i-^. ':^t ^»'-f/ ^^^^> University of California • Berkeley FttOM * ^ T> IV. A. li GARY'S (> CIIEVP BOOK STOIIE, ()i ^ portion of 2 to 1, the ^* times of vibration will be in the same propor- tion ; tliat is, whilst a b makes one vibration, c D passes through two, or the vibrations coincide shorter string. If the strings e lengths, and have the same tension or weight at* n ; but if one be double the thickness of the other, the time and number of vibrations in will be double those in the thicker. Or, wlien the strings are of equal length and thick- ness, but of different tension, the difference of time or vibration will be inversely as the square root of the weights N N , &c. ; that is, if the weights are as 1 to 4, the times of vibration will be as 1 to 2, the sq\iare roots of 1 and 4. at every second of the F and G H be of equal the smaller string 90 Speaking Trumpet, In wind instruments, where the sound proceeds, from the elasticity and compression of the air, the vibrations or pulses will be in proportion to the length and width of the tube that compresses it. As the vibrations of strings coincide at different intervals, the more frequently this coincidence hap- pens, the more agiecable is the sensation which is produced in the ear. The vibrations uniformly coincide when the strings are of the same length, magnitude, and tension, which produces perfect unison or concord. The next greatest lumiber of coincident vibrations is when the strings are in the proportion of 2 to 1; that is, when the shorter string is half the length of the longer, and makes two vibrations, whilst the siiorter makes but one : these are called octaves or eighths. M the vibrations be to one another as 2 to 3, the coincidence will be at the third of the shorter string, and in music it is called a fifth. If the vibrations be as 3 to 4, they produce a fourth, and so on through all compound vibrations, which form concords and discords, accordingly as the vibration of the different strings relate to one another. When two strings of equal tone are placed near to one another, on striking one, the pulse or undulatory motion of the air will produce a sympathetic sound in the other. In like manner, strings of different lengths which are in concord \^ ith each other, communicate a vibration throughout the whole when any individual string is put in motion. Speaking Trumpet. The advantages of this instrument in augmenting sound, arise from the reflection of the pulses on the sides of the tube as they are propagated by the mouth. The aerial pulses, which are thus driven through the Echoes, 91 tube, not only augment the sound by increasing the aerial density of the pulses, but also, by directing them more immediately to the object; likewise the reflection of the pulses on the sides of the trumpet receive additional force from the elasticity or rever- beration of the metal; or rather, every point of per- cussion may be considered as a part from which fresh pulses are perpetually generating. If a tube be continued to prevent the dispersion of the pulses, sounds may be carried to a very consider- able extent; even the softest whisper may be distinctly heard at the distance of 15 or 20 feet. The whispering tubes sometimes surprise and amuse, when the com- munication is concealed, and the ends of the tubes terminate in the mouth and ears of two figures set at some distance apart.* Echoes, In the transmission of sound, it is conceived that every point of impulse serves as a centre for genera- ting fresh impulses in every direction, and that sound passes through equal distances in equal times. Then, if the sum of the right lines with their reflections be equal to one another, the times will be equal ; that is, if the pulses which diverge in right lines from a given point, be variously reflected on diflferent sides, the sound will return in equal times to the generating point, or to any other where the distances become equal : this- return of the pulses is called an Echo. * As in Peale's museum. Ed. 92 Echoes. If aerial pulses be propagated from the point a, anA strike various points of the curve CDEFG, and the sums of the respec- tive lines taken together at b, be equal to one another; that is, ac + cb=ad4-db, and ad+db=ae + BE, &c. then the echo or reverbe- ration of sound will be heard at b , as a common point formed by the equal distances or tim€s of the re- spective quantities of sound. Sounds that follow one another are not distinctly heard if they exceed 9 or 10 in a second of timt. And as sound passes through 1142 feet in a second, the pulses of sound must precede each other by \ of 1142, which is about 127 feet, to be heard distinctly in suc- cession. So that if the various distances through which the sound is propagated do not exceed a b by 127 feet, the echo will not be formed clearly at b. If the sums of the lines do not exceed each other by more than 127 feet, the sound which is reflected from the different points of reverberation, will arrive so near the true time, that the difference will not be percep- tible to the ear. 93 HYDROSTATICS. Th I s useful and interesting part of Natural Philo- sophy treats of the motion, pressure, and equilibrium of fluids, and also of the art of weighing solids in them, to determine the different specific gravities or relations of bodies to one another. By the word Fluid is meant a body compounded of small particles, which easily give way to an impres- sing force, varying their place and mixing with one .another with great freedom and celerity. The consti- tuent parts, or the particles which form a fluid, are conceived to be exceedingly small, smooth, hard and spherical ; possessing the same nature and qualities as belong to bodies in general. A fluid is considered more or less perfect, as the particles which compose it move amongst themselves with more or less free- dom and celerity. Quicksilver is a more perfect fluid than water, and oil is more fluent than honey. Fluids are not perfectly dense, as a quantity of salt may be dissolved in water without augmenting the bulk of the water. This leads us to imagine that the particles of water are spherical, and that the interstices which are formed between them are occupied by the salt, in the same manner as when fine sand is poured into a case of shot, which fills up the vacuities without augmenting the bulk. Fluids, like most solid bodies, change their appear- ance by the* different modifications of heat; for a supe- rior quantity destroys the cohesive force of the par- ticles, and forms them into vapour, and an inferior quantity increases the cohesion, and forms them into a solid mass of ice. Metals, in like manner, are made fluid by an excess of heat, and become solid as it decreases. L 94 Hydrostatic Principles. This changeable quality, which belongs to bodies in general, leads us to suppose that all the particles of matter are constituted alike, and that the different ap- pearance of bodies arises from the various modifica- tions of the particles which compose them : be this as it may, one common property is clear; that all bodies, whether solid or fluid, consist of heavy particles, the gravity of which is always proportional to the quantity of matter which they contain. Hydrostatic Principles. As the principal part of the subject of fluids de- pends on certain laws, which are founded on reason and experiment, it will be our first object to explain them. All fluids, except air, are incompressible in any considerable degree. The Academy del Cimento, from the following ex- periment, supposed water to be totally incompressible. A globe made of Gold, which is less porous than any other metal, was completely filled with water, and then closed up: it was afterwards placed under a great compressive force, which pressed the fluid through the pores of the metal, and formed a dew all over its surface, before any indent could be made in the ves- sel. Now^ as the surface of a sphere will contain a greater quantity than the same surface under any other form whatever, the Academy supposed that the com- pressive power which was applied to the globe must either force the particles of the fluid into closer adhe- sion, or drive them through the sides of the vessel before any impression could be made on its surface ; for although the latter effect took place, it furnishes no proof of the incompressibility of water, as the Flo- rentines had no method of determining that the altera- Ht/drostatic Principles. 95 tion of figure in their globe of gold occasioned such a diminution of its internal capacity, as was exactly equal to the quantity of water forced into its pores; but this experiment serves to show the great minute- ness of the particles of a fluid in penetrating the pores of gold, which is the densest of all metals. Mr. Canton brought the question of incompressi- bility to a more decisive determination. He procured a glass tube, of about two feet long, with a ball at one end, of an inch and a quarter in diameter. Having filled the ball and part of the tube with mercury, and brought it to the heat of SO'' of Fahrenheit's thermo- meter, he marked the place where the mercury stood, and then raised the mercury by heat to the top of the tube, and there sealed the tube hermetically; then upon reducing the mercury to the same degi'ee of heat as before, it stood in the tube rh of an inch higher than the mark. The same experiment was re- peated with water exhausted of air instead of mercuiy , and the water stood in the tube tVo above the mark. Now, since the weight of the atmosphere on the out- side of the ball, without any counterbalance from within, will compress the ball, and equally raise both the mercury and water; it appears that the w^ater ex- pands ToV of an. inch more than the mercury, by re- moving the weight of the atmosphere. From this, and other experiments, he infers, that water is not only compressible but elastic, and that it is more capable of compressibility in winter than in summer. All fluids gravitate or weigh in proportion to their quantity of matter^ not only in the open air, or in vacuo ^ but in their own elements. Although this law seems so consonant to reason, it has been supposed by ancient naturalists, who were ignorant of the equal and general pressure of all fluids, that the component parts or the particles of the same element did not gravitate or rest on each other. So 9G Hydrostatic Principles. that the \veight of a vessel of water balanced in aii\ would be entirely lost when the fluid was weighed in its own element. The following experiment seems perfectly to decide this question. Take a common bottle, corked close, with some shot in the inside to make it sink, and fasten it to the end of a scale beam ; then immerse the bottle in water, and balance the weight in the opposite scale; after- wards open the neck of the bottle and let it fill Math water, which will cause it to sink; then weigh the bottle again. Now it will be found that the weight of the water which is contained in the bottle is equal to the difference of the w^eights in the scale, when it is balanced in air; which sufliciently shows that the weight of the water is the same in both situations. As the particles of fluids possess weight as a com- mon property of bodies, it seems reasonable that they should possess the consequent power of gravitation which belongs to bodies in general. Therefore, sup- posing that the particles which compose fluids are equal, their gravitation must likewise be equal; so that, in the descent of fluids, when the particles are stopped and supported, the gravitation being equal, one particle will not have more propensity than ano- ther to change its situation, and after the impelling force has subsided, the particles will remain at abso- lute rest. From the gravity ofjluids arises their pressure^ which is always proportional to the gravity. For if the particles of fluids have equal magnitude and weight, the gravity or pressure must be propor- tional to the depth, and equal in every horizontal line of fluid ; consequently the pressure on the bottom of vessels is equal in every part. The pressure of fluids upwards is equal to the pres- sure downwards at any given depth. Hydrostatic Principle.9. 97 I 4 8 For, suppose a column of water to consist of any given number of particles acting upon each other in a perpendicular direction, the first particle acts upon the second with its own weight only; and, as the second is stationary, or fixed by the surrounding particles, according to the third law of motion, that action and re- action are equal, it is evident that the action or gravity in the first is repelled in an equal degree by the reaction of the second; and in like manner the second acts on the third, with its own gravity added to that of the first, but still the reaction increases in an equivalent degree, and so on throughout the whole depth of the fluid. The particles of a fluid at the same depths press each other equally in all directions. This appears to rise out of the very nature of fluids, for as the particles give way to every impressive force, if the pressure amongst themselves should be unequal the fluid could never be at rest, which is contrary t experience ; therefore we conclude, that the particles press each other equally, which keeps them in their own. places. This principle applies to the whole of a fluid as well as a part. For if four or five glass tubes of different forms, be immersed in water, when the corks in the ends are taken out, the water will flow through the va- rious windings of the different tubes, and rise in all of them to the same height as it stands in the straight tube. Therefore the drops of fluids must be equally pressed in all directions during their ascent through the various angles of the tube, otherwise the fluid could not rise to the same height in them all. From the mutual pressure and equal action of the particles of fluids^ the surface will be perfectly smooth and parallel to the horizon. JJJ 98 Hydrostatic Principles. If from any exterior cause the surflice of water has some parts higher than the rest, these will sink down by the natural force of their own gravitation, and dif- fuse themselves into an even surfoce. Since fluids press equally every ruay^ the pressure of each particle against the sides of a vessels ivill be pro- portional to the depth of the particle from the surface of the fluid. For considering the first particle in the line c b to have no other weight upon it, there is no other pressure on the side of the square than that which arises from the particle itself; but the pressure of the second par- ticle is increased by the weight of the first, and the third by the weisfht of the first and second; DlaMioi^kogisaSlB thus the pressure keeps augment- ing arithmetically to the end of the series. Therefore, considering the pressure on the side as a series in arithmetical progression, be- ginning with (0), it is equal to half the pressure on the bottom: for as every particle on the bottom sustains 11 others, its pressure will be 11x12=132; but the sum of the series for the pressure on the side of the square is 66, which is equal to half 132. So that the pressure against the side is in proportion to the depth from the *surface, and the whole pressure on the base of the square is equal to the sum of the pressures on both sides. If the figure be made to represent a cubical vessel, the sum of the pressures against the four sides would be twice the pressure^on the bottom, consequently the whole force of the fluid would be three times the force of its gravity. Thus we perceive the difference between fluids and solids; the latter act solely by their gravity; but fluids are not only governed by gravity but by pressure Hydrostatic Principles. 99 likewise. Solids act in the perpendicular line of gra- vitation : fluids press equally in every direction. The force of solids is in proportion to the quantity of mat- ter; but the force of fluids is in proportion to the quantity and altitude. The pressure which the bottom of a vessel sustains from the fluid contained in it^ under every form of the vessel^ is equal to the weight of a column of the fluid] the base of which is equal to the area of the bottom, and the height the same with the perpendicular height of the fluid. From what has already been said, we find that the pressure amongst the particles of a fluid at the same depth is every way equal. Then, if the base and height of the ves- sel a d, (Fig. 2.) be equal to the base and height of a f, (Fig 1st.); the pressure on the base of the second is equal to the pressure on the base of the first, although the capacity or quantity contained in the first is considerably greater than that which is contained in the se- cond. Make i k l m, Fig. 1, equal to EGFH, Fig. 2; then the pressure on the bases l m and f h is evi- dently equal, and if i m be parallel to c D, there will be the same pressure upwards on every part of the line i m; and the pressure at i, where the column of water touches the side of the vessel, is equal to the pressure of any other column in the line i m ; therefore a column extended from i to l would be equal to ne, and the pressure on the side of the vessel at i, from the motion of fluids in eveiy direction, has a reaction equal to the weight of the column N E , which makes the pressure of i k on the base equal the whole column e f. This principle will KK H 100 Hydrostatic Principles. apply to any other lateral point in c e ; so that the whole, taken together, makes the pressure on the bottom c D equal to the pressure of the fluid on the bottom of the cylindrical vessel e f. It maybe shown that the lateral reaction at i is equal to the weight of the column n e , by inserting a glass tube in the side of the vessel, for the pressure upwards at i will fill up the tube to the surface of the vessel. From this it appears that the pressure on the bot- tom of a vessel, of what form soever, is not according to the quantity of fluid contained, but according to the perpendicular height. If c D, a hogshead full of water, be placed on it's end, and the brass tube a b inserted in the top ; on filling the tube with water, the compressive force on the sides of the vessel, consequently the danger of burst- ing, would be as great as if the whole column were carried up to the top of the tube. As the bottom of vessels supports a pressure proportional to the height of the fluid, so the sides near the bottom have the greatest force acting against them, which decreases in horizontal lines to the surface. What is called the Hydrostatic Bellows, is a ma- chine well calculated to show that the pressure of fluids arises more from the altitude than from the quantity contained. specific Gravity, ^e. 101 This is formed of two thick boards A and B, about 18 inches long and 16 inches wide; these are joined with strong leather in the manner of bel- lows, and to the under plank, b, is flistened a small brass tube, c d, which communicates with the interior of the machine. When the experi- ment is made, a small quantity of water is first poured in to keep the top and bottom asunder; then if 300 weight be placed on a, and the ma- chine be filled with water, till it stands about three feet high in the tub cd; the pressure will raise up the weights to the extent of the leather that joins the upper and lower surfaces together. This extraordinary power may be greatly increased by a forcing piston fixed in the tube. A similar method has been lately adopted by an ingenious mechanic, in forming a powerful machine to compress hay, cloths, or light packages of any de- scription, which are to be stowed in the hold of a vessel. On the Specific Gravity and Density of Bodies. Dejiiiitions: The density of a body is the quantity of matter contained under a given bulk or magnitude, which is relative as the quantity of matter is to its magnitude: for the greater the number of particles which are contained in a given portion of space, the greater is the density of the body ; and the fewer the number contained in a like space, the less is the den- sity. The specific gravity of a body is its weight, com- pared with any other body of the same bulk or magni- tude. Thus the specific gravity of lead to water is as 10 to 1 ; that is, a cubic inch of lead is as heavy as ten cubical inches of water. M 102 Density and Specific The specific gravity of bodies is as their density. For as specific gravity is the weight ojf a given magni- tude, and as the weight of bodies is according to the given quantities of matter, the specific gravity is as the quantity of matter contained in a given magnitude, or as the density of that magnitude. The specific gravity of bodies is inversely as their bulk when their weights are equal. And as the specific gravity of bodies is as their density, the density of bodies is likewise inversely as their bulk when the weights are equal. The specific gravity of gold to lead being as 19 to 10: a bar of gold an inch square, and 10 inches in height, will possess the same weight as a bar of lead 19 inches high and of the same base. The magnitude or bulk of a body is expressed by a number, denoting its relation to some standard gene- rally used, as a cubical inch, foot, &c. The expression of the absolute gravity of a body is likewise relative, being determined by some arbitrary weight, as an ounce, pound; &c. Principles and Experiments demonstrating the Den- sity and specific Gravity of Bodies. When a body is immersed in a fluid, it loses just as much of its weight as is equal to the weight of an equal bulk of the fluid; but the weight lost by the body IS gained by the fluid, which will be increased in its weight by w^hat the body has lost. For, when a solid enters a fluid specifically lighter than itself, it displaces as many particles as are equal to its own magnitude, and these particles oppose its descent with a force equal to their pressure upwards, in a column of which the base is equal to the bulk of the solid : therefore the weight of the solid in the water must be diminished, by the weight of the pressure of the fluid: but in as much as the gravity of the solid exceeds the pressure of the column of fluid upwards, it Gravity of Bodies. 103 descends by the excess, losing a part of its weight equal to the repulsive force of the fluid. This principle may be more clearly understood by the following experiment. Let E be one end of a scale-beam, and b a bucket made to contain a quantity of fluid exactly equal to the magnitude of the cylinder a, which is fastened to the bucket and scale-beam. After ba- lancing the scale at the opposite end, immerse the cylinder a in the vessel of water c f; then the end with the weights will overbalance the oppo- site end with the cylinder; but when the bucket b is filled with a quantity of water equal to the mag- nitude of the cylinder a, the balance will become equipoised. This evi- dently shows that the cylinder loses as much of its own weight as is equal to the weight of its magnitude of the fluid ; .for on adding this weight, the inequality ceases, and the balance is re- stored. But the weight of the fluid is increased as the weight of the body is decreased ; for the action or pressure on the bottom of the vessel is augmented in proportion to the bulk of the solid; and as the gravitation of fluids is according to their heights, the power of the fluid is increased by the difference of the height, before and after the immersion of the body: for if the immersion of A raise the water from d to c, the accumulated weight on the bottom of the vessel will be equal to the area of d c, which is equal to the magnitude of the body. If any body, lighter than an equal bulk of the fluid, be placed on its surface, it will sink, or descend in it, _.;iF 104 Density and Specific till it has removed or displaced as much of the fluid as is equal to the weight of the body. When a solid which is specifically lighter than a fluid is placed on its surface, it sinks till the pressure upwards is equal to the pressure downwards; then the respective powers are in equilibrio, and the weight of the fluid displaced is equal to the whole weight of the solid. Let a globular piece of wood, the specific gra- vity of which is less than that of the water, be set afloat in a vessel of water ; then take the exact weight of the whole, and observe the point to which the water rises by the immersion of the wood. Now, if the wood be taken out, and the vessel filled up to this point, on weighing the vessel again it will be found to have the same weight as when the wood was immersed, which shows that the weight of the water which is displaced, is equal to the weight of the whole body ; therefore, the whole solid is to the part immersed as the specific gra- vity of the fluid is to that of the solid. All solids of equal magnitude, though of different specific gravities, lose an equal weight when they are immersed in the same fluid. For as the weight that all bodies lose in water, is according to the quantity of water displaced; the same bulk will displace the same quantity, consequently it will lose an equal weight. Thus a piece of brass loses as mu(*h weight in water as a piece of gold of the same magnitude, although the specific gravity of the gold is twice as much' as that of the brass. Bodies that have the same weight, but different specific gravities, lose unequal parts of their weights when they are placed in the same fluid. If a piece of gold and a piece of brass be balanced in opposite scales, and afterwards weighed in water, it will be found that the gold ovtrweighs the brass: for when their weights are equal their magnitudes are as their specific gravities; now as the specific gravity Gravity of Bodies, 105 of the gold is more than twice that of the brass, the bulk of the gold is much less than the bulk of the brass of the same weight, therefore the bulk of the gold displaces less water and loses less of its own weight. If these two bodies be first balanced in water, and then in air, the brass in this case will overweigh the gold; for each of them loses a part of their weight proportionate to their bulk ; and as the bulk of the brass is greater than that of the gold, it loses more weight in the water; but this difference is restored when the bodies are weighed in air, which causes the brass to preponderate. If a solid which is equal in weight to an equal bulk of fluid be immersed therein; it will remain indiffer- ently in any part of the fluid. For as bodies descend by their own gravity, if a column of water of equal gravity and power be opposed in the descent of the body, the forces will destroy each other or become equal, so that the body which is im- mersed will remain suspended in any part of the fluid. If a body which is heavier than an equal bulk of the fluid be immersed in it, it will descend by the excess of its gravity above that of the fluid. When a body is immersed it loses a portion of its weight from the resistance of the medium, and descends by the excess; that is, if the gravity of the descending body be equal to 3, and the resistance of the medium equal to 2, the excess or power of descent will be 1. This relative gravity of solids by which they sink or swim, is amusingly shown by the ascent and de- scent of glass images in a jar of water. The images are made nearly of the same specific gravity with the water, but rather lighter, with their weights a little varied, to make them take different situations in the vessel. As the bodies of the images 106 Density and Specific are hollow, they contain a quantity of air, and when they are immersed in the fluid A B, the air comrhunicates with the wa- ter, by means of a small hole in the heel of each image. The top of the vessel c is covered with a bladder, which includes a quantity of air in the upper part a d ; on pressing the bladder, the elasticity of the air presses on the water, and causes it to com- press the air in the bodies of the images, which suf- fers a small portion of water to enter the heel, and in- crease their gravities ; this causes the images to de- scend : and when the pressure of the air is relaxed on the surface, by taking up the hand, the air which is contained in the images forces the water out of their bodies and they rise in the vessel: thus by varying the pressure the images may be made to ascend and descend at pleasure. It is by the pressure of fluids upwards, that bodies which are speciflcally lighter than water rise in it. For if any solid body lighter than water be im- mersed to a certain depth e, the pressure upon the water underneath the body is equal to the body d e, added to the co- lumn of water a i), which extends from the top of the body to the surface of the B| fluid : but the ascending pressure of the column of water c e is equal to a column of water a e ; therefore inasmuch as the body is specifically lighter than the fluid, so is the pressure e c greater than e a , consequently this superiority of pressure forces the body upwards to the surface of the fluid. By the reverse of this principle, bodies specifically heavier than water sink to the bottom : for if the body be of greater specific gravity than the water, the pres- Gravity of Bodies, 107 sure of the column a e is greater than the power of resistance e c; therefore the body decends. Bodies which are Hghter than water will not rise in it, if the pressure of the water underneath the body can' be taken off. For example, if a smooth and even plate be fitted on the end of the wooden cylin- der A c, and placed exactly on another plate B, at the bottom, of a vessel of water: the cylinder and plate a c, though specifi- cally lighter than the water, will not as- cend; for the action or pressure of the fluid upwards being taken away, the body is kept down, not only by its own natural gravity but by the weight of a column of water of the same base w^ith the plate, from the top of the body to the surface of the fluid. Bodies specifically heavier than water may be made to swim on it when the water is kept from the upper surface, till the descent of the body meets with a co- lumn of fluid the pressure of which is equal to the specific gravity of the body. As the specific gravity of water to brass is as 1 to 9 ; if a plate of brass be fitted into an open glass cylinder a c, which is sus- pended in a vessel of water d e , so that no part of the fluid can get on the upper surface of the brass; when it is sunk nine times its own thickness, it will re- main floating on the surface of the water in the cylinder ; pressed upwards by a force equal to the weight of a column of water, the height of Avhich is nine times the thick- ness of the plate, so that it remains supported by a resistance equal to its pressure. 