Education Library A SYSTEM OF NATURAL PHILOSOPHY IN WHICH THE PRINCIPLES OF MECHANICS, HYDROSTATICS, HYDRAULICS, PNEUMATICS, ACOUSTICS, OPTICS, ASTRONOMY, ELECTRICITY, MAGNETISM, STEAM ENGINE, AND ELECTRO-MAGNETISM, ARE FAMILI ARLY EXPLAINED, AND ILLUSTRATED BY MORE THAN TWO HUNDRED ENGRAVINGS. TO WHICH ARE ADDED, aUESTlONS FOR THE EXAMINATION OF PUPILS. DESIGNED FOR THE USE OF SCHOOLS AND ACADEMIES. BY J, L eOMSTOCK, M, D, Author of Introduction to Mineralogy, Elements of Chemistry, Introduction to Botany, Outlines of Geology, Outlines of Physiology, Nat. Hist. Birds. &c. STEREOTYPED FROM THE FIFTY-THIRD EDITION. NEW-YORK: ROBINSON, PRATT, & CO. 63 WALL-STREET. 1840. ENTERED, ^ According to act of Congress, in the year 183S, by J. L. COMSTOCK, in the Clerk's Office of the District Court of Connec.,- r ' v JJ./. ADVERTISEMENT THE necessity of reprinting the Author's Natural Philosophy, has given him an opportunity of reviewing and correcting the whole, and of making many changes, which could not have been done on stereotype plates. In addition to these corrections, he has added about forty pages of letterpress, and more than twenty new cuts, chiefly on the subjects of the Steam Engine and Electro- Magnetism. Both these subjects the Author has taken reat pains to explain and illustrate, in such a manner as to make them understood by the pupil. The mechanical principles on which this engine acts, it will be allowed, have been comprehend- ed only by a very few ; while the subject of electro- magnetism has become exceedingly interesting, on account of recent attempts to make its force a motive power. The whole work has been newly stereotyped ; and on all accounts, therefore, it is believed, will be much more acceptable to the public than formerly. J. L. C. Hartford, Ct., May, 1838. QCQiV PREFACE. WHILE we have recent and improved systems of Geogra- phy, of Arithmetic, and of Grammar, in ample variety, and Reading and Spelling Books in corresponding abundance, many of which show our advancement in the science of edu- cation, no one has offered to the public, for the use of our schools, any new or improved system of Natural Philosophy. And yet this is a branch of education very extensively studied at the present time, and probably would be much more so, were some of its parts so explained and illustrated as to make them more easily understood. The author therefore undertook the following work at the suggestion of several eminent teachers, who for years have regretted the want of a book on this subject, more familiar in its explanations, and more ample in its details, than any now in common use. The Conversations on Natural Philosophy, a foreign work now extensively used in schools, though beautifully written, and often highly interesting, is, on the whole, considered by most instructors as exceedingly deficient particularly in wanting such a method in its explanations, as to convey to the mind of the pupil precise and definite ideas ; and also in the omission of many subjects, in themselves most useful to the student, and at the same time most easily taught. It is also doubted by many instructors, whether Conversa- tions is the best form for a book of instruction, and particu - larly on the several subjects embraced in a system of Natu- ral Philosophy. Indeed, those who have had most experi- ence as teachers, are decidedly of the opinion that it is not; and hence, we learn, that m those parts of Europe where the subject of education has received the most attention, and, consequently, where the best methods of conveying instruc- tion are supposed to have been adopted, school books, in the form of conversations, are at present entirely out of use. 1* INDEX. J. Juno, 241. Jupiter, 242. Perkins' experiments, 95 Prismatic spectrum, 219. Properties of bodies, 9. Pneumatics, 124. L. Pumps, 139. Latitude and Longitude, 294. common, 140. how found, 296. forcing, 141. Leyden jar, 313. Pulley, 82. Lenses, various kinds of 194. / * . Lever, 66. R. compound, 73. Rainbow, 221. Level, water, 101-108 Rarity, 21. Lightning-rods, 310. Rockets, how moved, 40. Light, refraction o f i72. Reflection by lenses, 193. reflection jf, 175. Longitude, 294. S. how found, 297. Seasons, 260. heat and cold of, 2(55 M. Screw, 89. Magic lantern, 217. Magnetism, 316. perpetual, 92. Archimedes', 119. electro, 324. Matter, inertia of, 13. Malleability, 23. Sound, propagation of, 161 reflection of, 163. Spring, intermitting, 112. Mars, 240. Solar system, 228. Magnetic needle, 319. Steelyards, 70. Magnets, revolution of, 332. Solar and siderial time, 271 . Mechanics, 64. Stars, fixed, 299. Metronome, 63. Steam-engine, 144. Mercury, 238. Savary's, 144. Microscope, 208. Newcomen's, 147. solar, 209. . Watt's, 151. compound, 211. low and high presstuio l!ff Momentum, 38. Sun, 235. Mechanical powers, 92. Syphon, 111. Mirrors, 176. convex, 179. T. concave, 186. Telescope, 211. metallic, 192. reflecting, 214. Moon, 240. refracting, 211. time of falling to the earth, 31. Tides, 292. phases of, 284. surface of, 2S6. ' U. Motion denned, 36. Uniting wire, what, 326. absolute and relative, 37. velocity of, 37. V. reflected, 40. Velocity of falling bodies, 31. compound, 43. Venus, 238. circular, 43. Vision, 199. crank, 155. perfect, 202. curvilinear, 53. imperfect, 202. resultant, 58. angle of, 203. Musical strings, 165. Vesta, 241. instruments, 164. Volta's pile, 323. Musk, scent of, 11. W. 0. Optics, 169. Wedge, 88. Windlass, 76. definitions in, 170. Optical instruments, 208. Orbit, what, 230. Water, elasticity of, 95. equal pressure of, 96. bursting power of, 100. P. raised by ropes, 122. Wood, composition of, 12. Pallas, 241. Planets, density of, 234. situation of, 247. Whispering gallery, 164. Wind, 166. trade, 168. motions of, 248. Pendulum, 60. Z. ' Penumbra, 29L Zodiac, 232. NATURAL PHILOSOPHY. THE PROPERTIES OF BODIES. 1. A BODY is any substance of which we can gain a knowledge by our senses. Hence air, water, and earth, in all their modifications, are called bodies. 2. There are certain properties which are common to all bodies. These are called the essential properties of bodies. They are Impenetrability, Extension, Figure, Divisibility, Inertia, and Attraction. 3. IMPENETRABILITY. By impenetrability, it is meant that two bodies cannot occupy the same space at the same time, or, that the ultimate particles of matter cannot be pene- trated. Thus, if a vessel be exactly filled with water, and a stone, or any other substance heavier than water, be dropped into it, a quantity of water will overflow, just equal to the size of the heavy body. This shows that the stone only separates or displaces the particles of water, and therefore that the two substances cannot exist in the same place at the same time. If a glass tube open at the bottom, and closed with the thumb at the top, be pressed down into a vessel of water, the liquid will not rise up and fill the tube, because the air already in the tube resists it ; but if the thumb be re- moved, so that the air can pass out, the water will instantly rise as high on the inside of the tube as it is on the outside. This shows that the air is impenetrable to the water. 4. If a nail be driven into a board, in common language, it is said to penetrate the wood, but in the language of philoso- phy it only separates, or displaces the particles of the wood. What is a body 7 Mention several bodies. What are the essential properties of bodies'? What is meant by impenetrability ? How is it proved that air and water are impenetrable 1 When a nail is driven into a board or piece of lead, are the particles of these bodies penetrated or separated ? 10 PROPERTIES OF BODIES. The same is the case, if the nail be driven into a piece of lead ; the particles of the lead are separated from each other, and crowded together, to make room for the harder body, but the particles themselves are by no means penetrated by the nail. 5. When a piece of gold is dissolved in an acid, the par- ticles of the metal are divided, or separated from each other, and diffused in the fluid, but the particles of gold are suppo- sed still to be entire, for if the acid be removed, we obtain the gold again in its solid form, just as though its particles had never been separated. 6. EXTENSION. Every body, however small, must have length, breadth, and thickness, since no substance can exist without them. By extension, therefore, is only meant these qualities. Extension has no respect to the size, or shape of a body. The size and shape of a block of wood a foot square is quite different from that of a walking stick. But they both equally possess length, breadth, and thickness, since the stick might be cut into little blocks, exactly resembling in shape the large one. And these little cubes might again be divided until they were only the hundredth part of an inch in diameter, and still it is obvious, that they would possess length, breadth, and thickness, for they could yet be seen, felt, and measured. But suppose each of these little blocks to be again divided a thousand times, it is true we could not measure them, but still they would possess the quality of ex- tension, as really as they did before division, the only differ- ence being in respect to dimensions. 7. FIGURE, or form, is the result of extension, for we can- not conceive that a body has length and breadth, without its also having some kind of figure, however irregular. 8. Some solid bodies have certain or determinate forms which are produced by nature, and are always the same wherever they are found. Thus, a crystal of quartz has six sides, while a garnet has twelve sides, these numbers being invariable. Some solids are so irregular, that they cannot be compared with any mathematical figure. This is the case with the fragments of a broken rock, chips of wood, fractured glass, &c. Are the particles of gold dissolved, or only separated, by the acid ? What is meant by extension 1 In how many directions do bodies pos- sess extension 1 Of what is figure, or form, the result 1 Do all bodies possess figure ? What solids are regular in their forms 1 What bo- dies are irregular 1 PROPERTIES OF BODIES. 11 9. Fluid bodies have no determinate forms, but take their shapes from the vessels in which they happen to be placed. 10. DIVISIBILITY. By the divisibility of matter, we mean that a body may be divided into parts, and that these parts may again be divided into other parts. 11. It is quite obvious, that if we break a piece of marble into two parts, these two parts may again be divided, and that the process of division may be continued until these parts are so small as not individually to be seen or felt. But as every body, however small, must possess extension and form, so we can conceive of none so minute but that it may again be divided. There is, however, possibly a limit, beyond which bodies cannot be actually divided, for there may be reason to believe that the atoms of matter are inidvisible by any means in our power. But under what circumstances this takes place, or whether it is in the power of man during his whole life, to pulverize any substance so finely, that it may not again be broken, is unknown. 12. We can conceive, in some degree, how minute must be the particles of matter from circumstances that every day come within our knowledge. 13. A single grain of musk will scent a room for years, and still lose no appreciable part of its weight. Here, the particles of musk must be floating in the air of every part of the room, otherwise they could not be every where per- ceived. 