected in a point beyond the focus of parallel rays. There are concave lenses as well as convex, and the refraction which takes place by means of these differs from that which I have already explained. | C. What will the effect of refraction be when! parallel rays fall upon a double concave lens ? 1 * For an explanation of these terms, see Coiiver. XV. 1 on the Eye. I OF CONCAVE LENSES g63 T. Suppose the parallel rays, a a, h, Cy d, &c. pass through the lens A B, they will diverge after they have passed through the glass. J , Is there any rule for ascer- taining the degree of divergency ? Fig. 14. T. Yes, it will be precisely so much as if tlie rays had come from a radiant point x, which is the centre of the concavity of the glass. C. Is that point called the focus ? T. It is called the virtual or imaginary focus. Thus the ray a, after passing through the glass a b will go on in the direction ^ /z, as if it had come from the point x, and no glass been in the way ; the ray b w^ould go on in the direction m n, and the ray e in the direction r s, and so on. The ray cx in the centre suffers no refraction, but proceeds precisely as if no glass had been in the way. / . Suppose the lens had been concave only on one side, and the other side had been flat, how would the rays have diverged 1 T. They would have diverged after passing through it, as if they had come from a radiant point at the distance of a whole diameter of the convexity of the lens. C. There is then a great similarity in the refrac- tion of the convex and concave lens. T. True : the focus of a double convex is at the distance of the radius of convexity, and so is the imaginary focus of the double concave; and the focus of the plano-convex is at the distance of the diameter of the convexity, and so is the imaginary focus of the plano-concave. You will find that images formed by a concave leiis, or those formed by a convex lens, where the object is ivithin its principal focus, are in the same position with the objects they represent :— they are also imaginary, for the refracted rays never meet at the foci whence they seem to diverge. 364 OPTICS. But the images of objects placed beyond the focus of a convex lens are inverted, and i^eul, for the re- fracted rays do meet at their proper foci. Remember, convex lenses render the rays which pass through them convergent, and bring them to- gether into a focus. Concave lenses render the rays transmitted through them more divergent. CONVERSATION VIII. OF THE NATURE AND ADVANTAGES OF LIGHT OF THE SEPARATION OF THE RAYS OF LIGHT BY MEANS OF A PRISM — AND OF COMPOUND RAYS, &C. T. We cannot contemplate the nature of light without being struck with the great advantages which we enjoy from it. Without that blessing our con- dition would be truly deplorable. C. I well remember how feelingly Milton de- scribes his situation after he lost his sight • With the year Seasons retura ; but not to me returns Day, or the sweet approach of even or morn. Or sight of vernal bloom, or summer's rose. Or flocks, or herds, or human face divine ; But cloud instead, and ever-during- dark Surrounds me, from the cheerful ways of men Cut off, and for the book of knowledge fair Presented with an universal blank Of Nature's works, to me expunged and razed, And wisdom, at one entrance, quite shut out. T. Yet his situation was rendered comfortable by means of friends and relations, who all possessed the advantages of light. But if our world were deprived of light, what pleasure or even comfort could we en- joy] How," says a good writer, ** could we provide ourselves with food, and the other necessaries of life? How could we transact the least business? How could we correspond with each other, or be of the THE BLESSINGS OF LIGHT. 305 least reciprocal service, without light, and those ad- mirable organs of the body which the Omnipotent Creator has adapted to the perception of this inesti- mable benefit ?" J. But you have told us that the light would be of comparatively small advantage without an atmos- phere, T. The atmosphere not only refracts the rays of the light so that we enjoy longer days than we should without it, but occasions that twilight which is so beneficial to our eyes, for without it the appearance and disappearance of the sun would have been in- instantaneous ; and in every twenty-four hours we should have experienced a sudden transition from the brightest sun-shine to the most profound darkness, and from thick darkness to a blaze of light. C. I know how painful that would be, from having slept in a very dark room and having suddenly opened the shutters when the sun was shining extremely bright. T. The atmosphere reflects also the light in every direction, and if there were no atmosphere, the sun would benefit those only who looked towards it, and to those wliose backs were turned to that luminary it would all be darkness. Ought we not, therefore, gratefully to acknowledge the wisdom and goodness of the Creator, who has adapted these things to the advantage of his creatures ? J. I saw in some of your experiments that the rays of light after passing through the glass were tinged with diflferent colours ; what is the reason of this ? 2\ Formerly light was supposed to be a simple and uncompounded body ; Sir Isaac Newton, how- ever, discovered, that it was not a simple substance, but was composed of several parts, each of which has in fact a different degree of refrangibiiity. C. How is that shewn ? T. Let the room be darkened, and let there only be a very small hole in the shutter to admit the sun's rays ; instead of a lens I take a triangular piece or 3C6 OPTICS. glass, called a prism ; now, as in this there is nothing to bring the rays to a focus, they will, in passing through it, suffer different degrees of refrac- tion, and be separated into the different coloured rays, which, being received on a sheet of white paper, will exhibit the seven following colours, red, orange, yellow, green, blue, indigo, and violet, J, Here are all the colours of the rainbow : the image on the paper is a sort of oblong. T. That oblong image is usually called a spectrum, and if it be divided into 360 equal parts, the red will occupy 45 of them, the orange 27, the yellow 48, the green and the blue 60 each, the indigo 40, and the violet 80. C. The shade of difference in some of these colours seems very small indeed. T. You are not the only person who has made this observation ; some experimental philosophers say there are but three original and truly distinct colours, viz. the red, yellow, and blue, C. What is called the orange is surely only a mix- ture of the red and yellow, between which it is situated. T. In like manner the green is said to be a mixture of the yellow and blue, and the violet is but a fainter tinge of the indigo. J. How is it then that light, which consists of several colours, is usually seen as white? T. By mixing the several colours in due proportion white may be produced. J. Do you mean to say that a mixture of red, orange, yellow, green, blue, indigo, and violet, in any proportion, will produce a white ] T, If you divide a circular surface into 360 parts, and then paint it in the proportion just mentioned, that is, 45 of the parts red, 27 orange, 48 yellow, ^ &c. and turn it round with great velocity, the whole ^ will appear of a dirty white, and if the colours were more perfect the white v/ould be so too. OF COLOURS. 367 J. Was it then owing to the separation of the different rays, that I saw the rainbow colours about the edges of the image made with the lens? T. It was : some of the rays were scattered, and not brought to a focus, and these were divided in the course of refraction. And I may tell you now, though I shall not explain it at present, that the rainbow in the heavens is caused by the separation of the rays of light mto their component parts. C. And was that the cause of the colours which we saw on some soap bubbles which James was making with a tobacco-pipe ? T. It was. These bubbles are merely thin bladders of the solution, whose thickness continually varying produces the variety of colours which they exhibit. CONVERSATION IX. OF COLOURS. C. After what you said yesterday, I am at a loss to know the cause of different colours : the cloth on this table is green ; that of which my coat is made is blue : what makes the difference in these? T. All colours are supposed to exist only in the light of luminous bodies, such as the sun, a candle, &c. and that light falling incessantly upon different bodies is separated into its seven primitive colours, some of which are absorbed, while others are reflected. J. Is it from the reflected rays that we judge of the colour of objects ? T. It has generally been thought so; thus the cloth on the table absorbs all the rays but the green, which it reflects to the eye : but your coat is of a different texture, and absorbs all but the blue rays. C. Why is paper white, or the snow 7 T. The whiteness of paper is occasioned by its re- flecting the greatest part of all the rays that fall upon it. And every flake of snow, being an assemblage of frozen globules of water sticking together, reflects and OPTICS. refracts the light that falls upon it in all directions, so as to mix it very intimately, and produce a white image on the eye. J. Does the whiteness of the sun's light arise from a mixture of all the primary colours? T, It does, as may be easily proved by an experi- ment, for if any of the seven colours be intercepted at the lens, the image in a great measure loses its white- ness. With the prism I will divide the ray into its seven colours ;* I will then take a convex lens in order to re-unite them into a single ray, which will exhibit a round image of a shining white ; but if only fiv^e or six of these rays be taken with the lens, it will pro- duce a dusky white. C, And yet to this white colour of the sun we are indebted for all the fine colours exhibited in nature. T. Yes, and without light even the diamond would lose all its beauty. J. The diamond, I know, owes its brilliancy to the power of reflecting almost all the rays of light that fall on it : but are vegetable and animal tribes equally indebted to it? T. What does the gardener do to make his endive and lettuces white? C. He ties them up. T. That is, he shuts out the light, and by this means they become blanched. I could produce you a thou - sand instances to shew, not only that the colour, but even the existence, of vegetables depends upon light. Close wooded trees have only leaves on the outside, such is the cedar in the garden. Look up the inside of a yew tree, and you will see that the inner branches are almost oi altogether barren of leaves. Gera- niums and other green-house plants turn their flowers to the light; and plants in general, if doomed to dark- ness, soon sicken and die. J, There are some flowers, the petals of which are, * A figure will be given on this subject Avith explana tions, Conversation XVI i I. on the Rainbow. OF COLOURS. in different parts, of different colours ; how do you ac- count for this? T. The flower of the heart's-ease is of this kind, and if examined with a good microscope it will be found that the texture of the blue and yellow parts is very dif- ferent. The texture of the leaves of the white and red rose is also different. Clouds also, which are so vari- ous in their colours, are undoubtedly more or less dense, as well as being differently placed with regard to the eye of the spectator ; but they all depend on the light of the sun for their beauty. C. Are we to understand that all colours depend on the reflection of the several coloured rays of light 1 T. This seems to have been the opinion of Sir Isaac Newton; but he concluded from various experiments on this subject, that every substance in nature, provided it be reduced to a proper degree of thinness, is trans- parent. Many transparent media reflect one colour, and transmit another: gold-leaf reflects the yellow, but it transmits a sort of green colour by holding it up against a strong light. Mr. Delaval, a gentleman who a few years since made many experiments to ascertain how colours are produced, undertakes to shew that they are exhibited by transmitted light alone, and not by reflected light. /. I do not see how that can be the case with bodies that are not transparent. T. He infers, from his experiments, which you may hereafter examine for yourselves, that the original fibres of all substances, when cleared of heterogeneous matter, are perfectly white, and that the rays of light are reflected from these white particles through the colouring matter with which they are covered, and that this colouring matter serves to intercept certain rays in their passage through it, while a free passage being left to others, they will exhibit, according to these cir- cumstances, different colours. — The red colour of the shells of lobsters after boiling, he says, is only a super- ficial covering spread over the white calcareous earth R 2 370 OPTICS. of which the shells are composed, and maybe removed by scraping or filing. Before the application of heat it is so thick as to appear black, being too thick to admit the passage of light to the shell and back again. The case is the same with feathers, which owe their colours to a thin layer of transparent matter on a white giiDund. CONVERSATION X. REFLECTED LIGHT, AND PLAIN MIRRORS. T. We come now to treat of a different species of glasses, viz. of mirrors, or, as they are sometimes called, specula. J. A looking-glass is a mirror, is it not? T. Mirrors are made of glass, silvered on one side; they are also made of highly-polished metal. There are three kinds of mirrors, the plain, the convex, and the concave. C. You have shewn us that, in a looking-glass or plain mirror, "The angle of reflection is always equal to the angle of incidence."* T. This rule is not only applicable to plain mirrors, but to those which are convex and concave also, as I shall shew you to-morrow. But I wish to make some observations first on plain mirrors. In the first place, if you wish to see the complete image of yourself in a plain mirror, or looking-glass, it must be half as long as you are high. J. I should have imagined the glass must have been as long as I am high. T. In looking at your image in the glass, does it not seem to be as far behind the glass as you stand before it? /. Yes : and if I move forwards or backwards, the image behind the glass seems to approach or recede. T. Let a 6 be the looking-glass, and a the spectator^ * See Conversation II. OF PLAIN MIRRORS. 371 Fig. 15. standing opposite to it. The ray from his eye will be reflected in the same line a a, but the ray c b, flowing from his foot, in order to be seen at the eye, must be reflected by the line h a. C. So it will; for if a? 6 be a line perpendicular to the glass, the incident angle will hQ g b x, equal to the reflected angle a b x. T. And therefore the foot will appear behind the glass at D along the line a 6 d, because that is the line in which the ray last approaches the eye. /.Is that part of the glass a b intercepted by the lines A B and a d, equal exactly to half the length b d, or AC? T. It is ; Aa b and a b d may be supposed to form two triangles, the sides of which always bear a fixed proportion to one another; and if a b is double of a a, as in this case it is, b d will be double of a b, or at least of that part of the glass intercepted by a b and a d. C. This will hold true, I see, stand at what distance we please from the glass. T. If you walk towards a looking-glass your image will approach, but with a double velocity, because the two motions are equal and contrary. But if while you stand before a looking-glass, your brother walk up to you from behind, his image will appear to you to move i&t the same rate as he walks, but to him the velocity of the image will appear to be double; for with re- gard to you, there will be but one motion, but with re- gard to him, there will be two equal and contrary ones. J. If I look at the reflection of a candle in a look- ing-glass, I see in fact two images, one much fainter than the other; what is the reason of this'? ST2 OPTICS. r. The same may be observed of any object that is strongly illuminated, and the reason of the double image is, that a part of the rays are immediately re- flected from the upper surface of the glass, which form the faint image, while the greater part of them are re- flected from the farther surface, or silvering part, and form the vivid image. To see these two images you must stand a little side- ways, and not directly before the glass. C. What is meant by the expression of *'an image being formed behind a reflector?" T. It is intended to denote that the reflected rays come to the eye with the same inclination as if the ob- ject itself were actually behind the reflector. If you, standing on one side of the room, see the image of your brother, who is on the other side, in the looking-glass, the image seems to be formed behind the glass, that is, the rays come to your eye precisely in the same way as they would if your brother himself stood in that place, without the intervention of a glass. J. But the image in the glass is not so bright or vivid as the object. T. A plain mirror is in theory supposed to reflect all the light which falls upon it, but in practice nearly half the light is lost on account of the inaccuracy of tlie polish, &c. C. Has it not been said, that Archimedes, at the siege of Syracuse, burnt the ships of Marcellus by a machine composed of mirrors'? T. Yes : but we have no certain accounts that may be implicitly relied on. M. Buflbn, about fifty or sixty years ago, burnt a plank at the distance of seventy feet, with forty plain mirrors. ./. 1 do not see how they can act as burning-glasses. T. A plain mirror reflects the light and heat coming from the sun, and will illuminate and heat any sub- stance on which they are thiown, in the same manner as if the sun shone upon it. Two mirrors will reflect on it a double quantity of heat ; and if 40 or 100 mirrors could be so placed as to reflect from each the OF CONCAVE MIRRORS. 373 heat coming from the sun, on any particular substance, they would increase the heat 40 or 100 times. OF CONCAVE MIRRORS THEIR USES HOW THEY ACT, J. To what uses are concave mirrors applied? T. They are chiefly used in reflecting telescopes : that is, in telescopes adapted to viewing the heavenly bodies. And as you like to look at Jupiter's little moons and Saturn's ring through my telescope, it may be worth your while to take some pains to know by what means this pleasure is afforded you. T. I shall not object to any attention necessary to comprehend how these instruments are formed. T. A B represents a concave mirror, and a b, c d, e f, three A.# 3? the mirror a b in all its parts. J. Then all the lines drawn from c to the glass will be equal to one another, as c c d, and cf? T. They will : and there is another property be- longing to them J they are all perpendicular to the glass in the parts where they touch. C. That is, c b and 0^' are perpendicular to the glas at b and /, as well as c c/ at . T. Yes, they are : — c d is an hicident ray, but as it passes through the centre of concavity, it will be re- flected back in the same line ; that is, as it makes no angle of incidence, so there will be no angle of reflec- tion : a 6 is an incident ray, and I want to know what will be the direction of the reflected ray ] C. Since c 6 is perpendicular to the glass at the angle of incidence is a'b c\ and as the angle of re- CONVEKSATION XT parallel rays of light falling upon it. c is the centre of concavity, that is, one leg of your com- passes being placed on c, and then opening them to the length c d, and the other leg will touch B Fig. 16. 474 OPTICS. fiect'on is always equal to the angle of incidence, I must make another angle, as c b m equal to a b c,* and then the line b m is that in which the incident ray will move after reflection, 7'. Can you, James, tell me how to find the line in which the incident ray e /"will move after reflection? J. Yes: I will make the angle c /" m equal to c /*^, and the line / m will be that in which the reflected ray will move ; therefore, ef is reflected to the same point m as a 6 was. T. If^ instead of two incident rays, any number were drawn parallel to c d, they would every one be reflected to the same point m; and that point, which is called the focus of parallel rays, is distant from the mirror equal to half the radius c d. J. Then we may easily find the point without the trouble of drawing the angles, merely by dividing the radius of concavity into two equal parts. T. You may. The rays, as we have already ob- served, which proceed from any point of a celestial object, may be esteemed parallel at the earth, and therefore the image of that point will be formed at m. C. Do you mean that all the rays flowing from a point of a star, and falling upon such a mirror, will be reflected to the point 7n, where the image of the star will appear ? T. I do, if there be any thing at the point m to re- ceive the image. /. Will not the same rule hold with regard to ter- restrial objects ? T. No : for the rays which proceed from any terres- trial object, however remote, cannot be esteemed strictly parallel ; they therefore come diverging, and will not be converged to a single poi)it, at the distance * To make an angle c b m, equal to another given one, as « & c. From h, as a centre with any radius h x, de- scribe the arc x o which will cut c bin z : take the dis tance .r z in your compasses, and set off with it z o, and then draw the line bom, and the angle w 6 c is equal to the angle a b c. OF CONCAVB MIRRORS. of half the radius of the mirror's concavity from thp -LTdlZce"/ "".V" -'--^ atTli 5: C Can vn, ^T-""!-"";" ^ ''^If the radius. U Can you explam this by a figure? ■( . I will endeavour to do so. Let a b be a con- Kg. 17. part ofwMc'h'°'^ y ^"^""^^ from every rairror. that is, from the point m ravs will flow tn every point of the mirror, and so they will i^^m E ai d st whiriyr'"' '^'^^ extremities L^t us part of fhp'''!/'" "if' P™"^^'' "'to different r V ^^^J^''^^:}'^ '-eflected to a single points point. T 'i f ^ ^""^ the difficulty is to find that the glat;. ' ""^ ^ '■^'^ of concavity of dicuiar^to'tJ Y'^^ZV' be perpen- now g[vfn ani ftt t P?'"'/-' - ^ r ^7 j' ' the angle of incidence. did before.^"" ^l"^'! to it, as you tend' If 7 • ^ ^ ^qtial to M a c, and ex- tend the line A X to any length you please. M c and^h^ ' " " '"^''^ ^"h the ray of incMe^ce.^''^'"'""'" " "'^'^'^ ^"°*«r ^"g"^ to k Ind 'Jh^f ' °f '•'^''^^tion c c . equal 1 X 'nTl ^""^ P''°'l"^^^de for the use of 1 A i f' ^^^"^ •'y stretching the eye to Alps towering on Alps, can by their mirror brin' ti e e subhme objects into a narrow compass, and gratify the sight by pictures which the art of man in vain attempts to imitate." * Concave mirrors have been used for many other and different purposes, for by tnem, with a little in- genuity, a thousand illusions may be practised on the Ignorant and credulous. f cu on me in RoL^ir^^' P'?" exhibition in Bond Street, which you said depended on a con- cave mirror ; I was desired to look into the glass I did so, and started back, for I thought the point of a mot h^f /, ''"^ '"'PP'^ > then I slw a most beautiful nosegay, which I wished to grasp, but It vanished in an instant * ^ T. 1 will explain how these deceptions are ma- Fig. 23. naged : let e p be a concave mirror, 10 or 12 inches in diameter, placed in one room ; ^ b the wain cot that eparates the spectator from it, but in this there is a square or circular opening which faces the mirror ex- • See Economy of Nature, Vol. I., p. 26, 2d Edition. 384 OPTICS. actly. A nosegay, for instance, is inverted at c, which must be strongly illuminated by means of an Argand's lamp ; but no direct light from the lamp is to fall on the mirror. Now a person standing at g will see an image of the nosegay at d. J. What is to make it vanish 1 T. In exhibitions of this kind there is always a per- son behind the wainscot in league with the man that attends the spectator, who removes the real nosegay upon some hint understood between them. C. Was it then upon the man behind the scene that the approaching sword, and the advancing death's head, &c. depended'' T. It was : and persons have undertaken to exhi- bit the ghosts of the dead by contrivances of this kind ; for if a drawing of the deceased be placed in- stead of the nosegay, it may be done. But such ex- hibitions are not to be recommended, and indeed ought never to be practised, without explaining the whole process to the astonished spectator afterwaids. If a large concave mirror be placed before a blazing fire, so as to reflect the image of the fire on the flap of a bright mahogany table, a spectator suddenly introduced in the room will suppose the fire to be on the table. If two large concave -^ ^x. mirrors a and b be placed 1> opposite each other at the |V ^xl distance of several feet, Y\o-. 24. and red hot charcoal be ° put in the focus d, and some gunpowder in the other focus c, it will presently take fire. The use of a pair of bellows may be necessary to make the charcoal burn strongly. — This experiment may be varied by placing a thermometer in one focus, and lighted char- coal in the other, and it will be seen that the quick- silver in the thermometer will rise as the fire increases, though another thermometer at the same distance from the fire, but not in the focus of the glass, will not be aflfected by it. OF THiS EYE. gg. se™ to astonish those who a^etS CONVERSATION XV. OF THE DIFFERENT PABTS OP THE EYE. :xs^t:irtr---^^^^^^^ ^. The eye, when taken from the socket, is^ of a Fig. 25. 