108 To Determine the Specific Gravity of Bodies. As specific gravity implies the relation of bodies to one another, some standard or given quantity must be adopted by which these relations may be determi- ned, and, for the sake of experiment, it is necessary that this standard should be fluid: for this reason, as well as for conveniency, water is used. It is likewise found that a cubical foot of distilled wa er weighs one thousand ounces avoirdupois, which may be taken as a thousand, or as unity, to show the comparative relation, or the specific gravity of bodies. To make this subject more clear, it may be neces- sary to repeat, what we have before stated, that "when any body is immersed in a fluid it loses just as much of its weight as is equal to the weight of an equal bulk of the fluid ; but the weight lost by the body is gained by the fluid, which will be increased in its weight by as much as the body has lost." According to this principle we shall have .three terms given to find a fourth; that is, the weight of a body in air, the diflference of its weight in air and in water, and the given specific gravity of the water, to find the comparative relation of the body. Suppose a piece of gold weighs 38 grains in air, but when it ip balanced in water it weighs only S&, then it loses two grains by immersion, which is equal to the weight of the water displaced by the gold. Now by proportion, as the weight of the dis])Iaced fluid is to the weight of the gold in air, so is the given number, to the specific gravity of the gold; or As 2 : 38 :: 1000 : 19000, or 1 to 19. That is, if the specific gravity of the water be 1000, or 1, the specific gravity of gold will be 19000 ot 19; or, in other terms, gold is nineteen times heavier than water. To Determine, ^c. 109 By this means, with the assistance of the hydrosta- tic balance, the specific gravity of bodies in general may be determined. The hydrostatic scales are of various construc- tions, according to the accuracy which is re- quired in the experi- ment ; but it will be suf- ficient for our purpose to describe those that are the least complex yet sufficiently accurate for common experiments. They are made neai'ly like common scales, but with much greater nicety, and the strings to the scale at one end of the beam are shortened, so as to admit the water cylinder and body underneath it. When the experiment is to be performed, the body is fastened by a horsehair to the hook under the shorter scale, and then balanced in air ; it is afterwards weighed in the water cylinder, and the difference of weight of the body in the two mediums, shows the difterence of the spe- cific gravities, which is equal in weight to a bulk of water of the same magnitude as the body immersed. The relative proportion is then found in figures, by stating the question as we have already shown. By an improved balance of this kind the different qualities of gold, or of any other metal, may be ascer- tained with considerable exactness; for, as all bodies weigh in proportion to the gravitating matter which they possess under the same bulk, and as the specific gravity of fine gold is greater than that of any other metal, except platina, it will possess a greater weight under the same magnitude. In determining the quality ©f gold bv the balance, it is necessary to fix on some N 110 To Determine the Specific Gravity ^ ^c, criterion for its puriti' ; suppose it to be the common standard gold. First find the specific gravity of this standard by the preceding method; then, by taking the specific gravity of any alloyed quantity of gold, the diffei'ence between them will show the quantity of alloy. Thus, suppose a new standard guinea weighs 129 grains in air, and when it is weighed in water it requires 7^ grains in the water scale to balance it, its weight being only 122 - grains in the denser medium; it is evident, from what has been said of bulk and gra- vity, that any inferior alloy would require a greater weight in the water scale to restore an equal balance. If a suspected guinea should weigh 129 grains in air, but on trying it in water it requires 8 t grains in the water scale to produce an equilibrium, it shows the gold to be inferior to the standard, which only took Vi grains: thus the difference of purity may be known in every kind of metal, by the difference of gravity from the standard of that metal. The specific gravity of those bodies which are lighter than water may be determined in the following manner. Weigh the substance in air, then fasten it to the bottom of the water scale with a stiff wire, and then take the weights out of the opposite scale, allowing for the weight of the wire; afterwards immerse the body in the water cylinder, adding weights to the water scale till the balance is equal; then add the weights of the two scales together, and say, as the sum of the weights is to its weight in air, so is the specific gravity of the water to that of the body. For as light bodies do not rise in water by reason of their own levity, but from the superior density of the body in w^hich they are placed, the difference of these gravities will be according to their difference of weight. Suppose a piece of wood weighs in common air 59.5 grains, and when it is fastened to the water scale and immersed it requires 16.7 grains in the water scale to Hydrometer, 111 balance it; then add 59.5 to 16.7= 76.2, and say, as 76.2 : 59.5 :: 1000 : 781, the specific gravity of the wood. To find the specific gravity of fluids is only to find their different degrees of density, which may be done by fastening a weight to the water scale^ and afterwards immersing it in the different fluids, noting the weight of the body in each, and the difference of the weights will be the comparative gravity of each. To find the specific gravity of a fluid in relation to water (suppose brandy). Let a solid be fastened to the water scale and weigh- ed in air; suppose the weight to be 1464 grains, but on weighing it in water it loses 445 grains, so that the balance weight for this fluid must be 1464 — 445 =«1019. Now place 1019 grains in the weight scale, and immerse the other end in the brandv, and the bodv will descend, requiring 38.2 grains at the opposite end to restore the equilibrium; then sav, as 445 : 38.2 :: 1000 : 86, which, taken from 1000, leaves 914 for the relative gravity of the spirit to the water ; so that an equal quantity of the brandy is about tV lighter than water. Hydrometer. This is an instrument principally used by brewers and distillers to determine the strength of their liquors. The neck a b is a piece of brass, or any o ^ other metal which is graduated, to show the diflferent depths to which the instrument de- scends in different gravities of fluids, b is a brass bulb to which the neck is fastened ; and c is a weight which is sometimes hung from the bottom to keep the instrument in an erect position when the bulb is immers- ed in the fluid; and at a is a small shouldc. to receive the weights which are laid on the instrument, to adjust it to any particular depth on the graduated neck. 1 12 Hydrometer. Now, as the resistance of fluids is according to their density, it is obvious that the instrument will sink deepest in those fluids that are the lightest, and this variation is shown by the scale or neck. When the instrument is immersed, the fluid whkh is displaced by it is equal in bulk to that part of the instrument which is covered by the water, and in weight to the whole instrument. Then, supposing its weight to be 4000 grains, the diflferent bulks of fluids containing the weight of 4000 grains may be compared, so that if a difference of iV of an inch take place in the neck by immersing it in two different fluids, it shows that the same weight of the liquors diflfers in bulk by the magnitude of iV of an inch of the stem of the instru- ment. The specific gravity of fluids may be found by putting an ounce, or any other weight, of distilled water into a glass phial, and marking the height; then empty the bottle and fill it up to the same height ex- actly with any other fluid, and weigh them both in a nice balance ; the difference of these weights will be the difference of their specific gravities, for their bulks are equal. U3 TABLE OF SPECIFIC GRAVITIES^ Supposing Rain Water 1000. Refined Gold . . . 19,640 Refined Silver . . . 11,091 Lead 10,130 Copper 9,000 Iron 7,645 Tin 7,550 Copper Ore . . . 3,775 Lead Ore .... 6,800 Adamant, or Diamond 3,400 Cornelian . . . . 2,568 Lapis Lazuli . . . 3,054 Lapis Calaminaris . 5,000 Common Glass . . 2,620 Chalk, 2,370 Common Sea Coal . 1,272 Ivory 1,826 Boxwood . . * . 1,030 Oak 925 Elm 600 Ash 734 Fir 546 Cork 240 Wheat 757 Oats 472 Dry Pease .... 807 Barley 658 Crude Mercury . . 13,593 Mercury distilled 1 14110 5 1 1 times 3 ' Alum 1,714 Nitre 1,900 Myrrh 1,250 Verdigris . . . . 1,714 Opium ..... 1,365 Bees Wax .... 960 Pitch 1,190 Honey 1,450 Resin 1,100 Human Blood . . . 1,126 Distilled Water . . 993 Spring Water ... 999 Sea Water .... 1,030 Aquafortis .... 1 ,300 Oil of Vitriol . . . 1,700 Oil of Turpentine . . 874 Rectified Spirit of Wine 840 Burgundy Wine . . 955 114 HYDRAULICS. Hydraulics treat of the motion of fluids, and their application in forming water engines of every description. Hydrostatics show the weight or pressure of fluids upon solids, or the particles of a fluid upon one ano- ther when they remain at rest; and hydraulics treat of the power of fluids when they are in motion, and therefore of the force and construction of engines, pumps, mills, fountains, and every otiier description of hydraulical machines. Although the motion of fluids is sufl^ciently known to make tjfie effects eminently useful, yet we are still ig- norant of the mass, figure, and number of the particles which are in motion. There is little doubt, if we were better acquainted with the elements of fluidity, so as to determine absolute laws for the motion of fluids, that the advantages which we already possess from their power might be greatly extended; but, situated as we are, the mathematician must be content to draw his deductions from hypothesis, and leave the natu- ralist to found his principles on experiment. The general effects of fluids are considered as pro- ceeding from the following causes, viz. their own na- tural gravity or pressure, the spring of compressed air, or the compression of bodies on their surface. The most natural motion of fluids arises from their own gravity, which always causes them to attain a horizontal position when the course is left oj^en ; for a fluid will rise to the same . height whether it passes j- j 9 -- P through the regularly curv- \ /I /^ I ed conduit A B, or through ^^^ %/ ^^ the various turnings of c d, The Siphon. 115 Ciii'^^iiii^^iw when the intermediate heights are below the level of its extremities. To supply a reservoir with water at b, from a, the source the spring ; may be con- ducted between the two places by pipes laid on the surface of the earth; and, notwithstanding the ob- struction of the intervening hill, it will flow into the cistern with a velocity equal to that which it would have attained if it had been conducted through the more direct course a d b ; for the velocity of fluids is uniformly as their height, which is here represented by the dotted line a c. This modern mode of conveying water by pipes is a great saving both in time and expenditure, when it is compared to those stupendous aqueducts Avhich were constructed by die Romans; for in that sera, either from their ignorance of the pressure of fluids, or from their love of magnificence, they conducted water across hills and vallies by straight-lined ducts,, which were supported by immense arches or columns. The Siphon, or Crane, Th e siphon is a bent pipe or tube, which is used for emptying vessels of fluids, and sometimes for con- veying water from one place to another, over hills or obstacles that are higher than the surface of the fluid. If a small bent tube e f, the legs of which are of equal length, be filled with water and turned do^vnwards, with the ends suspended horizontal- ly, th^ fluid will not run out; for the 116 The Siphon. gravitating power of the water is equal in each leg, and the upper pressure of the atmosphere is kept off by the form of the machine, which causes the opposing resistance of the air that presses on the surface of the water in the extremities of the pipe to prevent its descent. But as the weight of a column of the atmosphere is equal to a column of water about 34 feet high of the same base, and to a column of mercury 29 inches high in a medium state of the air; if the inverted legs of the siphon ex- ceed these measures for the respec- tive fluids, the gravitating weight will overcome the resistance of the at- mosphere, and the fluid will run out. If the legs are of unequal length like those in the siphon G H, and the shorter leg be immersed in a ves- sel of fluid, on sucking the longer end with the mouth to produce a vacuum, or by inverting the tube full of water, the fluid will run out of the vessel till it reaches the bottom of the shorter leg ; for the orifice h of the longer leg is exposed to the pressure of the atmo- sphere; and, as the fluid is supported in the shorter leg by the surrounding fluid in the vessel, it is likewise supported by the pressure of the atmosphere which acts on the fluid in the vessel. Now the atmospheri- cal pressures are equal; but these pressures are coun- teracted by unequal columns of fluid c i and i h ; therefore the shorter column g i is more pressed against h i at the vertex i, than the column i h is pressed against i g ; consequently the longer column must give way to the greater pressure, and the fluid will run out of the orifice h. The crane which is used bv brewers or distillers for Fountains. 117 emptying hogsheads, is sometimes made with a cock and small pipe at the end of the longer leg to suck the air out of the tube, and sometimes with a cock only; for when the cock is shut before the shorter end is immersed in the fluid, the air which is pent up in the crane prevents the fluid from rising to the same height as that which surrounds it; but on opening the cock to emit the air, the pressure of the exterior fluid gives such velocity to the interior in rising to the general surface, that it is carried beyond it ; and if the curved part of the siphon be not too high above the liquor in the vessel, the interior fluid will fall over it and con- tinue to flow. Natural and Artificial Fountains. These are formed either by the pressure of a supe- rior fluid, or the pressure of condensed air on the sur- face of water. According to the motion of fluids it has been already stated, that the gravitation of the upper parts presses upon the lower, till the whole comes to a state of rest in a horizontal plane. Thus the water which descends from a reservoir at A would acquire such velocity ^ from its gravity as would carry it ^S<^- — -^ up to its level at b, if the pipe or ]m \ tube were continued; but as the j 1 ^ pipe terminates at d, it will issue j 1 |||| at the adjutage or aperture with I % ^fl a velocity that would have car- C»-.--->^..--"^ ried it up to b, and equal to that ^^^ which it has acquired in falling from a to c ; so that the velocity of fountains at their adjutage is in pro- portion to the perpendicular height of their extremi- ties ; but the resistance of the air at the lower extre- O / 118 Natural and mity breaks the column of the fluid and destroys its force, \\'hich, joined to friction and other impedi- ments, prevents the fluid from reaching the height of its source. In spouting fluids, or when water issues from a hole in the bottom or sides of vessels, it is found that the velocity of the fluid is equal to that which a body acquires by falling perpendicularly through a space equal to the distance between the surface of the water and the aperture in the vessel. * When the height of the fluid is kept up by a constant supply, the velocity will be equal, whatever may be the density of the fluid; for if tlie pressure be increased by a denser fluid, the issuing quantity will be greater, as velocities are always equal when the moving forces are proportional to the masses which they put in motion. Therefore the quan- tity of fluid which issues through the same hole in the same time is in proportion to the celerity of its motion; and as the velocity of bodies in falling through a given space is according to the squares of the distance fallen through, the velocity of issuing fluids must be accord- ing to the square root of their height or pressure. From the equal pressure of fluids in every direction, the issuing velocity will always be the same at the same depth, whether it proceed from a hole sideways, downwards, or upwards. The greatest distance to which water spouts from different holes in the side of a vessel, is from that hole which is placed exactly in the middle, between the top and bottom of the fluid ; and at the first and third quarters the projected distances will nearly be equal. Artificial fountains are formed by the compression of air on the s rface of a fluid. * Owing to the resistance of the iiir, and other catises of obstruction, the velocity of the spouting fluid will always be less than that assigned above, from theory. Ed. Artificial Foimtains, 119 If the vessel b be partly filled with water, and a. the upper part of the vessel, be filled with compressed air by means of an injecting syringe, the pressure of the air on the • surface of the fluid will force it up the pipe, and out of the adjutage, with a force proportional to the power of compression. , But as this subject has already been explained in Pneumatics, w^e will only describe a machine which acts by the compression of air for raising liquors from the cellar to the bar of taverns, &c. a is call- ed the receiving vessel, which is made perfectly air tight, and sunk about half its depth in the floor of the cellar; the leather hose D, is occasionally used to empty butts of liquor e into the receiv- ing barrel, through which it runs by its own natural gravity. After the re- ceiver is filled to a proper height the communication is stopped between the vessels, and the air is injected into the upper part of the receiver by means of the forcing piston and pipe b, which is placed near the bar. This compressive power on the surface compels the liquor to ascend through c c, which is a leaden pipe with a cock that passes from the bottom of the receiver to some convenient place where the liquor is to be drawn : When the velocity of the fluid decreases at the cock, it may be instantly renewed by three or four strokes with the handle of the piston. The most common machine which is now used for raising beer, is constructed like the following pump. 120 The Common Lifting Pump. This useful and domestic machine was invented about one hundred and twenty years before the birth of Christ; but it has been greatly improved, even since the time of Galileo, when the pressure of the atmo- sphere became more perfectly known. This pump is formed of a long cylinder of wood or lead, one end of which stands in the water at the bottom of the well. It con- tains two valves, or hollow pieces of wood, which fit close to the cylinder, with lids opening up- wards; the lower valve c remains fixed, but the upper valve b is fastened to the piston rod, and moves up and down by the action of the handle or lever. The mode of operation. This description supposes that the wa- ter in the cylinder of the pump stands no higher than the water in the well, and that the remainder of the cylinder is empty, or rather occupied by air. Now, when the handle of the pump is raised up, the piston b sinks towards c, which condenses the air between B and c, till its resistance forces open the valve or lid; then the air escapes into the upper and open part of the cylinder. As the piston rises, the air which is contained between b and c becomes rarefied, and the elasticity of that portion of air which is contained in the cyliijder, between the lower valve c and the surface of the water in the well, forces open the lower lid, and a part of it escapes into the rarefied space be- tween B and c^ which has been formed by the rising Common Lifting Pump, 121 of the piston. Thus, by a few strokes of the handle, if the wood or metal of the cylmder be sufficiently close to exclude the air, and the piston and valves be well fitted to the sides of the pipe, the compressive power of the atmosphere will be removed from the surface of that part of the fluid which is contained within the cylinder, and the atmospherical pressure on the general surface of the well will force it up the barrel to any height less than 33 or 34 feet. Then, supposing the lower valve to be placed at a less distance than S^ feet from the surface, the ascending water will force it open and get admitted into the cylin- der between c and b . . When the piston descends, the weight of the water upon the lower valve closes it, and the fluid, is forced through the upper by the sinking of the piston ; so that, when the handle is returned, the water, which now rests on the upper lid, is carried to- wards the top of the cylinder, and flows out of the spout E ; and the supply from the well, by the com- pression of the atmosphere upon its surface, forces through the valve c into the cylinder, as the upper piston raises the water by the power of the handle. After the pump has been worked, if the barrel and pistons be good, the water will stand in the cylinder close to the spout, and ready to flow on the first stroke of the handle. As it is the pressure of the atmosphere alone that forces the water up the barrel of the pump, when the lower valve is more than 33 or 34 feet from the sur- face of the water in the well, the pressure of the air cannot raise it to the valve, consequently the machine would be useless ; but this is prevented by sinking the lower piston in the cylinder till it be actually with- in the height of the pressure, and by lengthening the piston rod of the upper in proportion to the depth of the lower; this gives an additional weight of fluid to be lifted each stroke, and the power must be propor^ 122 Common Lifting Pump, ^ tionate at the handle. But conveniency requires that this operation should be performed by one person; therefore to lessen the weight of the column of water which extends from the upper piston-lid to the mouth of the pump, the diameter of the cylinder must be de- creased and made proportional to the depth of the well, so that the power may be equal to the operation ; but the quantity of water which is raised in an equal time will be less than when the diameter of the cylinder is greater. The following table shows the diameters of the bar- rels, and the quantity which is discharged in a minute at different depths, by the power of a man of ordinary strength; supposing him capable of discharging 272 gallons of water in a minute, by a pump 30 feet high and four inches in diameter; admitting that the power of the man was increased five times by the length of the lever. 123 Heights of the Pump above the Surface of the WeU. 1 Diameter of the 1 Bore where the Piston works. Qiiantity of Wa- ter discharg- ed in a Mi- nute. Feet. 10 Inches. 6.93 Galls. Pts. 81 6 15 5.66 54 4 20 4.90 40 7 25 4.38 32 6 30 4. 27 4 35 3. 70 23 3 40 3.46 20 3 45 3.27 18 1 50 3. 10 16 3 55 2.95 14 7 60 2.84 13 5 65 2.72 12 4 70 2.62 11 5 75 2.53 10 7 80 2.45 10 2 85 2.38 9 5 90 2.31 9 1 95 2.25 8 5 100 2. 