14. Gold is hammered so thin, as to take 282,000 leaves to make an inch in thickness. Here, the particles still ad- here to each other, notwithstanding the great surface which they cover, a single grain being sufficient to extend over a surface of fifty square inches. 1 5. The ultimate particles of matter, however widely they may be diffused, are not individually destroyed, or lost, but under certain circumstances, may again be collected into a body without change of form. Mercury, water, and many other substances, may be converted into vapor, or distilled in close vessels, without any of their particles being lost. In What is meant by divisibility of matter 7 Is there any limit to the divisibility of matter 1 Are the atoms of matter divisible 1 What ex- amples are given of the divisibility of matter 1 How many leaves of gold does it take to make an inch in thickness 1 How many square inches may a grain of gold be made to cover"? Under what circum- stances may the particles of matter again be collected in their original form 1 12 PROPERTIES OF BODIES. such cases, there is no decomposition of the substances, but only a change of form by the heat, and hence the mercury and water assume their original state again on cooling. 16. When bodies suffer decomposition or decay, their el- ementary particles, in like manner, are neither destroyed nor lost, but only enter into new arrangements or combina- tions with other bodies. 17. When a piece of wood is heated in a close vessel, such as a retort, we obtain water, an acid, several kinds of gas. and there remains a black, porous substance, called charcoal. The wood is thus decomposed, or destroyed, and its particles take a new arrangement, and assume new forms, but that nothing is lost is proved by the fact, that if the water, acid, gasses, and charcoal, be collected and weighed, they will be found exactly as heavy as the wood was, before distillation. 18. Bones, flesh, or any animal substance, may in the same manner be made to assume new forms, without losing a particle of the matter which they originally contained. 19. The decay of animal or vegetable bodies in the open air, or in the ground, is only a process by which the particles of which they were composed, change their places, and as- sume new forms. 20. The decay and decomposition of animals and vegeta- bles on the surface of the earth form the soil, which nou- rishes the growth of plants and other vegetables ; arid these, in their turn, form the nutriment of animals. Thus is there a perpetual change from death to life, and from life to death, and as constant a succession in the forms and places, which the particles of matter assume. Nothing is lost, and not a particle of matter is struck out of existence. The same mat- ter of which every living animal, and every vegetable, was formed, before and since the flood, is still in existence. As nothing is lost or annihilated, so it is probable that nothing has been added, and that we, ourselves, are composed of par- ticles of matter as old as the creation. In time, we must, in our turn, suffer decomposition, as all forms have done before us, and thus resign the matter of which we are composed, to form new existences. 21. INERTIA. Inertia means passiveness or want of When bodies suffer decay, are their particles lost 1 What becomes of the particles of bodies which decay 7 Is it probable that any matter has been annihilated or added, since the first creation 1 What is said of the particles of matter of which we are made 1 What does inertia mean? PROPERTIES OP BODIES. 13 power. Thus matter is, of itself, equally incapable of put- ting itself in motion, or of bringing itself to rest when in motion. 22. It is plain that a rock on the surface of the earth, never changes its position in respect to other things on the earth. It has of itself no power to move, and would, there- fore, for ever lie still, unless moved by some external force. This fact is proved by the experience of every person, for we see the same objects lying in the same positions all our lives. Now, it is just as true, that inert matter has no pow- 3r to bring itself to rest, when once put in motion, as it is, that it cannot put itself in motion, when at rest, for having no life, it is perfectly passive, both to motion and rest, and therefore either state depends entirely upon circumstances. 23. Common experience proving that matter does not put itself in motion, we might be led to believe, that rest is the natural state of all inert bodies, but a few considerations will show, that motion is as much the natural state of mat- ter as rest, and that either state depends on the resistance, or impulse, of external causes. 24. If a cannon ball be rolled upon the ground, it will soon cease to move, because the ground is rough, and pre- sents impediments to its motion ; but if it be rolled on the ice, its motion will continue much longer, because there are fewer impediments, and consequently, the same force of im- pulse will carry it much farther. We see from this, that with the same impulse, the distance to which the ball will move must depend on the impediments it meets with, or the resistance it has to overcome. But suppose that the ball and ice were both so smooth as to remove as much as pos- sible the resistance caused by friction, then it is obvious that the ball would continue to move longer, and go to a greater distance. Next suppose we avoid the friction of the ice, and throw the ball through the air, it would then continue in motion still longer with the same force of projection, be- cause the air alone, presents less impediment than the air and ice, and there is now nothing to oppose its constant mo- tion, except the resistance of the air, and its own weight, or gravity. 25. If the air be exhausted, or pumped out of a vessel by Is rest or motion the natural state of matter 7 Why does the ball roll farther on the ice than on the grqund 1 What does this prore? Why, with the same force of projection, will a ball move farther through the air than on the ice 1 2 14 PROPERTIES OF BODIES, means of an air pump, and a common top, with a small, haid point, be set in motion in it, the top will continue to spin Cor hours, because the air does not resist its motion. A pendu- lum, set in motion, in an exhausted vessel, will continue to swing, without the help of clock work, for a whole day, be- cause there is nothing to resist its perpetual motion, but the small friction at the point where it is suspended, and gravity. 26. We see, then, that it is the resistance of the air, of fric- tion, and of gravity, which causes bodies once in motion to cease moving, or come to rest, and that dead matter, of itself, is equally incapable of causing its own motion, or its own rest. 27. We have perpetual examples of the truth of this doc trine, in the moon, and other planets. These vast bodies move through spaces which are void of the obstacles of air and friction, and their motions are the same that they were thousands of years ago, or at the beginning of creation. 28. ATTRACTION. By attraction is meant that property, or quality in the particles" of bodies, which make them tend toward each other. 29. We know that substances are composed of small atoms or particles of matter, and that it is a collection of these, united together, that forms all the objects with which we are acquainted. Now, when we come to divide, or separate any substance into, parts, we do not find that its particles have been united or kept together by glue, little nails, or any such mechanical means, but that they cling together by some power, not obvious to our senses. This power we call at- traction, but of its nature or cause, we are entirely ignorant Experiment and observation, however, demonstrate, that this power pervades all material things, and that under different modifications, it not only makes the particles of bodies adhere to each other, but is the cause which keeps the planets in their orbits as they pass through the heavens. 30. Attraction has received different names, according to the circumstances under which it acts. 31. The force which keeps the particles of matter to- Why will a top spin, or a pendulum swing, longer in an exhausted vessel than in the air 1 What are the causes which resist the perpetual motion of bodies 7 Where have we an example of continued motion without the existence of air and friction 1 What is meant by attrac- tion'? What is known about the cause of attraction 7 Is attraction common to all kinds of matter, or not 1 What effect does this power have upon the planets 7 Why has attraction received different namp * PROPERTIES OF BODIES. 15 gether, to form bodies, or masses, is called attraction of co- hesion That which inclines different masses towards each other, is called attraction of gravitation. That which causes liquids to rise in tubes, is called capillary attraction. That which forces the particles of substances of different kinds to unite, is known under the name of chemical at- traction. That which causes the needle to point constantly towards the poles of the earth is magnetic attraction ; and that which is excited by friction in certain substances, is known by the name of electrical attraction. 32. The following illustrations, it is hoped, will make each kind of attraction distinct and obvious to the mind of the student. 33. ATTRACTION OF COHESION acts only at insensible distances, as when the particles of bodies apparently touch each other. 34. Take two pieces of lead, of a round form, an inch in diameter, and two inches long ; flatten one end of each, and make through it an eye-hole for a string. Make the other ends of each as smooth as possible, by cutting them with a sharp knife. If now the smooth surfaces be brought to- gether, with a slight turning pressure, they will adhere with such force that two men can hardly pull them apart by the two strings. 35. In like manner, two pieces of plate glass, when their surfaces are cleaned from dust, and they are pressed to- gether, will adhere with considerable force. Other smooth substances present the same phenomena. 36. This kind of attraction is much stronger in some bodies than in others. Thus, it is stronger in the metals than in most other substances, and in some of the metals it is stronger than in others. In general, it is most powerful among the particles of solid bodies, weaker among those of liquids, and probably entirely wanting among elastic fluids, such as air, and the gases. 37. Thus, a small iron wire will hold a suspended v^eight of many pounds, without having its particles separated ; the How many kinds of attraction are there ? How does the attraction of cohesion operate 1 What is meant by attraction of gravitation 1 What by capillary attraction 1 What by chemical attraction 1 What is that which makes the needle point towards the pole 1 ? How is elec- trical attraction excited 1 Give an example of cohesive attraction 1 In what substances is cohesive attraction the strongest 1 In what sub- stance is it weakest 1 J.6 PROPERTIES OF BODIKS. particles of water are divided by a very small force, whil* those of air are still more easily moved among each othei. These different properties depend on the force of cohesion with which the several particles of these bodies are united. 