386 OPTICS. globular form, and it is composed of three coats or skins, and three other substances called humours. The first figure represents the section of an eye, that is, an eye cut down the middle : and Fig. 26. the front view of an eye as it appears in the head. C. Have these coats and humours aU different names ? T. Yes : the external coat, which is represented by the outer circle A BCD E, is called the sclerotica: the front part of this, namely, cxb, is perfectly tran- sparent, and is called the cornea j beyond this, to- wards B and E, it is white, and called the white of the eye. The next coat, which is represented by the second circle, is called the choroides, J. This circle does not go all round. T, No : the vacant space ab is that which we call the pupil, and through this alone the light is allowed to enter the eye. C. What do you call that part which is of a beau- tiful blue in some persons, as in cousin Lydia j and in others brown, or almost black ? T. That, as ac, be, is part of the choroides, and i called the iris. C. The iris is sometimes much larger than it is at another. T» It is composed of a sort of net-work, which con tracts or expands according to the force of light i which it is placed. Let James stand in a dark cor ner for two or three minutes : — now look at his eyes. C. The iris of each is very smalt, and the pupil large. T. Now let him look, steadily, pretty close to thei candle. j C. The iris is considerably enlarged, and the pupili of the eye is but a small point in comparison of v. haa it was before. ^ T, Did you never feel uneasy after sitting some time in the dark, when candles were suddenly brought into the room 1 s /. Yes: 1 lemember last Friday evening we hadj OP THE HUMOURS OF THE EYE. 3B7 b3ea fitting half an hour almost in the dark at Mr onpTfThT^'.'' candles were introduced every one ot the company complamed of the pain which the suddea light occasioned. ^ fr J;»i^^ ''"'"^ ? ■^^'-'^ the iris was con- tacted very much; of course, the pupil being laZ more hght was admitted than it could well befr afd he efore, t.U hme was allowed for the iris to adfus Itself, the uneasmess would be felt ^^^'^ 'f"^ "'"d "coat, which, from in ;l . f"^- '^"^ net-work, which serves efr, r ' f\ ""I-^'' P^duced by The refraction of the different humours of the eye and pamted, as it were, on the surface. ^ ino^'tht'''' ""^ ''f intended forrefract- iefses ' '° '''^ ^^""^ manner as glass nl'f u^^^ ^""^ '''^y ^'•^ ""e «(r««„s, the " Vrta«,«. and the ayu.ow humours. The vitreous humour fills up all the space zz, at the back of the cje; ,t IS nearly of the substance of melted glass Idou2 '^^«Pr«^ented by df, in the shape of a double convex lens: and the aqueous, or waterJ humour, fills up all that part of the eye betwein the V^llT ^°<1 cornea c x rep'^esm f " eye irt,/^ ^-^ 'l^*''^ optic nerve, which serves to convey to the brain the sensations produced on the retina. ^ ! y. Does the retina extend to the brain' ^. It does: and we shall, when we meet next endeavour to explain the office of these humours fn affecting vision. In the mean time, I woul"reques" you to consider again what I have told you of the ItXCVasVtr^ — -'-h-a'ml ;| ^. 1 intended to have reserved this to another 388 OPTICS. opportunity : but I may now say, that the eye-brows defend the eye from too strong a light ; and they pre- vent the eyes from injuries by the sliding of substances down the forehead into them. The eye-lids act like curtains to cover and protect the eyes during sleep ; when we are awake, they diffuse a fluid over the eye, which keeps it clean and well adapted for transmitting the rays of light. The eye-lashes, in a thousand instances, guard the eye from danger, and protect it from floating dust, with which the atmosphere abounds. CONVERSATION XVI. OF THE EYE, AND THE MANNER OF VISION. C. I do not understand what you meant, when you said the optic nerve served to convey to the brain the sensations produced on the retina. T, Nor do I pretend to tell you in what manner the image of any object painted on the retina of the eye is calculated to convey to the mind an idea of that object : but I wish to shew you, that the images of the various objects which you see are painted on the retina. Here is a bullock^s eye, from the back part of which I cut away the three coats, but so as to leave the vitreous humour perfect : I will now put against the vitreous humour a piece of white paper, and hold the eye towards the window; what do you see ? /. The figure of the window is drawn upon the paper ; but it is inverted. T*. Open the window, and you will see the trees in the garden drawn upon it in the same inverted state, or any other bright object that is presented to it. C. Does the paper in this instance represent the innermost coat, called the retina ? T, It does, and I have made use of paper because it is easily seen through, whereas the retina is opaque ; transpaiency would be of no advantage to it. The IMAGES ON THE RETINA. 389 retina, by means of the optic nerve, is conveyed to the brain, or, in other w^ords, the optic nerve is an extension of the retina. J. And does it carry the news of every object that is painted on the retina? T. So it should seem j for w^e have an idea of whatever is drawn upon it. I direct my eyes to you, and the image of your person is painted on the retina of my eye, and I say I see you. So of any thing else. C. You said the rays of light proceeding from ex- ternal objects were refracted in passing through the different humours of the eye. T. They are, and converged to a point, or there would be no distinct picture drawn on the retina, and of course no distinct idea conveyed to the mind. I will shew you what I mean by a figur.e, taking an arrow again as an illustration. Fig. 27. As every point of an object a b c sends out rays in all directions, some rays from each point on the side next the eye, will fall upon the cornea between x y, and by passing through the humours of the eye, they will be converged and brought to as many points on the retina, and will form on it a distinct inverted picture c a of the object. J. This is done in the same manner as you shewed us by means of a double convex lens. T. All three of the humours have some effect in refracting the rays of light, but the crystalline is the most powerful, and that is a complete double convex lens : and you see the rays from a are brought to a OPTICS. point at a : those from b will be converged at h, and those from c at c ; and, of course, the intermediate ones between a and b, and b and c, will be formed between a and 6, and b and c. Hence the object be- comes visible by means of the image of it being drawn on the retina. C. Since the image is inverted on the retina, how is it that we see things in the proper position ? T. This is a proper question, but one that is not very readily answered. It is well known that the sense of touch or feeling very much assists the sense of sight ; some paintings are so exquisitely finished, and so much resemble sculpture, that the eye is com- pletely deceived ; we then naturally extend the hand to aid the sense of seeing. Children, who have to learn the use of all their senses, make use of their hands in every thing ; they see nothing which they do not wish to handle, and, therefore, it is not improbable, that by the sense of the touch, they learn, unawares, to rectify that of seeing. The image of a chair, or table, or other object, is painted in an inverted posi- tion on the retina ; they feel and handle it, and find it erect; the same result perpetually recurs, so that, at length, long before they can reason on the subject, or even describe their feelings by speech, the inverted images give them an idea of an erect object. C. I can easily conceive that this would be the case with common objects, such as are seen every day and hour. But will there be no difficulty in supposing that the same must happen with regard to any thing which I had never seen before ? I never saw ships sailing on the sea till within this month ; but when I first saw them, they did not appear to me in an inverted position. T. But you have seen water and land before, and they appear to you, by habit and experience, to be lowermost, though they are painted on the eye in a different position : and the bottom of the ship is next the water, and consequently, as you refer the water to the bottom, so you must the hull of the ship which is OBJECTS NOT SEEN DOUBLE. 391 connected with it. In the same manner all the parts of a distant prospect are right with respect to each other ; and therefore, though there may be a hundred objects in the landscape entirely new to you, yet as they all bear a relation to one another, and to the earth on which they are, you refer them, by experi- ence, to an erect position. /. How is it that in so small a space as the retina of the eye, the images of so many objects can be formed? T. Dr. Paley* tells us, The prospect from Kamp- stead Hill is compressed into the compass of a six- pence, yet circumstantially represented. A stage coach travelling at its ordinary rate, for half an hour, passes in the eye only over one twelfth part of an inch, yet the change of place is distinctly perceived throughout its whole progress." Now what he asserts we all know is true : go to the window and look steadily at the prospect before you, and see how many objects you can discern without moving your eye. /. I can see a great number very distinctly indeed, besides which I can discern others on both sides, which are not so clearly defined. C. I have another difficulty ; we have two eyes, on both of which the images of objects are painted'; how is it that we do not see every object double 1 T. When an object is seen distinctly with both eyes, the axes of them are directed to it, and the object appears single ; for the optic nerves are so framed, that the correspondent parts, in both eyes, lead to the same place in the brain, and excite but one sensation. But, if the axes of both eyes are not directed to the object, that object seems double. J, How does that appear I * See Paley's Natural Theology, p. 35, 7th edit, or p. 13, in the Analysis of that work, by the Author of these Dialogues. 3D2 OPTICS. T. Look at your brother, while I push your right eye a little out of its place towards the left. J. I see two brothers, the one receding to the left hand of the other. T. The reason is this ; by pushing the eye out of its natural place, the pictures in the two eyes do not fall upon correspondent parts of the retina, and therefore the sensations from each eye are excited in different parts of the brain. When any point of an object is seen distinctly with both eyes, the axes of both are directed to that point, and, meeting there, the object appears single, though looked at with both eyes. Seeing with both eyes at once likewise enables us to judge more accurately of distances than we could if we saw with only one. CONVERSATION XVIT. OF SPECTACLES, AND OF THEIR USES. C. Why do people wear spectacles ? T. To assist the sight, which may be defective from various causes. Some eyes are too flat, others are too convex : in some the humours lose a part of their transparency, and on that account a deal of light that enters the eye is stopped and lost in the passage, and every object appears dim. The eye, without light, would be a useless machine. Spectacles are intended to collect the light, or to bring it to a proper degree of convergency. C. Are spectacle-glasses always convex ? T. No : they are convex when the eyes are too flat ; but if the eyes are already very convex, then concave glasses are used. You know the properties of a convex glass ? J. Yes ; it is to make the rays of light converge sooner than they would without. OF SPECTACLES. 393 f Fig. 28. T. Suppose then a person is unable to see objects distinctly, owing to the cornea c d, or to the crystal- line a h, or both, being too flat. The focus of rays proceeding from any object, Xy will not be on the retina, where it ought to be, but at z beyond it. C. How can it be beyond the eye 1 T, It would be beyond it, if there were any thing to receive it ; as it is, the rays flowing from x will not unite at so as to render vision distinct. To remedy this, a glass m w is placed between the object and the eye, by means of which the rays are brought to a focus sooner, and the image is formed at d. J. Now I see the reason why people are obliged, sometimes, to make trial of many pairs of spectacles before they get those that will suit them. They can- not tell exactly what degree of convexity is necessary to bring the focus just to the retina. T. That is right ; for the shape of the eye may vary as much as that of their countenance ; of course, a pair of spectacles that might suit you, would not be adapted to another, whose eyes should require a similar aid. — What is the property of concave glasses 1 C. They cause the rays of light to diverge. T. Then for very round and globular eyes, these will be useful, because, if the cornea c d, or crystal- line humour a 6, be too convex, the rays flowing from X will unite into a focus before they arrive at the retina, as at z, C. If the sight then depend on sensations produced on the retina, such a person will not see the object S2 3:>1 OPTICS. at all, because "the image of it does not reach the retina. Fig. 29. T. True : t)ut at z the rays cross one another, and pass on to the retina, where they will produce some sensations, but not those of distinct vision, be- cause they are not brought to a focus there. To remedy this, the concave glass m n is interposed between the object and the eye, which causes the rays coming to the eye to diverge; and being more divergent when they enter the eye, it requires a very convex cornea or crystalline to bring them to a focus at the retina. J. I have seen old people, when examining an object, hold it a good distance from their eyes. T. Because their eyes being too flat, the focus is throvv'n beyond the eye, and therefore they hold the object at a distance to bring the focus z (Fig. 28.) to t'le retina. C. Very short-sighted people bring objects close to their eyes. T, Yes ; I once knew a young man who was apt, in looking at his paper, to rub out with his nose what he had written with his pen. In this case, bringing the object near the eye produces a similar effect to that produced by concave glasses ; because the nearer the object is brought to the eye, the greater is the angle under which it is seen ; that is, the ex- treme rays, and, of course, all the others, are made more divergent. J . I do not understand this. OF SPECTACLES. 895 T. Well, then, let e be the eye, and the object a b seen at z, and also at .r, double the distance ; will not the same object appear under different angles to an eye so situated? J, Yes, certainly, a e 6 will be larger than c e J, and will include it. T. Then the object being brought very near the eye, has the same effect as magnifying the object, or of causing the rays to diverge ; that is, though a b and c d are of the same lengths, yet a b being nearest to the eye will appear the largest. C. YoQ say the eyes of old people become flat by age ; is that the natural progress ? T. It is ; and therefore people who are very short- sighted while young, will probably see well when they grow old. J. That is an advantage denied to common eyes. T. But people blessed with common sight, should be thankful for the benefit they derived while young. C. And I am sure we cannot too highly estimate the science of optics, that has afforded such assistance to defective eyes, which, in many circumstances of life, would be useless without them. T, Salvinus Armatus, a nobleman of Florence, claimed the honour of inventing spectacles ; he died iti 1317, and the fact was inscribed on his tomb. But it is generally allowed that Alhazen was really the inventor, about 50 years before. Fig. 30. OPTICS. CONVERSATION XVIII. OF THE RAINBOW. T. You have frequently seen a rainbow ? C. Oh, yes; and very often there are two at the same time, one above the other ; the lower one being by far the most brilliant. T. This is, perhaps, the most beautiful meteor in nature ; it never makes its appearance but when a spectator is situated between the sun and the shower. /. Is a rainbow occasioned by the falling drops of rain 1 T. Yes ; it depends on the reflection and refrac- tion of the rays of the sun by the falling drops. C. I know now how the rays of the sun are re- fracted by water, but are they reflected by it also ? T, Yes ; water, like glass, reflects some rays, while it transmits or refracts others. You know the beauty of the rainbow consists in its colours. J. Yes, the colours of the rainbow" is a very common expression ; I have been told there are seven of them, but it is seldom that so many can be clearly distinguished. r. Perhaps that is owing to your want of patience ; I will shew you the colours first by means of the prism. Fig. 31. If a ray of light s be admitted into a darkened room, through a small hole in the shutter a y, its natural coui-se is along the line to d : but if a glass prism a c be introduced, the whole ray will be bent upwards, OF THE RAINBOW. 397 and if it be taken on any white surface, as m n, it will form an oblong image p t, the breadth of which is equal to the diameter of the hole in the shutter. C. This oblong is of different colours in different parts. T. These are the colours of the rainbow. /. But how is the light which is admitted by a circular hole in the window spread out into an oblong? T. If the ray were of one substance, it would be equally bent upwards, and make only a small circular image. Since, therefore, the image or picture is oblong, it is inferred, that it is formed of rays differ- ently refrangible, some of which are turned more out of the way, or more upwards, than others ; those which go to the upper part of the spectrum being most refrangible, those which go to the lowest part are the least refrangible ; the intermediate ones possess more or less refrangibility, according as they are painted on the spectrum. Do you see the seven colours 1 C. Yes ; here is the violet, indigo, blue, green, yellow, orange, B.nd red. T. These colours will be still more beautiful if a convex lens be interposed, at a proper distance, be- tween the shutter and the prism. J. How does this apply to the rainbow \ Fig. 32. 398 OPTICS. T. Suppose A to be a drop of rain, and s a ray from the sun falling upon or entering it at d, it will not go to c, but be refracted to n, where a part will go out, but a part also will be reflected to q, where it will go out of the drop, which, acting like a prism, separates the ray into its primitive colours, and the v iolet will be uppermost, the red lowermost. C Is it at any particular angle that these colours are formed 1 T. Yes, they are all at fixed angles ; the least re- frangible, or red, makes an angle with the solar inci- dent ray, equal to little more than 42 degrees ; and the violet or most refrangible ray, will make with the solar ray an angle of 40 degrees. /. I do not understand which are these angles. T. The ray s d would go to/ c, therefore the angle made with the red ray is sfq, and that made with the violet ray is sc q ; the former is 42° 2', the latter 40« 17'. C. Is this always the case be the sun either high or low in the heavens 1 T. It is ) but the situation of the rainbow will vary accordingly as the sun is high or low, that is, the higher the sun, the lower will be the rainbow ; a shower has been seen on a mountain by a spectator in a valley, by which a complete circular rainbow has been exhibited. /. And I once remember standing on Morant's Court Ililj, in Kent, when there was a heavy shower, while the sun shone very bright, and all the landscape be- neath, to a vast extent, seemed to be painted with the prismatic colours. T. I recollect this well ; it was certainly the most beautiful one I ever beheld. C. You have not explained the principles of the upper or fainter bow. T, This is formed by two refractions and two reflections : suppose the ray t r to be entering the drop B at r. It is refracted at reflected at s, re- flected again at i, and refracted as it goes out at u, OF THE RAINBOW. 399 v^lience it proceeds being separated to the spectator at g. Here the colours are reversed ; the angle formed by the red ray is 51^ and that formed by the violet is 540. J. Does the same thing happen with regard to a whole shower, as you have shewn with respect to the two drops ? T. Certainly, and by the constant falling of the rain the image is preserved constant and perfect. Here is the representation of the two bows. The Fig. 33. rays come in the direction s a, and the spectator stands at e with his back to the sun, or, in other words, he must be between the sun and the shower. This subject may be shewn in another way ; if a glass globule filled with water be hung sufficiently high before you, when the sun is behind, to appear red, let it descend gradually, and you will see, in the descent, all the other six colours follow one another. Artificial rainbows may be made with a common watering pot, but much better with a syringe fixed to an artificial fountain ; and I have seen one by spirt- ing up water from the mouth : it is often seen in cascades, in the foaming of the waves of the sea, in fountains, and even in the dew on the grass. Dr. Langwith has described a rainbow, v/hich he saw lying on the ground, the colours of which were almost as lively as those of the common rainbow. It was extended several hundred yards, and the colours were so strong, that it might have been seen much farther, if it had not been terminated by a bank, and the hedge of a field. Rainbows have also been produced by the reflec- tion of the sun's beams from a river : and Mr, 400 OPTICS. Edwards describes one which must have been formed by the exhalations from the city of London, when the sun had been set twenty minutes.* CONVERSATION XIX. OF THE REFRACTING TELESCOPE. T, We now come to describe the structure of telescopes, of which there are two kinds ; viz. the refracting and the reflecting telescope. C. The former or refracting telescope depends, I suppose, upon leni^es for the operation ; and the re- flecting telescope acts chiefly by means of mirrors. T. Yes, these are the general principles upon which they are formed ; and we shall devote this morning to the explanation of the refracting telescope. Here is one completely fitted up. J. It consists of two tubes, and two glasses. T. The tubes are intended to hold the glasses, and to confine the boundary of the view. I will there- fore explain the principle by the following figure, in which is repre- sented the eye b, the two lenses, mn, oq, and the object xy. The lens 0 q, which is nearest to the object, is called the object-glass, and that m n nearest to the eye is called the eye-glass. C. Is the object-glass a double convex, and the eye-glass a double concave ? T. It happens so in this particu- lar instance, but it is not necessary that the eye-glass should be concave ; the object-glass must, however, in all cases, be convex. Fig. 34. * See Phil. Trans. Vol. VI. and L. REFRACTING TELESCOPE. 401 C. I see exactly, from the figure, why the eye-glass is concave : for the convex lens converges the rays too quickly, and the focus by that glass alone would be at E : and therefore the concave is put near the eye to make the rays diverge so much as to throw them to the retina before they come to a focus. T. But that is not the only reason: by coming to a focus at e, the image is very small, in comparison of what it is when the image is formed on the retina by means of the concave lens. Can you, James, explain the reason of all the lines which you see in the figure ? J. I think I can ; — there are two pencils of rays flowing from the extremities of the arrow, which is the object to be viewed. The rays of the pencil flowing from X go on diverging till they reach the convex lens 0 q, when they will be so refracted by passing through the glass, as to converge, and meet in the point X, Now the same may be said of the pencil of rays which come from ^/j and, of course, of all the pencils of rays flowing from the object between x and y. So that the image of the arrow would, by the con- vex lens, be formed at e. T. And what would happen if there were no other glass ? J. The rays would cross each other and be diver- gent, so that when they got to the retina, there would be no distinct image formed, but every point, as x or 3/, would be spread over a large space, and the image would be confused. To prevent this the concave lens m n is interposed; the pencil of rays which would, by the convex glass, converge at x, will now be made to diverge, so as not to come to a focus till they arrive at a ; and the pencil of rays which would, by the convex glass, have come to a point at 3/, will, by the interposi- tion of the concave lens, be made to diverge so much as to throw the focus of the rays to b instead oiy. By this means, the image of the object is magnified. T. Can you tell the reason why the tubes require to be drawn out more or less for different persons ? C. The tubes are to be adjusted in order to throw 402 OPTICS. the focus of rays exactly on the retina: and as some eyes are more convex than others, the length of the focus will vary in different persons, and by sliding the tube up and down this object is obtained. T, Refracting telescopes are used chiefly for viewing terrestrial objects; two things, therefore, are requisite in them; the first is, that it should shew objects in an upright position, that is, in the same position as we see them without glasses ; and the second is, that they should afford a large field of view. J. What do you mean, sir, by a field of view? T. All that part of a landscape which may be seen at once, without moving the eye or instrument. Now in looking on the figure again, you will perceive that the concave lens throws a number of the rays beyond the pupil c of the eye, on to the iris on both sides, but those only are visible, or go to form an image, which pass through the pupil ; and therefore, by a telescope made in this way, the middle part of the object is only seen, or, in other words, the prospect is by it very much diminished. J. Is not the image of the object in [/ the telescope inverted 1 "^"^ " T. Yes, it is : for you see the image Fig. 35. r. By substituting a double con- vex eye-glass g h instead of the con- cave one. Here the focus of the double convex lens is at e, and the gkss g h must be so much more con- vex than 0 p, as that its focus may be also at E : for then the rays flowing from the object x y, and passing through the object-glass o p, will form the inverted image m e d. Now by interposing the double convex g /i,the image is thrown on the retina, and it is seen under the large angle dec, that is, the image 7n B d will be magnified to the size C E D. C. How is that remedied ? REFRACTING TELESCOPE. 