19 8 1 124 The Forcing Pump. The forcing pump is not only used to raise water from the well to the surface of the earth, but likewise to force it into reservoirs on the tops of buildings, from which pipes are laid to convey it to different parts as conveniency requires. This macliine differs from the common pump by having a pipe c joined to the barrel, through which the water passes into the air vessel d f ; and by the com- pression of the air which is contained in the upper part h d, the fluid is forced up a pipe, fixed on g, to a considerable height. The operation. By moving the handle the air is exhausted out of the barrel i b, and forced into the air vessel, and the water, follows up the cylinder by atmospherical pressure, in the same manner as in the com- mon pump. But as the piston a is solid, and the closing of the lower valve prevents the water from return- ing into the well, it is forced through the pipe and valve c f into the air vessel D F, and the valve f closes again by the pressure which rests upon it, whilst the piston ascends to admit a fresh supply into the upper part of the cylinder. The upper part d of the air ves- sel is made perfectly air tight, and as the water rises in it, it condenses the air by pressing it upwards ; now the air, by its elasticity, reacts on the surface of the fluid in proportion to its density, and forces it up the pipe h g Avith a velocity proportional to the de- De la Hirers Pump. 125 grce of compression upon the surface; and as the elasticity of the air makes the pressure perpetual, the pipe produces a continued stream during the rising and failing of the piston. To gain force, and a perpetual discharge, the air vessel is now used in the construction of engines for extinguishing lire. The following is the construction of a pump for raising water both by the ascent and descent of the same piston. De la Hire's Pump. Th e barrel and pipe the common forcing pump, and likewise the conveyance pipe f, which carries the water into the air vessel ; there is also another convey- ance pipe E, which con- ducts the water into the air vessel that rises in the cylinder d g c, and the piston a k. works in a collar of leather at a, which totally excludes the air from the upper part of the cylinder. The operation. When the piston k "^descends it shuts the valve /r, and forces the water up the pipe F into the receiver L, the valvey of which immediately closes by the pressure of the fluid. A B are the same a3 those of 126^ Hair Rope Pump* Then, if the air be exhausted out of the adjoining barrel d c c, which communicates with the upper part of the cylinder a k, by the action of the piston, and if the height d c should not exceed 33 feet, it is evident the water will rise up the barrel and fall through the valve g into the cylinder a k as the pis- ton descends ; but as the piston ascends, the pressure of the water above it shuts the valve gy and forces open another at e in the air vessel, by which it enters the receiver; then the return of the stroke which forces the water up the pipe f closes the valve e and opens g again to admit the water into the cylinder. Thus the ascending and descending strokes of the piston force a continual supply of water into the air vessel, whence it is discharged through a conducting pipe by the elasticity and compression of the air, as in the preceding machine. Hair Rope Pump, The three hair ropes f pass in grooves over two pullies a b, and the lines are kept extended by a weight which is fastened to the lower pulley b; at c is a wheel and han- dle, over which the line passes that joins them to a small multiplying w^heel fastened to the well beam, and this acts on the uppermost pul- ley. When the machine is put in motion, as the hair ropes pass through the water in the well it sinks into their interstices, and by the quickness of their motion it is carried up the ascending ropes in considerable quan- tities, till it reaches the upper pulley, when it ialls into Archimedes^ Screw, 127 the reservoir e . This method, simple as it may appear, is now used to raise water from a well 90 feet deep, and by tolerable exertion it is capable of drawing up about 9 gallons a minute. Archimedes^ Screw for raising Water. This machine, which is now seldom used, is of very ancient date, and is formed by a long cylin- der, with a spiral tube from the bottom to the top, through which the water rises till it flows out of the pipe at its upper extremity as it passes the under side of the cylinder. The principle is as fbl- ; lows. When the machine is placed in an oblique di- rection in the water, the fluid enters at a, the mouth of the spiral, and by the surrounding pressure rises to c. When it has attained this point, it cannot after- wards occupy any other part of the spiral than that which is on the under side ; for it cannot move from c towards d, because it is situated higher above the horizon; and as this will always be the same in every similar part, it is evident that when the machine is in motion, the water, as it is raised by the spiral, will always remain on the uiider side till it flows out of Ae spouf. ^^V, 128 Steam Engines* The great improvement, as well as the complexity of the smaller parts of these powerful machines, would make a long description of the whole obscure and uninteresting ; it will therefore be sufficient, for the present purpose, to show the principle of their operation in the most simple state of their construc- tion. The beam a is placed between two large standards, and turns on its axis b, with a piston and rod fastened to each end, which work in the cylinders d k and g h ; the vessel c is partly filled with water, which is kept boiling by a fire underneath it, and this fills the upper part of the boiler with a very powerful elastic steam or vapour ; by turning the cock d the vapour passes through the neck of the vessel and presses against the bottom of the solid piston r, which forces it up to the top of the cylinder. Then the compression which raises the piston f compels the piston g, at the Steam Engines* 12& opposite end of the beam, to descend into the cylin- dei' G H, and work the pump, which is of the same construction as the forcing engine that we have al- ready described. The cock d is shut when the piston G is to be raised up to resume its stroke, and the steam in the cyhnder k d is instantly con- densed by letting in a small quantity of water from the reservoir through the pipe l, which, by destroy- ing the repulsive force of the vapour, suffers the pis- ton F to descend in the cylinder, by a superior gravi- tation which is given to that end of the beam in its construction. When the boilers are so large that the quantity of steam is sufficient to make 20 or 25 strokes in a mi- nute, and each of them 7 or 8 feet high in cylinders 9 inches in diameter, the engine will discharge about 320 hogsheads in an hour. In the improved construction of steam engines, the operation of turning the cocks is performed by the machine itself, which not only saves the attention of one or two persons, but likewise performs the duty with much more regularity, and causes less danger from the dreadful effects of the vapour when it is not properly discharged. To prevent the bursting of the boiler by any extraordinary expansion of steam, a valve or regulator is made in the side of the vessel, which is forced open by the vapour, to evacuate it when it has acquired a certain force in the boiler. The steam which is raised by the ordinary heat of boiling water is about 3000 times as rare as water, and 31 times as rare as air; and the expansive power of steam against the sides of a globe of copper four inches in diameter, when the water is boiling in it, has been calculated at 3825016s, weight. Great improvements have been made in steam en- gines^ by Messrs. Boulton and Watt, of Soho, near Birmingham. One of these powerful machines, which \ 130 Steam Engines. was constructed by them, now works a pump 18 inches in diameter, and 600 feet high; the piston makes 10 or 12 strokes, of seven feet long, in a minute, and raises a weight equal to SOOQQlbs. fifty feet high in the same time, which is performed with a fifth part of the coal that is usually consumed by a common engine. The present improvements in steam engines fit them for a variety of purposes where great power is requir- ed; such as raising water from mines, blowing large bellows to fuse ore, supplying towns with water, grin- ding corn, &c. Mr. Boulton has lately constructed an apparatus for coining, which moves by an improved steam engine. The machinery is so ingeniously con- structed, that four boys of ten or twelve years of age are capable of striking 30000 guineas in an hour, and the machine itself keeps an accurate account of the number which is struck. 131 ELECTRICITY. Electricity is that power or property which lsome bodies possess of attracting Hght substances when they are excited by friction. Amber, sealing wax, resin, glass, and the tourmalin (which is a red-coloured transparent fossil found in the island of Ceylon), are of this description. The attractive power of amber was known some centuries before the Christian sera, but it was then considered as a mere quality which was attached to that peculiar body. But electricity is now supposed to be a primary agent of nature, which is diffused throughout the w^hole atmosphere, and enters into the minutest pores of bodies in general. Thus, the electrical fluid, which had escaped investigation for many ages, is now become a principal object of science. About the year 1745, we find this subject diffusing widely under the splendid talents of Watson, Canton, and Priestley, in London; Franklin, in America; and the Abbe Nollet, in France. In the hands of these philosophers electricity has made more progress in a few years than it had gained in all the prececling ages. It was at this time that the mode of accumulating electrical fluid on the surface of glass was carried to its greatest height, by means of what is called the Leyden Phial, from the birth-place of the inventor, who was a native of Leyden; but the greatest disco- very that vtas ever made in electricity was reserved for Dr. Franklin, in America. It had been imagined that a similarity existed be- tween lightning and the electrical fluid; but Franklin brought this supposition to the test, and proved the truth of it by the simple m.eans of a boy's kite covered with a silk handkerchief instead of paper, and some 132 Electricity, wire fastened in the upper part, which served to collect and conduct the fluid. When he had raised this ma- chine into the atmosphere, he drew electrical fluid from the passing clouds, which descended through the flaxen string of the kite as a conductor, and Was afterwards drawn from an iron key which he tied to the line at a small distance from his hand. B}' this simple means he proved, that the fluid which pro- duces lightning was exactly the same as that which he obtained from his electrical machine. This important experiment immediately led to the formation of con- ductors to secure buildings, ships, &c. from the dread- ful effects of lightning. No subject in nature has given rise to a greater va- riet}^ of opinions than the theory of electricity ; even in the present day the ideas of philosophers are much divided with respect to the mode of its action; but this must remain undetermined, till we are better acquainted with its constituent principles; the dawn seems to arise, and a succeeding age may discover, that what was once considered as the individual quality of a bimple substance, is the first agent of nature; that it isdiftused from the sun, as its infinite source, through- out the immensity of our system, and that it not only produces light and fire, which give life and energy to matter, but that its reacting power checks the absorb- ing attraction of the sun, and gives motion and bounds to the revolving planets. The present opinions on electricity are principally divided into two parts; one relating to what is called vitreous and resinous, and the other to positive and negative electricity. The former of these opinions was first laid down by M. du Fay, and was afterwards new-modelled by Symer; but it is now generally re- jected. This theory supposes that electrical matter is formed by two distinct fluids, which are repulsive with respect to themselves, but attractive to one another, Electricity. 133 and the electricities are called vitreous and resinous, from their respective affinities to glass and resin. It is further supposed, that these fluids are attracted by all bodies, and exist in intimate union in their pores, with- out any exterior mark of existence, until the two fluids be brought into action by a separation of their parts, which is produced by friction. When these electrici-^ ties are collected and separated by the attrition of the rubber on the surface of the cylinder of an electrical machine, the vitreous passes over to the prime con- ductor, whilst the resinous is drawn to the rubber. In this state of separation they exert their respective quali- ties, so that by electrifying light bodies with each kind of fluid, those that possess the vitreous will repel each other, as well as those that are mutually electrified with the resinous; but if two bodies which are oppo^ sitely electrified be brought near together, they will attract each other, and give and receive at the same moment an equal portion of their respective electrici- ties. According to this theory the electric spark has a double current, and the electrified body will receive from any conductor in the electrical atmosphere an equivalent of the opposite fluid to that which it gives; so that if the finger be presented to the prime conductor of the machine, whilst the body inhales the vitreous stream from the conductor, it gives an equal stream of the resinous from the body; these quantities are so exactly alike, that a light body may be suspended by the opposing forces between the end of the finger and the conductor. The preceding subject embraces the immediate outline of the double current, or what is called vitre- ous and resinous electricity. The other theory, of positive and negative electricity, was first taken up by Watson, but was afterwards illustrated, digested, and confirmed by Franklin, and from thence it is called the Franklinian Theory. It supposes that the Q ■ 134 Electricity, whole phenomena of electricity depend on a subtile and elastic fluid, entirely of the same kind, repellent amongst its own particles, but attracted by all bodies, and universally disseminated throughout their pores. When bodies hold their own natural quantity undis- turbed, they are said to be in a nonelectrified state; but when the natural quantity of fluid in a bod} is dis- turbed, either by adding more to that which it natu- rally possesses, or by taking away a part of its natu- ral quantity, it has an electrical appearance, or it is in an acting state. When a body possesses more fluid than its natural quantity, it is called plus, or posi- tive; and when it contains less than its natural quan- tity, it is called minus, or negative. The progress of this fluid depends on the nature of the bodies through which it passes; those which give it the greatest facility in its course are ciilled con- ductors, and the fluid is instantaneously transmitted through them even to the greatest distances. Those bodies the pores of which will not admit the transmis- sion of electrical fluid, are called electrics, and are impermeable, so that there cannot be an accumulation on one side without a deficiency on the other; and when the two sides are joined together by a proper conductor, the superior quantity, or the positive, rushes through it to the inferior quantity, or negative, till the fluid on both sides of the body be in equilibrio, or in its natural state. When an electric is rubbed by a conductor, as the friction of the rubber upon the cylin- der of an electrical machine, the fluid is carried from one to the other, and the rubber will be electrified nega- tively ; but as an insulated cushion only affords a small portion of electric fluid, a conducting chain is con- nected with it, which gives a constant supply from the negative to the positive side. Thus it is conceived that bodies differently electrified will readily approach, Electricity, 135 but that those which are equally charged have an equal repellency. Having thus stated the principal features of the two prevailing theories, it would be unnecessary to follow them both; we have therefore preferred the latter, not only from its more general acceptation, but as it likewise possesses a simplicity of principle which ap- pears consonant to the general operations of nature. The compound quality of the fluid, double currents, and opposing action of the first theory, stand unsup- ported by any other phenomena of similar principles in the operations of nature; but the latter has a strong coincidence with the system of elastic fiuidsln general. Before we enter into the experimental proofs of positive and negative electricity, it will be necessary to introduce some preparatory knowledge on this subject. First, of the existence of the electrical fluid. If a glass tube, about an inch and a half in diameter and three feet in length, be rubbed briskly with a piece of leather in a darkened room, small divergent flames will fly off* with a crackling noise, and sometimes a spark of fire six or eight inches long may be seen following the hand upon the surface of the tube. If a brass ball be suspended from the tube by any conduc- ting body, such as a piece of wire, or thread, the electric fluid will descend through the conductor and electrify the ball, which will give a spark to the knuckle, or electrify any light body that is presented to it. When the ball is suspended from the tube by a silken string instead of Wire or thread, the excitation of the glass will produce no sign of electricity in the ball; and if the down of a feather, or any other light body, be presented to it, it will remain unmoved; for, as silk is not a conductor, the fluid cannot pass from the glass to the ball. Thus, with respect to electricity, all bodies are divided into two kinds, called conductors and non- 136 Electricity, conductors ; though, in point of fact, no body in nature can be considered purely in either point of view, so as completely to transmit or retain the electrical fluid. The general class of conductors comprehends metals, semimetals, ores, and fluids, in their natural state, ex- cept air and oils. Green wood is likewise a conductor, but when it is baked it becomes a nonconductor. Many electrics or nonconductors, such as glass, resin, air, &c. become conductors by being heated. A conductor cannot be electrified if it have any com- munication with the earth by means of immediate conductors, for then the excited fluid will pass through the conductmg bodies and be dispersed. Insulated conductors are conducting bodies sup- ported or surrounded by an electric or nonconductor, so that tlie communication with the earth is cut oflT. A brass ball and thread suspended from a glass tube is an insulated conductor; for on excitation the fluid passes from the nonconducting tube through the thread to the ball, where it is retained by the surrounding atmosphere as an electric ; or, if the ball be suspended by a silken string, and an excited tube be brought to the ball and afterwards taken away, the electrical fluid which is communicated will remain insulated by the air and the nonconducting body of the silk. The greatest quantity of electricity is obtained by the friction of a conducting body upon the surface of an electric. If the rubber be afterwards insulated, the nonconducting surface will remain charged with the electric fluid, and communicate electric sparks to any conducting body that is presented to it. If a conducting body be insulated and electrified, the whole of the fluid which is collected will be car- ried off* by a single spark drawn by a conducting body; for, as the fluid passes with the greatest facility through all parts of a conductor, the whole flies off* at the instant of communication; but nonconductors that are charged, Electrical Machine. 137 only part with that share of their fluid which lies on the surface next to the conductor. A mutual attraction exists between electrified and nonelectrified bodies; for, if a light substance be placed near an electrified body, it will fly towards it till it have obtained the same intensity of fluid, then it will be repelled and attracted by any nonelectrified body that is near it. If a nonelectrified body be set at a proper distance from an electrified body, and a feather be placed between them, the feather will be alternately attracted and repelled by each ; for when it is electrified it flies to the nonelectrified body, and delivers its electricity, it is then attracted and charged again by the other; and thus it will continue its course, backwards and forwards, till it have reduced the surplus of fluid in that which is electrified. Electrical Machine. As the excitation which is produced by the hand with a rubber on a tube or plate of glass, is not only very laborious, but inadequate to the production of any material quantity of electrical fluid, machines have been constructed of various forms for this purpose ; some with spherical glass electrics, some with cylin- ders, and others by the revolution of a circular plate of glass between cushions or rubbers placed near the edge ; but as the cylindrical machine is the most common, and perhaps the most useful, we have given a description of it previous to any further investiga- tion of electrical fluids. In the plate, a represents a glass cylinder called an electric, or nonconducting body, the axis of which is supported by the two sides m m, and these are fixed into the plank k, which is the basis of the machine ; 138 Electrical Machine, c is a common winch or handle by which the cylin- der is turned, and d is the cushion or rubber, which is supported and insulated by the glass pillar f. The lower end of f pass- es into a socket that is acted upon by the screw s, for the pur- pose of increasing or diminishing the pressure of the cu- shion on the surface of the cylinder, b is a piece of black silk which prevents the electrical fluid from flying off*, and reach- es nearly to the re- ceiving points fixed in the conductor e e ; for the closer the silk adheres to the cylinder, the greater will be the degree of excitation, e e is a metalhc body, which is called the prime conductor; this is made in various forms, therefore the present T form is not essen- tially necessary; the receiving points are fastened to the side opposite the cylinder, and the whole is supported from the frame by the insulating pillar of glass g, so that the electrical fluid which is collected on the prime conductor cannot disperse, but remains accumulated for the purpose of experiments. A small quadrant electrometer fixes into a small hole in the conductor, and shows the increase or decrease of the electrical fluid by the rising or falling of the index upon the td^Q^ of the quadrant. The chain l has one end fas- tened to the cushion, and the other lies upon the floor or table, to serve as a conductor for the electrical fluid in passing from the earth to supply the machine ; when this chain is taken oflT, or unconnected with the Electrical Machine. 139' earth, the machine becomes insulated, and it will re- tain the electricity that it has acquired during its ope- ration. Before the electrical machine is excited by turning the handle, it must be carefully wiped, or gently rub- bed, with an old silk handkerchief, to free it from dust, or any loose filaments which may have attached them- selves to it, and likewise to take away any damp that it may have acquired by standing in the room; all of which, particularly the latter, weaken the excitation, by serving as conductors to carry off the electrical fluid. In damp weather it will be of considerable advantage to the power of the machine, to place it in the gentle warmth of the fire for some little time before it is used. When the machine is perfectly dry and free from dust, grease the cylinder by turning it against a piece of greasy leather until the glass be uniformly obscured; then continue the motion till the silk have wiped oflf part of the tallow, and made the cylinder semitransparent. Now take an amalgam of zinc and mercury combined with a little tallow, which may be bought ready pre- pared, and spread a little of this composition smooth and even on a piece of leather; then apply it to the sur- face of the cylinder, and turn the handle till the friction become tolerably strong, which v/ill give it a great degree of excitation, and prepare it for the general purpose of experiment. A greater degree of excitation may be produced by rubbing the amalgamated leather against a clean cylinder and silk, which will make the machine act powerfully for the moment, but this soon passes ofl^" when the former mode is properly pursued, the machine will retain its effect for some days. 140 Positive and Positive and Negative Electricity. When the machine is put in motion, the electrical fluid which is collecting will produce a crackling noise, and in a darkened room the flame will be seen spread on the surface of the cylinder, but the course of the fluid cannot be actually determined. The advocates for positive and negative electricity consider it as an elastic fluid capable of condensation and rarefaction, and that it is drawn from the eartli through the chain which is attached to the rubber, af- terwards collected on the cylinder by the friction of the cushion against it, and thence taken up by the points and carried to the prime conductor, which, by being insulated, receives more than its natural quan- tity, and the fluid condenses as it augments. When the chain is taken off" and the cushion insulated, a very small quantity of electricity is produced from the fric- tion of the rubber and cylinder; this tends to show- that the fluid is received by the rubber from the earth ; therefore, when the rubber is insulated as well as the prime conductor, the quantity of fluid which is pos- sessed by the former being less than that which is ac- cumulated by the latter, it is called minus, or nega- tive; and the fluid which is collected by the conduc- tor is plus, or positive. Whilst this inequality exists in bodies which are brought within each other's elec- trical atmosphere, the superior power of the positive will attract the inferior resistance of the negative, till the force of the electricity become equal ; but when bodies, alike positive or alike negative, are opposed to each other, they resist with an equal power and clasticit3^ The preceding supposition derives considerable support from the following experiment. If the con- ductor (^(see the figure of the machine), with a small point at o, be placed on a brass rod connected with Negative Electricity, 141 the cushion, and another with the brass point p, be supported by the prime conductor, those bodies which are electrified by (^ will not only be attracted by the conductor r , but the electrical fluid will diverge in a conkal form from the point p, as emitting its electri- city, whilst a small, faint, globular fiame will be seen on the point o, as if it \vere imbibing the electric stream. But even this state of positive and negative electri- city is governed by the quality of the cylinder and rubber; for, if a glass tube be made rough by grind- ing the surface with emery, and excited by soft flan- nel, the electricity will be negative ; but if it be rub- bed by a dried oil- silk and whiting it becomes positive y even a polished cylinder may be rendered negative by rubbing it with the hairy side of a cat's skin. A cy- linder made of baked wood, rubbed with a smooth rubber of oiled silk, becomes negative ; but by rub- bing it with coarse flannel, it is rendered positive. If a cylinder be made of sulphur or resin, the electricity is the reverse of that which is produced by the smooth glass cylinder and rubber of the usual machines ; for the rubber in this case partakes of the positive, and the cylinder, or the prime conductor, is electrized with the negative. This difference between the resin and glass gave rise to what is called the double current, or vitreous and resinous electricity ; but it is now generally sup- posed that the difference arises more from the effect of the surfaces that act on each other, than from any pCr culiar qualities in the different bodies.