38. When the particles of fluids are left to arrange them- selves according to the laws of attraction, the bodies which they compose assume the form of a globe or ball. 39. Drops of water thrown on an oiled surface or on wax globules of mercury, hail stones, a drop of water ad- hering to the end of the finger, tears running down the cheeks, and dew drops on the leaves of plants, are all examples of this law of attraction. The manufacture of shot is also a striking illustration. The lead is melted and poured into a sieve, at the height of about two hundred feet from the ground. The stream of lead, immediately after leaving the sieve, separates into round globules, which, be- fore they reach the ground, are cooled and become solid, and thus are formed the shot used by sportsmen. 40. To account for the globular form in all these cases, we have only to consider that the particles of matter are mutually attracted towards a common centre, and in liquids being free to move, they arrange themselves accordingly. 41. In all figures except the globe, or ball, some of the particles must be nearer the centre than others. But in a body that is perfectly round, every part of the outside is exactly at the same distance from the centre. 42. Thus, the corners of a cube, or Fig. 1. square, are at much greater distances from the centre, than the sides, while the circumference of a circle or ball is every / where at the same distance from it. This [ difference is shown by fig. 1, and it is \ quite obvious, that if the particles of \ matter are equally attracted towards the common centre, and are free to arrange themselves, no other figure could possibly be formed, since then every part of the outside is equally attracted. 43. The sun, earth, moon, and indeed all the heavenly Whv are the particles of fluids more easily separated than those of Bohds 1 What form do fluids take, when their particles are left to their own arrangement 7 Give examples of this law. How is the globular form which liquids assume accounted for 1 ? If the particles of a body are free to move, and are equally attracted towards the centre, what must be its figure 1 Why must the figure be a globe? PROPERTIES OF BODIES. 17 Fil waik with so much difficulty, for after they have strength to stand, it requires considerable experience, so to balance the body, as to set one foot before the other without falling. 206. By experience in the ark of balancing, or of keeping the centre of gravity in a line over the base, men sometimes perform things, that, at first sight, appear altogether beyond human power, such as dining with the table and chair standing on a single rope, dancing on a wire, &c. 207. No form, under which matter exists, escapes the ge- neral law of gravity, and hence vegetables, as well as ani- mals, are formed with reference to the position of this centre, in respect to the base. It is interesting, in reference to this circumstance, to ob- serve how exactly the tall trees of the forest conform to this law. 208. The pine, which grows a hundred feet high, shoots up with as much exactness, with respect to keeping its cen- tre of gravity within the base, as though it had been direct- ed by the plumb line of a master builder. Its limbs towards the top are sent off in conformity to the same law ; each one growing in respect to the other, so as to preserve a due balance between the whole. 209. It may be observed, also, that where many trees grow near each other, as in thick forests, and consequently where the wind can have but little effect on each, that they always grow taller than when standing alone on the plain. The roots of such trees are also smaller, and do not strike so deep as those of trees standing alone. A tall pine, in the midst of the forest, would be thrown to the ground by the first blast of wind, were all those around it cut away. Thus, the trees of the forest, not only grow so as to pre- serve their centres of gravity, but actually conform, in a cer- tain senste, to their situation. CENTRE OF INERTIA. 210. It will be remembered that inertia, (21) is one of the inherent, or essential properties of matter, and that it is in consequence of this property, when bodies are at rest, that they never move without the application of force, and when In what does the art of balancing, or walking on a rope, consist ? What is observed in the growth of the trees of the forest, in respect to the laws of gravity 1 What effect does inertia have on bodies at rest 7 What effect does it have on bodies in motion 1 62 EQUILIBRIUM once in motion, that they never cease moving without some external cause. 211. Now, inertia, though, like gravity, it resides equally in every particle of matter, must have, like gravity, a centre in each particular body, and this centre is the same with that of gravity. 212. In a bar of iron, six feet long and two inches square, the centre of gravity is just three feet from each end, or ex- actly in the middle. If, therefore, the bar is supported at this point, it will balance equally, and because there are equal weights on both ends, it will not fall. This, there- fore, is the centre of gravity. Now suppose the bar should be raised by raising up the centre of gravity, then the inertia of all its parts would be overcome equally with that of the middle. The centre of gravity is, therefore, the centre of inertia. 213. The centre of inertia, being that point which, being lifted, the whole body is raised, is not, therefore, always at the centre of the body. 214. Thus, suppose the same bar Fig. 29. of iron, whose inertia was over- come by raising the centre, to have s~^\ balls of different weights attached \^J to its ends ; then the centre of iner- tia would no longer remain in the middle of the bar, but would be changed to the point a, fig. 29, so that to lift the whole, this point must be raised, instead of the middle, as before. EQUILIBRIUM. 215. When two forces counteract, or balance each other, they are said to be in equilibrium. 216. It is not necessary for this purpose, that the weights opposed to each other should be equally heavy, for we have just seen that a small weight, placed at a distance from the centre of inertia, will balance a large one placed near it. To produce equilibrium, it is only necessary, that the weights on each side of the support should mutually coun- teract each other, or if set in motion, that their momenta should be equal. Is the centre of inertia, and that of gravity, the same 1 Where is the centre of inertia in a body, or a system of bodies 1 Why is the point of inertia changed, by fixing different weights to the ends of the iron oar*? What is meant by equilibrium 1 To produce equilibrium, must the weights be equal ? CURVILINEAR MOTION. 53 A pair of scales are in equilibrium, when the beam is m a horizontal position. 217. To produce equilibrium in solid bodies, therefore, it is only necessary to support the centre of inertia, or gravity. 218. If a body, or several bod- Fig. 30. ies, connected, be suspended by a string, as in fig. 30, the point of support is always in a perpendic- ular line above the centre of in- ertia. The plumb line d, cuts the bar connecting- the two balls at this point. Were the two weights in this figure equal, it is evident that the hook, or point of support, must be in the middle of the string, to preserve the hori- zontal position. 219. When a man stands on his right foot, he keeps him- self in equilibrium, by leaning to the right, so as to bring his centre of gravity in a perpendicular line over the foot on which he stands. CURVILINEAR, OR BENT MOTION. 220. We have seen that a single force acting on a body, (153,) drives it straight forward, and that two forces acting 1 crosswise, drive it midway between the two, or give it a di- agonal direction, (160.) 221. Curvilinear motion differs from both these, the di- rection of the body being neither straight forward, nor di- agonal, but through a line which is curved. 222. This kind of motion may be in any direction, but when it is produced in part by gravity, its direction is al- ways towards the earth. 223. A stream of water from an aperture in the side of a vessel, as it falls towards the ground, is an example of a curved line ; and a body passing through such a line, is said to have curvilinear motion. Any body projected forward, as a cannon ball or rocket, falls to the earth in a curved line. 224. It is the action of gravity across the course of the stream, or the path of the ball, that bends it downwards, and When is a pair of scales in equilibrium 1 When a body is suspended by a string, where must the support be with respect to the point of in- ertia 1 What is meant by curvilinear motion 1 What are examples of this kind of motion 1 What two forces produce this motion ? 5* 54 CURVILINEAR MOTION. makes it form a curve. The motion is therefore the result of two forces, that of projection, and that of gravity. 225. The shape of the curve will depend on the velocity of the stream or ball. When the pressure of the water is great, the stream, near the vessel, is nearly horizontal, be- cause its velocity is in proportion to the pressure. When a ball first leaves the cannon, it describes but a slight curve, because its projectile velocity is then greatest. The curves described by jets of water, under different degrees of pressure, are readily illustrated by tapping a tall vessel in several places, one above the other. 226. Suppose fig. 31 be Fig. 31. such a vessel, filled with wa- ter, and pierced as represent- ed. The streams will form curves differing from each other, as seen in the figure, Where the projectile force is greatest, as from the lower orifice, the stream reaches the ground at the greatest distance from the vessel, this distance decreasing, as the pressure becomes less towards the top of the vessel. The action of gravity being always the same,^ the shape of the curve described, .as just stated, must depend on the velocity of the moving body; but whether the pro- jectile force be great or small, the moving body, if thrown horizontally, will reach the ground from the same height in the same time. 227. This, at first thought, would seem improbable, for, without consideration, most persons would assert, very posi- tively, that if two cannon were fired from the same spot, at the same instant, and in the same direction, one of the balls fall- ing half a mile, and the other a mile distant, that the ball which went to the greatest distance, would take the most time in performing its journey. 228. But it must be remembered, that the projectile force On what does the shape of the curve depend 1 How are the curves described by jets of water illustrated 1 What difference is there in re- spect to the time taken by a body to reach the ground, whether the curve be great or small 1 Why do bodies forming different curves from the same height, reach the ground at the same time 1 CURVILINEAR MOTION. 