403 on the retina stands in tlie same position as the object ; but we always see things by having the images inverted : and, therefore, whatever is seen by telescopes constructed as this is, will appear in- verted to the spectator, which is a very unpleasant circumstance with regard to terrestrial objects ; it is on that account chiefly used for celestial obser- vations. C, Is there any rule for calculating the magnifying power of this telescope ? T. It magnifies in proportion as the focal distance of the object-glass is greater than the focal distance of the eye-glass. Thus, if the focal distance of the ob- ject-glass is ten inches, and that of the eye-glass only a single inch, the telescope magnifies the diameter of an object ten times ; and the whole surface of the ob- ject will be magnified a hundred times*. C. Will a small object, as a silver penny for instance, appear a hundred times larger through this telescope than it would by the naked eye 1 T. Telescopes, in general, represent terrestrial ob- jects to be nearer and not larger: thus, looking at the silver penny a hundred yards distant, it will not appear to be larger, but at the distance only of a single yard. J. Is there no advantage gained if the focal dis- tance of the eye-glass and of the object-glass be equal ] T. None ; and therefore in telescopes of this kind we have only to increase the focal distance of the ob- ject-glass, and to diminish the focal distance of the eye-glass, to augment the magnifying power to almost any degree. C, Can you carry this principle to any extent ? T. Not altogether so : an object-glass of ten feet focal distance will require an eye-glass whose focal distance is rather more than two inches and a half : and an object-glass with a focal distance of a hundred feet, must have an eye-glass whose focus must be about 404 OPTICS. six inches from it. How much will each of these glasses magnify? C. Ten feet divided by two inches and a half, give for a quotient forty-eight; and a hundred feet divided by six inches, give two hundred : so that the former magnifies 48 times, and the latter 200 times. T. Refracting telescopes for viewing terrestrial ob- jects, in order to shew them in their natural posture, are usually constructed with one object-glass, and three eye-glasses, the focal distances of these last being equal. J, Do you make use of the same method in calcu- lating the magnifying power of a telescope constructed in this way, as you did in the last ? T. Yes ; the three glasses next the eye having their focal distances equal, the magnifying power is found by dividing the focal distance of the object-glass by the focal distance of one of the eye-glasses. We have now said as much on the subject as is necessary to our plan. C. What is the construction of opera-glasses, that are so much used at the theatre ? T, The opera-glass is nothing more than a short re- fracting telescope. The lught telescope is only about two feet long : it represents objects inverted, much enlightened, but not greatly magnified. It is used to discover objects not very distant, but which cannot otherwise be seen for want of sufficient light. CONVEHSATION XX. OF REFLECTING TELESCOPES. T, This is a telescope of a different kind, and is called a reflecting telescope. C. What advantages does the reflecting telescope possess over that which you described yesterday? T, The great inconvenience attending refracting REFLECTING TELESCOPE. 405 telescopes is their length, and on that account they are not very much used when high powers are required. A reflector of six feet long will magnify as much as a refractor of a hundred feet. J . Are these, like the refractmg telescopes, made in different ways? T, They were invented by Sir I. Newton, but have been greatly improved since his time. The following figure will lead to a description of one of those most Fig. 36. in use. You know that there is a great similarity between convex lenses and concave mirrors. C. They both form an inverted focal image of any remote object, by the convergence of the pencils of rays. T. In instruments, the exhibitions of which are the effects of reflection, the concave mirror is substituted for the convex lens, t t represents the large tube, and t t the small tube of the telescope, at one end of which is D F, a concave mirror, with a hole in the middle at p, the principal focus of which is at i k ; opposite to the hole p is a small mirror l, concave towards the great one ; it is fixed on a strong wire m, and may, by means of a long screw on the outside of the tube, be made to move backwards or forwards, a b is a reniote object: from which rays will flow to the great mirror d f. /. And 1 see you have taken only two rays of a pencil from the top, and two from the bottom. T. And in order to trace the progress of the reflec- tions and refractions, the upper ones are represented by full lines, the lower ones by dotted lines. Now the 40G OPTICS. rays at c and e, falling upon the mirror at d and r, are reflected, and form an inverted image at m. C. Is there anything there to receive the image ? T. No : and therefore they go on towards the re- flector L, the rays from different parts of the object crossing one another a little before they reach l. J. Does not the hole at p tend to distort the image? T, Not at all ; the only defect is, that there is less light. From the mirror l the rays are reflected nearly parallel through p ; there they have to pass the plano- convex lens R, which causes them to converge at a h, and the image is now painted in the small tube near the eye. C. What is the other plano-convex lens s for? T. Having, by means of the lens i?, and the two concave mirrors, brought the image of the object so nigh as at a h, we only want to magnify the image. J. This, I see, is done by the lens s. T. It is, and will appear as large as c d, that is, the image is seen under the angle c f d. C. How do you estimate the magnifying power of the reflecting telescope? T, The rule is this: '^Multiply the focal distance of the large mirror by the distance of the small mirror from the image m: then multiply the focal distance of the small mirror by the focal distance of the eye-glass ; and divide these two products by one another, and the quotient is the magnifying power." J. It is not likely that we should know all these in any instrument we possess. T. The following then is a method of finding the same thing by experiment. ** Observe at what dis- tance you can read any book with the naked eye, and then remove the book to the farthest distance at which you can distinctly lead by means of the tele- scope, and divide the latter by the former." C. Had not Dr. Herschel a very large reflecting telescope? r. He made many, but the tube of the grand tele- scope is nearly 40 feet long, and 4 feet ten inches in OF THE MICROSCOPE. 407 diameter. The concave surface of the great mirror is 48 inches of polished surface in diameter, and it magnifies 6000 times. This noble instrument cost the Doctor four years' severe labour ; it v^^as finished August 28, 1789, on which day w^as discovered the sixth satellite of Saturn. CONVERSATION XXI. OF THE MICROSCOPE ITS PRINCIPLE OF THE SINGLE MICROSCOPE — OF THE COMPOUND MICROSCOPE OF THE SOLAR MICROSCOPE. T We are nov^ to describe the microscope, which is an instrument for viewing very small objects. You know that, in general, persons who have good sight cannot distinctly view an object at a nearer distance than about six inches. C. I cannot read a book at a shorter distance than this ; but if I look through a small hole made with a pin or needle in a sheet of brown paper, I can read at a very small distance indeed. T. You mean, that the letters appear, in that case, very much magnified, the reason of which is, that you are able to see at a much shorter distance in this way than you can without the intervention of the paper. Whatever instrument, or contrivance, can render minute objects visible and distinct, is properly a mi- croscope. J. If I look through the hole in the paper at the dis- tance of five or six inches from the print, it is not magnified. r. The object must be brought near to increase the angle by which it is seen ; this is the principle of all microscopes, from the single lens to the most com- pound instrument, a is an object not clearly visible Fi^/ 37. Fig. 38. 408 OPTICS. at a less distance than a b; but if the same object be placed in the focus c of the lens d, the rays which proceed from it will become parallel, by passing through the said lens, and therefore thcobject is dis- tinctly visible to the eye at e, placed any where before the lens. There are three distinctions in microscopes; the single, the compound, and the solar. C. Does the single microscope consist only of a lens? T. By means of a lens a great number of rays proceeding from a point are united in the same sensi- ble point, and as each ray carries with it the image of the point from whence it proceeded, all the rays united must form an image of the object. J, Is the image brighter in proportion as there are more rays united? T. Certainly: and it is more distinct in proportion as their natural order is preserved. In other words, a single microscope or lens removes the confusion that accompanies objects when seen very near by the naked eye; and it magnifies the diameter of the object, in proportion as the focal distance is less than the limit of distinct vision, which we may reckon from about six to eight inches. C. If the focal distance of a reading-glass be four inches, does it magnify the diameter of each letter only twice 1 T. Exactly so : but the lenses used in microscopes are often not more than one-fourth, or one-eighth, or even one-twentieth part of an inch radius. J. And in a double convex the focal distance is always equal to the radius of convexity. T, Then tell me how much lenses of one-fourth, one-eighth, and one-twentieth of an inch will each magnify 7 /. That is readily done ; by dividing 8 inches, the limit of distinct vision, by one-fourth, one-eighth, and one-twentieth. C. And to divide a whole number, as 8, by a frac- tion, as J, &c. is to multiply the said number by the SINGLE MICROSCOPE. 409 denominator of the fraction : of course, 8 multiplied by 4 gives 32 ; that is, the lens, whose radius is a quarter of an inch, magnifies the diameter of the object 32 times. / . Therefore the lenses of which the radii are one eighth, and one-twentieth, v/ill magnify as 8 multi- plied by 8, and 8 multiplied by 20 ; that is, the for- mer will magnify 64 times, the latter 160 times, the diameter of an object. T, You see, then, that the smaller the lens, the greater its magnifying power. Dr. Hooke says, in his work on the microscope, that he has made lenses so small as to be able, not only to distinguish the par- ticles of bodies a million times smaller than a visible pomt, but even to make those visible of which a mil- lion times a million wojild hardly be equal to the bulk of the smallest grain of sand. C. I wonder how he made them. T, I will give you his description : he first took a very narrow and thin slip of clear glass, melted it in the flame of a candle or lamp, and drew it out into exceedingly fine threads. The end of one of these threads he melted again in the flame till it run into a very small drop, which, when cool, he fixed in a thin plate of metal, so that the middle o/ it might be directly over the centre of an extremely small hole made in the plate. Here is a very convenient single microscope. /. It does not seem, at first sight, so simple as those which you have just now described. Fig. 39. T 410 OPTICS. T. A is a circular piece of brass, or it may be made of wood, ivory, &c. in the middle of which is a very small hole ; in this is fixed a small lens, the focal distance of which is o d ; at that distance is a pair of pliers DE, which may be adjusted by the sliding screw p, and opened by means of two little studs a e ; with these any small object may be taken up, and viewed with the eye placed in the other focus of the lens at f, to which it will appear magnified as at iM. C. T see by the joint it is made to fold up. r. It is ; and may be put into a case, and carried about in the pocket, without any incumbrance or in- convenience. Let us now look at a double or com- pound microscope. J. How many glasses are there in this 1 T. There are two ; and the construction of it may be seen by this figure ; cd is called the object-glass, and e/the eye-glass The small object ab is placed a little farther from the glass cd than its prin- . cipal focus, so that the pencils of rays flowing from the different 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. This image is viewed by the eye-glass, ef, which is so placed that the image g h may be in the focus, and the eye at about an equal dis- tance on the other side ; the rays of ^ each pencil will be parallel after going out of the eye-glass, as at e and f, till they come to the eye at k, by the humours of which they will be converged and collected into points on the retina, and form the large inverted image a b. C. Pray, sir, how do you calculate the magnifying power of this microscope ? r. There are two proportions which, when found, Fio'. 40. COMPOUND MICROSCOPE. 411 are to be multiplied into one another: (1) As the distance of the image from the object-glass is greater than its distance from the eye-glass ; and, (2) as the distance from the object is less than the limit of distinct vision. Example. If the distance of the image from the object-glass be 4 times greater than from the eye- glass, the magnifying power of 4 is gained ; and if the focal distance of the eye-glass be one inch, and the distance of distinct vision be considered at 7 inches, the magnifying power of 7 is gained, and 7x4 gives 28 ; that is, the diameter of the object will be magnified 28 times, and the surface will be magnified 784 times. J. Do you mean that an object will, through such a microscope, appear 784 times larger than by the naked eye ? T. Yes, I do ; provided the limit of distinct vision be 7 inches ; but some persons, who are short- sighted, can see as distinctly at 5 or 4 inches as another can at 7 or 8 ; to the former the object will not appear so large as to the latter. Ex. 2. What will a microscope of this kind mag- nify to three different persons, whose eyes are so formed as to see distinctly at the distance of 6, 7, and 8 inches by the naked eye ; supposing the image of the object-glass to be five times as distant as from the eye-glass, and the focal distance of the eye-glass be only the tenth part of an inch 1 C. As five is gained by the distances between the glasses, and 60, 70, and 80, by the eye-glass, the magnifying powers will be as 300, 350, and 400. J. How is it that 60, 70, and 80, are gained by the eye-glass 1 C. Because the distances of distinct vision are put at 6, 7, and 8 inches, and these are to be divided by the focal distance of the eye-glass, or by one-tenth ; but to divide a whole number by a fraction, we must multiply that number by the denominator, or lower figure in the fraction : therefore, the power gained by 412 OFrics. the distance between the two glasses, or 5, must be multiplied by 60, 70, or 80. And the surface of the object will be magnified in proportion to the square of 300, 350, or 400, that is, as 90,000, 122,600, or 160,000. T. We come now to the solar microscope, which is by far the most entertaining of them all, because the image is much larger, and being thrown on a sheet, or other white surface, may be viewed by many spectators at the same time, without any fatigue to the eye. Here is one fixed in the window shutter ; but I can explain its construction best by a figure. J. There is a looking-glass on the outside of the window. Fig. 41. T, Yes, the solar microscope consists of a looking- glass s 0 without, the lens a h in the shutter d u, and the lens nm within the dark room. These three parts are united to, and in a brass tube. The looking- glass can be turned by the adjusting-screw, so as to receive the incident rays of the sun s s s, and reflect them through the tube into the room. The lens a h collects those rays into a focus at n m, where there is another magnifier ; here, of course, the rays cross, and diverge to the white screen on which the image of the object will be painted. C. I see the object is placed a little behind the focus. 2'. If it were m the focus it would be burnt to CAMERA OBSCURA. 413 pieces immediately. The magnifying power of this instrument depends on the distance of the sheet or white screen ; perhaps about 10 feet is as good a distance as any. You perceive, that the size of the image is to that of the object as the distance of the former from the lens n m is to that of the latter. J. Then the nearer the object to the lens, and the farther the screen from it, the greater the power of this microscope. T, You are right, and if the object be only half an inch from the lens, and the screen nine feet, the image will be 46,656 times larger than the object : do you understand this 1 C, Yes ; the object being only half an inch from the lens, and the image 9 feet, or 108 inches, or 216 half inches, the diameter of the image will be 216 times larger than the diameter of the object, and this number multiplied into itself will give 46,656. r. This instrument is calculated only to exhibit transparent objects, or such as the light can pass through in part. For opaque objects a different mi- croscope is used : and indeed there are an indefinite number of microscopes. CONVERSATION XXII. OF THE CAMERA OBSCURA, MAGIC LANTHORN, AND MULTIPLYING GLASS. T. We shall now treat upon some miscellaneous subjects; of which the first shall be the Camera Ob' scura. C. What is a camera obscura ? T. The meaning of the term is a darkened cham- ber : the construction of it is very simple, and will be understood in a moment by you, who know the pro- perties of the convex lens. A convex lens placed in a hole of a window-shutter will exhibit, on a white sheet of paper placed in tlie 414 OPTICS. focus of the glass, all the objects on the outside, as fields, trees, men, houses, &c. in an inverted order. J. Is the room to be quite dark, except the light which is admitted through the lens 1 T, It ought to be so ; and to have a very interest- ing picture, the sun should shine upon the objects. J. Is there no other kind of camera obscura ? T. A portable one may be made with a square box, in one side of vhich is to be fixed a tube, having a convex lens in it : within the box is a plane mirror reclining backwards from the tube, in an angle of forty-five degrees. C. On what does this mirror reflect the image of the object 1 T, The top of the box is a square of unpolished glass, on which the picture is formed. And if a piece of oiled paper be stretched on the glass, a landscape may be easily copied j or the outline may be sketched on the rough surface of the glass. /. Why is the mirror to be placed at an angle of 45 degrees exactly 1 T, The image of the objects would naturally be formed at the back of the box opposite to the lens ; in order, therefore, to throw it on the top, the mirror must be so placed that the angle of incidence shall be equal to the angle of reflection. In the box, ac- cording to its original make, the top is at right angles to the end, that is at an angle of 90 degrees, therefore the mirror is put at half 90, or 45 degrees. C. Now the incident rays falling upon a surface which declines to an angle of 45 degrees, will be re- flected at an equal angle of 45 degrees, which is the angle that the glass top of the box bears with respect to the mirror. J. If I understand you clearly, had the mirror been placed at the end of the box or parallel to it, the rays would have been reflected back to the lens ; and none would have proceeded to the top of the box. T. True : in the same manner as when one person stands before a looking-glass, another at the side of MAGIC LANTHORN. '415 the room cannot see his image in the glass, because the rays flowing from him to the looking-glass are thrown back to himself again j but let each person stand on the opposite side of the room, while the glass is in the middle of the end of it, they will both stand at an angle of 45 degrees, with regard to the glass, and the>jays from each will be reflected to the other. C. Is the tube fixed in this machine 1 T, No ; it is made to draw out, or push in, so as to adjust the distance of the convex glass from the mir- ror, in proportion to the distance of the outward ob- jects, till they are distinctly painted on the horizontal glass. J. Will you now explain the structure of the magic lanthorn, which has long afforded us occasional amusement '? T, This little machine consists, as you know, of a sort of tin box ; within which is a lamp or candle : the light of this passes through a great plano-convex lens, placed in a tube fixed in the front. This strongly illuminates the objects, which are painted on slips of glass, and placed before the lens in an inverted posi- tion. A sheet, or other white surface, is placed to re- ceive the images. C. Do you invert the glasses on which the figures are drawn, in order that the images of them may be erect 1 T. Yes : and the illumination may be greatly in- creased, and the effect much more powerful, by placing a concave mirror at the back of the lamp. C. Did you not tell us that the Phantasmagoria, which we saw at the Lyceum, was a species of the magic lanthorn 1 T. There is this difference between them : in com- mon magic lanthorns, the figures are painted on tran- sparent glass, consequently the image on the screen is a circle of light, having a figure or figures on it : but in the Phantasmagoria, all the glass is made opaque, except the figure only, which being painted in 41G OPTICS, transparent colours, the light shines through it, and no light can come upon the screen but what passes through the figure. J. But there was no sheet to receive the picture. T. No ; the representation was thrown on a thin screen of silk placed between the spectators and the lanthorn. C. What caused the images to appear approaching and receding? T. It is owing to removing the lanthorn farther from the screen, or bringing it nearer to it ; for the size of the image must increase, as the lanthorn is carried back, because the rays come in the shape of a cone ; and as no part of the screen is visible, the figure appears to be formed in the air, and to move farther off when it becomes smaller, and to come nearer as it increases in size. J. Here is another instrument, the construction of which you promised to explain: the multiplying glass* jT. One side of this glass is cut into many distinct surfaces, and in looking at an object, as your brother, through it, you will see not one object only, but as many as the glass contains plane surfaces. I will draw a figure to illustrate _ _ this : let AB represent a glass, flat at the side next the eye h, and cut into three distinct surfaces on the opposite side, as a h, h d, d b. The object c will not appear magnified, but as rays will flow from it to all parts of the glass, and each plane surface will refract these rays to the eye, the same object will appear to the eye in the direction of the rays which enter it through each surface. Thus a ray c / falling perpendicularly on the middle surface, will suffer no refraction, but shew the object in its true place at c : the ray from c b falling obliquely on the plane surface a h, will be refracted in the direc- tion be, and on leaving the glass at e, it will pass to J MULTIPLYING GLASS 417 the eye in the direction of e h, and therefore it appears at e: and the ray cd will, for the same reason, be refracted to the eye in the direction b h, and the object c will appear also in d. If instead of three sides, the glass had been cut into 6 or 20, there would have appeared 6 or 20 different objects differently situated. T2 MAGNETISM. CONVERSATION I. OF THE MAGNET J ITS PROPERTIES ; USEFUL TO MARI- NERS, AND others; iron rendered magnetic; PROPERTIES OF THE MAGNET. TUTO R CH A R LES JAMES . Tutor. You see this dark brown mineral body ; it is almost black, and you know it has the property of attracting needles and other small iron substances. James. Yes, it is called a load-stone, leading-stone, or magnet ; we have often been amused with it ; but you told us that it possessed a much more important property than that of attracting iron and steel. T. This is what is called the directive property, by which mariners are enabled to conduct their vessels through the mighty ocean out of the sight of land ; by the aid of this, miners are guided in their subter- ranean inquiries, and the traveller through deserts otherwise impassable. Charles. Were not mariners unable to make long and very distant voyages till this property of the magnet was discovered'; T. Till then, they contented themselves with mere coasting voyages : seldom trusting themselves from the sight of land. J. How long is it since this property of the magnet was first known ? T. About five hundred years; and it is not possible to ascertain, with any degree of precision, to whom we are indebted for this great discovery C. You have not told us in what the discovery consists. T. When a magnet, or a needle rubbed with a magnet, is freely suspended, it will always, and in all USE OF THE MAGNET. 