* * If the body rubb€d be less affected by the friction than the rubber, the former will be electrified- negatively, and the latter fiositivelij ; and vice versa. Ed. R 142 Poijits. It is experimentally found that points, attached to any conducting body, either receive or deliver elec- trical fluid more freely than flat or round bodies. For this reason the prime conductors of electrical machines are always furnished with points, to receive the fluid with the greater facility from the electrical atmosphere of the cylinder. To show^ the superior attraction of points. If tlie round knob of a brass conducting rod be held near to the prime conductor when the machine is in motion, the electric spark will be seen darting towards it ; but if a needle, or fiine pointed conductor, be presented even at twice the distance of the knob, the sparks will instantly cease, and the fluid will be silently drawn off* by the point; but when the point is withdrawn, the spark will immediately recommence and fly towards the brass knob. If this experiment be performed in a darkened room, a small globular spark appears at the end of the point when it is presented, which shows that the needle receives the fluid from the conductor. When the wire or needle is fixed towards the end of the prime conductor, on presenting a brass knob, or the finger, the fluid will pass off" without any visible appearance; but the electric stream will produce a current like wind, which may be sensibly felt. When the needle is fixed perpendicularly on the prime conductor, if crossed wires, with their ends all bent the same way, be balanced on the point, the resistance which the air gives to the electric current that issues from the points, will drive the fly round with considerable velocitv. Electrical Attraction, bV. 143 Another amusing experiment, called the electrical orrery, is performed by means of the current which issues from electrified points. A piece of bent wire is suspended by a needle in the top of a glass stand, and a small globe of glass is fixed in the centre ; at one end of the wire is another needle, which supports a short cross wire bent at each end; L is a pith ball placed upon it, which represents the earth, m is a smaller one at the end of the wire for the moon, and s, the small glass globe over the stand, may represent the sun. When tlie conducting chain is connected with the needle in the top of the stand, and the machine is excited, the sun turns on his axis, and the moon makes her monthly revolution round the earth, whilst the earth is carried in its annual orbit round the sun. Electrical Attraction and Repulsion. Electricity attracts all kinds of bodies, but repels them after they are electrized; for if a piece of light downy feather be suspended at, about the distance of a foot from an electrified conductor, it will be attract- ed or drawn towards it, and afterwards repelled or driven off; for the electric atmosphere which sur- rounds the prime conductor attracts the body till it becomes electrified with a portion of the same fluid, after which the atmosphere of the conductor and that of the feather repel each other, and the feather is driven off till it have discharged the fluid w^hich it had accumulated, then it returns on the same principle as before. This attracting and repelling power is whimsically 144 Electrical Attraction^ ^c. ^ illustrated by droll figures cut out in paper. The figures are laid on a metal plate and stand, which is placed exactly under another brass plate suspended by a chain from the prime con- | ductor. When the machine is excited the upper plate is electrified, and the attracting atmosphere draws the figures towards it but when the figures are electrified by tlv upper plate, they are repelled and fly back to the lower uninsulated stand to discharge their electricity, and then they are attract- ed again as before; thus the figures conti- nue jumping backwards^ and forwards till they have discharged the fluid from the upper plate and conductor. The electrified bells is another pleasing experiment, which show^s the attraction and repulsion of the elec- trical fluid. Four small bells are suspended by small w^ires from the end of two cross rods, and each arm suspends a clapper, hung by a silken string; the upper part of the stand is made of solid glass, and the conducting chain of the machine communicates with the brass knob and fly on the top; tow^ai'ds the lower part of the stand is another bell larger than the rest, which is uninsulated, and forms part of a condifctor with the earth. When this machine is put in motion, the fluid passes down the conducting wires and electrifies the oells; these attract their respective clappers, which are afterwards repulsed and driven off to the uninsulated bell in the centre, which receives their electricity and conve}s it throigh the bottom of the stand to the earth. Thus the five bells are kept continually ringing, which pro- Ley den Phial, 14$ duces a pleasant peal when the tones of the bells are properly varied. Leyden PhiaL , What is called the Leyden phial, is a glass jar coated inside and outside with tin-foil, except about two inches on each side from the top of the jar downwards, to prevent the connexion of the fluid between the inside and outside when the glass is charged. The mouth of the jar is covered by a piece of wood, which receives a thick brass wire; the upper end of the wire has a brass knob fastened to it, and the lower end, which goes hito the jar, has a small wire or brass chain fixed to it, that communicates with the bottom and sides, and serves as a conductor to charge the jar with electrical fluid. JVhen the jar is to be charged, it may be held in the hand, or placed upon a table with its knob against the knob of the conductor; on exciting the machine, the electrical fluid passes from the prime conductor through the knob and wire into the interior of the jar. The iiuid-, thus collected and condensed, will be of the same kind as that which surrounds the prime conduc- tor. The exterior part of the jar being uninsulated, gives its natural electricity to the earth through the medium of the conducting bodies that connect them, and it will acquire an electricity of the same kind as that which belongs to the rubber; thus the fluid is insu- lated, with respect to its connexion from the opposite sides of the jar, by the unfoiled part at the top, and the increase in the interior is in proportion *to the de- crease on the exterior side. When great force is required from the electric fluid, a number of jars of the above description are 146 Leyden Phial, placed on a metal frame which forms a communi- cation between their outside coatings and the earth, and the insides of the jars have conducting wires which pass to the prime conductor. In this manner imy number of jars may be charged with the same facility as a single one, and from the powerful effect of the electric fluid, when it is thus collected, it is called an electric battery. The bottle -form is not absolutely necessary in com- bining electrical fluid; glass plates were used for the same purpose before this invention w^as known ; indeed it may be combined by bodies of every form, but as cylindrical jars offer the largest surface and greatest conveniency in the smallest space, they are the more usually preferred. Dr. Franklin has shown, in his theory of the Leyden phial, that when one side of the jar is electrified posi- tively, the opposite side is electrified negatively; for whatever quantity be thrown on one side, that on the other is diminished in like proportion, so that the change of an electric jar is nothing more than drawing off the fluid from one side and carrying it to the other; for it is impossible to charge one side of a jar, unless the opposite side have a conductor to carry off the fluid "which it contains: in like manner an electrical jar can- not be discharged without a communication between the opposite sides to restore the electrical fluid to its natural quantity. To explain this subject more particu- larly : If the knob of a coated jar be held near thee on- ductor, on turning the machine it will be seen by the index of the electrometer when the jar has received its full charge ; then take a discharging rod with a glass handle, and bring one of its knobs to the knob of the jar, and the other to the outside coating, which forms a conducting circuit between the inside and outside of the jar, and the surcharge of the interior will fly through the conducting rod to the exterior, till the Leyden Phial liF -powers on each side are equal. A person may convince himself of the transmission and force of the fluid through the wires, by forming the conductor himself; for if he touch the coated side of the jar with one finger, and bring the other to the knob of the jar, he will then receive a strong shock, which will be parti- cularly felt, either m his wrists, elbows, or breast, according to the strength of the charge. If the electrical circuit be made by any number of persons joining hands, when the first and last touch the knob and side of the jar, or take hold of two pieces of wire which are joined to them, the whole number will receive the shock at the same instant, be it ever so great; for the passage of electricity is so instantaneous, that in a circuit of wire ten miles in length the shock was felt at the same moment that the jar was discharged. In charging a jar, if it be held by the knob, and the coated side presented to the conductor, the exterior will receive the surcharge of the fluid, and the interior will lose it; that is, the outside will be positive, and the inside negative, but the effects in the discharge w^ill be the same. If the jar be placed on an insulated stand, with m> knob near the prime conductor, the index will rise by the accumulation of the fluid upon the conductor; but, on trial, it will be found that no electricity has entered the jar. It has been already observed, that the elec- trical jar is charged by taking the fluid from one side and carrying it to the other, so that, in this case, as the outside has no communication with the earth, it cannot part with its natural electricity, therefore none can be accumulated on either side of the glass. When the jar is thus insulated, if one of its sides communicate with the rubber by means of a wire, and the other be connected with the prime conductor, the jar will be readily charged with its own fluid; for the electricity on one side passes through the wire to 148 Ley den PhiciL the rubber, and thence through the prime conductor to the opposite side of the jar. If the knob of the electric jar be placed about half an inch from the end of the prime conductor, and a pointed wire be pre- sented to the coated side of the jar, the fluid will be seen entering the jar from the conductor, and passing from the outside, vmder the appearance of a small star, upon the point of the wire which is held towards it ; but if a piece of wire be fastened round the jar, with an end projecting towards another conducting wire, the fluid will rush from the side by this point, and di- verge its luminous rays in a conical form, taking the point of the wire as the vertex. After having explained the operation of the Leyden phial, it may be entertaining as well as useful, to give an account of some of those experiments which are performed by means of accumulated fluid in an elec- trical jar. A hole may be struck through a card, by placing it against the tin-foil on the side of the jar; for if one end of the conducting rod be brought against the card, and the other to the knob, the fluid will rush tlirough the conductor, and pierce the card as it joins the other fluid on the exterior surface of the jar. To impregnate the surface of glass with gold or silver leaf. Take two slips of glass, about an inch broad and three or four inches long, and place a nar- row slip of gold leaf, about an inch longer than the glass plates, between them, so that each end of the leaf may project about half an inch beyond the ends of the glass; then lay the whole upon a non- conducting surface, with a weight upon it, and bring one end of the conducting chain or wire from the bottom of the jar into contact with that end of' the leaf which is opposite to it, then bring one knob of the discharging rod to the knob of the jar, and the other to the opposite end of the leaf; thus the electric circuit is completed, and the leaf will be melted and Leyden Phial, 149 driven into the surface of the glass by the force of the electrical fluid. What is called the spotted bottle, is fitted up like the Leyden phial, only the tin- foil coating is gummed on in little square pieces at some distance from each other; so that when the bottle is charged in the dark, the sparks will be seen flying across the spaces, from one square to another. If it be discharged gently, by bringing a pointed wire gradually to the knob of the jar, the fluid will pleasingly illuminate the uncoated parts, and make a crackling noise in passing the spaces. A double set of bells will show the course of the fluid in a Leyden phial. Let the bottle be placed horizontal- ly in the frame of an insulated stand, with a knob and wire in each end, communi- cating independently with the inside and outside of the jar, and let a set of bells be suspended from each wire ; then place the knob b against the conductor, hang up the chain that lies on the table, and charge the jar. Now, whilst the jar is receiving the fluid, the bells on the opposite wire a, which are connected with the out- side of the* jar, will continue ringing. After the jar has received its charge, unhook the chain at the end B, and let it lie on the table; then touch the opposite end A with the finger, and those bells at b will begin, and the others will cease ; if the finger be again pkiced on B, those at a will commence. Thus, by varying 150 Leyden Phial, the end, each set may be rung, till the whole of the fluid be discharged out of the jar. The following experiment shows how buildings may be set on fire by lightning, when combustible bodies are laid near metallic rods, plates of iron, Stc. Take some powdered resin, rub and mix it well in dry tow or cotton, and put it between two metallic balls, which are placed in a tin toy resembling a house; from these balls there are two wire conduc- tors, one of which passes to the outside of the charged jar, and the other is connected with one end of the discharging rod. When the jar is discharged, the fluid will instantly force its passage between the balls in the house and fire the tow or cotton, which blazes out of the windows, and shows some appearance of a house on fire. The grand object of scientifical pursuits is, to add to the benefits of society; then how much are we in- debted to Franklin for his great discovery of electrical conductors ! which avert the dreadful effects of light- ning, and convey it harmlessly to the earth. The fol- lowing experiment is formed to show the effect that metallic conductors have on lightning, and how ne- cessary they are for our security. A D represents the gable end of a building which is made of wood, with a square hole in the middle; and G B c H is a small piece of wood that fits loosely into the hole, with a piece of wire g h laid through its diagonal; a g and H D is the conducting rod, Avhich is joined by g h for the ]g sake of experiment ; i is the con- ducting chain which connects the bottom of the conductor with the bottom of the jar, to complete the circuit with the Electric Ba ttery . 151 discharging rod k. Now if die jar represent a diun- der cloud discharging its fluid in the direction k a towards the top of the house, it will be attracted by the rod and carried by the metallic conductor a g h d to the earth, without injuring the building. But if the jar be charged again, and the square piece of wood be reversed, placing g ii in the direction c b ; when the jar is discharged an explosion will ensue, and the piece of wood, which may be considered as a part of the building, will be driven out with considerable force, by the interruption v/hich is given to the fluid in passing through the conductor. Electric Batterij. In an electric battery, or a combination of jars, the accumulated fluid is capable of performing powerful experiments; but great care must be taken in using it, lest any person should chance to get into the elec- trical circuit, which would endanger his life if the bat- tery were laige. When the battery is used, it is like- wise liighly necessaiy to use the electrometer, to ascer- tain the height of the charge. If a quire of paper be suspended by a string, and two ends of a conducting wire be brought near each side of it, and the circuit completed; on discharging the battery, the electric fluid will pierce a hole through the paper without putting it in motion. Or, if a thick piece of glass be placed on an insula- ted stand, with a weight laid upon it, and the conduct- ing wire of the machine be brought into contact with the ends of the glass; on discharging the battery, ^part of the glass will be reduced to powder, or, if the glass be of tolerable thickness, it is sometimes coloured and shivered in a curious manner. When the coated surface of the glass jars in the battery contains about thirty square feet, the fluid will melt brass wire of considerable thickness. 152 METEOROLOGY. There is not a subject throughout the sciences that interests the general class of mankind so much as Meteorology, and yet, after all our researches into nature, this interesting knowledge is more vague and suppositionary, with respect to its first cause, than any other whatever. Our utmost knowledge only extends to the estab- lishment of a few facts which have been gathered from observation, and in reasoning upon these facts questions arise every moment which we are unable to resolve. However, all kinds of meteorological phenomena must chiefly depend on a circulation of fluid, or the change of water into new forms, which is afterwards regenerated or brought back into its original state. Rain Is produced by water which rises from the surface of the earth in the form of a rare, insensible, and ex- panded vapour. In the atmosphere its state is changed from vapour to aeriform fluid, and by some unknown cause it is again changed into mists and clouds, from which it is gathered into drops, and then falls to the ground to take its turn again in the common course of evaporation. But the agency in the formation of clouds into rain, and even of the vapour into clouds, has been very va- riously considered. Some suppose that the cold which constantly occu- pies the superior regions of the air chills or condenses the vesicles, or small bubbles into which vapour is formed, at their arrival from a warmer situation, and by bringing them nearer to each other, it causes several Raitu 153 to join together in little masses, which increasing their qu.uid^y of matter in a greater degree than their surface, they become too heavy for the air, and so descend in rain. But it seems difficult to ascribe tliis operation to cold, as rain as ofien happens in warm as in cold wea- ther; and another circumstance appears to destroy ihe probability of this supposition, which is a remarkable fact, that the drops of rain increase in size as they de- scend. On the top of a high hill, for instance, the drops of rain may be quite small, half way down its side they will be found much larger, and towards the bottom the drops will be very large, descending in impetuous rain; which proves that the atmosphere still condenses the vapours where it is warm as well as where it is cold.^ Some bring in the assistance of the wind to produce this effect, which, by beating against a cloud or congre- gated mass, drives the vesicles nearer together, by which means they combine and descend from their increase of gravity; so that when two winds blow to- wards the same point, the rain will descend in greater quantities. Others attribute this discharge of the clouds to different changes in the atmosphere, which alter the spring or elasticity of the air; for as this elasticity principally depends on the dry terrene exhalations, when these are weakened the atmosphere is unable to support the weight of the clouds, and they fall to- the ground. When these small vesicles are descending, * There seems to be no difficulty in ascribing the formation of clouds and rain to the agency of cold, or abstraction of heat ; the clouds being formed in the upper regions of the atmosphere, where the degree of cold may be sufficient to condense the va- pours, though the weathet be warm in the lower regions ; and that the drops of rain irii.rease in size as they fall is a natural consequence of cohesive attraction. Ed. 154 Rain. * , they will still continue to fall, although they may meet with an increasing resistance from the increasing density of the air towards the earth, and as they all tend to the same point, the centre of the earth, the farther they fall together the more they will congregate or fall into each other.* If such be the state of the atmosphere that these vesicles descend at a small distance from the surface of the earth, the coalitions in their fall will not be numerous, therefore the drops will be but small, form- ing what is called a dew; but when they rise higher they descend as a mist or fog, and still higher as small rain, so that when the drops are the greatest the rain descends from the greatest height. When the ascending vesicles meet with neither wind nor cold enough to o condense them, they will remain for several days, and sometimes weeks, before they disperse, which pro- duces what is called a heavy, thick, or cloudy sky. But since the great improvements which have been made in the science of electricity, rain has been con- sidered as an electrical phenomenon. Beccaria supposes that rain, hail, and snow, are produced by the effects of a moderate electricity in the air; and that clouds, that bring rain, are produced in the same manner as thunder clouds, only by a more moderate electricity. That previous to rain a quantity of electric matter escapes out of the earth where there is a redundancy ; and that, as it ascends into the higher regions of the at- mosphere, it collects and conducts into its path a great quantity of vapours ; and the same cause that collects them will condense them more and more, till in the places of the nearest intervals they come almost into contact, so as to form small drops, which, uniting with others as they fall, come down in the form of rain. * This convergence is certainly too small to produce any sen- sible effect. £d. 155 Clouds. Clouds are composed of a mass of vesicles, which may be seen in particular situations, and frequently on high mountains where die clouds float beneath the observer. These vesicles keep rising and falling in the air till they become in equilibrium with it, then they remain in that state till they be again agitated by a change of levity in that part of the atmosphere. When these vapours approach within a certain distance of each other, they lose the latent fire which they contain, and the vapours tend to unite ; for it is now determined that a separation of the latent heat from the fluid of which vapour is composed, is attended with a con- densation of that vapour in some degree. In such cases it will first appear like a smoke, mist, or fog; then as a cloud; and if this cause continue to operate, the cloud will produce either rain or snow, according to the degree of cold in the air; but it is not easy to discover why these clouds remain so long suspended without discharging themselves. For when the vapours which are formed from known causes get beyond a maximum in the temperature of the air, the vesicles are formed by a rapid decomposition of superfluous vapours, and as soon as this ceases the vesicles are dissipated, and the fluid immediately descends in drops. This, and other circumstances, teach us to believe that there is some other agent concerned in the forma- tion of clouds besides mere heat and cold ; this agent is now supposed to be electricity, not only in the forma- tion of clouds of every description, but in producing hail, snow, or rain. This is most certain, that the clouds which are formed by atmospheric vapours, whether they be rendered visible by electricity or not, 156 Clouds. contain prodigious quantities of electrical fluid, which frequently produce the most disastrous effects. The most extraordinary instance of this kind upon record happened in the island of Java, in the East Indies, in August 1772. On the llth of that month, at midnight, a bright cloud was observed covering a mountain in the district called Cheribou, and several reports like those of a gun were heard at the same time. The people who dwelt upon the upper parts of the mountain not being able to fly fast enough, a great part of the cloud, about eight or nine miles in circum- ference, detached itself under them, and was seen at a distance rising and falling like the waves of the sea, and emitting globes of fire so luminous that the night became clear as day. The effects of it were astonish- ing; every thing was destroyed for twenty miles round, the houses were demolished, and plantations were buried in the earth, and 2140 people lost their lives, beside 1500 head of cattle, and a vast number of goats, horses, and other animals. The height of the clouds from the surface of the earth is not very considerable, as we find persons fre- quently pass through them as they ascend the side of a high mountain, on the top of which they see the clouds rolling under their feet. Mr. de Luc particu- larly mentions, that he saw his own shadow, and that of the rock on which he sat, projected on a cloud beneath him, with a stratum of clouds extending to a considerable distance. Those clouds which are the most highly electrified are generally the lowest, not being more than seven or eight hundred yards from the earth, even some of them are so low that they appear to touch its surface, but the generality of clouds are suspended at about the height of a mile. The shape of a cloud is probably owing to its elec- tricitv, for at or near the time of thunder, the clouds Clouds. 