55 does not in the least interfere with the force of gravity. A ball flying horizontally at the rate of a thousand feet per second, is attracted downwards with precisely the same force as one flying only a hundred feet per second, and must therefore descend the same distance in the same time. 229. The distance to which a ball will go, depends on the force of impulse given it the first instant, and consequently on its projectile velocity. If it moves slowly, the distance will be short if more rapidly, the space passed over will be greater. It makes no difference, then, in respect to the descent of the ball, whether its projectile motion be fast, or slow, or whether it moves forward at all. 230. This is demonstrated by experiment. Suppose a cannon to be loaded with a ball, and placed on the top of a tower, at such a height from the ground, that it would take just three seconds for a cannon ball to descend from it to the ground, if let fall perpendicularly. Now suppose the can- non to be fired in an exact horizontal direction, and at the same instant, the ball to be dropped towards the ground. They will both reach the ground at the same instant, pro- vided its surface be a horizontal plane from the foot of the tower to the place where the projected ball strikes. 281. This will be made plain by fig. 32, where a is the perpendicular line of the descending ball, c b the curvilinear path of that projected from the cannon, and d, the horizon- tal line from the foot of the tower. Fig. 32. Suppose two balls, one flying at the rate of a thousand, and the other at the rate of a hundred feet per second, which would descend most during the second 1 Does it make any difference in respect to the de- scent of the ball, whether it has a projectile motion or not 1 Suppose, then, one ball be fired from a cannon, and another let fall from the same height at the same instant, would they both reach the ground at the same time 1 Explain fig. 32, showing the reason why the two balls will reach the ground at the same time. 66 CURVILINEAR MOTION. The reason why the two balls reach the ground at the same time, is easily comprehended. 232. During the first second, suppose that the ball which is dropped, reaches 1 ; during the next second it falls to 2; and at the end of the third second, it strikes the ground. Meantime, the ball shot from the cannon is projected for- ward with such velocity as to reach 4 in the same time thai the other is falling to 1. But the projected ball falls down- ward exactly as fast as the other, for it meets the line 1, 4, which is parallel to the horizon, at the same instant. During the next second, the projected ball reaches 5, while the other arrives at two 5 and here again they have both descended through the same downward space, as is seen by the line 2, 5, which is parallel with the other. During the third sec- ond, the ball from the cannon will have nearly spent its pro- jectile force, and, therefore, its motion downward will be greater, while its motion forward will be less than before. The reason of this will be obvious, when it is considered, that in respect to gravity, both balls follow exactly the same law, and fall through equal spaces in equal times. Therefore, as. the falling ball descends through the greatest space during the last second, so that from the cannon, having now a less projectile motion, its downward motion is more direct, and, like all falling bodies, its velocity is increased as it approaches the earth. 233. From these principles it may be inferred, that the horizontal motion of a body through the air, does not in the least interfere with its gravitating motion towards the earth, and, therefore, that a rifle ball, or any other body projected forward horizontally, will reach the ground in exactly the same period of time, as one that is let fall perpendicularly from the same height. 234. The two forces acting on bodies which fall through curved lines, are the same as the centrifugal and centripetal forces, already explained ; the centrifugal, in case of the ball, being caused by the powder the centripetal, being the ac tion of gravity. 235. Now, it is obvious, that the space through which a cannon ball, or any other body, can be thrown, depends on Why does the ball approach the earth more rapidly in the last part of the curve, than in the first part ? What is the force called which throws a ball forward ? What is that called, which brings it to the ground 1 On what does the distance to which a projected body may be thrown depend 1 Why does the distance depend on the velocity 1 CURVILINEAR MOTION. 57 the velocity with which it is projected, for the attraction of gravitation, and the resistance of the air, acting perpetually, the time which a projectile can be kept in motion, through the air, is only a few moments. 236. If, however, the projectile be thrown from an ele- vated situation, it is plain, that it would strike at a greater distance than if thrown on a level, because it would remain longer in the air. Every one knows that he can throw a stone to a greater distance, when standing on a steep hill, than when standing on the plain below. 237. Bonaparte, it is said, by elevating the range of his shot, bombarded Cadiz from the distance of five miles. Per- haps, then, from a high mountain, a cannon ball might be thrown to the distance of six or seven miles. 238. Suppose the cir- Fi^ 33. cle, fig. 33, to be the earth, and a, a high mountain on its surface. Suppose that this moun- tain reaches above the atmosphere, or is fifty miles high, then a can- non ball might perhaps reach from a to b, a dis- tance of eighty or a hundred miles, because the resistance of the at- mosphere being out of the calculation, it would have nothing to contend with, except the attraction of gravi- tation. If, then, one degree of force, or velocity, would send it to b, another would send it to c : and if the force was increased three times, it would fall at d, and if four times, it would pass to e. If now we suppose the force to be about ten times greater than that with which a cannon ball is pro- jected, it would not fall to the earth at any of these points, but would continue its motion, until it again came to the point &, the place from which it was first projected. It would now be in equilibrium, the centrifugal force being just equal to that of gravity, and therefore it would perform Explain fig. 33. Suppose the velocity of a cannon ball shot from a a mountain 50 miles high, to be ten times its usual rate, where would it stop 1 When vould this ball be in equilibrium 1 58 RESULTANT MOTION. another, and another revolution, and so continue to revolve around the earth perpetually. 239. The reason why the force of gravity will not ulti- mately bring it to the earth, is, that during the first revolu- tion, the effect of this force is just equal to that exerted in any other revolution, but neither more nor less; and, there- fore, if the centrifugal force was sufficient to overcome this attraction during one revolution, it would also overcome it during the next. It is supposed, also, that nothing tends to affect the projectile force except that of gravity, and the force of this attraction would be no greater during any other revolution, than during the first. 240. In other words, the centrifugal and centripetal forces are supposed to be exactly equal, and to mutually balance each other ; in which case, the ball would be, as it were, suspended between them. As long, therefore, as these two forces continued to act with the same power, the ball would no more deviate from its path, than a pair of scales would lose their balance without more weight on one side than on the other. 241. It is these two forces which retain the heavenly bodies in their orbits, and in the case we have supposed, our cannon ball would become a little satellite, moving perpetu- ally round the earth. RESULTANT MOTION. 242. Suppose two men to be sailing in two boats, each at the rate of four miles an hour, at a short distance opposite to each other, and suppose as they are sailing along in this manner, one of the men throws the other an apple. In re- spect to the boats, the apple would pass directly across, from one to the other, that is, its line of direction would be per- pendicular to the sides of the boats. But its actual line through the air would be oblique, or diagonal, in respect to the sides of the boats, because in passing from boat to boat, it is impelled by two forces, viz., the force of the motion of the boat forward, and the force by which it is thrown by the hand across this motion. Why would not the force of gravity ultimately bring the ball to the earth 1 After the first revolution, if the two forces continued the same, would not the motion of the ball be perpetual? Suppose two boats, sail- ing at the same rate, and in the same direction, if an apple be tossed from one to the other, what will be its direction in respect to the boats ? What would be its line through the air, in respect to the boats 1 RESULTANT MOTION. 59 243. This diagonal motion of the apple is called the re- sultant, or the resulting motion, because it is the effect, or result, of two motions, resolved into one. Perhaps this will be more clear by fig. 34, where Fig. 34. a b, and c d, are supposed to be the sides of the two boats, and & the line e, f, that of the apple. Now the apple when thrown, has a motion with the boat at the rate of four miles an hour, from c c towards d, and this motion is e supposed to continue just as though it had remained in the boat. Had it remained in the boat during the time it was passing from e to f, it would have passed from e to h. But we suppose it to have been thrown at the rate of eight miles an hour in the direction towards g, and if the boats are moving south, and the apple thrown towards the east, it would pass in the same time, twice as far towards the east as it did towards the south. Therefore, in respect to the boats, the apple would pass in a perpendicular line from the side of one to that of the other, because they are both in motion; but in respect to one perpendicular line, drawn from the point where the apple was thrown, and a parallel line with this, drawn from the point where it strikes the other boat, the line of the apple would be oblique. This will be clear, when we consider, that when the apple is thrown, the boats are at the points e and g, and that when it strikes, they are at h and / these two points being opposite to each other. The line e f, through which the apple is thrown, is called the diagonal of a parallelogram, as already explained under compound motion. 244. On the above principle, if two ships, during a bat- tle, are sailing before the wind at equal rates, the aim of the gunners will be exactly the same as though they stood still; whereas, if the gunner fires from a ship standing still, at another under sail, he takes his aim forward of the mark he intends to hit, because the ship would pass a little for- ward while the ball is going to her. And so, on the con- What is this kind of motion called 1 Why is it called resultant mo- tion'? Explain fig. 34. Why would the line of the apple be actually perpendicular in respect to the boats, but oblique in respect to parallel lines drawn from where it was thrown, and where it struck 1 How is this further illustrated 1 When the ships are in equal motion, where does the gunner take his aim'? Why does he aim forward of the mark; when the other ship is in motion 1 60 PENDULUM, trary, if a ship in motion fires at another standing still, the aim must be behind the mark, because, as the motion of the ball partakes of that of the ship, it will strike forward of the point aimed at. 245. For the same reason, if a ball be dropped from the topmast of a ship under sail, it partakes of the motion of the ship forward, and will fall in a line with the mast, and strike the same point on the deck, as though the ship stood still. 