419 places, stand nearly north and south ; and its devia- tion from either point, at any one place, remains the same, within narrow limits, for a long time. C. Is it known which end points to the north, and which to the south? T, Yes, or it would be of little use : each magnet, and each needle, or other piece of iron, that is made an artificial magnet by being properly rubbed with the natural magnet, has a north end and a south end, called the north and south 'poles : to the former a mark is placed, for the purpose of distinguishing it. / . Then if a ship were to make a voyage to the north, it must follow the direction which the magnet takes ; making a due allowance for the deviation you mentioned. T. True ; and if it were bound a westerly course, the needle always pointing north, the ship must keep in a direction at right ang-les to the needle. In other words, the direction of the needle must be across the ship. C. Could not the same object be obtained by means of the pole star? r. It might, in a considerable degree, provided you could always insure a fine clear sky ; but what is to be done in cloudy weather, which, in some latitudes, will last for many days together? C. I did not think of that. T. Without the use of the magnet, no persons could have ventured upon such voyages as those to the West Indies, and other distant parts; the know- ledge, therefore, of this instrument cannot be too highly prized. /. Is that a magnet which is fixed to the bottom of the globe, by means of which we set the globe in a proper direction with regard to the cardinal points, north, south, east, and west ? T. That is called a compass, the needle of which, being rubbed by the natural or real magnet, becomes possessed of the same properties as those which belong to the magnet itself. 420 MAGNETISM. C. Can any iron and steel be made magnetic? T. They may ; but iron is the most proper for the purpose. Bars of iron thus prepared are called a7^ti- ficial magnets. J. Will these soon lose the properties thus ob- tained 1 T. Artificial magnets will retain their properties almost any length of time, and since they may be rendered more powerful than natural ones, and can be made of any form, they are generally used, so that the natural magnet is kept more as a curiosity than for utility. C. What are the leading properties of the magnet 1 T. (1.) A magnet attracts iron. (2.) When placed so as to be at liberty to move in any direction, its north end points to the north pole, and its south end to the south pole : this is called the polarity of the magnet. (3.) When the north pole of one magnet is presented to the south pole of another, they will attract one another. But if the two soiith or the two 7iorth poles are presented to each other, they will repel. (4.) When a magnet is so situated as to be at liberty to move any way, the two poles of it do not lie in an horizontal direction ; it inclines one of its poles towards the horizon, and, of course, elevates the other pole above it ; this is called the inclination or dipping of the magnet. (5.) Any magnet may be made to impart its properties to iron and steel. But the properties of the magnet will be enlarged upon during our future Conversations. CONVERSATION II. MAGNETIC ATTRACTION AND REPULSION. T. Having mentioned the several properties of the magnet or loadstone, I intend, at this time, to enter more particularly into the nature of magnetic attrac- tion and repulsion. — Here is a thin iron bar, eight or nine inches long, rendered magnetic, and on that ac- ATTRACTION, &c. 421 count it is now called an artificial magnet : I bring a small piece of iron within a little distance of one of the poles of the magnet, and you see it is attracted or drawn to it. C. Will not the same effect be produced, if the iron be presented to any other part of the magnet ? T. The attraction is strongest at the poles, and it grows less and less in proportion to the distance of any part from the poles ; so that in the middle, between the poles, there is no attraction, as you shall see by means of this large needle. J. When you held the needle near the pole of the magnet, the magnet moved to that, which looks as if the needle attracted the magnet. T, You are right: the attraction is mutual, as is evident from the following experiment. T place this small magnet on a piece of cork, and the needle on another piece, and let them float on water, at a little distance from each other, and you observe that the magnet moves towards the iron, as much as the iron moves towards the magnet. C. If two magnets were put in this situation, what would be the effect? r. If poles of the same name, that is, the two north, or the two south, be brought near together, they will repel one another ; but if a north and south be presented, the same kind of attraction will be visible, as there was between the magnet and needle. J. Will there be any attraction or repulsion if other bodies, as paper or thin slips of wood, be placed between the magnets, or between the magnet and iron? r. Neither the magnetic attraction nor repulsion is in the least diminished, or in any way affected, by the interposition of any kind of bodies, except iron. Bring the magnets together within the attracting or repelling distance, and hold a slip of wood between them ; you see they both come to the wood. C, You said that iron was more easily rendered 422 MAGNETISM. magnetic than steel; does it retain the properties as long too 1 T. If a piece of soft iron and a piece of hard steel be brought within the influence of a magnet, the iron will be most forcibly attracted, but it will almost instantly lose its acquired magnetism, whereas the hard steel will preserve it a long time. J, Is magnetic attraction and repulsion at all like what we have sometimes seen in electricity? T. In some instances there is a great similarity : Ex. I. Tie two pieces of soft wire each to a separate thread, which join at top, ^ and let them hang freely from a hook x, 11"' ' If I bring the marked or north end of a I magnetic bar just under them, you will i \ see the wires repel one another, as they j \ are shewn in the figure hanging from s. | || -j, C. Is that occasioned by the repelling |j i power which both wires have acquired in 1" 1 I consequence of being both rendered mag- netic with the same pole 1 Fig. 1 . T, It is : and the same thing would have occurred if the south pole had been presented instead of the north. /. Will they remain long in that position ? T. If the wires are of very soft iron they will quickly lose their magnetic power ; but if steel wires be used, as common sewing needles, they will con- tinue to repel each other after the removal of the magnet. Ex. II. I lay a sheet of paper flat upon a table, and strew some iron filings upon it. I now lay this small magnet among them, and give the table a few IRON MAGNETIC. 423 gentle knocks, so as to shake the filings, and you ob- serve in what manner they have arranged themselves about the magnet. C. At the two ends or poles the particles of iron form themselves into lines, a little sideways ; they bend, and then form complete arches, reaching from some point in the northern half of the magnet to some other point in the southern half. — Pray how do you account for this 1 T, Each of the particles of iron, by being brought within the sphere of the magnetic influence, becomes itself magnetic, and possessed of two poles, and con- sequently disposes itself in the same manner as any other magnet would do, and also attracts with its extremities the contrary poles of other particles. Ex. III. If I shake some iron filings through a gauze sieve, upon a paper that covers a bar magnet, the filings will become magnets, and will be arranged in beautiful curves. J. Does the polarity of the magnet reside only in the two ends of its surface ? T. No : one half of the magnet is possessed of one kind of polarity, and the other of the other kind, but the ends, or poles, are those points in which that power is the strongest. Definition. A line drawn from one pole to the other is called the axis of the magnet. CONVERSATION III. the method of making magnets of the mariner's compass. r. I have already told you that artificial magnets, which are made of steel, are now generally used in preference to the real magnet, because they can be procured with greater ease, may be varied in their form more easily, and will communicate the magnetic virtue more powerfully. C. How are they made ] 424 MAGNETISM. T. The best method of making artificial magnets is to apply one or more powerful magnets to pieces of hard steel, taking care to apply the north pole of the magnet or magnets to that extremity of the steel which is required to be made the south pole, and to apply the south pole of the magnet to the opposite extremity of the piece of steel. J. Has a magnet, by communicating its properties to other bodies, its own power diminished 1 T. No, it is even increased by it. — A bar of iron three or four feet long, kept some time in a vertical position, will become magnetic, the lower extremity of it attracting the south pole, and repelling the north pole. But if the bar be inverted, the polarity will be reversed. C. Will steel produce the same effects ? T, It will not ; the iron must be soft, and hence bars of iron that have been long in a perpendiculai position are generally found to be magnetical, as fire irons, bars of windows, &c. — If a long piece of hard iron be made red hot, and then left to cool in the direction of the magnetical line, it usually becomes magnetical. Striking an iron bar with a hammer, or rubbing it with a file, while held in this direction, renders it mag- netical. An electric shock, and lightning, frequently render iron magnetic. J. An artificial magnet, you say, is often more powerful than the real one ; can a magnet, there- fore, communicate to steel a stronger power than it possesses 1 T. Certainly not: but two or more magnets^ joined together, may communicate a greater power to a piece of steel than either of them possesses singly. C. Then you gain power according to the number of magnets made use of? T. Yes ; very powerful magnets may be formed by first constructing several weak magnets, and then joining them together to form a compound one, and to act more powerfully upon a piece of steel. EXPERIMENTS. 425 The following methods are among the best for forming artificial magnets : Fig. 3. 1. Place two magnetic bars a and b in a line, so that the north or marked end of one shall be opposite to the south end of the other, but at such a distance, that the magnet c, to be touched, may rest with its marked end on the unmarked end of b, and its un- marked end on the marked end of a. Now apply the north end of the magnet l, and the south end of D, to the middle of c, the opposite ends being elevated as in the figure. Draw l and d asunder along the bar c, one towards a, the other towards b, preserving the same elevation : remove l d a foot or more from the bar when they are off the ends, then bring the north and south poles of these magnets together, and apply them again to the middle of the bar c as before : the same process is to be repeated five or six times, then turn the bar, and touch the other three sides in the same way, and with care the bar will acquire a strong fixed magnetism. fig. 4. 2. Upon a similar principle, two bars, a b, c d, may be rendered magnetic. These are supported by two 426 MAGNETISM. bars of iron, and they are so placed that the marked end B may be opposite to the unmarked end d ; then place the two attracting poles g i on the middle of A B, as in the figure, moving them slowly over it ten or fifteen times. The same operation is to be per- formed on c D, having first changed the poles of the bars, and then on the other faces of the bars ; and the business is accomplished. The touch thus com- municated may be farther increased by rubbing the different faces of the bars with sets of magnetic bars, disposed thus : J. I suppose all the bars should be very smooth. Fig. 5. 2\ Yes, they should be well polished, the sides and ends made quite flat, and the angles quite square. There are many magnets made in the shape of horse- shoes, these are called horse-shoe magnets, and they retain their power very long by taking care to join a piece of iron to the end as soon as it is done with. C. Does that prevent its power from escaping 1 T. It should seem so ; the power of a magnet is even increased by suffering a piece of iron to remain attached to one or both of its poles. Of course a single magnet should always be thus left. J, How is magnetism communicated to compass needles 1 T, Fasten the needle down on a board, and draw magnets about six inches long, in each hand, from the centre of the needle outwards ; then raise the bars to a considerable distance from the needle, and bring them perpendicularly down on its centre, and draw them over again, and repeat this operation about twenty times, and the ends of the needle will point to the poles contrary to those that touched them. VARIATION OF THE COMPASS. 427 C. 1 remember seeing a compass when I was on board the frigate that lay off Worthing : the needle was in a box, with a glass over it. T. The mariner's compass consists of the box, the card or fly, and the needle. The box is circular, and is so suspended as to retain its horizontal position in all the motions of the ship. The glass is intended to prevent any motion of the card by the wind; the card or fly moves with the needle, which is very nicely balanced on a centre. It may, however, be noticed, that a needle, which is accurately balanced before it is magnetized, will lose its balance by being magnetized, on account of what is called the dipping, therefore a small weight, or moveable piece of brass, is placed on one side of the needle, by the shifting of which the needle will always be balanced. CONVERSATION IV. OF THE VARIATION OF THE COMPASS. C. You said, I think, that the magnet pointed nearly north and south ; how much does it differ from that line 1 T. It rarely points exactly north and south, and the deviation from that line is called the variation of the compass, which is said to be east or west. /. Does this differ at different times ? r. It does ; and the variation is very different in different parts of the world. The variation is not the same now that it was half a century ago, nor is it the same now at London that it is at Bengal or Kamt- schatka. The needle is continually traversing slowly towards the east and west. ° This subject was first attended to by Mr. Burrowes, about the year 1580, and he found the variation then, at London, about 11° 11' east. In the year 1657, the needle pointed due north and south : since which the variation has been gradually increasing towards the west; and in the year 1803 it was equal to some- 428 MAGNETISM. thing more than 24" west, and was then advancing towards the same quarter. C. That is at the rate of something more than ten minutes each year ? T. It is ; but the annual variation is not regular ; it is more one year than another. It is different in the several months, and even in the hours of the day. Its present mean variation at London is about 24^ 33' west. J. Then if I want to set a globe due north and south, to point out the stars by, I must move it about, till the needle in the compass points to 24^ 33' west ? r. Just so ; and mariners knowing this, are as well able to sail by the compass as if it pointed due north. C. You mentioned the property which the needle had 0^ dipping, after the magnetic fluid was communi- cated to it : is that always the same 1 T. It varies slightly. It was discovered by llobert Norman, a compass-maker, in the year 1576, and he then found it to dip nearly 72«, and from many observations made at the Royal Society, it is found to be now about 70° 32'. /. Does it differ in different places ? T. Yes: in the year 1773 observations were made on the subject in a voyage towards the north pole, and from these it appears that In latitude 60O 18' the dip was 75o 0' 70 45 — 77 52 — 80 12 • 81 52 80 27 82 2i The dip always increasing as the latitude is greater. I will shew you an experiment on this subject. Here is a magnetic bar, and a small dipping needle ; if I carry the needle, suspended freely on a pivot, from one end of the magnetic bar to the other, it will, when directly over the south pole, settle directly per- pendicularly to it, the north end being next to the SUMMARY, 429 south pole : as the needle is moved, the dip grows less and less, and when it comes to the magnetic centre it will be parallel to the bar ; afterwards the south end of the needle will dip, and when it comes directly over the north pole, it will be again perpen- dicular to the bar. The following facts are deserving of recollection. 1. Iron is the only body capable of being affected by magnetism. 2. Every magnet has two opposite points, called poles, 3. A magnet freely suspended arranges itself so that these poles point nearly north and south. This is called the directive property, or polarity, of the magnet. 4. When two magnets approach each other, the poles of the same names, that is, both north, or both south, repel each other. 5. Poles of different names attract each other. 6. The loadstone is an iron ore naturally possessing magnetism. 7. Magnetism may be communicated to iron and steel, 8. A steel needle rendered magnetic, and fitted up in a box, so as to move freely in any direction, con- stitutes the mariners* compass. C. I think there is a similarity between electricity and magnetism. T. You are right ; there is a considerable analogy, and a remarkable difference, also, between magnetism and electricity. Electricity is of two sorts, positive and negative ; bodies possessed of the same sort of electricity repel each other; and those possessed of different sorts attract each other. — In Magnetism, every magnet has two poles ; poles of the same name repel each other, and the contrary poles attract each other. ^ In Electricity, when a body, in its natural state, is brought near to one that is electrified, it acquires a contrary electricity, and becomes attracted by it.— In 430 MAGNETISM. MAGNETisM,*when an iron substance is brought near one pole of a magnet, it acquires a contrary polarity, and becomes attracted by it. One sort of electricity cannot be produced by itself. In like manner, no body can have only one magnetic pole. The electric virtue may be retained by electrics, but it pervades conducting substances. The magnetic virtue is retained by iron, but it pervades all other bodies. On the contrary : The magnetic povv^er differs from the electric, as it does not affect the senses with light, smell, taste, or noise, as the electric does. Magnets attract only iron, but the electric fluid attracts bodies of every sort. The electric virtue resides on the surface of electri- fied bodies, but the magnetic is internal. A magnet loses nothing of its power by magnifying bodies, but an electrified body loses part of its elec- tricity by electrifying other bodies. ELECTRICITY. CONVERSATION I. INTRODUCTION. THE EARLY HISTORY OF ELECTRICITY. TUTOR CHARLES JAMES. Tutor. If I rub pretty briskly with my hand this stick of sealing wax, and then hold it near any small light substances, as little pieces of paper, the wax will attract them 3 that is, if the wax be held within an inch or more of the paper, they will jump up and adhere to it. Charles. They do ; and I think I have heard you call this the effects of electricity, but 1 do not know what electricity is. T, It is the case with this part of science as with many others, we know it only by the effects which it produces. As I have not hitherto, in these Conver- sations, attempted to bewilder your minds with useless theories, neither shall I, m the present case, attempt to say what the electrical fluid is : its action is well known ; it seems diffused over every portion of matter with which we are acquainted, and, by the use of proper methods, it is as easily collected from sur- rounding bodies as water is taken from a river. James. I see no fluid attaching to the sealing wax when you have rubbed it. T. You do not see the air which you breathe, and with which you are surrounded, yet we have shewn you * that it is a fluid, and may be taken from any vessel, as certainly, though not with so much ease, as * See Conversations on Pneumatics. 432 ELECTRICITY, water may be poured from this glass. With the ex- ercise of a small degree of patience, you shall set such experiments as will not fail to convince you that there is as certainly a fluid, which is called the elec- tric fluid, as there are such fluids as water and air. C. Water must have been known since the crea- tion,' and the existence of the air could not long re- main a secret, but who discovered the electric fluid, which is not at all evident to the sense either of sight or feeling 7 • , r T, Thales, who lived six centuries betore the Christian era, was the first who observed the electri- cal properties of amber, and he was so struck with the appearances, that he supposed it to be animated. J. Does amber attract light bodies, like sealing wax 1 T. Yes, it does ; and there are many other sub- stances, as well as these, that have the same power. After Thales, the first person we read of that noticed this subject was Theophrastus, who discovered that iourmalin has the power of attracting light bodies. It does not, however, appear that the subject, though very curious, excited much attention until about the year 1600, when Dr, Gilbert, an English physician, examined a great variety of substances, with a view of ascertaining how far they might or might not be ranked among electrics. C. What is meant by an electric 7 T. Any substance which being excited or rubbed by the hand, or by a woollen cloth, or other means, and has the power of attracting light bodies, is called an electric. /. Is not electricity accompanied with a pecuhar kind of light, and with sparks 1 T. It is, of which we shall speak more at large hereafter : the celebrated iMr. Boyle is supposed to have been one of the first persons who got a glimpse of the electrical light, or who seems to have noticed it by rubbing a diamond in the dark. But he little imagined, at that time^ what astonishing effects would ELECTRIC FLUID. 43^3 afterwards be produced by the same power. Sir Isaac Newton was the first who observed that excited glass attracted light bodies on the side opposite to that on which it was rubbed. C. How did he make the discovery? T. Having laid upon the table a round piece of glass, about two inches broad, in a brass ring, by which it was raised from the table about the eighth of an inch, and then rubbing the glass, some little bits of paper .which were under it were attracted by it, and moved very nimbly to and from the glass. C. I remember standing by a glazier when he was cementing, that is, rubbing over some window-lights with oil, and cleaning it off with a stiff brush and whitmg, and the little pieces of whiting under the glass kept continually leaping up and down, as the brush moved over the glass. T, That was, undoubtedly, an electrical appear- ance, but I do not remember having ever seen it noticed by any writer on electricity. A complete history of this science is given by Dr. Priestley, which will, hereafter, afford you much entertainment and interesting instruction. To-morrow we shall enter into the practical part of the subject ; and I doubt not that the experiments in this part of science will be as interesting as those in any other which you have been studying. The electric light, exhibited in different forms ; the various signs of attraction and repulsion acting on all bodies ; the electric shock, and the explosion of the battery, will give you plea- sure, and excite your admiration. CONVERSATION II. OF ELECTRIC ATTRACTION AND REPULSION OF ELEC- TRICS AND CONDUCTORS, ^ T. You must for a little time, that is, till we ex» hibit before you experiments to prove it, take it for granted, that the earth and all bodies with which we U 434 ELECTRICITY, are acquainted, contain a certain quantity of exceed- ingly elastic and penetrating fluid, which philosophers call the electric fluid. C. You say a certain quantity ; is it limited ? T, Like other bodies it undoubtedly has its limits ; this glass will hold a certain quantity of water, but if 1 attempt to pour into it more than that quantity, a part will flow over. So it is with the electric fluid : there is a certain quantity which belongs to all bodies, and this is called their natural quantity, and so long as a body contains neither more nor less than this quantity, no sensible eflect is produced. J. Has this table electricity in it 7 T. Yes, and so has the inkstand, and every thmg else in the room ; and if I were to take proper means to put more into it than it now has, and you were to put your knuckle to it, it would throw it out in the shape of sparks. J. I should like to see this done. C. But what would happen if you should take away some of its natural quantity 1 T. Why then if you presented any part of your body to the table, as your knuckle, a spark would go from you to the table. J. But perhaps Charles might not have more than his natural share, and in that case he could not spare any. T. True 5 but to provide for this, the earth on which he stands would lend him a little, to make up for what he parted with to the table. • j i, ii J. This must be an amusing study ; I thmtc I shall like it better than any of the others. T. Take care that you do not pay for the amuse- ment before we have done. Here is a glass tube about eighteen inches long, and perhaps an inch or more in diameter ; I rub it up and down quickly in my hand, which is dry and warm, and now I will present it to these fragments of paper, thread, and gold-leaf: you see they all move to it. That is called electrical attraction. ELECTRICS AND CONDUCTOIIS. 