157 vary their shape continually, assuming grotesque and fanciful appearances ; we may often perceive the edge we are looking at dissipated in the place where it was first observed; sometimes the edge stretches itself out, whilst the cloud vanishes away. It frequently happens that, when one edge disappears, others are formed by which the cloud is enlarged; at other times the edges evaporate till the whole disappears. It is difficult to account for all these changes in the same cloud, unless we attribute it to the different changes in its electricity, by which it is supposed to be com- pounded. The motion of clouds is most frequently directed by the currents of air, though this is not always the case, particularly when thunder is expected ; that is, when the clouds are highly charged with electricity, then they are observed to move very slowly, and some- times appear quite stationary. In some cases the motion of the clouds evidently depends on their electricity, independently of any current whatever ; for, in calm and warm weather, small clouds are often seen moving in different directions, setting out and meeting each other at such short distances as to make it impossible that they should be governed by different currents of air. When clouds of this description join each other, they do not form a larger cloud ; but, on the contrary, become less, and sometimes totally vanish, which is supposed to arise from the discharge of the different electricities with which they were charged. This appears to throw a new light on the formation of clouds; for if two clouds, one electrified positively, and the other negatively, destroy each other on con- tact, it may follow that any quantity of vapour in the atmosphere is invisible ; unless it be electrified either positively or negatively, when it will be seen as a cloud. T 158 . Hall As snow is formed by a congelation of vapour in the upper regions of the atmosphere by an extraor- dinary degree of cold, we may fairly suppose, from the appearance of hail, that it is a congelation of a like kind arising from cold, but in a lower medium, as the descending drops of rain pass through it, which forms them into a kind of rarefied ice. But, however, this subject, like a great many others in meteorology, can- not be explained in a very satisfactory manner. Beccaria imagines that hail is formed in the higher regions of the air, where the cold is intense, and where the electric matter is very copious, by which a num- ber of aqueous particles are frozen, and that these par- ticles collect others in their descent, so that the den- sity of the substance of the hailstones grows less and less from the centre, that being the hardest which is first formed in the upper regions of the atmosphere. He likewise adds, in support of this principle, that the size of hailstones, like drops of rain, is the smallest on the tops of high mountains, which corresponds with the observation of different persons that have observed the descent of hail in such situations. Lightning and Thunder, It has been sufficiently ascertained by experiment, that lightning is the same as electricity, and that thun- der clouds are charged in a positive and negative state, and that they will even change this state many times in an hour; but whether this fluid be generated or col- lected in the atmosphere, is q, question that has not yet been properly resolved. Beccaria is of opinion, that the clouds receive their electricity from the earth; for, as clouds are formed from exhaled vapours, the same power in nature which attracts them from the earth draws them towards each other, so that a kind of aqueous conducting chain is Lightning and Thunder, 159 formed for the passage of the fluid from its grand re- ceptacle the earth. Thus may electricity be transmit- ted from one part of the earth that is surcharged, to another that is deficient. The quantity contained by the clouds is in such abundance, that although parti- cular clouds are perpetually discharging the collected matter to the earth, yet the unbounded stores of the earth instantly supply the deficiency, so that the clouds continue to discharge their electricity, with a short intermission, during the whole time of a thunder- storm, or till the electricity be restored to an equili- brium. The rumbling noise of thunder which follows the flash or discharge, most probably arises from the col- lapsing of the air w hich is rarefied in the electric cir- cuit, so that the sound is soonest heard from that part of the direction which is nearest to us, and continues successively according to .the distances. Some imagine that the clouds do not receive their electricity from the earth, but from the heating and cooling of the air; so that the clouds in passing through a rarefied part of the atmosphere, receive electricity from it, and give it back again to those parts, which are in a more condensed state. Others have conceived, from the sultry state of the atmosphere in thunder storms, that electric matter is generated by the fermentation of sulphurous vapours with mineral or acid vapours in the atmosphere. Whatever may be the cause that disturbs the na- tural equilibrium of the air, or the means by which it restores itself, the concussion of the elements at least proves the inequality of the atmosphere; and the dreadful consequences which frequently follow it, make every one anxious to guai'd against its effects. The Aurora Borealis, or Northern Lights, are sup- posed to be produced by the continual discharge of electric fluid; for, in the higher and more attenuated 16G Lightning and Thunder. parts of the atmosphere where they are always seen, a large quantity of the fluid cannot be collected by clouds to make a great concussion like thunder; but it is dispersed as fast as it is collected, which gives that perpetual flashing resembling the appearance of elec- trical fluid in a rarefied receiver. What are called Falling, or Shooting Stars, are sup- posed to be produced by electrical fluid passing through the air when its course is not disturbed by stormy clouds ; and by attaching itself to those con- ductors it may meet with in the air, it becomes visible in its passage to the earth. The Ignis Fatuus is a luminous body which is seen in the dark hovering over bogs and marshy places, seemingly both changeable in situation and varying in its appearances. This body is generally considered as inflammable air arising from the marsh, ignited by electricity. Even earthquakes are now supposed to be produced by electricity; what was once thought to proceed from subterranean vapours, is now attributed to the discharge of a cloud in a highly electric state, either in its passage to another cloud or to the earth. Earth- 4 quakes are most frequent in dry and hot countries, and hot climates are more subject to lightning and other electrical meteors, than those which are more remote from the equator. Another circumstance which seems to confirm this opinion is, that the concussion of an earthquake is not followed by any noxious smell, which would probably take place if it proceeded from the explosion of vapour; and the atmosphere is found to be highly charged with electricity for some time previous to this dreadful calamity. Much of the power of electricity still depends upon hypothesis, and, although great advances have been recently made, yet much remains to be done; we still see through a veil, but if ever time should withdraw this curtain from before our eyes, then the wonders of electricity will be completed ; we may then discover its effects, not only in the laws of planetary motion, but Hkewise trace it to its source in meteorology, magnetism, vegetation, muscular motion, and all the economy of nature. mnd. Wind is a current of air which usually blows from one part of the horizon to its opposite part. Winds may be reduced into three classes, called va- riable, periodical, and general. Variable winds are those w^hich are not subject to any particular period, either in duration or return. The stated or periodical winds are such as return at certain times ; these may be divided into two kinds, viz. the sea and land breezes, which are produced by the diurnal motion of the sun, and blow alternately from the sea and land ; and monsoons, which are caused by the annual revolution of the sun, blowing one way for a certain number of months, and the op- posite way for the rest of the year. General winds, which are usually called trade winds, blow always nearly in the same direction, as those between the tropics in the Atlantic and Pacific Oceans. The annual and diurnal revolution of the sun may be the general cause of winds, but this hypothesis can by no means sufficiently explain the phenomena, without having recourse to some other aid, as these causes could only produce regular winds, the pro- gress of which would correspond and be connected with the seasons. With respect to the effect of heat on the air, there is no doubt but that those places which receive the ^eatest force from the sun's rays, will have the air 162 IVind, more rarefied, and its elasticity more weakened, than those which receive less of his influence; therefore a wind will blow towards the rarefied place, as the re- sistance will not be able to oppose the adjoining pres- sure. For the spring of the air increases as the com- pressing weight increases, and compressed air is denser than that which is less compressed, so ihat all winds will blow into a rarer air out of that place which is filled with a denser. This principle is practically de- monstrated by the current of air that rushes in on all sides to a fire burning in an open situation; here the particles of air that surround it, being rarefied by the heat, ascend in a constant current, and the cooler or denser air presses in by its elasticity, and produces a very sensible effect on the bodies of those that are placed at a small distance from the fire. Dr. Hal ley, who has paid great attention to this subject, supposes that it is the action of the sun's beams that produces the general winds, as it passes over the air^ earth, and water; for, as the sun appears to be continually shifting to the Westward during a diurnal fevoiution, the lower air becomes attenuated by his rays, and the tendency of the whole body is to- wards this rarefied passage, which produces an easterly wind to about 30 degrees on each side of the equator; and as this motion is communicated to a vast ocean of air, the current continues during the night, till the sun appears again to give fresh impulse, and restore the motion that was lost in its absence, which causes the easterly wind to be perpetually in this situation. As the air towards the poles is less rarefied than that between the tropics, it will necessarily follow that a wind will blow from the north and south towards the equator. If the surface of the globe were covered with water alone, the winds would be perpetually the same as they are in the Atlantic and Pacific Oceans ; but tlit Wind, 163 large continents of land receiving a greater degree of heat than the water during the day, they communicate it to the air above them, which becoming more rare- fied than that part over the sea, the denser air passes towards the land. This accounts foif those westerly instead of easterly winds that blow towards the coast of Guinea from some distance on the sea. The sea and land breezes in the West Indies no doubt arise from the same cause. The breeze from the sea to the land begins to appear about nine o'clock in the morning, and keeps gradually increasing till noon, and dies away about four or five in the after- noon ; about six in the evening it changes into a fand breeze, which continues blowing towards the sea till near eight the next morning. These changes may be accounted for according to the preceding principles; for, as the heat takes more effect on the land than on the water during the day, the air over the land becomes more rarefied, which causes the cooler air to rush in to keep up the equili- brium. In the evening, as soon as the sun is set, the dews come on so excessively, that the air becomes suddenly cooled, consequently more dense than that which is over the water, and this causes the air to press from the land to the water, which produces the opposite current. The cause of the monsoons, or periodical winds, is owing to the course of the sun northwards of the equa- tor one half of the year, and southwards the other. While it passes through the six northern signs of the ecliptic, the various countries of Arabia, Persia, India, and China, are heated, and reflect great quantities of the solar rays into the atmosphere, by which means it becomes greatly rarefied, and the equilibrium is de- stroyed; in restoring it again, the air not only rushes in from the equatorial parts southwards where it is colder, but likewise that from the northern climes must necessarily have a tendency towards those parts, which produces the monsoons for the first six months. 164 PFind. Then, during the other six months, whilst the sun is traversing the ocean, and countries in the southern tropic, it heats and rarefies the air in those parts, con- sequently causes the equatorial air to alter its course, to veer quite about, and blow from the opposite points of the horizon. These are the general affections of constant and regular winds ; but the whole .ire subject to variations and exceptions, on account of different circumstances, local or otherwise. Some philosophers differ from Dr. Halley in his theory of the wind, and suppose that, as the earth turns on its axis eastwards, the particles of air being very light are left behind, so that with respect to the earth's surface they would seem to move westerly, which produces a constant easterly wind, which is the strong- est and most regular where the diurnal motion is the swiftest, that is between the tropics. Others conceive that the atmosphere is a gravitating fluid substance, subject to the attraction of the sun and moon as well as the earth; and, therefore, when their influence, either singly or conjointly, is opposed to that of the earth, the same effects will take place in the air as are produced upon water; and that there are tides of air which vary in their form and pressure, ac- cording to the different positions of the attracting bo- dies. But that, as various altitudes of the atmosphere may have an equality of weight or pressure, we can- not discover the aerial tides of ebb and flow by the barometer; yet the change which is produced by the difference of pressure, must create a motion in the parts which produces wind, either more or less, as these differences conspire with, or act against, each other. The force or velocity of the most vehement wind does not fly at the rate of more than fifty or sixty miles kn hour, and the medium velocity of wind is not more than twelve or fifteen milejs an hour. Sometimes thr Wind. 165 wind is so slow as not to exceed the velocity of a per- son walking or riding in it; in this case a person mov- ing with it feels no wind, as there is no difference of velocity, or no relative force, which is all we are sen- sible of whilst the body is in motion. U 106 LIGHT AND COLOURS. Light is that power by which objects are made perceptible to our sense of seeing, or the sensation oc- casioned in the mind by the view of luminous objects. Light, like many other effects in nature, the princi- ple and essence of which exceed the bounds of human understanding, has, for many ages, been a subject of much speculation and hypothesis; it has been consi- dered as a mere quality of particular bodies, or a fluid medium by which the vibrations of luminous bodies are carried to the eye ; but Newton demonstrates it to be an absolute body, composed of infinitely small particles of matter, which issue by a repulsive force from luminous bodies with wonderful velocity, di- verging in right lines in all directions. The particles of light issue from the sun as the pri- mary source, and keep a rectilinear motion, till they are inflected by the attraction of some other body, or refracted by passing obliquely through a medium of different density, or reflected by the intervention of an opposing body ; so that the small particles, of which light is composed, are governed by the power of at- traction in the same manner as the particles of the grossest bodies. That the particles of light are infinitely small, may be reasonably inferred from their penetrating the densest bodies; the pores of glass, crystal, or a dia- mond, cannot stop the subtilty of light; and yet the greatest collection of this matter has never been found to have any sensible weight, even in the finest scale. Considering the immense velocity of light, if the par- ticles were not infinitely small, and probably placed at a considerable distance from one another, the pres- sure on the tyt would be insufferable. Light. 167 It appears evident, that the rays of light are emitted in right lines, from the shadow which is thrown be- hind those bodies on which they fall, as the corre- sponding parts of the substance and shadow form right lines with the source of the ray. The velocity of light has been considered instan- taneous; nor can its passage from one visible object to another be marked by any difference of time, al- though the following discovery of Roemer's, sup- ported by Cassini and others, sufficiently proves a pro- gressive motion. He observed that the eclipses of Jupiter's satellites varied 16 2 minutes in time in some particular situations of the earth in its orbit, being 81 minutes sooner than the calculated time Avhen the earth wSHP^^Rarest to the planet, and 8t minutes later than the tables when the earth was in the opposite part of its orbit. From this observation, Cassini, and others, have concluded that the difference of time proceeds from the progressive motion of light in passing the orbit of the earth. This subject may be further explained by the following diagram. 168 Light, B Let A be the sun, or the centre of the system, from which the tables are calculated; B c the earth's orbit; e Jupiter, and D one of his satellites just en- tering his disk. Then an observer at A would find the time of im- mersion coincide with the tables ; but at B it takes place 8i minutes sooner than at a, and at c 8i mi- nutes later; which, taken together, gives a difference of 16 J minutes for the progressive time of the re- flected light of the satellite in pass- ing from B to c, the diameter of / the earth's orbit. Now if we take / the diameter of the earth's orbit at I 190 millions of miles, and divide \ it by 990, the seconds in 16i mi- \ nutes, the result will be about .200000 miles, which is the won- derful velocity of light in a second of time. As the sun is placed in the centre of the earth's orbit, the distance of these two bodies from each other is equal to the semidiameter of the earth's or- bit, or 95 millions of miles ; so that the particles of light aie transmitted from the sun to the earth in 8t minutes, which is near two millions of times swifter than the velocity of a cannon ball, supposing it to fly at the rate of 450 miles in an hour. Natural philosophers were formerly of opinion that the solar light was simple and uniform, without any difference or variety of part^ and that the different colours of objects were made by refraction, reflection, or shadows. But the illuminated mind of Newton has demonstrated, from clear and decisive experiments, that light is not similar, but compounded of dissimi- lar rays, some more and some less refrangible, or #A •^c" Prism. 169 capable of being turned out of their course ; and that those rays which are the most refraiigible are like- wise the most reflexible, or most easily turned back, and accordingly as the rays differ in these qualities they excite in us the sensation of different colours. To explain this subject more fully: the sun's light consists of rays which have a considerable inequality of refraction as they are transmitted through the me- dium of the atmosphere, and impress a sensation of colours as they are more or less bent in their course, which we call violet, indigo, blue, green, yellow, orange, and red. Those rays which pass the most directly, or are the least refrangible, produce red; the next rays in refrangibility produce orange, then yellow, &c. The rays which produce violet are the most broken, or have the greatest refrangibility. These are all the primary colours in nature; but by blending or mixing different rays together they mu- tually alloy each other, and constitute intermediate colours, and shades of colours, of every description. Whiteness is a peculiar production; it is not com- posed of any particular ray of light, but produced by a copious reflection and due proportion of the rays of all sorts of colours. Blackness proceeds from the peculiar quality of a body whi^h stifles and absorbs the rays of light that fall upon it; so that instead of reflecting them out- wards, they are reflected and refracted inwards, till the incident rays are lost. Prism, The principle of colours is beautifully illustrated and explained by what is called a prism, which is a solid piece of glass, with three flat sides, through which the sun's rays are refracted. 170 Prism. Let ABC represent a glass prism, and e a small hole in the window-shutter of a darkened room, by which the pencil of rays e g enters and falls on the side of the prism at c. If the medium of the glass did not obstruct the rays, thdf'^ would pass on in a straight line and parallel direction to h, and there illuminate a small circle in the side of the room h i ; but when the rays enter and pass out of the denser medium of the glass they are refracted or bent towards l ; there- fore if the rays were equally refracted by the prism, they would pass on from f to l in parallel lines, and enlighten a circular spot at l, similar to that which was formed at h. But if the pencil of liglijb. be com- posed of rays which are not equally refrangible, then those which are the least refrangible windfall near- est to the right-lined direction e g h, and* hose that are the most refrangible will be the most distant, and tl^e intermediate degrees of refrangibility will issue in different rays between the two extremes. This ac- cords with experiment; for after the rays quit the side of the prism f, they diverge according to their refrangibility, and form an oblong spectrum, va- riously coloured, on the side of the wall between i and k; the lower part of which, being the least bent, prodtit^es a lively red colour; this changes by grada- tion into an orange, thence into a yellow, and, as the rays rise higher, into a green, blue, indigo, and vio- Prism. 171 let, which is the most distant, as being the most broken. There is a remarkable analogy between colour and sound; for it is found, that the divisions of the un- compounded colours on the spectrum agree with the different divisions of a musical chord. As an additional proof that the refrangibility of the sun's rays is various, and that the different rays re- flect their own colours ; if the spectrum be received on a perforated board, so that the uncompounded co- lours may pass distinctly through the hole, it will be found that they still preserve their individual colour, whether it be received on a sheet of white paper be- hind the hole, or be again refracted by another prism upon some other surface. The prism likewise shows, that those rays which are the most refrangible are also the most reflexible. For if the prism a b e be placed in a darkened room, in such a manner that the pencil of light which passes through the hole D may be re- flected from the point E in the base; the violet rays will be first seen reflected in the upper line e f, and the other rays will continue their refraction through e c and E G, Sec; but if the prism be gently moved on its axis, the indigo will be reflected after the violet, then the blue, green, and so on, till the red be reflected, which is the last. Now, as the rays of light differ both in refrangibi- lity and reflexibility, in accounting for the different colours of bodies, it is supposed that different bodies are endued with a power or aptitude to reflect the 172' Prism. rays of one particular colour, and to imbibe the rest. This opinion is likewise supported by experiment; for, if any body be placed in the uncompounded light of the spectrum, so that a pure and distinct colour may fall upon it, the body will appear of the same colour; only with this difference, that it will reflect that colour most brightly, which is the same as the body itself reflects in full light. But as all bodies re- flect other colours, in some degree, besides the pre- dominant colour which belongs to them, they cannot appear so full and clear as the colour is seen in the spectrum where it is uncompounded; but they reflect their colour feebly and weak in proportion to the Compound of the rays which are reflected. 173 OPTICS. This science, in its extended signification, em- braces a considerable part of the philosophy of na- ture ; connected with vision, it not only comprehends the whole doctrine of light and colours, but extends to the phenomena of visible objects in general. The word optics is here taken in a stricter sense, and implies an explanation of that part of vision which causes objects to produce different effects on the eye ; why they appear further off, or nearer, than they really are ; and why they appear more or less distinctly by the modifications and inflections of the rays of light in passing through different mediums. This subject is divided into two parts, called Re- flected and Refracted Vision. X 174 Refracted Vision, This shows the different directions and effects of rays of light in passing from one medium into ano- ther; which appai'ently increases the magnitude of a body, brings distant objects nearer to the sight, and makes those minute parts of nature visible that the unassisted eye could never discover. Rays or pencils of light pass in right lines from lu- minous objects, each carrying an impression to the eye of that part of the object whence it was emitted. When the direction of the ray keeps in the same me- dium, the object, image, and eye, will be in a straight line ; but when the ray is transmitted obliquely through different mediums, such as air, water, glass, or any other transparent body, as it enters into each it changes its direction, inclining more or less to the perpendi- cular of the medium, according to the density of the body. This deflection is supposed to proceed from the attraction of the denser medium, which acts in right lines perpendicular to the surface; thus the attractive j)Ower impedes the rays in their oblique course, and draws them towards the axis of the medium; when rays fall perpendicular they have no refraction. Definitions, — The diagram d h e represents a ves- sel filled with water, or any transparent body denser than air, and d b e is its surface. The line a b is the course of the ray of light from the object or radiant at a, which is called the line of incidence, and b is the point of incidence ; the line G B is the line of refraction, or the course of the ray, when the ^ direction is changed at b, by entering into a denser A C B E V r \j/^ Refracted Visiofi. 175 medium than that of the air through wliich it has passed; the right line d e, or the surface of the me- dium, where the lines of incidence and refraction meet each other, is called the plane of refraction ; b h , the perpendicular to that plane, is the axis of refrac- tion ; and the continuation of the same line, c b, above the surface of the medium, is called the axis of incidence. The angle a b c, which is formed by the line of incidence and the perpendicular c b , is called the angle of incidence ; and a c is the sine of the an- gle of incidence. The angle h b g , which is formed by the line of refraction and its axis, is the angle of refraction ; k g is its sine ; and the angle g b f, which is the difference between the angles of incidence and refraction, is called the angle of deviation. The course of rays, or pencils of light, is divided into three kinds, viz. parallel, converging, and di- verging rays. Bays are called parallel, when they would pass to infinity at equal distances from each other, as n l o m. When rays of light issue from bodies at immense distances, they are considered as parallel to one another ; so that those rays which proceed from the sun and pass through our atmosphere, are taken as parallel, though each point of K the sun is a radiant, diverging rays of light on the earth; but as the distance is so immense, and the angle of divergency so infinitely small, the rays may be fairly considered as passing in the same parallelism. Converging rays are those which, in passing from a rare to a denser medium, are refracted or bent towards the perpendicular, and meet in a common centre called the focus, as n p o. Diverging rays recede from the axis or perpendi- 176 Refracted Vision. cular, in passing from a dense into a rarer medium ; or when converging rays have crossed each other in their focus, or burning point, then they diverge or pass off from one another, as q^ p r represents in the figure. A ray of light in its oblique passage out of a rare into a denser medium, as from air into water, is re- fracted towards the perpendicular of the water; and the angle of incidence is alvcjys greater, in a given ratio, than the angle of refraction, except when the ray falls perpendicular to the denser medium, then they become equal. If a ray of light, which passes from a in air, fall on the denser medium of the water b, it does not pro- ceed in a right line to i ; but is attracted, or drawn out of its course, towards the perpendicular b d, and pas- ses in the direction b g, which is nearer to the axis than the straight line i b . If G B be a ray of light passing from a denser into a rarer medium, or from water into air, it will not pro- ceed in a right-lined direction to k ; but when it comes to b at the surface, its course wi]' be refracted, and the ray will recede from the perpendicular d c, and pass on in the direction b a . It may be practically found, by the following ex- periment, that light, in passing out of a rare into a denser medium, approaches nearer to the perpendi- cular, and that it recedes in passing from a denser to a rarer. Refracted Fision, 177 A piece of money being placed at the bottom of the cylindrical vessel o r, let the ]^ (y eye of the observer be so placed at N, as just to lose sight of the object at p ; or so that a ray shall pass from the remote part of the piece to the eye in the direction PON. Whilst the eye con- tinues in this situation, if the vessel be filled with water the object will become vi- sible; for the rays which pass from s in a right line will be bent at o, the surface of the water, and fall into the eye at n, or be refracted from the perpendi- cular o q. It may here be necessary to observe, that impressions are received in the eye by the rays which proceed from the object; so that in the above experiment, when the vessel is filled with water, the rays pass from a denser into a rarer medium, conse- quently recede from the axis of refraction when they enter the eye. If we consider the ray as issuing from N, that is, from the rare into the denser medium, it will pass in the direction o s, approaching the per- .pendicular ; and, as the water is drawn out of the vessel, the ray will recede till it falls into the straight line NOP, at the outer edge of the coin. The sine of the angle of incidence is in a given ratio to the sine of the angle of refraction ; this may be found experimentally in the following manner. 