246. If a man upon the full run drops a bullet before him from the height of his head, he cannot run so fast as to over- take it before it reaches the ground. 247. It is on this principle, that if a cannon ball be shot up vertically from the earth, it will fall back to the same point ; for although the earth moves forward while the ball is in the air, yet as it carries this motion with it, so the ball moves forward also, in an equal degree, and therefore comes down at the same place. 248. Ignorance of these laws induced the story-making sailor to tell his comrades, that he once sailed in a ship which went so fast, that when a man fell from the mast- head, the ship sailed away and left the poor fellow to strike into the water behind her. PENDULUM. 249. A pendulum is a heavy body, such as a piece of brass, or lead, suspended by a wire or cord, so as to swing backwards and forwards. When a pendulum swings, it is said to vibrate ; and that J . part of a circle through which it vibrates, is called its arc. 250. The times of the vibration of a pendulum are very nearly equal, whether it pass through a greater or less part of its arc. Suppose a and b, fig. 35, to be two pendulums of equal length, and suppose the weights of each are carried, the one to c, and the other to d t and both let fall at the same in- If a ship in motion fires at one standing still, where must be the aiml Why, in this case, must the aim be behind the mark 1 What other il- lustrations are given of resultant motion 7 What is a pendulum? What is meant by the vibration of a pendulum ? What is that part of a circle called, through which it swings 1 Why does a pendulum vibrate in equal time, whether it goes through a small or large part of its arc 1 PEND JLUM. mstant ; their vi- Pig. 35. brations would be equal in re- spect to time, the one pass- ing through its arc from c to e, and so back again, in the same time that .he other passes from dtof, and back again. 251. The reason of this appears to be, that when the pen- dulum is raised high, the action of gravity draws it more directly downwards, and it therefore acquires, in falling, a greater comparative velocity than is proportioned to the trifling difference of height. 252. In the common clock, the pendulum is connected with wheel work, to regulate the motion of the hands, and with weights, by which the whole is moved. The vibra- tions of the pendulum are numbered by a wheel having sixty teeth, which revolves once in a minute. Each tooth, there- fore, answers to one swing of the pendulum, and the wheel moves forward one tooth in a second. Thus the second hand revolves once in every sixty beats of the pendulum, and as these beats are seconds, it goes round once in a minute. By the pendulum, the whole machine is regulated, for the clock goes faster, or slower, according to its number of vibrations in a given time. The number of vibrations which a pendu- lum makes in a given time, depends upon its length, because a long pendulum does not perform its journey to and from the corresponding points of its arc so soon as a short one. 253. As the motion of the clock is regulated entirely by the pendulum, and as the number of vibrations are as its length, the least variation in this respect will alter its rate of going. To beat seconds, its length must be about 39 inches. In the common clock, the length is regulated by a screw, which raises and lowers the weight. But as the rod to which the weight is attached, is subject to variations of Describe the common clock. How many vibrations has the pendu- lum in a minute 1 On what depends the number of vibrations which a pendulum makes in a given time 1 What is the medium length of a pendulum beating seconds ? Why does a common clock go faster in winter than in summer"? 62 PENDULUM. length in consequence of the change of the seasons, being contracted by cold and lengthened by heat, the common clock goes faster in winter tha^ in summer. 254. Various means have been contrived to counteract the effects of these changes, so that the pendulums may con- tinue the same length the whole year. Among inventions for this purpose, the gridiron pendulum is considered the best. It is so called, because it consists of several rods of metal connected together at each end. 255. The principle on which this pendulum is construct- ed, is derived from the fact, that some metals dilate more by the same degrees of heat than others. Thus, brass will di- late twice as much by heat, and consequently contract twice as much by cold, as steel. If then these differences could be made to counteract each other mutually, given points at each end of a system of such rods would remain stationary the year round, and thus the clock would go at the same rate in all climates, and during all seasons. This important object is accomplished by the Fig. 36. following means. 256. Suppose the middle rod, fig. 36, to be l^ made of brass, and the two outside ones of steel, all of the same length. Let the brass rod be firmly fixed to the cross pieces at each end. Let the steel rod a, be fixed to the lower cross piece, and Z>, to the upper cross piece. The rod a, at its upper end, passes through the cross piece, and, in like man- ner, b passes through the lower one. This is done to prevent these small rods from playing backwards and forwards as the pendulum swings. 257. Now, as the middle rod is lengthened by the heat twice as much as the outside ones, and the outside rods together are twice as long as the middle one, the actual length of the pendulum can neither be increased nor diminished by the variations of temperature. What is necessary in respect to the pendulum, to make the clock go true the year round 1 What is the principle on which the gridiron pen- dulum is constructed 1 What are the metals of which this instrument is made 1 Explain fig. 36, and give the reason why the length of the pendulum will not change by the variations of temperature 1 PENDULUM. 63 258. To make this still plainer, sunpose the Fig. 37. lower cross piece, fig. 37, to be standing on a ta- ble, so that it could not be lengthened downwards, and suppose, by the heat of summer, the middle rod of brass should increase one inch in length. This would elevate the upper cross piece an inch, but at the same time the steel rod a, swells half an inch, and the steel rod b, half an inch, there- fore, the iwo points, c and d, would remain exact- ly at the same distance from each other. 259. As it is the force of gravity which draws the weight of the pendulum from the highest point of its arc down- wards, and as this force increases, or diminishes, as bodies approach towards the centre of the earth, or recede from it, so the pendulum will vibrate faster, or slower, in proportion as this attraction is stronger or weaker. 260. Now, it is found that the earth at the equator rises higher from its centre than it does at the poles, for towards the poles it is flattened. The pendulum, therefore, being more strongly attracted at the poles than at the equator, vi- brates faster. For this reason, a clock that would keep exact time at the equator, would gain time at the poles, for the rate at which a clock goes, depends on the number of vibrations its pendulum makes. Therefore, pendulums, in order to beat seconds, must be shorter at the equator, and longer at the poles. For the same reason, a clock which keeps exact time at the foot of a high mountain, would move slower on its top. 26 1 . Metronome. There is a short pendulum, used by mu- sicians for marking time, which may be made to vibrate fast or slow, as occasion requires. This little instrument is call- ed a metronome, and besides the pendulum, consists of seve- ral wheels, and a spiral spring, by which the whole is moved. This pendulum is only ten or twelve inches long, and instead of being suspended by the end, like other pendu- lums, the rod is prolonged above the point of suspension, and there is a ball placed near the upper, as well as at the lower extremity. Explain fig. 37. What is the downward force which makes the pen- dulum vibrate 1 Explain the reason why the same clock would go faster at the poles, and slower at the equator. How can a clock which goes true at the equator be made to go true at the poles 1 Will a clock keep equal time at the foot, and on the top of a high mountain 1 Why will it not 1 What is the metronome 1 How does this pendulum differ from common pendulums? 64 MECHANICS. 262. This arrangement will be Fig. 38. Tiederstood by fig. 38, where a is the axis of suspension, b the upper ball, and c the lower one. Now when this pendulum vibrates from the point a, the upper ball constantly retards the motion of the lower one, by in part counterbalancing its weight, and thus preventing its full velocity downwards. 263. Perhaps this will be more apparent, by placing the pendulum, fig. 39, for a moment on its side, and across a bar, at the point of suspen- Fig. 39. sion. In this position, it will be seen, that the little ball would prevent the large one Q- from falling with its full weight, since, were it moved to a cer- tain distance from the point of suspension, it would balance the large one, so that it would not descend at all. It is plain, therefore, that the comparative velocity of the large ball, will be in proportion as the small one is moved to a greater or less distance from the point of suspension. The metronome is so constructed, the little ball being made to move up and down on the rod, at pleasure, and thus its vi- brations are made to beat the time of a quick, or slow tune as occasion requires. By this arrangement, the instrument is made to vibrate every two seconds, or every half, or quarter of a second, at pleasure. o MECHANICS. 264. Mechanics is a science which investigates the laws and effects of force and motion. 265. The practical object of this science is, to teach the best modes of overcoming resistances by means of mechan- ical powers, and to apply motion to useful purposes, by means of machinery. How does the upper ball retard the motion of the lower one ? How is the metronome made to go faster or slower, at pleasure 1 What is mechanics'? What is the object of this science? MECHANICS. 65 266. A machine is any instrument by which power, mo* (ion, or velocity, is applied, or regulated. 267. A machine may be very simple, or exceedingly com- plex. Thus, a pin is a machine for fastening clothes, and a steam engine is a machine for propelling mills and boats. 268. As machines are constructed for a vast variety of purposes, their forms, powers, and kinds of movement, must depend on their intended uses. 269. Several considerations ought to precede the actual construction of a new or untried machine ; for if it does not answer the purpose intended, it is commonly a total loss to the builder. 270. Many a man, on attempting to apply an old princi- ple to a new purpose, or to invent a new machine for an old purpose, has been sorely disappointed, having found, when too late, that his time and money had been thrown away, for want of proper reflection, or requisite knowledge. 271. If a man, for instance, thinks of constructing a ma- chine for raising a ship, he ought to take into consideration the inertia, or weight, to be moved the/orcgtobe applied the strength of the materials, and the space, or situation, he has to work in. For, if the force applied, or the strength of the materials, be insufficient, his machine is obviously useless; and if the force and strength be ample, but the Space be wanting, the same result must follow. 