435 C. They jump back again now j and now thev re- turn to the glass. ^ r. They are, in fact, alternately attracted and re- pelled, and this will last several minutes if the dass be strongly excited. I will rub it again ; present your knuckle to it in several parts one after another. /. What is that snapping? I feel, likewise, some- thing like the pricking of a pin. The snapping is occasioned by little sparks which come from the tube to your knuckle, and these give the sensation of pain. ment ^ ^^^^ ^ ^^^^ ^^^^^^ experi- C The sparks are evident enough now, but I do not know where they can come from. J. The air and every thing is full of the fluid which appears in the shape of sparks ; and whatever be^ he cause which I do not attempt to explain, the rubbing of the glass with the hand collects it from the air, and having now more than its natural share. It parts with It to you, or to me, or to any body else that may be near enough to receive it. substance besides the hand ex- cite the tube 7 nrJ* lT^'^^^''^^?^^''^ ^""^ t^^^s science, are called the rubbers ; and the glass tube, or what- dectric ^^'""^ ^^""^ ^^""'^^^^ '^^"^'^ being 4dtedV'" ''''' ""^^ substances capable of T. You may rub this poker, or the round ruler, lor ever, without obtaining an electric spark from it. J. J3ut you said one might get a spark from the mahogany table if it had more than its share. 1 •/ ^"^^ ""^^ ^^^^ sparks from the poker o^f thtdecttZ^^^^^ "^^^ ^^^^^ commonLare P.rfL^'"'^^'!? you distinguish between bodies that can be, and those that cannot be, excited ? 436 ELECTRICITY. r. The former, as I have told you, are called dec- tries, as the glass tube : the latter, such as the poker, the ruler, your body, and a thousand other substances are denominated condyle tors. C. I should be glad to know the reason of the dis- tinction, because I shall be more likely to remem- ber it. T. That is right: when you held your knuckle to the glass tube, you had several sparks from the different parts of it : but if I, by any means, over- charged a conductor, as this poker, all the electri- city will come away at a single spark, because the superabundant quantity flows instantaneously from every part to that point where it has an opportunity of getting out. I will illustrate this by an experiment. But first of all let me tell you, that all electrics are called also non-conductors, J. Do you call the glass tube a non-conductor, be- cause it does not suffer the electric fluid to pass from one part of it to another 1 T, I do : silk, if dry, is a non-conductor. With this skein of sewing silk I hang the poker, or other me- tal substance, a, to a hook in the ceiling, so as to be about twelve inches from it ; under- neath, and near the extremity, are some small substances, as bits of paper, &c. I will ex- cite the glass tube, and present it to the upper part of the po- ker. C. They are all attracted; but now you take away the glass they are quiet. T. It is evident that the elec- tric fluid passed from one part of the tube through the poker, which is a conductor, to the Fig. 1. ELECTRICS AND CONDUCTORS. 437 paper, and attracted it: — if the glass be properly excited you may take sparks from the poker. Would not the same happen if another glass tube were placed in the stead of the poker ] T, You shall try.—- now I have put the glass in the place of the poker, but let me excite the other tube as much as I will, no effect can be produced on the paper .--—there are no signs of electrical attraction, which shews that the electric fluid will not pass through glass. C. What would have happened if any conducting- substance had been used, instead of silk, to suspend the iron poker 1 r. If I had suspended the poker with a moistened hempen string, the electric fluid would have all passed away through that, and there would have been no (or very trifling) appearance of electricity at the end of the poker. You may vary these experiments till you make yourselves perfect with regard to the distinction be- tween electrics and conductors. Sealing-wax is an electric, and may be excited as well as a glass tube and will produce similar effects. I will give you a list of electrics, and another of conductors, disposed according to the order of their perfection, beginning in each list with the most perfect of their class ; thus glass is a better electric than amber ; and gold a bet- ter conductor than silver. TABLE. Electrics. Conductors. Glass of all kinds. All the metals in the fol- All precious stones, the most lowing order :~ transparent the best. Gold ; silver ; copper ; pla- T^^^"-'' tina; brass; iron; tin; ^"^P^"^- quicksilver; lead. All resinous substances. The semi-metals.* Wax of all kinds. Metallic ores.* Silk and cotton. Charcoal. Electrics. Dry external substances, as feathers, wool, and hair. Paper ; loaf sugar. Air, when quite dry. Oils and metallic oxides. * Ashes of animal and vege- table substances. Most hard stones. ELECTRICITY. Conductors. The fluids of an animal body. Water, especially salt wa- ter, and other fluids, ex- cept oil. Ice, snow. Most saline substances. Earthy substances. Smoke ; steam, and even A vacuum. CONVEUSATION III. OF TPIE ELECTRICAL MACHINE. T. I will now explain to you the construction of the electrical machine, and shew you how to use it. C. For what purpose is it used 1 r. Soon after the subject of the electric fluid en- gaged the attention of men of science, they began to contrive the readiest methods of collecting large quan- tities of it. By rubbing this stick of sealing-wax I can collect a small portion ; if I excite or rub the glass tube, I get still more. The object, therefore, was, to find out a machine by which the largest quan- tities can be collected, with as little trouble and ex- pense as may be. J. You get more electricity from the tube than from the sealing-wax, because it is five or six times as large ; by increasing the size of the tube you would increase the quantity of the electric fluid, I should think. T. That is a natural conclusion. But if you look * This and other chemical terms are explained and familiarly illustrated in a work, by the author of the Scientilic Dialogues, entitled " Dialogues on Chemis- try," &c. ELECTRICAL MACHINE. 439 to the table of electrics which I made out yesterday, you will see, that had the wax been as large as the glass tube, it would not have collected so much of the electric fluid, because, in its own nature, it is not so good an electric. C. In that table glass stands as the most perfect electric, but there are several substances between it and wax, all of which are, I believe, more perfect electrics than wax. r. They are ; electricians, therefore, had no hesi- tation as to the nature of the substance : they fixed on glass, which, being easily melted and run, or blown mto all sorts of forms, is, on that account, very valuable. The most common form that is now used is that of a glass cylinder, from five or six inches in diameter to ten or twelve. Here is one completely fitted up. Fig. 2. The cylinder a b is about eight inches in diameter, and twelve in length ; this I turn round in the frame- work with the handle dc. J. What is the piece of black silk k for ? r. The cylinder would be of no use without a rub- ber you know : on which account you see the glass pillar R s, which, being cemented into a piece of hard 440 ELECTRICITY. wood, is made to screw into the bottom of the ma- chine ; on the pillar is a cushion to which is attached the piece of black silk. C. And I perceive the cushion is made to press very hard against the glass. T. This pressure, when the cylinder is turned round fast, acts precisely like the rubbing of the tube by the hand, though in a still more perfect manner. I will turn it round. J. Here is not much sign of electricity yet. T. No : the machine is complete, but it has no means of collecting the fluid from the surrounding bodies : for you see the cushion or rubber is fixed on a glass pillar, and glass will not conduct the electric fluid. C. Nevertheless it does, by turning round, shew some signs of attraction. T. Every body in nature with which we are ac- quainted possesses a portion of this fluid, and there- fore the signs which are now evident arise from the small quantity which exists in the rubber itself, and the atmosphere that immediately surrounds the ma- chine. C. Would the case be different if the rubber were fixed on a conducting substance instead of glass 1 T. It would ; but there is a much easier method ; I will hang on this brass chain to the cushion at r, which being several feet long lies on the table, or on the floor, and this you know is connected, by means of other objects, with the earth, which is the grand reservoir of the electric fluid. Now see the effect of turning round the cylinder : but I must make every part of it dry and rather warm, by rubbing it with a dry warm cloth. J. It is indeed very powerful. What a crackling noise it makes ! T, Shut the window-shutters. C. The appearance is very beautiful j the flashes from the silk dart all round the cylinder. T. I will now bring to the cylinder the tin con- ELECTRICAL MACHINE. 441 ductor L, which is also placed on a glass pillar fn fixed in the stand at f. r > /. What are the points in the tin conductor for ? T. They are intended to collect the fluid from the cylinder ; I will turn the cylinder, and do you hold your knuckle within four or five inches of the con- ductor. C. The painful sensations which these sparks oc- casion, prove that the electric fluid is a very powerful agent when collected in large quantities. T. To shew you the nature of conducting bodies, 1 will now throw another brass chain over the con- ductor, so that one end of it may lie on the floor : see now if you can get any sparks while I turn the ma- chine. ^ /. No, I can get none, put my knuckle as near to It as I will :— does it all run away by the chain 1 It does : a piece of brass or iron wire would do as well; and so would any conducting substance which touched the conductor with one end, and the floor with the other : your body would do as well as the chain. Place your hand on the conductor while I turn round the cylinder: and let- your brother bring his knuckle near the conductor. C. I can get no spark. T It runs through James to the earth, and you see his body is a conductor as well as the chain. With a very little contrivance I can take sparks trom you or James, as well as you did from the con- ductor. / . I should like to see how that is done. T. Here is a small stool, having a mahogany top and glass legs. If you stand on that, and put your hand on the conductor, the electricity will pass from the conductor to your body. C. Will the glass legs prevent it from running from him to the earth ? T, They will : and therefore what he receives from the conductor, he will be ready to part with to any U2 ^ 442 ELECTRICITY. of the surrounding bodies, or to you if you bring youi hand near enough to any part of him. J. The sparks are more painful in coming through my clothes than when I received them on my bare hand. T, You understand, I hope, the process. C. By means of the chain trailing on the ground, the electric fluid is collected from the earth on the glass cylinder, which gives it through the points to the conductor ; from this it may be conveyed away again by means of other conductors. T. Whatever body is supported, or prevented from touching the earth, or communicating with it, by means of glass or other non-conducting substances, is said to be insulated. Thus a body suspended on a silk line is insulated, and so is any substance that stands on glass, or resin, or wax, provided that these are in a dry state, for moisture will conduct away the electric fluid from any charged body. CONVERSATION IV. OF THE ELECTRICAL MACHINE. C. What is that shining stuff" which I saw you put on the rubber yesterday ? , T. It is called amalgam ; the rubber, by itself, would produce a very slight excitation ; but its power is greatly increased by laying upon it a little of this amalgam, which is made of quicksilver, zinc, and tin-foil, with a little tallow or mutton suet. J. Is there any art required in using this amal- gam 1 T. When the rubber and silk flap are very clean and dry, and in their place, then spread a little of the amalgam upon a piece of leather, and apply it to the under part of the glass cylinder, while it is revolving from you ; by this means particles of the amalgam will be carried by the glass itself to the lower part of the rubber, and will increase the excitation. ELECTHICAL MACHINE. 443 C. I think I once saw a globe instead of a cylinder for an electrical machine. T. You might : globes were used before cylinders, but the latter are the most convenient of the two! The most powerful electrical machines are fitted with flat plates of glass. In our experiments we shall be content with the cylinder, which will answer every purpose of explaining the principles of the science. /. As I was able to conduct the electricity from the tm conductor to the ground, could I likewise act the part of the chain by conducting the fluid from the earth to the cushion 1 T, Undoubtedly : I will take ofF the chain, and now do you keep your hand on the cushion while I turn the handle. /. I see the machine works as well as when the chain was on the ground. T, Keep your present position, but stand on the stool with glass legs ; by which means there is now all communication cut ofF between the cushion and the earth ; in other words, the cushion is completely insulated, and can only take from you what electricity It can get from your body. Go, Charles, and shake hands with your brother. C. It does not appear that the machine had taken all the electricity from him, for he gave me a smart spark. T. You are mistaken; he gave you nothing, but he took a spark from you. C. I stood on the ground, I was not electrified • how then could I give him a spark ? ' r. The machine had taken from James the electri- city that was in his body, and by standing on the stool, that IS, by being insulated, he had no means of re- ceiving any more from the earth, or any surrounding objects; the moment, therefore, you brought your hand near him, the electricity passed from you to him. C. I certainly felt the spark, but whether it went 444 ELECTRICITY. out of, or entered into, my hand, I cannot tell ; have I then less than my share now ? r. No : what you gave to your brother was sup- plied immediately from the earth. Here is another glass-legged stool ; do you stand on this, but at the distance of a foot or two from your brother, who still keeps his place. I take the electricity from him by turning the machine, and, as he stands on the stool, he has now less than his share. But you have your natural share, because, though you also are insu- lated, yet you are out of the influence of the machine ; extend, therefore, your hand, and give him a part of the electric fluid that is in you. C. I have given him a spark. T. And, being yourself insulated, you have now less than your natural quantity, to supply which you shall have some from me : give me your hand. Why you draw it back without my touching it ! C. I did, but it was near enough to get a strong spark from you. T, When a person has less electricity than his natural share, he is said to be electrified minus, or ne- gatively : but if he has more than his natural share, he is said to be electrified plus, or positively. J. Then before Charles gave me the spark I was electrified minus, and when he had given it me he was minus till he received it from you. T. That is right. Suppose you stand on a stool and hold the rubber, and Charles stand on another stool, and touch the prime conductor l while I turn the machine, which of you will be plus, and which minus electrified? /. I shall be minus, because I give to the rubber : and Charles will be plus, because he receives from the conductor what I gave to the rubber, and which is carried by the cylinder to the conductor. T. You then have less than your share, and your brother has more than he ought to have. Now if I get another glass-legged stool, I can take from ELECTRICAL MACHINE. 443 Charles what he has too much, and give it to you who have too little. C. Is it necessary that you should be insulated for this purpose ? T, By being insulated I may perhaps carry back to J ames the very electricity which passed from him to you. But if I stand on the ground, the quantity which I take from you will pass into the earth, be- cause I cannot, unless I am insulated, retain more than my natural share. /. And what is given by you to me is likewise in- stantaneously supplied by the earth ? T. It is. Let us make another experiment to shew that the electric fluid is taken from the earth. Here are some little balls made of the pith of elder : they are put on thread, and, being very light, are well adapted to our purpose. While the chain is on the cushion, and I work the machine, do you bring the balls near the conductor by holding the thread at D. ^..^ / . They are attracted by it, and , ^ now the two balls repel each other, Fig. 3. as in the figure x. T. I ought to have told you, that the upper part d of the line is silk, by which means you know the balls are insulated, as silk is a non-conductor. I take the chain off from the cushion, and put it on the con- ductor, so as to hang on the ground, while I turn the machine. Will the balls be affected now, if you hold them to the conductor? /. No, they are not. T. Take them to the cushion. C. They are attracted and repelled now by being brought near the cushion, as they were before by being carried to the conductor. T. Yes, and you may now take sparks from the cushion as you did just now from the conductor ; in 446 ELECTRICITY. both cases it must be evident that the electric fluid is brought from the earth. Some machines are furnished with two conductors, one of which is connected with the cushion, the other such as we have described. Turn the cylinder, and both conductors will be electrified ; but any body which is brought within the influence of these, will be attracted by one of the conductors, and repelled by the other: and, if a chain or wire be made to con- nect the two together, neither will exhibit any electric appearances : they seem, therefore, to be in opposite states ; accordingly, electricians say, that the conductor connected with the cushion is negatively electrified, and the other is positively electrified. Machines of this kind have been used for medicinal purposes, but not hitherto with much success. They have been principally had recourse to in palsy, con- tractions of the limbs, rheumatism, St. Vitus's dance, and some cases of deafness and imoaired vision, to remove local pain CONVERSATION V. OF ELECTRICAL ATTRACTION AND REPULSION. J. What is this large roll of sealing-wax for 1 T. As I mean to explain, this morning, the prin- ciples of electrical attraction and repulsion, I have, besides the electrical machine, brought out for use a roll of sealing-wax, which is about fifteen inches long, and an inch and a quarter in diameter, and the long glass tube. C. Are they not both electrics, and capable of being excited? r. They are ; but the electricity produced by ex- citing them has different or contrary properties. J, Are there two kinds of electricities then ? T, We will shew you an experiment before we at- tempt to give any theory.^ — I will excite the glass tube, OF TWO ELECTRICITIES. 447 and Charles shall excite the wax ; now do you bring the pith-balls, which are suspended on silk (Fig. 3.) to the tube : they are suddenly drawn to it, and now they are repelled from one another, and likewise from the tube, for you cannot easily make them touch it again : — but take them to the excited wax. / . The wax attracts them very powerfully : now they fall together again, and appear in the same state as they were in before they were brought to the excited tube. Repeat the experiment again and again, because on this two different theories have been formed : one of which is, that there are two electricities, called by some philosophers the vitreous or positive electricity, and the resinous or negative electricity. C. Why are they called vitreous and resinous 7 T, The word vitreous is Latin, and signifies any glassy substance ; and the word resinous, used to denote that the electricity produced by resins, wax, &c. possesses different qualities from that produced by glass. /. Is it not natural to suppose that there are two electricities, since the excited wax attracts the very same bodies that the excited glass repels ? T, It may be easily explained, by supposing that every body, in its natural state, possesses a certain quantity of the electric fluid, and if a part of it be taken away, it endeavours to get it from other bodies ; or if more be thrown upon it than its natural quantity, it yields it readily to other bodies that come within its influence. C. I do not understand this. T. If I excite this glass tube, the electricity which it exhibits is supposed to come from my hand ; but if I excite the roll of wax in the same way, the effect is, according to this theory, that a part of the electric fluid naturally belonging to the wax passes from it through my hand to the earth : and the wax being surrounded by the air, which, in its dry state, is a 448 ELECTRICITY. non-conductor, remains exhausted, and is ready to take sparks from any body that may be presented to it. J. Can you distinguish that the sparks come from the glass to the hand ; and, on the contrary, from the hand to the wax 1 T. No : the velocity with which light, and of course the electric spark, moves, renders it impossible to say what course it takes ; but I shall shew you other ex- periments which seem to j ustify this theory : and as nature always works by the simplest means, it seems more consistent with her usual operations, that there should be one fluid rather than two, provided that known facts can be equally well accounted for by one as by two. C. Can you account for all the leading facts by either theory? T. Yes, we can. You saw when the pith-balls were electrified, they repelled one another. It is a general principle in electricity, that two bodies having more than their natural share of the electric fluid, will repel one another. But if one have more, and the other less, than its share, they will attract one another. J. How is this shewn? T. I vv'ill hold this ball, which is insulated, by a silk thread, to the conductor, and do you, Charles, do the same with the other. Let us now bring them together. C. No, we cannot : they fly from one another. T, I will hold mine to the insulated cushion, and you shall hold yours to the conductor, while the machine is turned : now I suspect they will attract one another, J. They do indeed. C. The reason is this ; that the cushion, and what- ever is in contact with it, parts with a portion of its electricity ; but the conductor and the adjoining bodies have more than their share j therefore, the ATTRACTION AND REPULSION. 449 ball applied to the cushion, being negatively electri- fied, will attract the one connected with the con- ductor, which is positively electrified. T. Here is a tuft of feathers, which I stick in a small hole in the conductor : now see what happens when I turn the cylinder. / . They all endeavour to avoid each other, and stand erect in a beautiful manner. Let me take a spark from the conductor ; now they fall down in a moment. T, When I turned the wheel they all had more than their share of the electric fluid, and therefore they repelled one another ; but the moment the elec- tricity was taken away, they fell into their natural position. A large plume of feathers, when electrified, grows beautifully turgid, expanding its fibres in all directions, and they collapse when the electricity is taken ofF. ^ J . Could you make the hairs of my head repel one another ? T. Yes, that I can. Stand on the glass-legged stool, and hold the chain that hangs on the conductor, in your hand, while I turn the machine. C. Now your hairs stand all on end. J. And I feel something like cobwebs over my face. T. There are, however, no cobwebs, but that is a sensation which a person always experiences if he is highly electrified. Hold the pith-ball, Charles, near your brother's face. /. It is attracted in the same manner as it was before with the conductor. r. Hence you may lay it down as a general rule, that all light substances coming within the influence [ of an electrified body are attracted by it, whether it ! is electrified positively or negatively. C. Because they are attracted by the positive elec- tricity to receive some of the superabundant quan- tity ; and by the negative to give away some that they possess. 