178 Refracted Vision. Let the quadrant c d e, which is graduated on its circular edge, have two moving indices, a and b, that turn in the point e, and let a e be pro- longed to F ; then set the index A to 15 degrees on the scale, and B to 19^°, and bring the edge of the quadrant d e to the surface of a vessel of clear wa- ter, immersing the lengthened part of the index a f ; now the immersed part f e will appear bent to G, which is in a right line with the index b e. In like manner if a be removed to 30^, and b to 41|<^, the refraction of e f will bring the apparent place of the limb in a right line with b e. Thus the angle of refraction may be found to any line of incidence, ei- ther out of a rare into a denser medium, or from a denser into a rarer; by placing the longer index to the angle of incidence on the quadrant, then immers- ing it in water, and afterwards moving the shorter index till it apparently coincide with the refracted limb of the other, and the number of degrees on the scale, opposite to the shorter index, will be the angle of refraction, whether the incidental ray issue from a denser or rarer medium. 1 But as the sines of the angles of incidence and re- fraction, between the same media, are in a constant ratio, if the angles of incidence and refraction of one incidental line be given, any other may be known by finding the sines, and saying, As the sine of the known ^ of incidence Is to the sine of its refraction. So is the sine of any other ^ of incidence To the sine of its refraction. 179 Reflected Vision, This part of optics shows the effect produced by rays of light falling on a polished surface, and thence returned to the eye, or thrown oif in a certain direc- tion from the plane, without penetrating its substance. Rays of light must either fall perpendicularly or obliquely upon a mirror or speculum, if it be per- fectly smooth and even. When the ray falls perpendicularly upon the sur- face, as D F on A B, the ray is reflected back again in the same direc- tion; but when it falls obliquel}^, like c f, on the plane, it is reflected from the surface, and A passes in the direction F E, making the angle of reflection e f d exactly equal to the angle of incidence c f d. This being universally the law in all oblique angles of reflected rays on a polished surface, it may not be improper to note again, that, in reflected lights the angle of incidence is ever equal to the angle of reflec- tion^ whether it be in plane or spherical surfaces^ con- cave or convex. Parallel rays falling on a plane mirror still keep parallel after reflection; for, as the angle of incidence is equal to the angle of reflection, and as the incidental rays e f and h i, are parallel to each other, the reflected rays c i and c f, will likewise be parallel. Diverging rays have the same divergency when 180 Reflected Fisioji, they are reflected, as they would have at an equal distance if continued in right lines. That is, if two diverging rays, a g and b h, be reflected to e and f ; the arc e f will be equal to the arc d c , which would have been the position of the rays, supposing them to have passed in a right-lined direction.* Converging rays which are reflected, have the same con- vergency at an equal distance as if they passed in straight lines. Thus; as the converging rays i n and k o have their angles of incidence equal to their angles of reflection, they converge to the point m, at a distance equal to l, where the rays would have met in an un- interrupted course, t By spherical or convex sur- faces parallel rays are rendered divergent. Every spherical sur- face may be considered as composed of an infinite number of straight lines. Suppose E F two of those straight lines which form part of the convex surface of a sphere, and that the rays a f, b e, fall parallel on the points e and f, thenJD it is evident that, as these lines are reflected in equal angles from the oblique lines E and F, they will diverge and lose their parallel direction. * The arc e f will equal the arc c d, only when the right line, »r reflecting surface g h, passes through the centre of the circle. t Supposing o N to pass through the centre of the circte. Ed. Reflected Vision . 181 By considering converging rays in the same man- ner it appears that they will become less conver- gent, and diverging rays more divergent. In concave surfaces parallel rays are made con- vergent. For, as this surface, like that of the convex mirror, may be supposed to be formed by an infinite num- ber of right lines, the parallel rays k h and I G which fall on the oblique lines G and H are reflected in equal angles through the lines g l and h m, tending towards each other till they meet in the vertex. In like manner it may be shown, on the same superficies, that rays already convergent become still more so, and that diverging rays are made less divergent. From the foregoing principles it will be easy to comprehend the effects of mirrors, and account for the principal phenomena which may occur, either with those that are plane, convex, or concave. A plane mirror does not alter the figure or change the size of objects; but the whole image is equal and similar to the whole object, and has the same situation on one side of the plane that the object has on the other. A spectator will see his own image as far beyond a mirror as he is before it, and as he moves to or from the mirror, the image will at the same time advance to, or recede from him, on the opposite side, but seemingly, with double velocity, because the two motions are equal and contrary. If a person view himself in a plane looking-glass, he will see himself completely, at any distance, in a part of the glass the length and breadth of which are equal to half the length and breadth of the correspon- ding parts of his body ; for, as the image appears as far behind the glass as he stands before it, the part Y 182 Lenses. of the mirror on which the rays fall will be equal to half the length or breadth of the object, or the rays will only spread half as much as they wou.d do at double the distance. In a convex mirror the image always appears smal- ler than the object, and the diminution increases as the object recedes. This will be easily understood, when it is considered that the reflecting convex sur- face of a mirror renders incident converging rays less convergent. The image does not appear so far behind a reflect- ing convex mirror as in a plane one ; for the diverging rays are reflected more divergent, consequently they have their imaginary focus much nearer, which makes the image appear nearer to the surface of re- flection. A concave mirror differs from the two preceding. It only shows bodies erect when the object is placed between its real focus and the mirror; then the rays are rendered convergent, and the image appears larger than the object; but when it is placed beyond the focus of the mirror, the rays cross each other at the focus, and the image appears inverted. Lenses, A L E N s is a piece of glass or crystal so formed, that rays of light in passing through it have their di- rection changed; it either converges them to a point or focus beyond the lens, or diverges them as if the rays had proceeded from a point before it, or brings converging and diverging rays parallel to each other. The lens marked > ^ 1, is called plano^ a ^ jL Ji^ cT^ convex^ having one A of its sides plane, and the other sphe- rical, which forms Lenses* 183 the segment of a sphere. 2. Is double convexy having both sides the same, and is like two equal seg- ments of a sphere joined together. 3. Is a plano-coit- cave lens; one of its sides being flat, and the other hollow, such as would be represented by the impres- sion of a small part of a sphere in soft wax. 4. Is double concave, having both its sides equally hollow. The 5th is called a meniscus, having one of its sides concave, and the other convex. The line b d, which passes through the middle of the lens perpendicular to the sides, is called the axis; the two points where it enters and passes out of the lens, the vertices; and the distance between them, the diameter. The focus, either of converging or, diverging rays, is situated somewhere in the axis of the lens. If the ray of light a e fall upon the plano-convex F lens at e, it will not pass on in tHe right line A H ; but as it is transmitted through the glass its course will be refracted into the line E d, approach- ing towards the per- pendicular of the convex side f g. Another ray fall- ing on c parallel to a e, and equidistant from the axis B, will be refracted in like manner, and will converge and meet a e in d, the focal point of the lens. All the intermediate rays which fall between e and c con- verge in the same way, only the rays will be less re- fracted as they approach towards b, till they fall into the centre and p^ss in a right line through the diame- ter to the converging point d. 184 Lenses. A- F c ::>& ^eS — ^I When parallel rays fall on the flat surface of the same figure, they , will tend from the perpendicular F G of the convex side, and converge with tlic pencil of rays from i £ and the intermediate pa- rallel rays to the point a. Considering the two mediums, air and glass, through which the rays pass, it appears that a ray from the spherical surface of the lens, whether in passing through the denser medium of the glass, or the rarer medium of the air, still converges .to a point somewhere in the axis. Therefore if both sides of the lens be spherical, the convergency will still be greater, and the rays will meet in some point nearer to the centre of the lens. Consequently if a b be a double convex lens, the parallel rays which fall upon it will converge to the point c , by the double convexity of its sides ; whereas, if one of its sides had been flat, as in the preceding example, the con- verging point would have been extended to d. If A B be a plano-concave lens, and c d the axis; let the ray a g fall upon the lens at a, then it will be refracted iy passing through the glass, and diverge from the direct line G I into a d, ap- proaching towards the perpendicular of the con- cave side. The ray at b being equidistant from c will diverge in the same manner, as well as the rest of the Lenses, 185 D-^gvii^i^^^^^^ intermediate rays, in proportion to their distance from the axis of the lens. Let the rays fall on the flat side of the lens: after having passed through ^ the denser medium of the C • glass, they will diverge on the opposite side as they pass into the rarer medium of the air, and the ray g a will be refract- ed from the perpendicular F E, as if it proceeded from d; likewise c, and all the mtermediate rays, will diverge in proportion to their distance from the axis. Consequently, if the rays equally diverge from the hollow surface, either in passing from or into the lens there will be an equal divergency from both sides of the double concave lens a b ; and those rays, which would have di- verged in a pla- no-concave lens as coming from c,^ now diverge C^«jS)Vg::::: with the addition- al concave side, as if they issued from the virtual focusEjinthe ax- is of the lens c D. When the radiant, or object, is at a considerable dis- tance from the lens, the rays issuing from it will fall upon the glass and converge" to the focal point; whence the image will appear inverted, but clear and distinct, as if the object was placed there in the same position. The image removes further from the lens as the radiant approaches; so that when it is brought within the focal point, the rays diverge to infinite' distances, passing through the lens in parallel lines. 186 Lenses. If A B represent a double convex lens, and c d an object at a considerable distance from the glass, the v;s c.. :::■ — r-~ I— :sjt»*t:'.„ — — rays d a and d b issuing from the point d, will be re- fracted at A and b, and converge to e, and form one ^ extremity of the object ; in the same manner c a and c b, in passing from c, will converge to f, the other extremity ; then if we conceive an infinite number of rays passing from every other point of the object c d, they will cross each other and meet somewhere be- tween E and F, the foci of the lens, forming a complete image of the object reversed, and the linear magnitude of the object and image will be relatively as their dis- tance from the lens. Camera Obscura. This machine is formed on the above principle; for, if G H represent a darkened room, and a b a lens fixed in the side of it, the object c d, with all its sha- dows and colours, will be distinctly seen on a white surface placed in the focus of the glass, but in an in- verted position. As the appearance of inverted objects is unpleasant to the eye, if the lens of the camera be placed in a short tube on the top of a small build- ing, and the image of the objects be reflected through the lens by an inverted mirror placed above it, the pic- ture will be presented in a proper position upon die receiving table in the focal point of the lens ; giving the most beautiful and animated representation of all the surrounding objects in their own colours. 187 ' > ■". The Magic Lantern. This amusing machine is made to magnify small pictures, which are pauited upon glass, and to throw the shadow upon the side of a darkened room; it is principally formed by a convex lens. A lighted candle or lamp is placed in the inside of a square box, which has the tube g b projecting from its side; g h is a thick plano-convex lens, which strongly illuminates the object e f when it is put in- verted mto the tube; k is a concave reflecting mirror to give additional force to the light; and a b is a dou- ble convex lens, placed in a moveable tube, which slides m the interior of the projecting tube g b : when this lens is properly adjusted it throws the shadow of the object large and upright against the side of the wall. The magnitude of the shadow c d is represented as much larger than the image e f, as the distance c a IS greater than e a. Burning Glass. This is a double convex, or plano-convex lens, which collects the sun's rays upon its surface, and con- verges them into a point called the focus: when the rays are thus concentrated they burn with great ardour and will melt the densest metals. ' 188 Telescopes. As all those rays which fall upon the surface of a lens are collected in its focus, the effect will be in pro- portion to the difterence between the surface of the lens and the surface of the focus; therefore, if a lens four inches in diameter collect the sun's rays at the distance of a foot from the glass, the image at the focus will not be more than one tenth of an inch broad, so that the surface is more than sixteen hundred times less than that of the glass; therefore the sun's rays are so many times more dense at that point than on the surface. Burning glasses have been made three feet in diameter, and the rays, after passing through them, have been collected again by another lens placed pa- rallel to the former, so as to converge them into a still smaller point at the focus. By the intense heat of the rays thus combined, gold has been fluxed in a few se- conds, and sheet iron melted in a moment. Telescopes, A TELESCOPE is an optical instrument for discover- ing those distant objects that are invisible to the naked eye, or for rendering more clear and distinct those that are discernible ; it is constructed to act either by re- fraction or reflection. No invention in the mechanic arts has ever proved more useful and entertaining than the production of the telescope ; its utility both by sea and land is too well known to need observation. With respect to the know- ledge of the heavenly bodies, we owe much to the in- vention of the telescope, for without such assistance the bcience of astronomy must have been far short of its present state. The first invention is attributed to John Baptista Porta, a Neapolitan, about two centuries and a half ago; but Galileo soon afterwards greatly improved it, and by this means added considerably to the catalogue of fixed stars. Galileo's telescope passed on for many Astronomical Telescope, 189 years without material alteration, till Gregory and New- ton undertook the construction of telescopes, and brought them to a considerable degree of perfection, which has been completed by Herschel and others in the present day. ' There are many kinds of telescopes ; but as it would greatly exceed our plan to enter into a description of them all, it will be sufficient to describe some of the most material, such as the Astronomical Telescope, the Day or Land Telescope, the Newtonian and Gre- gorian. First, let it be premised, that the object-glass is that lens which is placed at the end of the tube near- est the object; the eye-glass is that which is nearest the the eye, and when there are more lenses than one in the tube, beside the object-glass, they are called eye-glasses likewise. The Astronoinical Telescope, This consists of an object and eye-glass fitted into a long tube. The object glass, which is a segment of a large sphere, is made either double convex or plano- convex; the eye-glass is double convex, formed from a segment of a small sphere, and these glasses are placed in the tube at the common distance of their foci. Suppose rays of light issuing from every part of the object H I, fall upon the object-glass a b ; in passing H A y^v.«^r...... A I_. — through it they will be refracted and converged into tlie foci E K, where the inverted image of the object will be formed; then, if the eye-glass c d, which is of shorter Z 190 La)id Telescope* focal distance, be so placed as to include e k, the rays will pass on through c d, in a position nearly parallel, cross each other at f, and form a large but inverted image of the object on the retina at c. The objects will be magnified by this glass in proportion as the distance of the focus of the object-glass m e, exceeds the dis- tance of the focus of the eye-glass e l. Land Telescope. This instrument is used for viewing objects in the day time, on the surface of the earth, it is usually formed by three double convex eye-glasses, and a dou- ble convex or plano-convex object-glass; it exhibits the objects in an upright position, and the lenses are dis- posed in such a manner in the tube, that the distance between any two may be the aggregate of the distance of their foci; so that an eye placed in the focus of the first glass will see objects upright and distinct, and magnified in the ratio of the distance of the focus of the object-glass to the distance of the focus of the eye-glass at the opposite extremity. If A B be the object, the rays from which are received by the object-glass c d, they enter the first eye-glass G H, but instead of falling into the eye, as in the astro- nomical telescope, they pass on to i k, another lens equally convex, which is placed at double its focal dis- tance from G H, so that the rays are transmitted parallel through the interval between them, and cross each other in the common focus of g h and i k. After passing IK, the rays are again converged into the foci l m, where the image is formed in a position the reverse of Rejiecting Telescopes, 191 E f; these rays are again transmitted through n o, and are tliea collected on the retina of the eye at p, where the image is clearly formed, and in an upright position. As tiie addition of glasses in the land telescope does not magnify the object, an astronomical telescope may be used as a land telescope, by having an extra tube with eye-glasses made to slide into the end of the tele- scope; or that which is used in the day may be used for astronomical observations, by taking out two of the glasses. Land telescopes are sometimes made with three glasses only, and some with five ; but the dimness of the latter is equally inconvenient with the false repre- sentation of the former; so that four glasses, the me- dium, appears the best calculated to avoid the imper- fections of either. The aberration and colouring in the rays of light, as they are transmitted through lenses, was a great obsta- cle to the improvement of telescopes, till a late optician, DoUond, contrived to form the lenses of different kinds of glass, which mutually correct each other's refrangi- bility, and greatly remedy the defects. An instrument thus fitted up is called an Achromatic Telescope. Rejiecting Telescopes, Reflecting telescopes are those which are chiefly formed by mirrors, and reflect the object to the eye in- stead of refracting. They are principally confined to two kinds, called the Gregorian and Newtonian. 192 Meflecti?ig Telescopes* In the construction of the JVeivtonian, let a b c d be C EA I H TB a large tube, open at the end c d, and closed at a b;' the length b d being, at least, equal to the focus of the metallic reflector f e, which is placed near the end of the tube. The rays g ^, i i, that come from h, a distant object, being considered as parallel to each other, fall on the concave speculum e f, and are reflected back upon a small plane speculum l, which is inclined in an angle of 45*^; from this mirror they are again re- flected to a convex lens, which is fixed in the side of the tube, and converged into n , the focus of the glass, where the image appears magnified and distinct to the eye. By fixing an additional tube, with lenses to the side of the telescope al n, the object may be either in- creased farther or diminished, and brought into an up- right position. The Gregorian telescope a b c d, is a large brass Al?- HQI tube, in which is placed a concave metallic speculum E E, with a round hole, ee, perforated in the middle; F G is a small concave mirror fastened to the rod k, which is moved back\vards or forwards at pleasure. If L represent an object at a considerable distance, and Reflecting Telescopes. 193 its rays o o, p /?, enter the tube parallel to each other, they will fall on the larger speculum, e e; from which they are reflected into the foci at m , where an inverted image is formed; but after crossing each other, the rays fall on the concave speculum f g, the centre of which e is the axis of the tube z e. From f g the rays would again converge into the foci q^q^ with image upright; but in converging to bring them close to the eye, they fall upon a convex lens at s s, by which the image is formed in the focus betv^'een i i, and thence taken up and carried to the eye at jz by a meniscus h, where the image appears magnified, upright, clear, and distinct. The latter of these reflecting telescopes is now gene- rally used, as it shows all objects in their natural posi- tions, and is of a form the most convenient for portabi- lity and readiness in management. 194 SOLAR SYSTEM, The power of God being made manifest even in the smallest of his works, how much must the human mind be led to contemplate his infinite wisdom and power, when we survey the multitude of stars that are scattered through the infinity of space! Judging from analogy of the general purposes of creation, we can hardly conceive them to be placed for mere ornament, or even for the purpose of giving light in the absence of the sun, when the reflection of his rays from the moon, a single satellite, gives a thousand times more light to the earth than the whole of the stars. Rather let it be presumed, that the great number of stars, which have no revolutionary motion, are destined for more important purposes; and, like the sun in our system, are inexhaustible fountains of light and heat, which diffuse their vivifying powers to their own sur- rounding orbs ; the opacity of whose bodies, and the immensity of their distance from us, render them in- visible to our eyes. Thus the extent of imagination falls infinitely short in comprehending the greatness of God or his works. We form a comparative idea of the distance of a mile, a thousand or a million of miles ; but where are the bounds of that almighty power, which .created those innumerable systems that are scattered at such immense distances from each other through the infinity of space? The sun, and the planetary worlds which revolve round it, is one of those numerous systems that we are now about to consider. Various opinions have been adopted, at different times, with respect to the motion of the sun and pla- nets; but as the Copernican system is now establish- Solar System' 195 ed, and accepted by all enlightened nations as the most consonant to reason and the operations of nature, it will be sufficient for our purpose, before we enter into an explanation of it, merely to mention two others, which at different times have had their disciples. The first is called the Ptolemean, from Ptolemy its framer, who was born at Pelusium, in Egypt, and flourished as a great mathematician and astronomer soon after the commencement of the Christian sera. Guided by the sensible appearances of the heavenly bodies, without considering either their absolute or relative motion, he considered the earth as a stationary body, fixed in the centre of the system, and that the sun and planets were subordinate, and revolved round the earth in twenty-four hours. After the Ptolemean the other was formed by Tycho Brahe, a Dane; who considered the earth to be placed in the centre of the universe, and that the sun revolv- ed round it, whilst the rest of the planets revolved round the sun. In an improvement of this system, a diurnal motion was given to the earth round* its own axis to account for day and night, more naturally than by a revolution of the whole system in twenty-four hours. But this comphcated and ill-digested hypo- thesis soon fell into disrepute, to make way for that called the Copernican system : which is long likely to endure, as a monument of human ingenuity, and a rational system of the planetary motions. ^ Nicholas Copernicus was born at Thorn, in Prus- sia, in the year 1473. He rather revised and perfected the doctrine of Pythagoras, who existed about 600 years before Christ, than created any new system of his own. The Pythagorean idea of the universe at- tracted the mind of Copernicus, and, after a labour of twenty years, he brought it to perfection, and died just in time to save himself from the bigoted persecu- tion of the Romish church for his discovery. 196 The Copernican^ or Solar System, This supposes the sun placed in the centre of our system, and that the earth and the other planets, re- volve round it in different orbits at immense distances from each other. These planets, which we perceive by the reflection of the sun's rays from their opake bodies, are of three kinds, called primary, secondary, and comets. The primary planets are those which move round the sun in orbits nearly circular and concentric, but at different distances from it; they are seven in num- ber, called ^ Mercury, 9 Venus, © the Earth, o Mars, % Jupiter, I2 Saturn, and j^ the Georgium Sidus. Mars, Jupiter, Saturn, and the Georgium Sidus, are usually called superior planets, because their orbits include that of the earth. Venus and Mercury are called inferior planets, as their orbits are contained within the earth's orbit. In addition to these, are three or four others which have been lately discove ed, but their magnitude and revolutions have not yet been correctly defined. The secondary planets^ which are likewise called satellites or moons, are attendants on primary planets, and revolve round them whilst the primary planets cir- cle round the sun. The earth is accompanied by one moon, Jupiter by four, Saturn by seven, and the Geor^ gium Sidus by six. The comets are bodies which revolve round the sun in the planetary region, but their number and periodical revolutions have not yet been correctly detennined; they move in very eccentric orbits, suddenly appear and disappear, and are usually attended by a long train of light, which is called the tail of the comet. The adjoining diagram is the usual representation of the solar system in piano, but it is not strictly correct, as the planets move in elliptical orbits, die planes of which do not exactlv coincide with one another. Solar System. 