272. If he intends his machine for twisting the fibres of flexible substances into threads, he may find no difficulty in respect to power, strength of materials, or space to work in, but if the velocity, direction, and kind of motion he obtains, be not applicable to the work intended, he still loses his labour. 273. Thousands of machines have been constructed, which, so far as regarded the skill of the workmen, the in- genuity of the contriver, and the construction of the indi- vidual parts, were models of art and beauty; and, so far as could be seen without trial, admirably adapted to the intend- ed purpose. But on putting them to actual use, it has too often been found, that their only imperfection consisted in a stubborn refusal to do any part of the work intended. 274. Now, a thorough knowledge of the laws of motion, and the principles of mechanics, would, in many instances What is a machine? Mention one of the most simple, and one of the most complex of machines. 6* 66 LEVER. at least, have jTevented all this loss of labour and money, and spared him so much vexation and chagrin, by showing the projector that his machine would not answer the intend- ed purpose. 275. The importance of this kind of knowledge is there- fore obvious, and it is hoped will become more so as we proceed. 276. Definitions. In mechanics, as well as in other sciences, there are words which must be explained, either because they are common words used in a peculiar sense, or because they are terms of art, not in common use. All technical terms will be as much as possible avoided, but still there are a few, which it is necessary here to explain. 277. Force is the means by which bodies are set in mo- tion, kept in motion, and when moving, are brought to rest. The force of gunpowder sets the ball in motion, and keeps it moving, until the force of resisting air, and the force of gra- vity, bring it to rest. 278. Power is the means by which the machine is moved, and the force gained. Thus we have horse power, watei power, and the power of weights. 279. Weight is the resistance, or the thing to be moved by the force of the power. Thus, the stone is the weight to be moved by, the force of the lever, or bar. 280. Fulcrum, or prop, is the point or part an which a thing is supported, and about which it has more or less mo- tion. In raising a stone, the thing on which the lever rests, is the fulcrum. 281. In mechanics, there #re a few simple machines, called the mechanical powers, and however mixed, or com- plex, a combination of machinery may be, it consists only of these few individual powers. 282. We shall not here burthen the memory of the pu- pil with the names of these powers, of the nature of which he is at present supposed to know nothing, but shall explain the action and use of each in its turn, and then sum up the whole for his accommodation. THE LEVER. 283. Any rod, or bar, which is used in raising a weight, What is meant by force in mechanics 1 What is meant by powor ? What is understood by weight 1 What is the fulcrum 1 Are the me- chanical powers numerous, or only few in number ? LEVER. r surmounting a resistance, by being placed on a fulcrum, or prop, becomes a iever. 284. This machine is the most simple of all the mechani- cal powers, and is therefore in universal use. 285. Fig. 40repre- . Fig. 40. ents a straight lever, or handspike, called also a crow-bar, which is commonly used in raising and moving stone and other heavy bodies. The block b is the weight, or re- sistance, a is the lever, and c, the fulcrum. 286. The power is the hand, or weight of a man, applied at a, to depress that end of the lever, and thus to raise the weight. It will be observed, that by this arrangement, the applica- tion of a small power may be used to overcome a great re- sistance. 287. The force to be obtained by the lever, depends on its length, together with the power applied, and the distance of the weight and power from the fulcrum. 288. Suppose, fig. 41, that a Fig. 41. is the lever, b the fulcrum, d ^ the weight to be raised, and c the power. Let d be consider- ed three times as heavy as c, and the fulcrum three times as far from c as it is from d ; then the weight and power will ex- actly balance each other. Thus, if the bar be four feet long, and the fulcrum three feet from the end, then three pounds on the long arm, will weigh just as much as nine pounds on the short arm, and these pro- portions will be found the same in all cases. 289. When two weights balance each other, the fulcrum What is a lever 1 What is the simplest of all mechanical powers 1 Explain fig. 40. Which is the weight'] Where is the fulcrum 1 Where is the power applied"? What is the power in this case? On what does the force to be obtained by the lever depend 1 Suppose a lever 4 feet long, and the fulcrum one foot from the end, what number of pounds will balance each other at the ends 1 When weights balance each other, at what point between them must the fulcrum be? O 6 68 LEVER. is always at the centre of gravity between them, and there- fore, to make a small weight raise a large one, the fulcrum must be placed as near as possible to the large one, since the greater the distance from the fulcrum the small weight or power is placed, the greater will be its force. 290. Suppose me weight b, Fig. 42. fig. 42, to be sixteen pounds, and suppose the fulcrum to be placed so near it, as to be raised by the power a, of four y pounds, hanging equally dis- tant from the fulcrum and the end of the lever. If now the power a, be removed, and another of two pounds, c, be placed at the end of the lever, its force will be just equal to a, placed at the middle of the lever. 291. But let the fulcrum be moved along to the middle of the lever, with the weight of sixteen pounds still suspended to it, it would then take another weight of sixteen pounds, instead of two pounds, to balance it, fig. 43. 292. Thus, the power which Fig. 43. would balance 16 pounds, _ when the fulcrum is in one place, must be exchanged for another power weighing eight times as much, when the ful- crum is in another place. From these investigations, we may draw the following general truth, or proposition, concerning the lever : " That the force of the lever increases in proportion to the distance of the 'power from the fulcrum^ and diminishes in pro- portion as the distance of the weight from the fulcrum in- creases" 293. From this proposition may be drawn the following rults side, and goods are to be removed from the vessels into the upper stories. Instead of removing the goods into the saore, and hoisting them in the direction of a, it is only ne- cessary to carry the rope b, over the pulley c, which is at the end of a strong beam projecting out from the side of the store, and then the goods will be raised in the direction of d, thus saving the labour of moving them twice. The wheel and axle, under different forms? is applied to a variety of common purposes. 323. The capstan, in universal Fig. 57. use, on board of ships and other vessels, is an axle placed upright, with a head, or drum, a, fig. 57, pierced with holes, for the levers b, c, d. The weight is drawn by the rope e, passing two or three times round the axle to prevent its' slipping. This is a very powerful and convenient machine. When not in use, the levers are taken out of their places and laid aside, and when great force is required, two or three men can push at each lever. 324. The common windlass for drawing water, is another modification of the wheel and axle. The winch, or crank, by which it is turned, is moved around by the hand, and there is no difference in Fig. 58. the principle, whether [^ffTtrTTt a whole wheel is turn- r= ed, or a single spoke. |flJJUU.a-& The winch, therefore, answers to the wheel, "^ while the rope is taken up, and the weight rais- ed by the axle, as al- ready described. 325. In cases where great weights are to be raised, and it is required that the machine should be as small as possible, on account of room, What is the capstan 1 Where is it chiefly used 1 What are the pe- culiar advantages of this form of the wheel and axlel In the com- mon windlass, what part answers to the wheel 1 Explain fig. 58. *. iSJliaJ 3 WHEEL AND AXLE. 77 the simple wheel and axle, modified as represented by fig. 58, is sometimes used. 326. The axle may be considered in two parts, one of which is larger than the other. The rope is attached by its two ends, to the ends of the axle, as seen in the figure. The weight to be raised is attached to a small pulley, or wheel, round which the rope passes. The elevation of the weight may be thus described. Upon turning the axle, the rope is coiled round the larger part, and at the same time it is thrown off the smaller part. At every revolution, there- fore, a portion of the rope will be drawn up, equal to the circumference of the thicker part, and at the same time a portion, equal to that of the thinner part, will be let down. On the whole, then, one revolution of th machine will shorten the rope where the weight is suspended, ju3t as much as the difference between the circumference of the two parts. 327. Now, to understand the principle on which this machine acts, we must refer to fig. 59, where it is obvious that the two parts of the rope a and b, equally support, the weight d, and that the rope, as the ma- chine turns, passes from the small part of the axle e, to the large part h, consequently, the weight does not rise in a perpendicular line towards e, the centre of both, but in a line between the outsides of the large and small parts. Let us consider what would be the consequence of changing the rope a to the larger part of the axle, so as to place the weight in a line perpendicular to the axis of motion. In this case, it is obvious that the machine would be in equilibrium, since the weight d would be di- vided between the two sides equally, and the two arms of a lever passing through the centre c, would be of equal length, and therefore no advantage would be gained. But in the actual arrangement, the weight being sustained equally by the large and small parts, there is involved a lever power, the long arm of which is equal to half the diameter of the Why is the rope shortened, and the weight raised ? What is the de- sign of fig. 59 1 Does the weight rise perpendicular to the axis of mo- tion ] Suppose the cylinder was, throughout, of the same size, what would be the consequence 1 On what principle does this machine act 7 Which are the long and short arms of the lever, and where is the ful- 78 WHEEL AND AXLE. large part, while the short arm is equal to half the diameter of the small part, the fulcrum being between them. 328. System of Wheels. As the wheel and axle is only a modification of the simple lever, so a system of wheels acting on each other, and transmitting the power to the re- sistance, is only another form of the compound lever. 