450 ELECTRICITY. T, J ust so : and when they have received as much as they can contain, they are repelled by the electri- fied body. The same thing may be shewn in various ways. Having excited this glass tube, either by drawing it several times through my hand, or by means of a piece of flannel, I will bring it near this small feather. See how quickly it jumps to the glass. J. It does, and sticks to it. T. You will observe, that after a minute or two it will have taken as much electricity from the tube as it can hold, when it will suddenly be repelled, and ^ jump to the nearest conductor; upon which it will , discharge the superabundant electricity that it has acquired. J. I see it is now going to the ground, that being the nearest conductor. T, I will prevent it by holding the electrified tube between it and the floor. You see how unwilling it is to come again in contact with the tube : by pur- ' suing, I can drive it where I please without touch- . ing it. C, That is, because the glass and the feather are ' both loaded with the same electricity? t T. Let the feather touch the ground, or any other t conductor, and you will see that it will jump to the I tube as fast as it did before. - I will suspend this brass plate, which is about five / inches in diameter, to the conductor, and at the dis- ^ tance of three or four inches below I will place some ^ small feathers, or bits of paper cut into the figures of men and women. They lay very quiet at present ; observe their motions as soon as I turn the wheel. J. They exhibit a pretty country dance : they jump up to the top plate, and then down again. T. The same principle is evident in all these ex-i periments. The upper plate has more than its owuj share of the electric fluid, which attracts the littlel figures ; as soon as they have received a portion of itJ they go down to give it to the lower plate; and so *" ATTRACTION AND REPULSION. 451 will continue till the upper plate is discharged of its superabundant quantity. I will take away the plates, and hang a chain on the conductor, the end of which shall lie in several folds in a glass tumbler; if I turn the machine, the electric fluid will run through the chain, and will electrify the inside of the glass. This done, I turn it quickly over eight or ten small pith-balls, which lie on the table. C. That is a very amusing sight; how they jump about! They serve also to fetch the electricity from the glass and carry it to the table. T, If, instead of the lower metal plate, I hold in my hand a pane of dry and very clean glass, by the corner, the paper figures, or pith-balls, will not move, because glass being a non-conducting substance, it has no power of carrying away the superabundant electricity from the plate suspended from the con- ductor. But if I hold the glass flat in my hand, the figures will be attracted and repelled, which shews that the electric fluid will pass through thin glass. Take now the following results, and commit them to your memory : — • (1.) If two insulated pith-balls be brought near the conductor, they will repel each other. (2.) If an insulated conductor be connected with the cushion, and two insulated pith-balls be electrified by it, they will repel each other. (3.) If one insulated ball be electrified by the prime conductor, and another by the conductor con- nected with the cushion, they will attract each other. (4.) If one ball be electrified by glass, and another by wax, they will attract each other. (5.) If one ball be electrified by a smooth, and another by a rough, excited glass tube, they will attract one another. 452 ELECTRICITY. CONVERSATION VI. OF ELECTRICAL ATTRACTION AND REPULSION. T, I will shew you another instance or two of the effects of electrical attraction and repulsion. This apparatus consists of three bells suspended from a brass wire, the two outer ones by small brass chains; the middle bell, and the two clappers .t t, are suspended on silk. From the middle bell there is a chain n, which goes to the table, or any other conducting substance. The j'io-. 4. bells are now to be hung ° by c on the conductor, and the electrical machine to be put in motion. /. The clappers go from bell to bell, and make very pretty music : how do you explain this ? T. The electric fluid runs down the chains a and b ; to the bells a b, these, having more than their natural I quantity, attract the clappers x x, which take a por- I tion from a and b, and carry it to the centre bell n, i and this, by means of the chain, conveys it to the i earth. C. Would not the same effect be produced if the clappers were not suspended on silk ? T. Certainly not : nor will it be produced if the chain be taken away from the bell n, because then there is no way left to carry off the electric fluid to^ the earth, ■ Another amusing experiment is thus shewn : Let I there be two wires placed exactly one above another, ■ and parallel ; the upper one must be suspended fromB the conductor, the other is to communicate with thai table : a light ima^^e placed between these will, wheuB ELECTROMETERS. 453 the conductor is electrified, appear like a rope dancer. — This piece of leaf-brass is called the electric Jish ; one end is a sort of obtuse angle, the other is acute • if the large end be presented towards an electrified conductor, it will fix to it, and, from its wavering motion, it will appear to be animated. This property of attraction and repulsion has led to many inventions of instruments called electrometers. J. Is not an electrometer a machine to measure the strength of the electricity ? ^ T , Yes ; and this is one of the most simple, and it depends entirely upon the repulsion which takes place be- tween two bodies in a state of electrifi- cation. It consists of a lio-ht rod and a pith-ball, hanging parallel to the stem, but turning on the centre of a semi- circle, so as to keep close to its graduated limb. This is to be placed in a hole on the conductor l, (Fig. 2.) and according as the conductor is more or less electrified, the ball will fly Fig. 5. farther from the stem, C. If the circular part be marked with degrees, you may ascertain, I suppose, pretty accurately, the strength of any given charge. T, Yes, you may ; but you see how fast the air carries away the electricity; it scarcely remains a single moment in the place to which it was repelled. —Two pith-balls may be suspended parallel to one another, on silken threads, and applied to any part of an electrical machine, and they will, by their repul- sion, serve for an electrometer, for they will repel one fiSr^^^ machine acts more power- Has this any advantage over the other ? T, It serves to shew whether the electricity be I negative or positive ; for if it be positive, by applying an excited stick of sealing-wax, the threads will fall jtogether again ; but if it be negative, excited sealing- 454 ELECTRICITY. wax, or resin, or sulphur, or even a rod of glass (the polish of which is taken off), will make them recede farther. We have now, perhaps, said enough respecting electrical attraction and repulsion, at least for the present ; I wish you, however, to commit the follow- ing results to your memory: — 1. Bodies that are electrified positively repel each other. 2. Bodies that are electrified negatively repel each other. C. Do you mean, that if two bodies have either more or less of the electric fluid than their natural share, they will repel each other, if brought suf- ficiently near 1 T. That is exactly what I mean. 3. Bodies electrified by contrary powers, that is, two bodies, one having more, and the other less, than its natural share, attract each other very strongly. 4. Bodies that are electrified attract light sub- stances which are not electrified. These are facts which, I trust, have been made evident to your senses. To-morrow we will describe what is usually called the Leyden phial. CONVEKSATION VII. OF THE LEYDEN PHIAL, OR JAR. T. I will take away the wires and the ball from the conductor, and then remove the conductor an inch or two farther from the cylinder. If the machine act strongly, bring an insulated pith-ball, that is, yo know, one hanging on silk, to the end of the conduc tor nearest to the glass cylinder. C. It is immediately attracted. T. Carry it to the other end of the conductor, an see what happens. C. It is attracted again, but I thought it woul have been repelled. LEYDEN PHIAL. 455 T, Then as the ball was electrified before, and is still attracted, you are sure that the electricity of the two ends of the conductor are of different names ; that is, one is jflus, and the other minus. ' J. Which is the positive, and which the nerative end? ^ T. That end of the conductor which is nearest to the cylinder becomes possessed of an electricity dif- ferent from that of the cylinder itself. /. Do you mean, that if the cylinder is positively electrified, the end of the conductor next to it is electrified negatively 1 T, I do : and this you may see by holding an in- sulated pith-ball between them. C, Yes, it is now very evident, for the ball fetches and carries as we have seen it before. r. What you have seen with regard to the conduc- tor, is equally true with respect to non-conducting bodies :^ here is a common glass tumbler ; if I throw withinside it a greater portion of electricity than its natural share, and hold it in my hand, or place it on any conducting substance, as the table, a part of the electric fluid, that naturally belongs to the outside, I will make its escape through my body. C. Let me try this. I T, But you must be careful that you do not break !the glass. i C, I will hang the chain on the conductor, and let 'the other end lie on the bottom of the glass, and iJames will turn the machine. T. You must take care that the chain does not touch the edge of the glass, because then the electric fluid will, by that means, run from one side of it to the other, and spoil the experiment. I /. If I have turned the machine enough, take the chain out, and try the two sides with the insulated Ipith-ball. I C. What is this? something has pierced throuo-h |ny arms and shoulders. ° 456 ELECTRICITY. T, That is a trifling electrical shock, which you might have avoided, if you had vi^aited for my di- rections. C. Indeed it ^zs not trifling: I feel it now. T, This leads us to the Leyden phial : so called, because the discovery was first made at Leyden, in Holland, and by means of a phial or small bottle. J. Was it found out in the same manner as Charles has just discovered it] T. Nearly so : Mr. Cuneus, a Dutch philosopher, was holding a glass phial in his hand, about half filled with water, but the sides above the water and the outside were quite dry ; a wire also hung from the conductor of an electrical machine into the water. J. Did that answer to the chain ? T, Just so : and, like Charles, he was going to disengage the wire with one hand, as he held the bottle in the other, and was surprised and alarmed by a sudden shock in his arms, and through his breast, which he had not the least expected. C. I do not think there was any thing to be alarmed at. T. The shock which he felt was, probably, some- thing severer than that which you have just ex- perienced : but the terror was evidently increased by its coming so completely unexpected. When M. Muschenbroeck first felt the shock, which was by means of a thin glass bowl, and very slight, he wrote to Reaumur, that he felt himself struck in his arms, shoulders, and breast, so that he lost his breath, and was two whole days before he recovered from the effects of the blow. • fl C. Perhaps he meant the fright ] \ r. Terror seems to have been the effect of the shock : for he adds, '* I would not take a second shock for the whole kingdom of France." i Mr. Ninkler, an experimental philosopher a| I.eipsic, describes the shock as having given hidj convulsions, a heaviness in his head, such as he shoull OF THE LEYDEN JAR. 4.7 feel if a large stone were on it, and he had reason to dread a fever to preveat which he put himself 0", ^ course of coolmg medicines. " Twice " saTs he gave me a bleeding at the nose to wh'i.K T ' ' mclined; and my life, wLrc^-foslIy'tpasL" hel tears, received the shock twice and fmmH ^ ThfcoS ot fe""':! --W^^ atthX --^er shoe. J; Ts this called the Leyden phial ? Itis. 1 hey are now made in this manner, a is a glass jar, both inside and out being covered with tin-foil about three parts of the way up, as far as x. C, Does the outside coverino- answer to the hand, and the inside ^^^..nm covering to the water? j,-^ g , They do : the piece of wood * 2 IS placed on the top, merely to support the bras, wire and knob to the bottom of whl hanl 1 Cham that rests on the bottom of the ja^ I will fow set the jar m such a situation that it shall be withi^ i::zZ7/'''''' conductor whiie'Ti';;: the^knob' ''P^'^^^ ^'^^^ ^^"ductor to C. The brass wire touches the inside : if I, there- X 453 ELECTRICITY. fore, with one hand touch the knob, and with the other the outside covering, will it be sufficient '? T It will : but I had rather you would not, because the shock will be more power- ful than I should wish either myself or you ji to experience. Here is a brass wire with fj two little balls or knobs b s to it. I will brino- one of them, as s, to the outside, and l4g. /. the other, 6, to the ball v on the wire. , . . , J. What a brilliant spark and what a loud noise ! T The electric fluid that occasions the light and tlie noise, ran from the inside of the jar through the wire to s, and spread itself over the outside. C. Would it have gone through my arms, it i had put one hand to the outside, and touched the wire communicating with the inside with the other ? r. It would, and you may believe the ; , shock would have been in proportion to the quantity of fluid collected. Ihe in- strument I used may be called a discharg- ^ ^ incr rod : but here is a more convenient , ^ one : the handle a is solid glass, fastened into a brass socket, and the brass work is the same as fig. 7, only by turnmg on a Fig. 8, joint the arms may be opened to any extent. J, Why is the handle glass 1 T. Because glass being a non-conductor the elec. trie fluid passes through the brass work, withou affectinc. the hand; whereas with the other a small sensau^^^^ was percJived while I discharged the jar. C Would the jar never discharge itselt { T Yes : by exposure to the air for some time the charge of the jar will be silently and graduall dissipated, for the superabundant electric fluid of th ins rlil escape, by means of the air, to the outsid of the iar.-But dectricians make it a rule never kave a jar in its charged state, lest any person comm nto the room unawares should happen to touch and thereby receive a shock which might be attend with serious consequences. ELECTRICAL BATTERY. 459 CONVERSATION VIII. OF THE LEYDEN JAR— LANe's DISCHARGING FLEC^ TROMETER, AND THE ELECTRICAL BATTERY. C In discharging the jar yesterday, I observed that when one of the discharging- rods touched the outside of the jar, the flash and report took place betore the other end came in contact with the brass wire that communicates with the inside coating. les, It acts in the same manner as when vou take a spark from the conductor; you do not/for that purpose, bring your knuckle close to the tin J. .Sometimes, when the machine acts very power- l^^^l^J'''' get the spark at the distance of several nr j'. ^/ ^^^^ principle, the higher an electrical 01 Leyden jar is charged, the more easily, or at a greater distance, is it discharged. -UA^'T experiments it does not seem that it will discharge at so great a distance as that in which a spark may be taken from the conductor. i. Very frequently a jar will discharge itself, after It has accumulated as much of the electrical fluid asit cancontam: that is, the fluid which is thrown on the mside coating will make its way over the foating ^ non-conductor, on to the outside J. In a Leyden jar, after the first discharge, vou always I perceive, take another and smaller on^. rin.;./.! ; '''' ^^^^^ ^ Pe^-fect con. first ^;nrl'%r^ ' ^^^^^^^ pass at ca L r !/ ^^^^ remains is called the residuum, and this, in a large jar. would give you a considerable shock; therefore, I advise you always, in discharging an electrical jar, to take away the residuum before you venture to remove the apparatus. I will now describe an electrometer 460 ELECTRICITY. which depends, for its action, on the principles we have been describing. C. Do you mean upon the jar's discharging before the outside and inside coating are actually brought into contact? J". I do : the arm d is made of glass, and proceeds from a socket on the wire of the elec- trical jar F. To the top of the glass arm is cemented another brass socket e, through which a wire, with balls b and c at each end, will slide backwards and forwards. J. So that it may be brought Fig. 9. to any distance from the ball a, which is on the wire, connected with the inside of the jar? T. Just so. When thej arris set either in contact with, or very near, the conductor, as it is represented in the figure, and the ball b is set at the distance of the eighth of an inch from the ball a, let a wire c k be fixed between the ball c and the outside coating of the jar. Then as soon as the machine is worked, the jar cannot be charged beyond a certain point, for when the charge is strong enough to pass from a to the ball E, the discharge will take place, and the electric fluid collected in the inside will pass through the wire c K to the outside coating. C. If you remove the balls to a greater distance from one another, will a stronger charge be required before the fluid can pass from the inside of the jar to the ball b of the electrometer 1 T. Certainly : and therefore the discharge will be much stronger. This machine is called Lane's Dis- charging Electrometer, from the name of the person who invented it. It is very useful in applying the electric shock to medical purposes, as we shall see] hereafter. j ELECTRICAL BATTERY. 461 Fig. 10. This box contains nine jars or Leyden phials ; the wires, which proceed from the inside of each three of these jars, are screwed or fastened to a common horizontal wire e, which is knobbed at each extremity, and by means of the wires f f the inside coatings of three or six, or the whole nine, may be connected. J. Is it a common box in which the jara are placed 1 T, The inside of the box is lined with tin-foil ; sometimes very thin tin-plates are used, for the pur- pose of connecting, more effectually, the outside coat- ings of all the jars. C. What is the hook on one of the sides of the box for ? T, To this hook is fastened a strong wire, which communicates with the inside lining of the box, and, of course, with the outside coating of the jars. And, as you see, to the hook a wire is also fastened, which connects it with one branch of the discharging rod. J, Is there any particular art to be used in charg- ing a battery ? T. No: the best way is, to bring a chain, or piece of wire, from the conductor to one of the balls on the rods that rest upon the jars : and then set the machine to work. The electric fluid passes from the conductor to the inside of all the jars, till it is charged sufficiently high for the purpose. Great caution, however, must be used when you come to 462 ELECrRIClTY. make experiments with a battery, for fear of an accident, either to yourself, or to spectators. C. Would a shock from this be attended with any bad consequences 1 T. Yes : very serious accidents may happen from the electricity accumulated in a large battery, and even with a battery such as is represented in the figure, which is one of the smallest made : a shock may be given, which, if passed through the head or other vital parts of the body, may be attended with very mischievous effects. /. How do you know when the battery is properly charged ? T, The quadrant electrometer (Fig. 5.) is the best guide, and this may be fixed either on the conductor, or upon one of the rods of the battery. But if it is fixed on the battery, the stem of it should be of a good length, not less than 12 or 15 inches. C. How high will the index stand when the battery is charged 1 T. It will seldom rise so high as 90^, because a machine, under the most favourable circumstances, cannot charge a battery so high, in proportion, as a single jar. You may reckon that a battery is well charged when the index rises as high as 60^, or between that and 70''. J. Is there no danger of breaking the jars when the battery is very highly charged 1 T, Yes, there is j and if one jar be cracked, it is impossible to charge the others, till the broken one be removed : to prevent accidents, it is recommended not to discharge a battery through a good conductor, except the circuit is at least five feet long. C. Do you mean the wire should be so long ? T, Yes, if you pass the charge through that : but you may carry it through any conductor. j Before a battery is used, the uncoated part of the! jars must be made perfectly clean and dry, for the! smallest particles of dust will carry away the electricj EXPERIPJENTS. 463 fluid. And after an explosion, take care always to connect the wire on the hook with the ball, to pre- vent any residuum from remaining. /. Have not small animals been sometimes killed by an electrical battery ? T. Yes: small animals, such as mice, sparrows and pigeons, are instantly killed by a shock from thirty square inches of glass ; and even if a man receive a charge through the spine, he loses his power over the muscles to such a degree, as to cause him to fall prostrate on the ground, and the charge may be made sufficiently powerful to produce immediate death. C. Have any individuals ever been killed by over- charging the battery ? T, The celebrated Professor Richmann was acci- dentally killed by a stroke of lightning, whilst makino- experiments on the clouds. ° CONVERSATION IX, EXPERIMENTS MADE WITH THE ELECTRICAL BATTERY. r. I will now shew you some experiments with this large battery : to perform these in perfect safety, I must beg you to stand a good distance from it: this will prevent accidents. Ex. 1. I take this quire of writing-paper, and place It against the hook or wire that comes out of the box • and when the battery is charged, I put one ball of the discharging rod to a knob of one of the wires r and bring the other knob to that part of the paper that stands against the wire proceeding from the box: you see what a hole it has made through every sheet of the paper. Smell the paper where the perfora- tion is. C. It smells like sulphur. T. Or more like phosphorus: you observe in this experiment, that the electric fluid passed from the 464 ELECTRICITY, inside of the jars through the conducting rod and pa- per, to the outside. J. Why did it not pass through the paper in the same manner as it passed the brass discharging rod, in which it made no hole? T. Paper is a non-conducting substance, but brass is a conductor : through the latter it passes without any resistance, and in its endeavour to get to the in- side of the box, it burst the paper as you see : the same thing would have happened had there been twice or thrice as much paper. The electric fluid of a single jar will pierce through many sheets of paper. C. Would it serve any other non-conducting sub- stance in the same manner ? T. Yes ; it will even break a thin piece of glass, or of resin, or of sealing-wax, if they be interposed be- tween the dischargmg rod and the outside of the coating of the battery. Ex. 2. Place a piece of loaf sugar in the situation m which the quire of paper was just now, the sugar will be broken, and in the dark it will appear beau- tifully illuminated, and remain so for many seconds of time. Ex. 3. Let the small piece of wire, proceeding from the hole in the box, be laid on one side of a plate, containing some spirits of wine, and on the opposite side of the plate bring one of the knobs of the discharging rod, while the other is carried to the wires connected with the inside of the jars. C. I'hen the electric fluid will have a passage through the spirit 1 T. It will set it on fire instantly. Ex. 4. Take two slips of common window-glass, about four inches long, and one inch broad ; put a slip of gold leaf between the glasses, leaving a small part of it out at each end ; then tie the glasses toge- ther, or press them with a heavy weight, and send the charge of the battery through it, by connecting one end of the glass with the outside of the jars, and EXPERIMENTS. 405 bringing the discharging rod to the other end, and to the wires of the inside of the battery. /. W ill it break the glass ? r. It probably will; but whether it does or not, the gold leaf will be forced into the pores of the glass, so as to appear like glass stained with gold, which nothing can wash away. Ex. 5. If the gold leaf be put between two cards, and a strong charge passed through it, it will be com- pletely fused or melted, the marks of which will ap- pear on the card. This instrument, called a universal discharger, is Fig. 11. very useful for passing charges through many sub- stances. B B are glass pillars cemented into the frame A. To each of the pillars is cemented a brass cap, and a double joint for horizontal and vertical mo- tions ; on the top of each joint is a spring tube, which holds the sliding wires x, x, so that they may be set at various distances from each other, and turned in any direction ; the extremities of the wires are pointed, but with screws, at about half an inch from the points, to receive balls. The table ed, inlaid with a piece of ivory, is made to move up and down in a socket, and a screw fastens it to any required height. The rings c c are very convenient for fixing a chain or wire to them, which proceeds from the conductor. C. Do you lay anything on the ivory, between the balls, when you want to send the charge of a battery through it ? T. Yes ; and by drawing out the wires, the balls X 2 406 ELECTRICITY. may be separated to any distance less than the length of the ivory. The little figure ri repre- sents a press, which may be substituted Jf in the place of the table e d : it consists (^IIjiZZ2; of two flat pieces of mahogany, which I I may be brought together by screws. Fig. 12. J. Then, instead of tying the slips of glass together in Ex. 4, you might have done it bet- ter by making use of the press 1 T. I might ; but I was willing to shew you how the thing might be done if no such apparatus as this were at hand. The use of the table and press, which, in fact, always go together, is for keeping steady all descriptions of bodies, through which the charge of a single jar, or any number of which a battery consists, is to be conveyed. We will now proceed with the experiments. Ex. 6. I will take the knobs from the wires of the universal discharger, and having laid a piece of very dry writing-paper on the table e, I place the points of the wires at an inch or more from one another ; then, by connecting one of the rings c with the outside wire or hook of the battery, and bringing the discharging rod from the other ring c to one of the knobs of the battery, you will see that the paper will be torn to pieces. Ex. 7. The experiment which I am now going to make, you must never attempt by yourselves : I put a little gunpowder in the tube of a quill, open at both ends, and insert the pointed extremities of the two wires in it so as to be within a quarter of an inch or less from each other. I now send the charge of the battery through it, and the gunpowder, you see, is in- stantly inflamed. Ex. 8. Here is a very slender wire, not a hun- dredth part of an inch in diameter, which I connect with the wires of the discharger, and send the charge of a battery through it, which will completely melt it, and you now perceive the little globules of iron in- stead of the thin wire. EXPERIMENTS. 