197 o represents the sun placed in the centre of the sys- tem, M 1 is the orbit of Mercury round the sun, v 2 the path of Venus, e 3 the orbit of the earth, m 4 the orbit of Mars, j 5 the course of Jupiter, s 6 that of Sa- turn, and c s 7 is the orbit of the Georgium Sidus. All the planets are supposed to have a compound, motion, like the earth, called the annual and diurnal. The annual motion is that with which they pass through their orbits from west to east, forming a year by one complete revolution round the sun. The diurnal motion is the rotation of a planet round an imaginary line passing through its centre, called its axis, whilst it is moving through its annual orbit ; one complete rotation is called a day. This compound pla- netary motion may be conceived by the rolling of a ball 2A 198 Solar System. upon a table, which is perpetually turning round whilst it passes from one extremity to the other. The sun, moon, planets, and fixed stars, all appear to be placed in the same concave sphere, of which the eye of the spectator seems to be the centre ; so that the bodies apparently differ in magnitude, but not in their distances. We estimate the distance of objects on the surface of the earth by some given measure or compa- rative proportion with some other objects less remote; but in viewing celestial bodies, the immensity of their distance affords us no relative means of forming our judgment of their respective positions. Therefore the optical sense is deceived; for demonstrations show us that the sun is nearer to us than the fixed stars, the ocu- lar proof from eclipses convinces us that the moon is nearer the earth than the sun, and our i^eason teaches us to believe that some of the stars are many millions of niiles nearer to us than others. But an easy experi- ment will show how unable we are to judge of distances by our sight alone; for, in a dark night, ijf afew lighted candles be placed at different distances from a spectator, they will all appear equally remote, but the flames will vary in magnitude according to the distance, and the person will be unable to judge, with any correctness, how far he is from them. The planets in their annual courses cross the eclip- tic, or sun's apparent path, in two opposite points called the nodes; but the planets do not move in the same plane with each other, as they cross the ecliptic in dif- ferent parts of the heavens : this may be properly repre- sented by placing different sized hoops within each other in different directions; considering the centre as the sun's place, and the hoops themselves as the orbits of the planets. It has been already observed, that the orbits of the planets are not accurately represented by the circum- ference of a circle, for their course is elliptical, with clitferent eccentricities. Solar System. ' 199 A E B F represents the orbit of a planet with the sun in one of the foci c d; the axis a b is called the line of the apsides; and when the sun is at d, and the planet at a, its greatest dis- tance from it, the planet is said to be in its aphelion, or higher apsis; when it is at the other extremity b, or nearest the sun, it is then in its perihelion, or lower apsis. The mean distance of a planet from the sun is, when the planet is in either extremity of the conju- gate diameter e f. Two planets are said to be in conjunction when they both appear equally advanced in the same part of the heavens ; and when they are in opposite points they are said to be in opposition. Each planet has its peculiar course, which it al- ways pursues without deviation ; the whole courses of the planets are'' included in a certain zone or belt of the heavens, extending between 18^ and 19^ in breadth, which is called the Zodiac, containing the constellations Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. 200 TJie Sun and Planets. The Sun is the great luminary which dispenses light and heat to all the planetary system. It has been usually reckoned amongst the planets, but it more properly belongs to the fixed stars, as one of those central bodies dispersed through the infinity of space which have their subordinate orbs revolving round them. The sun is placed nearly in the centre of our system, and revolves round its own axis in 25i days; the axis has an inclination of about eight degrees with the ecliptic. Although its apparent diameter is seen from the ^arth under an angle of 32' \2" only, the real diame- ter of this beautiful luminary is not less than 890 thousand miles, and it is about 1392500 times bigger than our earth, which is near 96 millions of miles distant from it. It appears to us tohave a revolving motion through an orbit from east to west; but this apparent motion will be hereafter shown to arise from the diurnal motion of the earth from west to east, whilst it is passing through its annual orbit. When the sun is viewed through a telescope it seems to have dark spots on its disk, which, from its globular form and revolving motion, alter their shape and oc- casionally disappear: the various opinions that are given with respect to these spots leave us still in con- siderable doubts. Mercury is the least of all the planets and nearest the sun, which makes it seld(>m visible to us, as its reflected light is absorbed in the sun's more powerful rays. Its greatest elongation or distance from the sun, as viewed from the earth, is not more than 28^; it is computed to be about thirty-seven millions of miles distant from the sun, and revolves round its orbit in ^7 d. 23 h. which forms its year. The diameter of The Sun and Planets^ 201 Mercury is three thousand miles ; it contains 28274000 squ&re miles on its surface, and moves at the rate of 110680 miles in an hour. When it is seen through a telescope, its edge appears clear and dis- tinct. Its body is opake, and, like the moon, reflects a borrowed light, and changes its phases or appear- ance, according to its several positions. When it passes over the sun's face, or is between us and the sun, this is called its transit, and the planet appears like a black spot in the sun's disk. Femis has generally a larger and brighter appear- ance than any other planet, which makes it easily dis- tinguishable from the rest. Its diameter is 7699 miles, and its distance is 69500000 miles from the sun ; it revolves through its orbit, or completes its year in 224 d. 6 h. and moves at the rate of 80955 miles in an hour. Venus forms its day, or turns round on its own axis in 23 h. 22 m. and its greatest elongation from the sun is about 48^. Like Mercury, it is invisible at midnight, and is only seen for two or three hours in the morn- ing or evening when it passes before or after the sun. The Earth is placed next to Venus in the planetary- sphere; its diameter is 7920 miles, and it is about 96 millions of miles distant from the sun. It makes one complete revolution in its orbit in 365 d. 5 h. 48 m.; moving at the rate of 68856 miles in an hour. The earth turns round its own axis from west to east in 23 h. 56 m. which produces the apparent diurnal mo- tion of the sun and all the heavenly bodies from east to west, in.the same time ; the diurnal motion of the earth likewise causes what we call the rising and set- ting of the sun, and the length of days and nights. The axis of the earth is inclined 23|° to the plane of its orbit, and as this axis is always parallel to itself, or in the same direction in every part of its course, it causes the sun at one time of the year to enlighten more of the northern parts of the globe, and at ano- 202 The Sun and Planets. ther time of the southern, which produces the various seasons of spring, summer, autumn, and winter. The Moon is a secondary planet, and an attendant of the earth, revolving in an elliptical orbit, or rather the earth and the moon both revolve round a com- mon centre of gravity, which imaginary point is as much nearer to the earth as the mass of the earth ex- ceeds that of the moon. The moon makes its revolu- tion in its orbit round the earth in 27 d. 7 h. moving at the rate of 2299 miles in an hour. Its time in go- ing round the earth, reckoning from one new moon to another, or when it overtakes the sun again, is 29 d. 12 h. It is 2161 miles in diameter, and 240000 miles distant from the earth, turning round its own axis in the same time that it revolves round the earth, so that its days and nights are of the same length as our lu- nar months. The moon's orbit is inclined to the plane of the ecliptic in an angle of about 5^, and crosses it in two opposite points, called the nodes: lunar eclipses take place when the moon is in or near these points. Mars is the first of the superior planets, and is placed on the outside of the earth's orbit: it is 5309 miles in diameter, and its distance from the suri is about 146 millions of miles; it performs its revolution round the sun in 1 y. 321 d. 23 h. moving at the rate of 55287 miles in an hour, and revolves round its own axis in 24 h. 39 m. This planet has a greater analogy to the earth than any other planet: the diur- nal motion and the obliquity of its ecliptic have very small variation from those of the earth. When it is in opposition to the sun it is five times as near to us as when it is in conjunction, which has a very visible effect on its magnitude : it has a dusky and reddish hue, which is supposed to arise from the nature of the atmosphere that surrounds it. Jupiter is a primary planet, placed between Mars and Saturn, 90228 miles in diameter, and it is about The Siin and Planets, 203 a thousand times bigger than the earth : its distance from the sun is 499750000 miles, and it revolves round its own orbit in 11 y. 314 d. 12 h. moving at the rate of 29000 miles an hour: it has a diurnal mo- tion round its axis in 9 h. S6 m. which carries the equatorial parts of its surface with a velocity of 25000 miles an hour; this is about twenty-five times faster than the revolution of the same parts of the earth. Jupiter has four satellites revolving round it, which enlighten it in the absence of the sun, as the moon enlightens the earth; beside these attendants, it i^ surrounded by faint bodies, which are called its zones or belts ; these appearances are frequently changing, and are ascribed to clouds in its atmosphere. As the axis of Jupiter is nearly perpendicular to its orbit, there is hardly any difference in the seasons, and the days and nights are five hours each. Saturn is the sixth primary planet, and has been considered for many ages as the last and most remote planet in our system until some recent discoveries. In consequence of Saturn's immense distance from the earth, it casts but a feeble light of a dusky colour, although it surpasses all the rest, Jupiter excepted, in actual magnitude. Its diameter is computed to be 79979 miles, and its distance from the sun 916500000 miles, which is near ten times the dis- tance that the earth is from the same luminary. It takes 29 y. 167 d. to make one complete revolution in its orbit, and moves at the rate of 22298 miles in an hour. This planet is surrounded by two rings, one within ihe other, and beyond these rings are se- ven attendant moons, two of which were discovered by Herschel. Georgium Sidiis, This planet was discovered by Herschel in the year 1781: its light is of a bluish white colour, and may sometimes be seen by the naked eye in a clear night, without moonlight. The time of its annual revolution is about 80 years, and 204. The Sun and Planets, its diameter 34299 miles, which is more than four times the diameter of the earth : its distance from the sun is about 1832 millions of miles, and it has an inclination of 43^ 35' to its orbit. To assist the memory and form an idea of the pro- portional distance of each planet from the sun ; if the greatest extent of the Georgium Sidus from the sun were divided into 190 parts, the proportional dis- tance of the rest of the orbits would be, Mercury 5, Venus 7, the Earth 10, Mars 15, Jupiter 52, and Saturn 95. 205 The Earth and Moon distinctly considered; with an explanation of Seasons and Eclipses. The Earth. In the early ages, the opinions of mankind were much divided concerning the form of the earth; some, being guided by visual appearance, conceived it to be a stationary plane, bounded by the horizon, and that the whole universe was contained in that part of the heavens which was presented to their view. How- ever ill conceived this opinion may seem in the pre- sent day, it has had its supporters, even in the ages of Christianity. But as a strong proof that all the sages of antiquity were not equally ignorant of its real form, we find the ancient Babylonians and Greeks calculated eclipses both of the sun and the moon, which may be taken as a fair argument to show that they were not unacquainted with the rotundity of the earth. The number of convincing proofs which are pro- duced in the present day, cannot leave a doubt of its globular form, even in the commonest minds: for those who stand on the seashore, and observe a ship making out for sea, will perceive the hull first de- cline, as it approaches the horizon, till it totally dis- appear, and leave the mast and sails still in sight, and these gradually decline till the top of the mast sink from the eye ; even then, if the spectator as- cend the top of a hill or building, he will perceive the vessel again, till the convexity again hide it from his sight. It is perfectly clear, that if the earth were a plane, the hull of the vessel, which is the largest part of the body, would be seen the longest, and the mast and sails would first disappear as the inferior objects of 2B '206 The Earth, sight: but observation proves the reverse: then what else, than the sphericity of the earth, can produce this effect? When a ship is sailing at sea, either northwards or southwards, those stars which are placed nearly op- posite to the poles of the earth, appear to have no diurnal motion but remain fixed in the extreme parts of the heavens: therefore, if the earth were a plane, considering the immense distance of the stars, a ship in sailing either directly north or south would still observe them under the same angle, or with the same altitude; but daily experience teaches us the contrary, for vessels sailing northwards observe a gradual elevation of those stars which are in the north polar regions, and a depression of those towards the south : in sailing southwards, the appearance is the reverse, for the southern stars are elevated and the others depressed. This appearance is rationally ex- plained by the convexity of the earth, which increases the angle of observation as the ship sails towards the star, and decreases it as the vessel moves the oppo- site way. Eclipses of the moon are caused by the earth's sha- dow falling upon it, when the earth's body is inter- posed between the sun and the moon; yet, we always iind, that, in whatever position the earth may be placed at that time, its shadow falls with a circular edge upon the disk of the moon, which could not always happen if the earth were not of a globular form. The irregularities on the surface of the earth have no visible effect upon its shadow on the moon, for the highest mountain on its surface, considering the mag- nitude of the earth and its distance from the moon, would cause no more visible effect in its shadow, thim the finest grain of sand would produce on that of a billiard ball. The globular form of the earth is likewise practi- cally known by those circumnavigators who have Motion of the Earth. 207 sailed round it, by always continuing an easterly or westerly course, which has brought them again to the same port whence they set out. - These and various other proofs in the higher de- partments of science, leave no doubt of the sphericity of the earth, although it differs, in some measure, from a sphere, as it is flatted towards its poles, some- thing resembling an orange, so that the diameter of the earth at the equator exceeds that at the poles by about thirty-four miles : this was discovered, by ob- serving that a pendulum moved slower as it approach- ed the equator, and faster as it advanced towards the poles ; and this difference is caused by the centrifugal motion of the earth on its axis, which diminishes the force of gravity towards the equatorial parts of the globe, and flattens the earth towards its polar extre-- mities. Motion of the Earth, Independently of a small motion which occa- sions what is called the precession of the equinoxes, the earth has two general motions ; one round its own axis in twenty-four hours, which is called its diurnal motion, and causes the succession of day and night; the other is its annual motion or revolution round the sun as its centre, keeping its axis always inclined to its path in an angle of about 23^ degrees, which produces the various seasons of spring, summer, au- tumn, and winter. To the visual sense it would seem that the earth is fixed as the centre of our planetary system, and that the sun and the rest of the celestial bodies have a daily motion round it. It is not extraordinary that men in early ages should have considered this as the system of planetary motion, when, at the present hour, the uninformed, who judge only from sight, will not be persuaded to give up their opinions; but to those who ?08 Motion of the Earth. are susceptible of conviction, there are so many proofs of this error, that nothing but ignorance or obstinacy can hesitate to beUeve them. The following observation may tend to show re- lative motion, and how easily our senses may be de- ceived. If a person be placed under the deck of a vessel when it is sailing gently down the side of a coast with a fair wind, he will be perfectly insensible of its mo- tion; but if he cabt his eyes on the shore, he will see all the objects pass him with a rapidity equal to the velocity of the vessel, and the vessel itself will be ap- parently at rest. But here our reason tells us rhat our senses are deceived, and that the motion is in the ship and not in the objects. Then why cannot we suffer ourselves to believe that our sight is deceived by the apparent motion of the sun, and simplify the system of the universe by admitting a diurnal revolution of the earth from west to east, rather than force such a monstrous hypothesis as would drive the whole uni- verse round the earth from east to w^est to form the period of a day. By the diurnal motion of the earth, the same phe- uomena appear as if all the celestial bodies turned round it: so that in its rotation from west to east, when the sun or a star just appears on the eastern side of the horizon, it is said to be rising, and, as the earth continues its revolution, it seems gradually to ascend till it has reached the meridian, which is di- rectly south of the observer; here the object has its greatest elevation, and begins to decline till it set, or become invisible on the western side. In the same manner the sun appears to rise and run his course to the western horizon, where he disappear^ and night ensues, till he again illuminate the same part of the earth in another diurnal revolution. All the heavenly bodies do not appear to rise and set ; those that are placed in the two poles, or are op- Motion of the Earth. 209 posite to the imaginary axis of the earth, can have no apparent motion from its daily revolution, therefore al- ways appear in the same part of the heavens. In summing up the diurnal motion of the earth, it is considered as a globular opake body turning round on an imaginary axis from west to east, and that it is en- lightened by the sun's rays, which perpetually illumi- nate one half of its surface : the imaginary great circle which separates the illuminated part from that which is dark, or turned from the sun's rays, is called the termi- nator, and when any point on the eastern side of the globe comes to this line, it is sunrise, and when the point advances to the edge of the terminator on the op- posite or western side, it is called sunset. The light which gradually appears before the rising of the sun and gradually decreases after sunset, is called the crepus- culum, or twilight, and is occasioned by the reflection of the sun's rays from the atmosphere that surrounds the earth. Before we speak of the annual motion of the sun, it may be seen, by the following diagram, that if the earth be at rest and the sun in motion, or, if the sun be at rest and the earth in motion, the same general ap- pearance and effect will be produced; that is, either of them would apparently describe a great circle in the heavens in the plane of the ecliptic. For, if the earth be supposed at rest, and the sun re- volving round it, in the orbit a b c d, it would appear to a spectator on the earth, to describe the great circle f e g h in the heav^ens ; for, if a spectator on the earth at o view the sun at A, it will be referred to e, and at B to F, and so on of the rest. But, if the sun be placed at o, and the earth in the orbit A B G p; when the earth is at a, the sun will appear in a great 210 Changes of the Seasons. circle of the heavens at h ; when it is at b the sun will be seen at g, and so on with the other two points. Therefore, whatever regards the sun's place, with respect to its appearance in the heavens, it may be con- sidered, instead of the earth, as moving in an infinitely- great circle, called the ecliptic, having its centre in the eye of the observer. Changes of the Seasotis, The earth is s'lpposed to be divided into two equal parts, by a great circle drawn at equal distances from the' poles, which is called the equator : smaller circles drawn parallel to the equator, approaching the poles, are called the parallels of latitude, and great circles in- tersecting the equator at right angles and passing through the poles, are called meridians of longitude. The ecliptic is the earth's orbit^ or the apparent annual course of the sun, making an angle of about 23i degrees with the equator, from which the various seasons are derived, and the terminator is a great circle which bounds the illuminated part of the earth, or that half qf its surface which is always turned towards the sun. Let A B c D represent the earth, with its poles a b, so placed, that the terminator of the sun's rays passes through each pole; then it will likewise divide every circle or parallel of latitude Ji ^, ^, f, &c. into two equal C] parts, so that one half will be enlightened by the sun's rays, and the other will be dark or turned from them; but during the diurnal mo- tion, every part of each circle, or each meridian, will be brought to the terminator, and carried through the illu- minated part in the same time, which makes the days Changes of the Seasons, 211 and nights of equal length on every part of the globe, except at the poles a and b, where the sun would be seen during the whole day, just skimming above the horizon. But if the axis of the earth a b do not coincide with the great circle of the termi- ^y^ nator e f; the great circle "^ c D, called the equator, will be divided into t^o equal parts at g, and the inhabi- tants under that line, will have their days and nights of equal length; but the pa- rallels aa, b b, c c, he, will be divided into unequal parts by the terminator e f, and the inhabitants under those parallels which have the greatest part of their circle illuminated, will have their days longer, and nights shorter, than those which lie on the opposite side of the equator, where the dark and illuminated parts are just the reverse. It is likewise clear, from inspection, that as the diurnal and nocturnal parts of these parallels respectively increase from the equator, those places that lie under them will have a greater disproportion of day and night, so that, those that are at or near the pole a, will have constant day, or the sun always above the horizon ; and those at the op- posite pole B, will have continual darkness, or the sun always beneath the horizon, for six months together. In the present position of the axis, those who lie on the northern side of the equator have their summer, and those on the southern, their winter. The difference of light is not the only cause of sum- mer and winter. The sun appears much higher above the horizon to those places which have the longest days, consequently the rays fall more perpendicularly upon the earth, which, joined to the superior quantity of heat that is communicated by the greater length of the day, 212 Changes of the Seasons. day, causes the summer to be much liotter than winter. Having shown that the length of day and night is produced by the different relations of the axis of the earth to the terminator, we will next explain how this varies at different times of the year, and produces the variety of seasons. We find, by common observation, that the sun de- scribes different parallels, or daily appears at different heights above the horizon, which shows, that the plane of the ecliptic, or sun's apparent path, does not coincide with the plane of the equator, otherwise the sun would have the same altitude daily,-' and days and nights would always be of an equal length. Let it be remembered, that the effect is the same, whether the motion be in the earth or the sun ; therefore, we more properly conclude, that the axis of the earth is inclined to its orbit, or that the plane of the equator does not coincide with the plane of the ecliptic. Now it is found by observation, that this inclination of the planes forms an angle of 23^ degrees, which remains invariable throughout the whole of the earth's annual course, or that the axis of the earth al- ways moves parallel to itself. Therefore, if the earth's axis be inclined to the plane of the ecliptic, as in the last figure, one of its poles will be towards the sun, and the other will be removed from it in an equal proportion: but when the earth has made half of its revolution, and has arrived in the opposite point of its orbit, still retaining the inclination of its axis, the pole which was towards the sun will now be removed from it, and the opposite pole which was dark- ened, will be presented to it. As the earth passes from one of tliese extremes to the other, the plane of the ecliptic wdll coincide with the plane of the equator, near the intermediate distance between the extreme points', and then the terminator passes through the two poles of the earth, as in figure the first. From this revolution the seasons are produced; for Changes of the Seasons, 21: when the northern pole is the most inclined towards the- sun, it is midsummer to all the inhabitants on the north side of the equator; who then have their longest day, and shortest night: but when the earth is in the oppo- site pare of the ecliptic, or 180*^ distant, then the soudi- ern pole has its greatest inclination to the sun, and the inhabitants on that side of the equator have their mid- summer and longest dciys, while those in the norUiern hemisphere have their shortest days and midwinter. When the earth is in the intermediate part of its orbit, or 90 "^ distant from either extreme, that is, when the plane of the ecliptic coincides with the equator, the days and nights are of equal length all over the world, which is in the vernal and autumnal equinoxes, or spring and autumn. The following experiment will -show the effects which are produced by the inclination of the earth's axis to its orbit through all the twelve signs of the zodiac. Let the frame c p represent the elliptical orbit of the earth, which inter- sects the equator A B in the two points, or nodes E F ; making an an- gle of 23i^, which Hj is the inclination of the earth's axis to the ecliptic; and let s be a lighted can- ^ die, placed in the centre of the frame, representing the sun in the middle of the planetary system; with the ecliptic divided in- to signs, and the corresponding month marked against each. 2C 214 Changes of the Seasons. If a small globe, or terrella, representing the earthy be suspended by a string from e, or Libra, where the circles intersect each other, and the eye be placed a little above the light in the centre, the hemisphere of the globe will be illuminated, including both poies ; the apparent situation of the sun will be in the opposite side of the ecliptic f, or in the sign Aries, and this is called the vernal equinox, m hich kippens about the 20th of March, when the terminator passes through the poles, and makes the days and nights equal all over the earth. Whilst the terrella is moving through Libra, Scorpio, and Sagittarius, the terminator, or edge of the light, keeps increasing beyond the upper or north pole ; where it has attained its greatest distance, then the globe is in Capricornus, g, and the sun is apparently in the opposite sign Cancer, "h, which happens on the 21st of June. Now all the parallels of latitude in the northern hemisphere have the greatest part of their circles illu- minated, and their days are the longest ; but the dura- tion of light, or the length of day, is in proportion to the distance of the parallels from the equator, increas- ing from it to the nordi pole, and decreasing in like ra- tio from the equator to the south pole. But with respect to the terrella. Whilst we trace the return of the terminator towards the north pole through Capricornus, Aquarius, and Pisces, we perceive it advance towards the south, till the terrella is in the be- ginning of Aries, and the apparent place of the sun is in Libra, then the terminator passes through the poies, and the days and nights are again of an equal length, which happens at the autumnal equinox, about the 22d of September. Let the terrella be moved on through Aries, Taurus, and Gemini, till it reach the first degree in Cancer, and the sun's apparent place will be in Capricornus; during its progress through these signs, the illuminated part, or the terminator, leaves the northern pole in darkness, and enlightens the re- gions about the southern. Now the greater part of the Changes of the Seasons. 