329. Such a combi- nation is shown in fig. 60. The first wheel, a, by means of the teeth, or cogs, around its axle, moves the se- cond wheel, b, with a force equal to that of a lever, the long arm of which extends from the centre of the wheel and axle to the cir- cumference of the wheel, where the pow- er p is suspended, and the short pjrn from the same centre to the ends of the cogs. The dotted line c, passing through the centre of the wheel a, shows the position of the lever, as the wheel now stands. The centre on which both wheels turn, it will be obvious, is the fulcrum of this lever. As the wheel turns, the short arm of this lever will act upon the long arm of the next lever by means of the teeth on the circumference of the wheel b, and this again through the teeth on the axle of A, will transmit its force to the circurr ference of the wheel d, and so by the short arm of the third lever to the weight w. As the power or small weight falls therefore, the resistance, w, is raised, with the multiplied force of three levers, acting on each other. 330. In respect to the force to be gained by such a ma chine, suppose the number of teeth on the axle of the wheel a, to be six times less than the number of those on the cir- cumference *of the wheel b, then b would only turn round once, while a turned six times. And, in like manner, if the number of teeth on the circumference of d, be six times greater than those on the axle of b, then d would turn once, )n what principle does a system of wheels act, as represented in fig, 60 7 Explain fig. 60, and show how the power p is transferred by the action of levers to w. WHEEL AND AXLE. 79 viiile b turned six times. Thus, six revolutions of a would make b revolve once, and six revolutions of b would make d revolve once. Therefore, a makes thirty-six revolutions while d makes only one. 331. The diameter of the wheel a, being three times the diameter of the axle of the wheel d, and its velocity of mo- tion being 36 to 1, 3 times 36 will give the weight which a power of 1 pound at p would raise at w. Thus 36X3=108. One. pound at p would therefore balance 108 pounds at w. 332. No machine creates force. If the student has attend- ed closely to what has been said on mechanics, he will now be prepared to understand, that no machine, however simple or complex it may be, can create the least degree of force. It is true, that one man with a machine, may apply a force Avhich a hundred could not exert with their hands, but then it would take him a hundred times as long. 333. Su ppose there are twenty blocks of stone to be moved a hundred feet ;* perhaps twenty men, by taking each a block, would move them all in a minute. One man, with a capstan, we will suppose, may move them all at once, but this man, with his lever, would have to make one revolution for every foot he drew the whole load towards him, and therefore to make one hundred revolutions to perform the whole work. It would also take him twenty times as long to do it, as it took the twenty men. His task, indeed, would be more than twenty times harder than that performed by the twenty men, for, in addition to moving the stone, he would have the friction of the machinery to overcome, which commonly amounts to nearly one third of the force em- ployed. 334. Hence there would be an actual loss of power by the use of the capstan, though it might be a convenience for the one man to do his work by its means, rather than to call in nineteen of his neighbours to assist him. 335. The same principle holds good in respect to other machinery, where the strength of man is employed as the power, or prime maver. There is no advantage gained, except that of convenience. In the use of the most simple of all machines, the lever, and where, at the same time, there What weight will one pound at p balance at w 1 Is there any actual power gained by the use of machinery 1 Suppose 20 men to move 20 stones to a certain distance with their hands, and one man moves them back to the same place with a capstan, which performs the most actual labour ? Why 1 Why, then, is machinery a convenience 1 SO WHEEL AND AXLE. is the least force lost by friction, there is no actual gain of Dower, for what seems to be gained in force is always lost in velocity. Thus, if a lever is of such length to raise 100 pounds an inch by the power of one pound, its long arm must pass through a space of 100 inches. Thus, what is gained in one way is lost in another. 336. Any power by which a machine is moved, must be equal to the resistance to be overcome, and, in all cases where the power descends, there will be a proportion be- tween the velocity with which it moves downwards, and the velocity with which the weight moves upwards. There will be no difference in this respect, whether the machine be simple or compound, for if its force be increased by increasing the number of levers, or wheels, the velocity of the moving power must also be increased, as that of the resistance is diminished. 337. There being, then, always a proportion, between the velocity with which the moving force descends, and that with which the weight ascends, whatever this proportion may be, it is necessary that the power should have to the resistance the same ratio that the velocity of the resistance has to the velocity of the power. In other words, " The power multiplied by the space through which it moves, in a vertical direction, must be equal to the weight multiplied by the space through which it moves in a vertical direc- tion" 338. This law is known under the name of " the law of virtual velocities," and is considered the golden rule of mechanics. 339. This principle has already been explained, while treating of the lever (292) ; but that the student should want nothing to assist him in clearly comprehending so import- ant a law, we will again illustrate it in a different manner. 340. Suppose the weight of ten pounds to be suspended on the short arm of the lever, fig. 61, and that the ful- crum is only one inch from the weight ; then, if the le- In the use of the lever, what proportion is there between the force of the short arm, and the velocity of the long arm 1 How is this illus- trated 1 It is said, that the velocity of the power downwards, must be in proportion to that of the weight upwards 1 Does it make any dif- ference, in this respect, whether the machine be simple or compound 1 What is the golden rule of mechanics'? Under wha't name is this law known 1 WHEEL AND AXLE. 81 ver be ten inches long, on the other side Fig. 61. of the fulcrum, one pound at a would raise, or balance, the ten pounds at b. But in raising the ten pounds one inch in a ver- > tical direction, the long arm of the lever must fall ten inches in a vertical direction, and therefore the velocity of a would be ten times the velocity of b, 341. The application of this law, or rule, is apparent. The power is one pound, and the space through which it falls is ten inches, therefore 10X1=10. The weight is 10 pounds, and the space through which it rises is one inch, therefore 1X10=10. 342. Thus, the power, multiplied by the space through which it moves, is exactly equal to the weight, multiplied by the space through which it moves. Fig. 62. 343. Again, suppose the lever, fig. 62, to be thirty inches long from the ful- crum to the point where the power p is suspended, and that the weight w is two inches from the ful- crum. If the power be 1 pound, the weight must be 15 pounds, to produce equi- librium, and the power p must fall thirty inches, to, raise the weight w 2 inch- es. Therefore the power being one pound, and the space 30 inches, 30X1=30. The weight being 15 pounds, and the space 2 inches, 15X2=30. Thus, the power, multiplied by the space through which it falls, and the weight multiplied by the space through which it rises, are equal. However complex the machine may be, by which the force of a descending power is transmitted to the weight to be raised, the same rule will apply, as it does to the action of the simple lever. Explain fig. 61, and show how the rule is illustrated by that figure. Explain fig. 62, and show how the same rule is illustrated by it. What is said of the application of this rule to complex machines ? 82 PULLET. Fig. 63. THE PULLEY. 344. A pulley, consists of a wheel, which is grooved on the edge, and which is made to turn on its axis, by a chorfc passing over it. 345. Fig. 63 represents a simple pulley, with a single fixed wheel. In other forms of the machine, the wheel moves up and down, with the weight. 346. The pulley is arranged among the simple mechanical powers ; but when several are connected, the ma- chine is called a system of pulleys, or a compound pulley. 347. One of the most obvious ad- vantages of the pulley is, its enabling men to exert their own power, in places where they cannot go themselves. Thus, by means of a rope and wheel, a man can stand on the deck of a ship, and hoist a weight to the topmast. By means of two fixed pulleys, a weight may be raised upward, while the power moves in a horizontal direction. The weight will also rise vertically through the same space that the rope is drawn horizontally. 348. Fig. 64 represents two fixed pulleys, as they are a'/ranged for such a purpose. In the erection of a lofty edi- fice, suppose the upper pulley to be suspended to some part of the building ; then a horse, pulling at the rope a, would raise the weight w vertically, as far as he went horizon- tally. 349. In the use of the wheel of the pulley, there is no mechanical advantage, except that which arises from re- moving the friction, and diminishing the imperfect flexibi- ity of the rope. What is a pulley 1 What is a simple pulley 1 What is a system of pulleys, or a compound pulley *? . What is the most obvious advan- tage of the pulley 1 How must two fixed pulleys be placed to raise a weight vertically, as far as the power goes horizontally 1 What is ht advantage of the wheel of the pulley 7 Fig. 64. S~\ PULLET. 83 350. hi the mechanical effects of this machine, the result would be the same, did it slide on a smooth surface with the same ease that its motion makes the wheel revolve. 351. The action of the pulley is on a different principle from that of the wheel and axle. A system of wheels, as Fig. G5. already explained, acts on the same prin- ciple as the compound lever. But the mechanical efficacy of a system of pul- leys, is derived entirely from the division of the weight among the strings employed in suspending it. In the use of the single fixed pulley, there car. be no mechanical advantage, since the weight rises as fast as the power descends. This is obvious by fig. 63 ; where it is also apparent that the power and weight must be exactly equal, to balance each other. 352. In the single moveable pulley, fig. 65, the same rope passes from the fixed point a, to the power p. It is evident here, that the weight is supported equally by the two parts of the string between which it hangs. There- fore, if we call the weight w ten pounds, five pounds will be supported by one string, and five by the other. The power, then, will sup- port twice its own weight, so that a person pulling with a force of five pounds at p, will raise ten pounds at w. The mechanical force, therefore, in respect to the power, is as two to one. In this example, it is supposed there are only two ropes, each of which bears an equal part of the weight. 