467 C. Will other wires besides iron be melted in the same manner 1 T. Yes ; if the battery be large enough, and the wires sufficiently thin, the experiment will succeed with them all ; even with a single jar, if it be pretty large, very slender wire may be fused. But the charges of batteries have been used to determine the differeBt conducting powers of the several metals. J. If the charge is not strong enough to melt the wire, will it make it red hot ? T. It will : and when the experiment is properly done, the course of the fluid may be discerned by its effects : for if the wire is about three inches long, it will be seen that the end of it which is connected with the inside of the battery, is red hot first, and the redness proceeds towards the other. C. That is a clear proof that the superabundant electricity accumulated in the inside is carried to the outside of the jars. T, Ex. 9. We have already discussed the subject of magnetism : by discharging the battery through a small sewing-needle, it will become magnetic ; that is, if the needle be accurately suspended on a small piece of cork in a bason of water, one end will, of itself, point to the north, and the other to the south. Ex. 10. I will lay this chain on a sheet of writing- paper, and send the charge of the battery through the chain ; and you will see black marks will be left on the paper in those places where the rings of the chain touch each other. Ex. 10. Place a small piece of very dry wood between the balls of the universal discharger, so that the fibres of the wood may be in the direction of the wires, and pass the charge of the battery through them, and the wood will be torn in pieces. The points of the wires being run into the wood, and the shock passed through them, will effect the same thing. Ex. 12. Here is a glass tube, open at both ends, six inches long, and a quarter of an inch in diameter. 4G8 ELECTRICITY. These pieces of coik, with wires in them, exactly fit the ends of the tube. I put in one cork, and fill the tube with water, then put the other cork in, and push the wires so that they nearly touch, and pass the charge of the battery through them ; you see the tube is broken, and the water dispersed in every direc- tion.* C. If water is a good conductor, how is it that the charge did not run through it without breaking the tube? T. The electric fluid, like common fire, converts the water into a highly elastic vapour, which, occu- pying very suddenly a much larger space than the .water, bursts the tube before it can effect any means of escape. When a succession of electric discharges from a powerful electrical machine are sent through water, a decomposition of that fluid takes place, and it is re- solved into its two elements of oxygen and hydrogen, which immediately assume the gaseous form. CONVERSATION X. OF THE ELECTRIC SPARK, AND MISCELLANEOUS EXPERIMENTS. T. I wish you to observe some facts connected with the electric spark. By means of the wire in- serted in this ball, I fix it to the end of the conductor, and bring either another brass ball or my knuckle to it, and if the machine act pretty powerfully, a long, crooked, brilliant spark will pass between the two balls, or between the knuckle and ball. If the con- ductor is negative, it receives the spark from the * To prevent accidents, a wire cage, such as is used in some experiments on the air-pump, should be put over the tube before the discharge is made ; young per- sons should not attempt this experiment by themselves. EXPERIMENTS. body ; but if it is positive, the ball or the knuckle re- ceives the spark from the conductor. C. Does the size of the spark depend at all on the size of the conductor 1 T, The longest and largest sparks are obtained from a large conductor, provided the machine act very powerfully. When the quantity of electricity is small, the spark is straight; but when it is strong, and capable of striking at a greater distance, it as- sumes what is called a zig-zag direction. J. If the electric fluid is fire, why does not the spark, which excites a painful sensation, burn me, when I receive it on my hand ? T. Ex. 1. I have shewn you that the charge from a battery will make iron wire red hot, and inflame gunpowder. Now stand on the stool with glass legs, and hold the chain from the conductor with one hand. Do you, Charles, hold this spoon, which contains some spirit of wine, to your brother, while I turn the machine, and a spark taken from his knuckle, if large, will set fire to the spirit. C. It has indeed : did you do nothing with the spirit ? T. I only made the silver spoon pretty warm before I put the spirit into it. Ex. 2. If a ball of box-wood be placed on the conductor instead of the brass ball, a spark taken from it will be of a fine red colour. Ex. 3. An ivory ball placed on the conductor will be rendered very beautiful and luminous if a strong- spark be taken through its centre. Ex. 4. Sparks taken over a piece of silver leather appear of a green colour, and over gilt leather of a red colour. Ex. 5. Here is a glass tube, round which, at small 470 ELECTRICITY, distances from each other, pieces of tin-foil are pasted in a spiral form from end to end ; this tube is inclosed in a larger one fitted with brass caps at each end, which are connected with the tin-foil of the inner tube. — I hold one end a in my hand, and while one of you turn the machine, I will present the other end B to the conductor, to take sparks from it : but first shut the window-shutters. C. This is a very beautiful experiment. T. The beauty of it consists in the distance which is left between the pieces of tin-foil, and by increasing the number of these distances, the brilliancy is very much heightened. Ex. 6. The following is another experiment of the same kind : here is a word, with which you are ac- Fig. 14. quainted, made on glass, by means of tin-foil pasted on glass, fixed in a frame of baked wood. I hold the frame in my hand at h, and present the ball g to the conductor, and at every considerable spark the word is beautifully illuminated. Ex. 7. A piece of sponge fi'lled with water, and hung to a conductor, when electrified in a dark room, exhibits a beautiful appearance. Ex. 8. This bottle is charged : if I bring the brass knob that stands out of it to a basin of water which is insulated, it will attract a drop ; and on the removal of the bottle it will assume a conical shape, and if brought near any conducting substance, it will fly to it in luminous streams. Ex. '9. Place a drop of water on the conductor, and work the machine ; the drop will afford a long spark, assume a conical figure, and carry some of the water with it. EXPERIMENTS. 47I Ex. 10. On this wire I have fixed a piece of seahng-wax, and having fixed the wire into the end of the conductor, I will light the wax, and the moment the machme is worked the wax will fly oflT in the finest filaments imaginable. Ex. 11. I will wrap some cotton-wool round one of the knobs of my discharging rod, and fill the wool with finely bruised resin ; I now discharge a Leyden jar, or a battery, in the common way, and the wool is instantly m a blaze. The covered knob must touch the knob of the jar, and the discharge should be efl'ected as quickly as possible. You will remember, that the electric fluid always chooses the nearest road, and the best conductors to travel by ; in proof of which take the following ex- periment : — Ex. 12. With this chain I make a sort of W; the wire u touches the outside of a charged jar, and the wire x is brought to the ^ ^^v* knob of the jar, and in the dark a bril- \ /\ f liant W is visible. But if the wire u \ / \ / is continued to m, the electric fluid ° takes a shorter road to x, and, of course, Fio-. 15. only half of the VV is seen, viz. that " part marked mz y: but if, instead of the wire n m, a dry stick be laid in its place, the electric matter will prefer a longer circuit, rather than go through a bad conductor, and the whole W will be illu- minated. Ex. 13. Here is a two ounce phial, half full of salad oil; through the cork is passed a piece of slender wire, the end of which, within the phial, is so bent as to touch the glass just below the surface of the oil. I place my thumb opposite the point of the wire in the bottle, and in that position take a spark from the charged conductor. You observe that the spark, to get to my thumb, has actually perforated the glass. In the same way I can make holes all round the phial. 472 - ELECTRICITY. C. Would the experiment succeed with water instead of oil ? T. No, it would not. J. At any rate we see the course of the electric fluid in this experiment, for the spark comes from the conductor down the wire, and through the glass to the thumb. r. Its direction is, however, better shewn in this way : — Ex. 14. At that end of the conductor which is farthest from the machine, I fix a brass wire about six inches long, having a small brass ball on its ex- tremity. To this ball, when the machine is at work, I hold the flame of a wax taper. C. The flame is evidently blown from the ball in the direction of the electric fluid ; it has a similar ef- fect to the blast of a pair of bellows. Ex. 15. I will fix a pointed wire upon the prime conductor, with the point outward, and another like wire upon the insulated rubber : shut the window- shutter, and I will work the machine : — now observe the points of the two wires. J. They both are illuminated, but diflPerently. The point on the conductor sends out a sort of brush of fire, but that on the rubber is illuminated with a star. r. You see, then, the difference between the positive and negative electricity. The appearance of a star on the point of the wire will shew that the electricity is positive ; while, on the contrary, a luminous brush indicates that the electricity is negative. CONVERSATION XI. MISCELLANEOUS EXPERIMENTS OF THE ELECTRO- PHORUS ^ OF THE ELECTROMETER AND THE THUNDER HOUSE. T. I shall proceed this morning with some other experiments on the electrical machine. EXPERIMENTS. 473 Ex. 1. Here are two wires, one of which is con- nected with the outside of this charged Leyden jar, the other is so bent as easily to touch the knob of the jar. The two straight ends I bring within the dis- tance of the tenth of an inch of one another, and press them down with my thumb, and in this position, having darkened the room, I discharge the jar : do you look upon my thumb. C. It was so transparent that I think I even saw the bone of the thumb ; but did it not hurt vou very much ? ^ T, With attention, you might observe the principal blood-vessels, I believe, and the only inconvenience that I felt was a sort of tremor in my thumb, which IS by no means painful. Had the wires been at double the distance, the shock would have probably made my thumb the circuit, which must have caused a more powerful and unpleasant sensation, but being so close, the electric fluid leaped from one wire to the other, and during this passage it illuminated my thumb, but did not go through it. Ex. 2. If, instead of my thumb, a decanter full of water, having a flat bottom, were placed on the wires, and the discharge made, the whole of the water will be beautifully illuminated. Ex. 3. This small pewter bucket is full of water, and I suspend it from the prime conductor, and put m a glass syphon, with a bore so narrow that the water^will hardly drop out. See what will happen when I work the machine ; but first make the room dark. J. It runs now in a full stream, or rather in several streams, all of which are illuminated. T. Ex. 4. If the knob a commu- ^p^i^^ nicate with the outside of a charged ^ ^.B \ Leyden jar, and the knob b with the 'jj inside coating, and each be held about ^ two inches from the lighted candle Fig. 16. and opposite to one another, the flame will spread towards each, and a discharge will be 474 ELECTRICITY, made through it: this shews tlie conducting power of flame. This instrument, which consists of two circular plates, of which the largest b is about fifteen inches in diameter, and the other A 14 inches, is called an electropho- rus. The under plate b is made of glass, or sealing-wax, or of any other non-con- Fig. 17. ducting substance ; 1 have made one with a mixture of pitch and chalk boiled together. The upper plate a is sometimes made of brass, and sometimes of tin-plate, but this is of wood, covered very neatly with tin -foil : x is a glass handle fixed to a socket, by which the upper plate is removed from the under one. C. What do you mean by an electrophorus ? T. It is, in fact, a sort of simple electrical machine, and is thus used. Rub the lower plate b with a fine piece of new flannel, or with rabbits', or hares', or cats' skin, and when it is well excited, place upon it the upper plate a, and put your finger on the upper plate ; then remove this plate by the glass handle ^, and if you apply your knuckle, or the knob of a coated jar, you will obtain a spark. This operation may be repeated many times without exciting again the under plate. J. Can you charge a Ley den jar in this way? T. Yes, it has been done, and by a single excita- tion, so as to pierce a hole through a card. Here is another kind of electrometer, which is by far the most sensible that has | been yet invented ; that is, it is capable of ^ discovering the smallest quantities of ^ { |x electricity, a is a glass jar, b the cover 1 1 ^\ of metal, to which is attached two pieces of gold leaf x, or two pith-balls sus- pended on threads ; on the sides of the glass jar are two narrow strips of tin-foil. C. How is this instrument used ? Fig. 18. 2\ Any thing that is electrified is to be LIGHTNING. 475 brought to the cover, which will cause the pieces of gold leaf, or pith-balls, to diverge ; and the sen- sibility of this instrument is so great, that the brush of a feather, the throwing of chalk, hair-powder, or dust, against the cap b, evinces strong signs of electricity. Ex. 5. Place on the cap b a little pewter, or any other metallic cup, having some water in it ; then take from the fire a live cinder, and put it in the cup, and the electricity of vapour is very admirably ex- hibited. A thunder-cloud passing over this instrument will cause the gold leaf to strike the sides at every flash of lightning. Ex. 6. I will excite this stick of sealing-wax, and bring it to the cover b — you see how often it causes the gold leaf to strike against the sides of the glass. J . Are the slips of tin-foil intended to carry away the electric fluid communicated by the objects pre- sented to the cap b ? T, They are j and by them the equilibrium is re- stored. CONVERSATION XII. \ OF ATMOSPHERICAL ELECTRICITY. C. You said, yesterday, that the electrometer was affected by thunder and lightning : are lightning and ' electricity similar ? T. They are, undoubtedly, the same fluid ; and that they are the same was discovered by Dr. Franklin, in June, 1752. /. How did he ascertain this fact 1 ! T. He was led to form the theory, from observing the power which uninsulated yoints have in drawing off the electricity from bodies. And having made his system, he was waiting for the erection of a spire, in Philadelphia, to carry his views into execution, when I it occurred to him that a boy's kite would answer his r i. 47G ELECTRICITY. purpose better than a spire. He therefore prepared a kite, and having raised it, he tied to the end of the string a silken cord, by which the kite was completely insulated. At the junction of the two strings he fastened a key, as a good conductor, in order to take sparks from it. C. Did he obtain any sparks 1 T. One cloud, which appeared like a thunder- cloud, passed without any effect; shortly after, the loose threads of the hempen string stood erect, in the same manner as they would if the string had been hung on an electrified insulated conductor. He then presented his knuckle to the key, and obtained an evident spark. Others succeeded before the string was wet, but when the rain had wetted the string, he collected the electricity very plentifully. J. Could I do so with our large kite 1 T. I hope you will not try to raise your kite during a thunder-storm, because, without very great care, it may be attended with the most serious danger ; your kite is, however, quite large enough, being four feet high, and two feet wide; every thing depends on the string, which, according to Mr. Cavallo, who has made many experiments on the ^subject, should be made of two thin threads of twine, twisted with a copper thread. And to Mr. Cavallo's work on elec- tricity, vol. 2, such persons as are desirous of raising kites, for electrical purposes, should be referred, in which they will find ample instruction. C. How do the conductors, which I have seen fixed to various buildings, act in dispersing lightning ? r. You know how easy it is to charge a Leyden jar : but if, when the machine is at work, a person hold a point of steel, or other metal, near the con- ductor, the greater part of the fluid will run away by that point instead of proceeding to the jar. Hence it was concluded, that pointed rods would silently draw away the lightning from clouds passing over any building. J, Is there not a particular method of fixing them? LIGHTNING. 477 Yes : the metallic rod must reach from the ground, or the nearest piece of water, to a foot or two above the building it is intended to protect, and the iron rod should come to a fine point : some elec- tricians recommend that the point should be of gold, to prevent its rusting. C. What effects would be produced if lightning should strike a building without a conductor ? T, That may be best explained by informing you of what happened, many years ago, to St. Bride's church. The lightning first struck the weather-cock : from thence, descending in its progress, it beat out a number of large stones of different heights, some of which fell upon the roof of the church, and did great damage to it. The mischief done to the steeple was so considerable, that eighty-five feet of it was obliged to be taken down. / . The weather-cock was probably made of iron j why did not that act as a conductor ] T, Though that was made of iron, yet it was com- pletely insulated by being fixed in stone, that had be- come dry by much hot and dry weather. When therefore the lightning had taken possession of the weather-cock, by endeavouring to force its way to another conductor, it beat down whatever stood in its way. C. The power of lightning must be very great. T. It is irresistible in its effects ; the following experiment will illustrate what I have been saying Ex. 1. A is a board representing the gable end of a house ; it is fixed on another board b : a b c d is a square hole, to which a piece of wood is fitted ; a d represents a wire fixed diagonally on the wood a b c d ; X b, terminated by a knob x, represents a weather-cock, and the wire c z is fixed to the board a. It is evident, that in the state in which it is drawn in the figure, there Fig. 19. 478 ELECTRICITY, is an interruption in the conducting rod ; accordingly, if the chain m is connected with the outside of a Ley den phial, and then that phial is discharged through X, by bringing one part of the discharging rod to the knob of the Leyden phial, and the other to within an inch or two of x, the piece of wood, a, 6, c, d, will be thrown out with violence. J. Are we to understand by this experiment that if the wire x h had been continued to the chain, that the electric fluid would have run through it without dis- turbing the loose board 1 T. Ex. 2. Just so ; for if the piece of wood be taken out, and the part a be put to the place b, then d will come to c, and the conducting rod will be com- plete, and continued from x through a and d to z, and now the phial may be discharged as often as you please, but the wood will remain in its place, because the electric fluid runs thiough the wire to z, and makes its way by the chain to the outside of the phial. C. Then if x be supposed the weather-cock of the church, the lightning having overcharged this, by its endeavours to reach another conductor, as c s, forced away the stone or stones represented by a 6 c d ? T. That is what 1 meant to convey to your minds by the first experiment ; and the second shews very clearly, that if an iron rod had gone from the weather-cock to the ground, without interruption, it would have conducted away the electricity silently, and without doing any injury to the church. J. How was it that all the stones were not beat down ? T, Because, in its passage downwards, it met with many other conductors. I will read part of what Dr. Watson says on this fact, who examined it very attentively : — j "The lightning," says he, "first took a weather- cock, which was fixed at the top of the steeple, and was conducted without injuring the metal or anything FALLING STARS. 479 else as low as where the large iron bar or spindle which supported it terminated : there the metallic communication ceasing, part of the lightning ex- ploded, cracked, and shattered the obelisk, which ter- minated the spire of the steeple, in its whole diameter, and threw off, at that place, several large pieces of Portland stone. Here it likewise removed a stone from its place, but not far enough to be thrown down. From thence the lightning seemed to have rushed upon two horizontal iron bars, which were placed within the building across each other. At the end of one of these iron bars it exploded again, and threw off a considerable quantity of stone. Almost all the damage was done where the ends of the iron bars had been inserted into the stone, or placed under it; and, in some places, its passage might be traced from one iron bar to another." CONVERSATION XIII. ON ATMOSPHEinC ELECTRICITY OF FALLING STARS— OF THE AURORA BOREALIS OF WATER-SPOUTS AND WHIRLWINDS OF EARTHQUAKES. C. Does the air always contain electricity 1 T. Yes ; and it is owing to the electricity of the atmosphere that we observe a number of curious and interesting phenomena, such as falling stars ; the aurora borealis, or northern lights ; the ignis fatuus, or Will-with-the-wisp. J . I have frequently seen what people call falling stars, but I never knew that they were occasioned merely by electricity. T. These are seen chiefly in clear and calm weather : it is then that the electric fluid is probably not very strong, and, passing through the air, it be- comes visible in particular parts of its passage, ac- cording to the conducting substances it may meet with. One of the most striking phenomena of this kind 480 ELECTRICITY. is recorded by Signior Beccaria : — As he was sitting with a friend in the open air, an hour after sun-set, they saw a falUng, or, as it is sometimes called, a shooting star, directing its course towards them, grow- ing, apparently, larger and larger, till it disappeared not far from them, and, disappearing, it left their faces, hands, and clothes, with the earth and neigh- bouring objects, suddenly illuminated with a diffused and lambent light, attended with no noise at all. C. But how did he know that this was only the effect of electricity 1 T. Because he had previously raised his kite, and found the air very much charged with the electric matter : sometimes he saw it advancing to his kite like a falling star ; and sometimes he saw a kind of glory round it, which followed it as it changed its place. /. Since lofty objects are exposed to the effects of lightning, or the electric fluid, do not the tail masts of ships run considerable risk of being struck by it ? T. Certainly : we have many instances recorded of the mischief done to ships. One of which is re- lated in the Philosophical Transactions ; it happened on board the Montague, on the 4th of November, 1748, in lat. 42" 48' and 9° 3' west longitude, about noon. One of the quarter-masters desired the master of the vessel to look to the windward, when he ob- served a large ball of blue fire, rolling apparently on the surface of the water, at the distance of three miles from them : it rose almost perpendicular when it was within forty or fifty yards from the main-chains of the ship, it then went off with an explosion, as if a hundred cannon had been fired at one time, and left so great a smell of sulphur, that the ship seemed to contain nothing else. After the noise had subsided, the main-top-mast was found shattered to pieces, and the mast itself was rent quite down to the keel. Five men were knocked down, and one of them greatly burnt by the explo^ijii. C. Did it not seem to be a very large ball to have produced such cfTects? AURORA EOREALiS. 