215 circles or parallels of latitude in the northern hemi- sphere lYt in darkness, whilst those in the southern have their greatest portion of light, which produces midwinter to the former, and midsummer to the latter; and this takes place about the 21st of December. By moving the globe through the three remaining signs, Cancer, Leo, and Virgo, the terminator again ap- proaches towards the north, illuminates both poles, regains its first position in Libra, and completes its annual revolution. What is usually called summer, that is, from the vernal till the autumnal equinox, is nearly eight days longer than from the autumnal till the vernal; for the sun in passing through the six northern signs Aries, Taurus, Gemini, Cancer, Leo, and Virgo, performs its apparent motion in 186d. llh. 51m,; but in passing through the winter signs of Libra, Scorpio, Sagitta- rius, Capricornus, Aquarius, and Pisces, it only takes up 178d. 17h. 58m. which makes a difference of 7d. 17h. 53m. Let A B c D represent the earth's orbit, and s the sun in one of its foci; when the earth is at c the sun ap- pears at H the first sign Aries, and as the earth moves through c B to d, the sixy, southern signs; the sun ap- pears to move through the six northern h e r. In like manner, whilst the earth pas- ses through the northern signs D A c, the sun passes through the southeni"jF g h, the corresponding circle in the heavens to half of the earth's orbit c b d. Thus the line f h divides the ecliptic into two equal parts, and the elliptical orbit of the earth into two unequal parts ; the gi'eater part c a r>^ is that which the earth describes in summer, and the 216 The Moon's Motion, less is its Avinter course. Beside these unequal divisions of the earth's orbit, it apparently moves slower in its aphelion, or the distant part of its course, than in its perihelion, or when it is nearest the sun. 1 he apparent diameter of the sun is greater in win- ter than in summer, for the sun is considerably nearer the earth whilst the earth passes through the winter signs c B D, than whilst it passes through the summer signs D A c. It may naturally be asked, why the winter is colder than the summer, as the sun is nearer the earth? In summer, as before observed, the sun rises much higher above the horizon, therefore, its rays fall in a greater quantity, and more directly upon the earth than in winter; likewise the length of day, or the time that the sun is above the horizon in summer, being much longer than in winter, the earth and atmosphere receive more heat in the day than they lose in the night, so that we have a gradualaccumulationof heatduriiig the summer months, which makes it generally hotter after the sun has passed the summer solstice, or tropic of Cancer, than in any of the preceding signs. The Moon's Motion, The moon is one of those heavenly bodies, which we call satellites ; it is secondary to the earth, and re- volves round it, whilst the earth performs its annual course. The moon's apparent place, viewed by a spectator on the earth, is extended to a great circle of the heavens, and seems to move through the twelve signs of the zo- diac, in a month or lunar day. The plane of the moon's orbit, if it were extended, would intersect the ecliptic in two points, making an angle with it of about five degrees ; but this inclination vaines, being greatest when the moon is in its quadra- The Moon's Motion, 217 tiire, and least when it is in conjunction or opposition with the sun. The two points where the moon's orbit cuts the ecliptic are called the nodes: when the moon ascends from the south to the north side of the ecliptic, it is called the ascending node, and from the north to the south, the descending node. The line of the nodes is not always directed to the same point, but has a motion contrary to the order of the signs, and, by this retro- grade course, it completes its circuit in 18y. 225d., at which time the line of the nodes returns to the same point in the ecliptic. When the moon crosses the ecliptic, it is in its nodes, but in all other parts of its orbit it is in north or south latitude, according as it is above or below the ecliptic. The mean time of a revolution of the moon about the earth, that is, from one new moon to another, is called a sy nodical month, or lunation, and consists of 29d. 12h. 44m. The line of its revolution round the earth, from any point in the zodiac to the same point again, is called a periodical month, and contains 27d. 7h. 43m. The moon moves in its orbit about 2290 miles in an hour, and only turns once round its own axis whilst it makes a revolution round the earth, which causes it always to present the same side towards the earth, and makes its day and night of the same length as our lunar month. If the earth Avere stationary, the periodical and sy« nodical months would be the same; but as the earth keeps moving forwards in its orbit, whilst the moon is performing its revolution, it has not only to pass through its own orbit, but has likewise to overtake the earth again in its passage through the ecliptic. 218 Phases of the Moon . For if s be the sun in the centre of the system; e part of the earth's orbit, and M A the orbit of the moon ; when the moon is in conjunc- tion at A, if the earth remain- ^ ed at B, whilst it made its revolution a, a, m, by a, the periodical and synodical months would be the same: but, during this revolution, the earth has passed on in its orbit to c ; therefore the moon must advance to c, before the earth and moon can come into conjunction again; but it is obvious, by inspection, that when the moon has arrived at f, it will have completed its revolution round its orbit, and the time of performing the remaining arc f c, will be the difference of time between the periodical and synodical month, which is about two days and five hours. Phases of the Moon, ^ The moon is a dark opake body moving round the earth in a small orbit, and shines by a borrowed light from the sun, which illuminates one half of its body, and leaves the other in darkness. We perceive differ- ent degrees of this illumination, according to the vari- ous positions of the moon, with respect to the sun and the earth: hence we see one half of its body enlighten- ed, or a full face, when it is placed in opposition, or in that part of its orbit which is the most remote from the sun. When the moon is in conjunction, or in that part of its orbit which is between the earth and the sun, its enlightened face is turned from us, which renders it invisible; this is the time of new moon. When the moon appears in the intermediate part of its orbit, be- tween the conjunction and opposition, it is in its quad- ratures, and about half of its illuminated surface is Phases of the Moon. 2 1^ turned towards us. Its phases and appearances are par- ticularly explained by the figure. Let s represent the sun, k the earth, a b c d, &c. the moon in its orbit, with the sun's rays falling on that half of its surface which is opposite to the sun, and the outer circle c, b, c, d, &c. the various phases of the moon, as they appear to a spectator on the earth, during the whole of a lunar month. When the moon is in conjunction with the sun at A, the darkened side is presented towards the earth; therefore, being an opake body, it becomes invisible, and it is then called the time of new moon: when it has passed an eighth of its orbit and arrived at b, a quarter of its enlightened surface will be turned to- wards the earth, and it will appear horned as at b. In passing another eighth of its orbit, it arrives at c, and 220 Eclipses of the Moon. it is then in it^ quadrature ; one half of its surface ap- peared illuminated at c; after having passed on to d, it appears gibbous, and more than one half of its face is illuminated as appears at d; at e, the whole face is seen bright, which is called opposition, or the full moon. Thus, after having attained its fullest appear- ance, it again begins to decline from e to f, and ap- pears gibbous atyV thence it passes to g its quadrature, and is seen at g, half illuminated, then, after being horned at /z, it completes its revolution and falls into conjunction at a, with the sun. The earth serves to enlighten the moon, in the same manner as the moon enlightens us; but its ap- pearance must be much larger than that of the moon to the earth, and the changes take place in contrary order ; that is, when the moon appears full to us, the earth must be in conjunction with the sun, which turns the darkened surface to the lunarians. Soon after the new moon, the whole body is dimly seen, independently of the illuminated crescent on its outer surface, which proceeds from the light that is reflected on it from the earth; for at our new moon the earth appears as a full moon to the lunarians, and part of the light whidi they receive from us, is again re- flected back to the earth. Eclipses of the Moo?i, An eclipse of the moon is a privation of light, caused by the interposition of the earth directly be- tween the sun and the moon, vvhich intercepts the sun's rays, and prevents them from illuminating her surface. Or it may be considered as proceeding from the conical shadow of the earth, when the moon enters between the base and the vertex. As the earth's orbit is in the plane of the ecliptic when it is viewed from the sun, it is evident that the earth's shadow must tend Eclipses of the Moon. 221 directly to that part of the heavens ; and as the moon's orbit has an inchnation of about five degrees with the ecHptic, and only crosses it in two points, called its nodes; the shadow of the earth cannot fall upon th^p moon, except it is in or near one of its nodes. Let the line a d represent a part of the ecliptic, the plane of which coincides with that of the earth's orbit, and c B part of the orbit of the moon, crossing the ecliptic at h, which is called its node. Then if e f g h represent the earth or its shadow, in four different po- sitions in its orbit; when the moon i, approaches its node H, and the shadow of the earth is at e, it has no part of the sun's rays intercepted; but if the earth be at F and the moon at k, a small obscuration takes place, and the moon is partially eclipsed. When the moon is at L and the shadow at g, it enters wholly into it, which is called a total eclipse : but when the moon's centre passes through the centre of the earth's sha- dow, as at H, which can only happen when the moon is directly in one of its nodes, it is called a total and central eclipse. The duration of a central eclipse, or the time that the moon takes, from entering the shadow to quitting it, is about four hours ; during two hours of this time the moon passes through three times the length of its diameter totally eclipsed. The moon's diameter is supposed to be divided into twelve equal parts called digits, and the magnitude of a partial eclipse is denominated by the number of parts 2D ^22 Eclipses of the Moon, that are obscured ; thus, if the shadow pass through a quarter of the moon's diameter, it has three digits eclipsed. l^he earth, like all other opake globular bodies^ which receive the sun's rays, not only throws a dark converging, or conical shadow behind it; but has likewise a thin diverging shadow on each of its sides, called the penumbra, which is occasioned by a partial obscuration of light, from the sun* For if s be the sun, and e the earth, receiving its rays on its surface ; there will be a dark shadow or total obscuration in the cone m, a, n, which cannot receive a ray of light from any part of the sun, and a penum- bra or thinner shade will fall on each side, in the angu- lar parts B M A, c N A, increasing in darkness towards the sides ma, na. For the penumbra b m a can only receive a partial light from the upper part of the sun towards g, which keeps decreasing till it terminates at the side of the cone in the line g m a. In like manner the penumbra A N c is deprived of any rays from the upper part of the sun, and is only partially illuminated by the lower towards h, till the rays terminate in the line h n a. The moon passes through the penumbra before it enters the dark shadow, and afterwards traverses the opposite shade before it resumes its ordinary bright- ness: it may be distantly perceived when it is in the outer side of the penumbra, but when it approaches near to the dark cone, its surface is much more ob- scured. Eclipses of the Moon, 22^ Lunar eclipses are visible over every part of the earth, that has the moon at that time above the hori- zon ; and the eclipse appears of the same magnitude to all from the beginning to the end. On the northern side of the equator, the eastern side of the moon enters the v^restern side of the shadow and passes out by the eastern. Total central eclipses are of the longest du- ration; that is, when the diameter of the earth's sha- dow passes through the centre of the moon in its nodes; as the moon quits its nodes, either into north or south latitude, the eclipses become more partial and of less duration. The length of an eclipse, even in the nodes, is not always the same; for if it happen that the moon is in apogee, and the earth in aphelion, their greatest distances from each other, the length of the eclipse will be about 3 h. 57m.; but if it take place when the moon is in perigee, and the earth in perihe- lion, their nearest distance, then the duration will be 8h. 37 m. only. The moon in the midst of an eclipse has usually a faint copperish appearance ; this is supposed to pro- ceed from the rays of light, which are refracted by the earth's atmosphere, and fall upon the surface of the moon. The moon's nodes have a motion from the conse- quent to the antecedent signs, which move about 19^- degrees in a year; so that in 18 y. 225 d. it passes through all the signs in the ecliptic, and returns again to the same point. If the moon's nodes were fixed in the same part of the ecliptic, there would be just half a year between the times of the sun's conjunction with the nodes; therefore, in whatever sign or month of the year an eclipse should take place, it would always happen at the same time in every succeeding yeai'f but as the moon changes the situation of its nodes in the ecliptic for 18 y. 225 d. this w^ill be the period of succession before the same eclipses fal] in the same part of the ecliptic. 224 Eclipses of the Sun. What is called an eclipse of the sun is caused by the interposition of the moon between the sun and the earth ; which can only happen when the moon is in or near its conjunction. This seems more properly call- ed an eclipse of the earth, as the sun loses no part of its brightness ; but ,the intervention of the moon be- tween the sun's face and the earth, causes a partial darkness upon a small part of the earth's surface, ac- companied by a penumbra, or thinner shade, like that which was explained in the preceding subject. In a solar eclipse, let s represent the sun, l the earth, and m the moon in that part of its orbit which is called in its conjunction, or between the earth and the sun ; then r , s, t, u, is the moon's conical shadow, which passes over z, s, a small part of the earth, and produces an eclipse, or withholds the sun's rays from that part of its surface; and on each side of the conical shadow is the penumbra, or shade, which is caused by the partial deprivation of the rays of the sun. Now as the moon is so much less than the earth, it can only cover a small part by its shadow, therefore those parts that are out of its shade can perceive no appearance of an echpse; even a central eclipse, that is, when the moon's centre passes through the diame- iter of the sun, can only be visible to all those who have the moon above the horizon. In solar eclipses, the moon's shado^v upon the sur- face of the earth does not, in general, exceed 180 miles in diameter; though the penumbra extends se^ Eclipses of the Sun. 225 V eral hundred miles round. If the ecHpse happen when the moon is exactly in its nodes, it will cast a circular shadow on the earth, but when the moon has northing or southing the shade is elliptical. The course of the moon's shadow on the earth is generally from east to west, inclining towards the north, if it be in its ascending node, and towards the south in descending. The whole time that the shadow and penumbra take to pass any given point, is called the general eclipse; the total eclipse is only whilst the darkest part passes the place. In solar eclipses, the face of the moon appears co- vered with a faint light, which is attributed to the re- flection of the illuminated parts of the earth. When the moon changes in its apogee, or greatest distance from the earth, its shadow is not sufficiently long to reach to its surface, and the sun appears like a lumi- nous ring round the dark body of the moon, and forms what is called an annular eclipse. liXPLANATION OF TERMS IN THE PRECEDING WORK, *'i BERRJTIOJ\f\ in optics, the deviation or dispersion of the rays of light when reflected by a speculum or refracted by a lens, by which they are prevented from meeting or uniting in the same point, and then produce a confusion of images. Acceleration^ the increasing velocity of heavy bodies as they fall towards the centre of the earth, by the force of gravity. ^chromatic telescofie^ a species of refracting telescope which pro- duces the images of objects bright, distinct, and uninfected with colours about the edges, through the whole extent of a very large field or compass of view. Analogy^ the comparison of several ratios of quantities or num- bers one to another. Afihelion^ that point in the orbit of a planet in which it is at its greatest distance from the sun. Apogee., that point of the orbit of a planet which is farthest from the earth. Apses^ in astronomy, are the two points in the orbits of the pla- nets, where they are at their greatest and least distances from the sun or the earth. Atmosphere^ a term used to signify the whole of the fluid mass, consisting of air, aqueous and other vapours, electric fluids. Sec, which surrounds the earth to a considerable height. Attenuate^ to weaken or rarefy. Attrition^ the action or rubbing of one body upon another. Caloric^ supposed to be that elastic fluid which produces heat. Capillary tubes^ extremely fine tubes, like hairs. Catoptrics^ the science of reflex vision, or that part of optics which explains the laws and properties of light reflected from mir- rors or specula. Centrifugal force, is that by which a body revolving about a cen- tre, or about another body, endeavours to recede from it. 228 EXPLANATION OF TERMS. Ccntrijietal force, is that by which a moving body is perpetually urged towards a centre, and made to revolve in a curve in- stead of a right line. Cohesion^ that principle by which the p^ticlos of matter in all bo- dies combine and stick together. Collafising, falling together. Coliidun, the dashing or striking together of two bodies. Concave, an appellation used in speaking of the inner surface of hollow bodies, more especially of spherical or circular ones. Concentric, having the same centre. Condensation, the art of compressing or reducing a body into a less bulk or spape, by which means it is rendered more dense and compact. Congelation or freezing, the act of fixing the fluidity of any liquid by cold, or the application of cold bodies. Conical, of the form of a cone or sugar loaf. Contact, the relative state of two things that touch each other, but without cutting or entering, or where surfaces join each other without any inierstice. Convergi.ng, tending to one point. , Convex, round or curved, and protuberant outwards, as the out- side of a globular body. Corputicles, the minute parts or particles that constitute natural bodies. Curvilinear, bounded by curved lines, as the circumference of a circle, ellipsis or oval, 8cc. Densitij, that property of bodies by which they contain a certain quantity of matter under a certain bulk or magnitude. Diagram, a scheme for the explanation or demonstration of any figure or its properties. Diolitrics, the doctrine of refracted vision, or that -part of optics which explains the effects of light, as refracted by passing through different mediums, as air, water, glass, &c. and espe- cially lenses. Disk, the body or face of the sun or moon, which appears to us as a circular plane, although it is a spherical body. Diverging, in optics, is particularly applied to rays which, issuing from a radiant point, or having in their passage undergone a refraction or reflexion, do continually recede farther from each other. Eccentricity is the distances between the centres of two circles or spheres which have not the same centre, or the distance from the centre of an ellipse to one of its foci. Effluvium, a flux or exhalation of minute particles from any body, or an emanation of subtile corpuscles from a mixed sensible body, by a kind of motion or transpiration. EXPLANATION OF TERMS. 229 Klongation^ the removal of a planet to the farthest distance it can be from the sun, as it appears to an observer on the earth. Equilibrium^ equality of weight, equal balance between two forces acting in opposite directions. Evafioration, the act of dissipating the humidity of a body in fumes or vapour. Expandon^ the swelling or increase of the bulk of a body when acted upon by a superior degree of heat, or the effect produced by rarefaction. Ferruginous^ partaking of the nature and quality of iron. Fixity^ a property which enables a body to endure fire and other violent agents. Focus^ in optics, is the point of convergency, or that where the rays meet after refraction, or reflection. Friction^ the rubbing together of two bodies. Fulcrum^ a fixed point about which a lever, 8cc. turns and moves. Gibbous^ a term applied to the moon when she appears more than half full or enlightened, to distinguish her from the state when she is less than half full, or a crescent. Globules, very small spherical bodies. Gravity, weight, or that quality by which all heavy bodies tend towards the centre of the earth. Hemisphere, half a globe or sphere. Heterogeneous, composed of different kinds, natures, or qualities. Horizontally, parallel to the horizon. Horizon, a circle dividing the visible part of the earth and hea- vens from that which is invisible. Hypothesis, m philosophy, denotes a kind of system laid down from our own imagination, by which to account for some phe- nomena or appearances of nature. Ignite, to kindle or generate fire. Impulsion, XhvvisiiYi^ forwards or driving on. injiection, in optics, called also diffraction and deflection of the rays of light, is a property of them, by reason of which, when they come within a certain distance of any body, they will be either bent from or towards it, being a kind of imperfect re- flection or refraction. Intensity, the degree or rate of the power or energy of any quality. Interstices, spaces between, or where parts are not in contact. Lens, a piece of glass or other transparent substance, so formed that the rays of light in passing through it have their direction changed. 2E 230 EXPLANATION OF TERMS. Maximum, the greatest quantity, force, 8cc. which can take place under certain circumstances. Medium, denotes that space, or region, of fluid, &c., through which a body passes in its motion towards any point. MeHdian, in astronomy, is a great circle of tlie celestial sphere passing through the poles of the world, and both the zenith and nadir, crossing the equinoctial at right angles, and dividing the sphere into two equal parts or hemispheres, the one eastern and the other western. Meridian, in geography, is a great circle passing through the poles of the earth, and any given place the meridian of which it is. Momentum, the quantity of motion in a moving body. Oblate, flatted at the poles. Ofiake, dark, thick, not transparent. Orbit, the course in which any planet moves. Oscillation, or vibration, is the reciprocal ascent and descent of a pendulum. Pendulum, any heavy body so suspended as that it may swing backwards and forwards about some fixed point by the force of gravity. Penumbra, a faint or partial shade in an eclipse, observed be- tween the perfect shadow and the full light. Perilielion, that point in the orbit of a pUmet or comet which is nearest to the sun. In which sense it stands opposed to Aphe- lion, which is the highest or most distant point from the sun. Perigee, is that point in the heavens in which the sun or any planet is nearest to the earth. Phases, the various appearances or quantities of illumination of the Moon, Venus, Mercury, and the other planets, by the sun. Phenomenon, a singular appearance in nature. Porosity, the quality of being porous or full of small holes. Quadrature, in astronomy, that aspect or position of the moon when she is 90° distant from the sun. Radiant, any point from which rays proceed. Ramous, full of branches or fibres. Ratio, the rate or proportion which several quantities or numbers bear to each other. Refrangible, capable of being refracted or bent. Reflection of the rays of light, is their motion after being repel- led or reflected from the surface of bodies. R<^^ ^•?-t- -^i^i. ':y - ^iV ^., ^^