48i T. Yes; the person who noticed it said it was as Dig as a millstone. The Aurora Borealis is another electrical pheno- menon : this IS admitted without any hesitation, be- cause electricians can readily imitate the appearance with their experiments. sca^e "^^^^ ^ ^^^"^^ ^ ^^^^ ^"""^^ T, True ; there is a glass tube about thirty inches long and the diameter of it is about two inches ; it is nearly exhausted of air, and capped on both ends with brass. I now connect these ends, by means of a Cham, with the positive and negative part of a ma- chine, and in a darkened room you will see, whe*n the machine is worked, all the appearances of the northern lights in the tube. C. Why is it necessary nearly to exhaust the tube ^ i . Because the air, in its natural state, is a very bad conductor of the electric fluid ; but when it is perhaps, rendered some hundred times rarer than it usually is, the electric fluid darts from one cap to the other with the greatest ease. /. But we see the natural aurora borealis in the air. T. We do so, but it is in the higher regions of the atmosphere, where the air is much rarer than it is near the surface of the earth. The experiment which you have just seen accounts for the darting and un- dulatmg motion which takes place between the oppo- site parts of the heavens. The aurora borealis is the most beautiful and brilliant in countries in the hioh northern latitudes, as in Greenland and Iceland. ° The aurora borealis that was seen in this country on the 2t3d of October, 1804, is deserving of notice At seven m the evening a luminous arch was seen trom the centre of London, extending from one point ot the horizon, about s.s. w. to another point n.n w and passing the middle of the constellation of the Great Bear, which it in a great measure obscured. It appeared to consist of shining vapour, and to roll 482 ELECTRICITY, from the south to the north. In about half an hour its course was changed ; it then became vertical, and about nine o'clock it extended across the heavens from N. E. to s. w. ; at intervals the continuity of the luminous arch was broken, and there then darted from its south-west quarter, tov/ards the zenith, strong flashes and streaks of bright red, similar to what appears in the atmosphere during a great fire in any part of the metropolis. For several hours the atmosphere was as light in the south-west as if the sun had set but half an hour ; and the light in the north resembled the strong twilight which marks that part of the horizon at midsummer. J. How do you account, sir, for the Will-with-the- wisp, or Jack-a-lanthorn, that is close to the ground, where the air is thickest ? T. This is a meteor which seldom appears more than six feet above the ground ; it is always about bogs and swampy places, and these, in hot weather, emit what is called inflammable air, which is easily set fire to by the electric spark. These, therefore, as you shall see in our chemical experiments, we can as readily imitate as the aurora borealis. — In some parts of Italy meteors of this kind are frequently very large, and give a light equal to that of a torch. Waterspouts, which are sometimes seen at sea, are supposed to arise from the power of electricity. C. I have heard of these ; but I thought that ,i water-spouts at sea, and whirlwinds and hurricanes ? by land, were produced solely by the force of the wind. . T. The wind is, undoubtedly, one of the causes, but! it will not account for every appearance connected with them. Water-spouts are often seen in calm weather, when the sea seems to boil, and send up a smoke under them, rising in a sort of hill towards the spout. A rum.bling noise is often heard at the time of their appearance, which happens generally in those months that are peculiarly subject to thunder-storms, and they are commonly accompanied or followed by WATERSPOUTS. 483 lightning. When these approach a ship, the sailors present and brandish their swords to disperse them, which seems to favour the conclusion^ that they are electrical. J, Do the swords act as conductors ? T. They may, certainly : and it is known that by these pointed instruments they have been efFectually dispersed. The analogy between the phenomena of water- spouts and electricity, may be made visible by hang- ing a drop of water to a wire, communicating with the prime conductor, and placing a vessel of water under it. In these circumstances, the drop assumes all the various appearances of a water-spout, in its rise, form, and mode of disappearing. Water-spouts, at sea, are undoubtedly very like whirlwinds and hurricanes by land. These some- times tear up trees, throw down buildings, make caverns ; and in all the cases they scatter the earth, bricks, stones, timber, &:c. to a great distance in every direction. Dr. Franklin mentions a remarkable ap- pearance which occurred to Mr. Wilkie, a consider- able electrician. On the 20th of July, 1758, at three o'clock in the afternoon, he observed a great quantity of dust rising from the ground and cove^ring a field and part of the town in which he then was. "There was no wind, and the dust moved gently to- wards the east, where there appeared a great black cloud, which electrified his apparatus positively to a very high degree. This cloud went towards the west, the dust followed it, and continued to rise higher and higher, till it composed a thick pillar, in the form of a sugar-loaf, and at length it seemed to be in contact with the cloud. At some distance from this, there came another great cloud, with a long stream of smaller ones, which electrified his apparatus negatively, and when they came near the positive cloud, a flash of lightning was seen to dart through the cloud of dust, upon which the negative clouds 48^1 ELECTRICITY, spread very much and dissolved in rain, wbicn pre- sently cleared the atmosphere. C. Is rain, then, an electrical phenomenon I T. The most enlightened and best informed elec- tricians reckon rain, hail, and snow, among the effects produced by the electric fluid. J. Do the negative and positive clouds act in the same manner as the outside and inside coatings of a charged Leyden jar? T. Thunder-clouds frequently do nothing more than conduct or convey the electric matter from one place to another. C. Then they may be compared to the discharging YOdl T. And perhaps, like that, they are intended to re- store the equilibrium between two places, one of which has too much, and the other too little, of the electric fluid. The following is not an uncommon appearance: a dark cloud is observed to attract others to it, and when grown to a considerable size, its lower surface swells in particular parts towards the earth. During the time that the cloud is thus forming, flashes of lightning dart from one part of it to the other, and often illuminate the whole mass ; and small clouds are observed moving rapidly beneath it. When the cloud has acquired a sufficient ex- tent, the lightning strikes the earth in two opposite places. J. I wonder the discharge does not shake the earth, as the charge of a jar does anything through which it passes. T. Every discharge of clouds through the earth may do this, though it is imperceptible to us. Earthquakes are probably occasioned by vast dis- charges of the electric fluid : they happen most fre- quently in dry and hot countries, which are subject to lightning and other electric phenomena ; they are even foretold by the electric corruscations, and other appearances in the air for some days preceding the MEDICAL ELECTRICITY. 485 event. Besides, the shoek of an earthquake is in- stantaneous to the greatest distances. Earthquakes are usually accompanied with rain, and sometimes by the most dreadful thunder-storms. CONVERSATION XIV. MEDICAL ELECTRICITY. T. If you stand on the stool with glass legS;, and hold the chain from the conductor while I work the machine a few minutes, your pulse will be increased, that is, it will beat more frequently than it did before. From this circumstance physicians have applied elec- tricity to the cure of many disorders : in some of which their endeavours have been unavailing — in others the success has been very complete, C. Did they do nothing more than this 1 T, Yes ; in some cases they took sparks from their patients — in others they gave them shocks. /. This would be no pleasant method of cure, if the shocks were strong. T. You know by means of Lane's electrometer, described in our seventh Conversation, the shock may be given as slightly as you please. C. But how are shocks conveyed through any part of the body ? T. There are machines and apparatus made pur- posely for medical purposes, but every end may be answered by the instrument just referred to. Suppose the electrometer to be fixed to a Leyden phial, and the knob at a to touch the conductor, and the knob B to be so far off, as you mean the shocks to be weak or strong, one chain or wire is to be fixed to the ring c of the electrometer, and another wire or chain to the outside coating : the other ends of these two wires are to be fastened to the two knobs of the dis- charging rod. J. What next is to be done, if I wish to electrify my knee, for instance 1 488 ELECTRICITY. T. All you have to do is to bring the balls of the discharging rod close to your knee, one on the one side, and the other on the opposite side. C. And, at every discharge of the Leyden jar, the superabundant electricity from withinside will pass from the knob at a to the knob b, and will pass through the wire and the knee, in its way to the out- side of the jar, to restore to both sides an equilibrium. J. But if it happen that a part of the body, as the arm, is to be electrified, how is it to be done, because in that case I cannot use both my hands in conduct- ing the wires 1 T. Then you may seek the assistance of a friend, who will, by means of two instruments called direc- tors, be able to conduct the fluid to any part of the body whatever. C. What are directors? T, A director consists of a knobbed brass wire, which by means of a brass cap is cemented to a glass handle. So the operator, holding these directors by the extremities of the glass handle, brings the balls, to which the wires or chains are attached, into contact with the extremities of that part of the body of the patient through which the shock is to be sent. If I feel rheumatic pains between my elbow and wrist, and a person hold one director at the elbow and another about the wrist, the shocks will pass through, and probably will be found useful in removing the complaint, J. Is it necessary to stand on the glass-footed stool to have this operation performed ? T. By no means : when shocks are administered, the person who receives them may stand as he pleases, either on the stool or on the ground ; the electric fluid, taking the nearest passage, will always find the other knob of the other director, which leads to the outside of the jar. C. Is it necessary to make the body bare? T. Not in the case of shocks, unless the coverings be very thick : but when sparks are to be taken, then THE TORPEDO. 48T the person from whom they are drawn must be insu- lated, and the clothes should be stripped off the part affected. J. For what disorders are the shocks and sparks chiefly used? . r. Shocks have been found useful in paralytic dis- orders ; in contractions of the nerves ; in sprains, and in many other cases ; but great attention is necessary in regulating the force of the shock, because, instead of advantage, mischief may occur if it be too violent. C. Is there less danger with sparks ? r. Yes ; for unless it be in very tender parts, as the eye, there is no great risk in taking sparks ; and they have proved very effectual in removing many complaints. The celebrated Mr. Ferguson was seized, at Bristol, with a violent sore throat, so as to prevent him from swallowing any thing : he caused sparks to be taken from the part affected, and in the course of an hour he could eat and drink without pain. This, in some instances, has been found an excel- lent method in cases of deafness, ear-ache, tooth-ache, swellings inside the mouth, &c. J. Would not strong sparks injure the ear ? T. They might ; and therefore the electric fluid is usually drawn with a pointed piece of wood, to which it comes in a stream, or, when sparks are taken, a very small brass ball is used, because, in propor- tion to the size of the ball is the size of the spark. CONVERSATION XV. OF ANIMAL ELECTRICITY OF THE TORPEDO— OF THE GYMNOTUS ELECTRICUS OF THE SILURUS ELECTRICUS. T. There are three kinds of fish which have been discovered, that are possessed of the singular property of giving shocks very similar to those experienced by means of the Leyden jar. 488 ELECTRICITY. C. I should like much to see them ; are they easily obtained ? T. No, they are not : they are called the torpedo, the gymnotKS electricus, and the silurus electricus, J. Are they all of the same genus 1 T, No ; the torpedo is a flat fish, seldom twenty inches long, and is common in various parts of the sea coast of Europe. The electric organs of this fish are placed on each side of the gills, where they fill up tlie whole thickness of the animal, from the lower to the upper surface, and are covered by the common skin of the body, C. Can you lay hold of the fish by any other part of the body with impunity ? T. Not altogether so : for if it be touched with one hand, it generally communicates a very slight shock ; but if it be touched with both hands at the same time, one being applied to the under, and the other to the upper surface of the body^ a shock will be received similar to that which is occasioned by the Leyden jar, J. Will not the shock be felt if both hands be put on one of the electrical organs at the same time ? T. No : and this shews that the upper and lower surfaces of the electric organs are in opposite states of electricity, answering to the positive and negative sides of a Leyden phial. C. Are the same substances conductors of the electric power of the torpedo, by which artificial electricity is conducted ? T. Yes, they are : and if the fish, instead of being touched by the hands, be touched by conducting substances, as metals, the shock will be communicated through them. The circuit may also be formed by several persons joining hands, and the shock will be felt by them all at the same time. But the shock will not pass where there is the smallest interruption ; it will not even be conducted through a chain. J. Can you get sparks from it 1 T. No spark was ever obtained from the torpedo. THE GYMNOTUS. 489 nor could electric repulsion and attraction be pro- duced by it. C. Is it known how the power is accumulated ? T. It seems to depend on the will of the animal, for each effort is accompanied with a depression of its eyes, and it probably makes use of it as a means of self-defence. J. Is this the case also with the other electrical fishes ? T, The gymnotus possesses all the electric pro- perties of the torpedo, but in a very superior degree. This fish has been called the electrical eel, on account of its resemblance to the common eel. It is found in the large rivers in South America. C. Are these fishes able to injure other fishes by this power 1 T, If small fishes are put into the water in which the gymnotus is kept, it will first stun, 'or perhaps kill them, and if the animal be hungry, it will then de- vour them. But fishes stunned by the gymnotus may be recovered, by being speedily removed into another vessel of water. The gymnotus is said to be possessed of a new kind of sense, by which it knows whether bodies that are brought near it are conductors or not. C. Then it possesses the same knowledge by in- stinct which philosophers have gained by experiment. T. You are right. The following experiment, among others, is very decisive on this point. Ex. The extremities of two wires were dipped into the water of the vessel in which the animal was kept ; they were then bent, extended a great way, and ter- minated in two separate glasses full of water. These wires, being supported by non-conductors, at a con- siderable distance from each other, the circuit was incomplete : but if a person put the fingers of both hands into the glasses in which the wires terminated, then the circuit was complete. While the circuit was incomplete, the fish never went near the extremities of Y 2 490 ELECTRICITY. the wires, as if desirous of giving the shock ; but the moment the circuit was completed, either by a person, or any other conductor, the gymnotus immediately went towards the wires, and gave the shock, though the completion of the circuit was out of his sight. /. How do they catch these kinds of fish ; the man would, probably, let them go on receiving the shock 1 T, In this way the property was, perhaps, first discovered. The gymnotus, as well as the others, may be touched, without any risk of the shock, with wax or with glass ; but if it be touched with the naked finger, or with a metal, or a gold ring, the shock is felt up the arm. C Does the silurus electricus produce the same effects as the others? T. This fish is found in some rivers in Africa, and it is known to possess the property of giving the shock, but no other particulars have been detailed respecting it. With regard to the torpedo, its power of giving the benumbing sensation was known to the ancients, and from this it probably took its name. In Firmin^s Natural History of Surinam is some account of the tremhling eel, which Dr. Priestley conjectures to be different from the gymnotus ; it lives in marshy places, from whence it cannot be taken, except when it is intoxicated. It cannot be touched with the hand, or with a stick, without feeling a terrible shock. If trod upon with shoes, the legs and thighs are affected in a similar manner. Humboldt, the celebrated traveller, when journey- ing across the Llanos, in South America, was in- formed, that in the neighbourhood of the small town of Calaboza, at a place called the Cano de Bera, the gymnoti were very numerous ; and being desirous to obtain some of them to make experiments upon, he was conducted to a small piece of water, shallow, stagnant, and muddy, but of the heat of 79 degrees. Understanding from the natives that the only way in SUMMARY, i&c. 491 which they could be cauglit was by driving horses and mules into the water to disturb them, and cause them first to expend their electric power, he ordered about thirty horses and mules to be collected and driven into the water ; the natives, by means of long bamboos or harpoons, preventing their escape. The gymnoti, roused from their slumbers by this noise and tumult, mount near the surface, and swimming like so many livid water serpents, briskly pursue the in- truders, and, gliding under their bellies, discharge through them the most violent and repeated shocks. The horses, convulsed and terrified, their manes erect, and their eyes staring with pain, make in- effectual struggles to escape. In a few minutes Iwo of them sunk under the water and were drowned: but the surviving horses gradually recovered from the shocks, and became more composed and vigorous. In a quarter of an hour the gymnoti had expended their power, and were then in such a state of languor and exhaustion, that they were easily taken. CONVERSATION XVI. GENERAL SUMMARY OF ELECTRICITY, WITH EXPERIMENTS, r. You now understand what electricity is ? C. Yes; it is a fluid which seems to pervade all substances, and, when undisturbed, it remains in a state of equilibrium. J. And that certain portion which every body is supposed to contain is called its natural share. r. When a body is possessed of more, or retains less, than its natural share, it is said to be charged, or electrified. C. If it possess more than its natural share, it is said to be positively electrified, but if it contain less than its natural share, it is said to be negatively electrified. 492 ELECTRICITY. T, Does it not sometimes happen, that the same substance is both positively and negatively electrified at the same time? J. Yes: the Ley den jar is a striking instance of this, in which, if the inside contain more than its natural share, the outside contains less than its natural quantity. T. What is the distinction between conductors and non-conductors of electricity? C. The electric fluid passes freely through the former, but the latter oppose its passage. r. You know that electricity is excited in the greatest quantities by the friction of conducting and non-conducting substances against each other. Ex. Rub two pieces of sealing-wax, or two pieces of glass, together, and only a very small portion of electricity can be obtained ; therefore, the rubber of a machine should be a conducting substance, and not insulated. Every electrical machine, with an insulated rubber, will act in three different ways ; the rubber will product negative electricity : the conductor will give out positive electricity ; and it will communicate both powers at once to a person or substance placed between two directors connected with them. /. How does the rubber produce negative elec- tricity ? T. If you Stand on a stool with glass legs, or upon any other non-conducting substance, and lay hold of the rubber, or a chain that communicates with it, the working the machine will take away from you a quantity of your natural electricity; therefore you will be negatively electrified. C. Will this appear by the nature of the electric fluid, if I hold in my hand a steel point, as a needle ? T. If you. Standing on a non-conducting sub- stance, are connected with the rubber, and your brother, in a similar situation, connected with the conductor, hold points in your hands, and I, while I SUMMARY, &c. 493 stand on the ground, first present a brass ball, or other substance, to the needle in your hand, and then to that in his hand, the appearance of the fluid will be different in both cases ; to the needle in your hand it will appear like a star, but to that in your brother's it will be rather in the form of a brush. — What will happen if you bring two bodies near to one another that are both electrified J. If they are both positively or both negatively electrified, they will repel each other, but if one is negative and the other positive, they will attract one another till they touch, and the equilibrium is again restored. T. If a body, containing only its natural share of electricity, be brought near to another that is elec- trified, what will be the consequence ? C. A quantity of electricity will force itself through the air in the form of a spark. T, When two bodies approach each other, one superabundant electricity rushes violently from one to the other to restore the equilibrium. What will happen if your body, or any part of it, form part of the circuit 1 J. It will produce an electric shock, and if, instead of one person alone, many join hands, and form a part of the circuit, they will all receive a shock at one and the same instant. r. If I throw a larger quantity of electricity than its natural share on one side of a piece of glass, what will happen to the other side ? C. The other side will become negatively electri- fied ; that is, it will have as much less than its natural share, as the other has more than its natural share. T. Does electricity, communicated to glass, spread over the whole surface? J. No, glass being an excellent non-conductur, the electric fluid will be confined to the part on which it is thrown ; and for that reason, and in order to apply electrified positively negatively, the 494 ELECTRICITY. it to the whole surface, the glass is covered with tin- foil, which is called a coating, T. And if a conducting communication be made between both sides of the glass, what takes place then ? ^ C, A discharge ; and this happens whether the glass be flat, or in any other form. T, What do you call a cylindrical glass vessel thus coated for electrical purposes 1 J, A Leyden jar; and when the insides, and also the outsides, of several of these jars are connected, it is called an electrical battery. ' r. Electricity, in this form, is capable of pro- ducmg the most powerful effects, such as melting metals, firing spirits, and other inflammable sub- stances.— What efl'ect has metallic points upon electricity? ^ C. They discharge it silently, and hence their great utility in defending buildings from the dire efl'ects of lightning.— Pray what is thunder ? T. As lightning appears to be the rapid motion of vast masses of electric matter, so thunder is the noise produced by the motion of lightning: and when electricity passes through the higher parts of the atmosphere, where the air is very much rarefied, it constitutes the aurora borealis. Ex. If two sharp-pointed wires be bent, with the four ends at right angles, but pointing differ- ent ways, and they be made to turn upon a wire x fixed on the conductor, the moment it is elec- trified a flame will be seen at the Fig. 21. points a bed; the wire will begin to turn round in the direction opposite to that to which the points are turned, and the motion will become very rapid. If the figures of horses, cut in paper, be fastened upon these wires, the horses will seem to pursue one another, and this is called the electrical horse-race. SUMMARY, &c. 495 Of course, upon this principle, many other amusing and very beautiful experiments may be made : and upon this principle several electrical orreries have been contrived, shewing the motions of the earth and moon, and the earth and planets round the sun. J. Hovi^ do you account for this 1 . c T Fix a sharp-pointed wire into the end ot the large conductor, and hold your hand near it :---no sparks will ensue; but a cold blast wi come from the point, which will turn any light mills, wheels, i