353. If the number of ropes be increased, the weight may be increased with the same power; or the power may be diminished in proportion as the number of ropes is increas- ed. In fig. 66, the number of ropes sustain- ing the weight is four, and therefore, the weight may be four times as great as the power. How does the action of the pulley differ from that of the wheel and axle ? Is there any mechanical advantage in the fixed pulley '* What weight atp, fig. 65, will balance ten pounds at w 7 Suppose the num- ber of ropes be increased, and the weight increased, must the power bo increased also 7 r4 PULLEY. Tills principle must be evident, since it is plain that each rope sustains an equal part of the weight. The weight may therefore be considered as divided into four parts, and each part sustained by one rope. 354. In fig. 67, there is a system of pulleys represented, in which the weight is sixteen times the power. .355. The tension of the rope Fig. 67. d, e, is evidently equal to the' power, p, because it sustains it : d, being a moveable pulley, must sustain a weight equal to twice the power ; but the weight which it sustains, is the tension of the second rope, d, c. Hence the ten- sion of the second rope is twice that of the first, and, in like manner, the tension of the third rope is twice that of the second, and so on, the weight being equal to twice the tension of the last rope. 356. Suppose the weight ?, to be sixteen pounds, then the two ropes, 8 and 8, would sustain just 8 pounds each, this being g the whole weight divided equally between them. The next two ropes, 4 and 4, would evidently sustain but half this whole, weight, because the other half is already sustained by a rope, fixed at its upper end. The next two ropes sustain but half of 4, for the same reason-; and the next pair, 1 and 1, for the same reason, will sustain only half of 2. Lastly, the power p, will balance two pounds, because it sustains but half this weight, the other half being sustained by the same rope, fixed at its upper end. 357. It is evident, that in this system, each rope and pul- ley which is added, will double the effect of the whole. Thus, by adding another rope and pulley beyond 8, the Suppose the weight, fig. 66, to be 32 pounds, what will each rope bear 1 Explain fig. 67, and show what part of the weight each rope sustains, and why 1 pound atp will balance 16 pounds at w. Explain the reason why each additional rope and pulley will double the effect of the whole, or why its weight may be double by that of 'all the others, with the same power. W INCLINED PLANE. 85 weight w might be 32 pounds, instead of 16, and still be balanced by the same power. 358. In our calculations of the effects of pulleys, we have allowed nothing for the weight of the pulleys themselves, or for the friction of the ropes. In practice, however, it will be found, that nearly one third must be allowed for friction, and that the power, therefore, to actually raise the weight, must be about one third greater than has been allowed. 359. The pulley, like other machines, obeys the laws of virtual velocities, already applied to the lever and wheel. Thus, " in a system of pulleys, the ascent of the weight, or re- sistance, is as much less than the descent of the power, as the weight is greater than the power} 1 If, as in the last example, the weight is 16 pounds, and the power 1 pound, tb^ weight will rise only one foot, while the power descends 16 feet. 360. In the single fixed pulley, the weight and power are equal, and, consequently, the weight rises as fast as the power descends. 361. With such a pulley, a man may raise himself up to the mast head by his own weight. Suppose a rope is thrown over a pulley, and a man ties one end of it round his body, and takes the other end in his hands ; he may raise himself up, because, by pulling with his hands, he has the power of throwing more of his weight on that side than on the other, and when he does this his body will rise. Thus, al- though the power and the weight are the same individual, still the man can change his centre of gravity, so as to make the power greater than the weight, or the weight greater than the power, and thus can elevate one half his weight in succession. THE INCLINED PLANE. 362. The fourth simple me- Fig. 68. chanical power is the inclined plane. This power consists of a plain, smooth surface, which is inclined towards, or from the earth. It is represented by fig. 68, where from a to b is the inclined plane ; the line from d to a, is its height, and that from b to d, its base. In compound machines, how much of the power must be allowed for the friction 1 How may a man raise himself up by means of a rope and single fixed pulley 1 What is an inclined plane 1 d 86 INCLINED PLANE. A board, with one end on the ground, and the othe end resting on a block, becomes an inclined plane. 363. This machine, being both useful and easily con- structed, is in very general use, especially where heavy bodies are to be raised only to a small height. Thus a man, by means of an inclined plane, which he can readily con- struct with a board, or couple of bars, can raise a load into his wagon, which ten men could not lift with their hands. 364. The power required to force a given weight up an inclined plane, is in a certain proportion to its height, and the length of its base, or, in other words, the force must be in proportion to the rapidity of its inclination. 365. The power p, Fig. 69. fig, 69, pulling a weight up the inclined plane, from c to d, only raises it in a perpendicular di- rection from e to d, by acting along the whole length of the plane. If the plane be twice as long as it is high, that is, if the line from c to d be double the length of that from e to d, then one pound at p will bal- ance two pounds any where between d and c. It is evident, by a glance at this figure, that were the base, that is, the line from e to c, lengthened, the height from e to d being the same, that a less power at p, would balance an equal weight any where on the inclined plane ; and so, on the contrary, were the base made shorter, that is, the plane more steep, the power must be increased in proportion. 366. Suppose two inclined Fig. 70. planes, fig. 70, of the same height, with bases of differ- ent lengths ; Ihen the weight and power will be to each other as the length of the planes. If the length from a' a to b, is two feet, and that On what occasions is this power chiefly used 7 Suppose a man wants to load a barrel of cider into his wagon, how does he make an inclined plane for this purpose 1 To roll a given weight up an inclined plane, to what must the force be proportioned 1 Explain fig. G9. If the length of the long plane, fig. 70, be double that of the short one, what must be the proportion between the power and the weight 1 INCLINED PLANE. 87 from b to c, one foot, then two pounds at d will balance four pounds at w, and so in tnis proportion, whether the planes be longer or shorter. 367. The same principle, with respect to the vertical ve- locities of the weight and powers, applies to the inclined plane, in common with the other mechanical powers. Suppose the inclined plane, Fig. 71. fig. 7 1 , to be two feet from a to b, and one foot from c to b, then, as we have already seen by fig. 69, a power of one pound at p, would balance a weight of two pounds at w. Now, in the fall of the power to draw up the weight, it is obvious that its ver- tical descent must be just twice the vertical ascent of the weight ; for the power must fall down the distance from a to b, to draw the weight that distance ; but the vertical height to which the weight w is raised, is only from c to b. Thus the power, being two pounds, must fall two feet, to raise the weight, four pounds, one foot; and thus the power and weight, multiplied by the several velocities, are equal. 368. When the power of an inclined plane is considered as a machine, it must therefore be estimated by the proportion which the length bears to the height ; the power being in- creased in proportion as the elevation of the plain is dimin- ished. Hilly roads maybe regarded as inclined planes, and loads drawn upon them in carriages, considered in reference to the powers which impel them, and subject to all the con- ditions which we have stated, with respect to inclined planes. 369. The power required to draw a load up a hill, is in proportion to the length and elevation of the inclined plane. On a road, perfectly horizontal, if the power is sufficient to overcome the friction, and the resistance of the atmosphere, the carriage will move. But if the road rise one foot in fifteen, besides these impediments, the moving power will have to lift one fifteenth part of the load. 370. If two roads rise, one at the rate of a foot in fifteen feet, and another at the rate of a foot in twenty, then the What is said of the application of the law of vertical velocities to the inclined plane? Explain fig. 71, and show why the power must fall twice as far as the weight rises. 88 THE WEDGE, same power that would move a given weight fifteen feet on the one, would move it twenty feet on the other, in the same time. In the building of roads, therefore, both speed and power are very often sacrificed to want of judgment, or ignorance of these laws. 371. A road, as every traveller knows, is often continued directly over a hill, when half the power, with the increase of speed, on a level road around it, would gain the same dis- tance in half the time. v Besides, where is there a section of country in which the traveller is not vexed with roads, passing straight over hills, when precisely the same distance would carry him around them on a level plane. To use a homely, but very perti- nent illustration, " the bale of a pot is no longer, when it lies down, than when it stands up." Had this simple fact been noticed, and its practical bearing carried into effect by road makers, many a high hill would have been shunned for a circuit around its base, and many a poor horse, could he speak, would thank the wisdom of such an invention. THE WEDGE. 372. The next simple mechanical power is the wedge. This instrument may be considered as two inclined planes, placed base to base. It is much employed for the purpose of splitting or dividing solid bodies, such as wood and stone, Fig. 72 represents such a wedge as is usually Fig. 72. employed in cleaving timber. This instrument is also used in raising ships, and preparing them to launch, and for a variety of other purposes. Nails, awls, needles, and many cutting instru- ments, act on the principle of this machine. There is much difficulty in estimating the power of the wedge, since this depends on the force, or the number of blows given it, together with the obliquity of its sides. A wedge of great obliquity would require hard blows to drive it forward, for the same reason that a plane, much inclined, requires much force to roll a heavy body up it. But were the obliquity of the wedge, and the force of each blow given, still it would be On what principle does the wedge act 1 In what case is this power useful 1 What common instruments act on the principle of the wedge 1 What difficulty is there in estimating the power of the wedge ? SCREW 89 difficult to ascertain the exact power of the wedge in ordi- nary cases, for, in the splitting of timber and stone, for in- stance, the divided parts act as levers, and thus greatly in- crease the power of the wedge. Thus, in a log of wood, six feet long, when split one half of its length, the other half is divided with ease, because the two parts act as levers, the lengths of which constantly increase, as the cleft extends from the wedge. THE SCREW. 373. The screw is the fifth and last simple mechanical power. It may be considered as a modification of the in- clined plane, or as a winding wedge. It is an inclined plane running spirally round a Fig. 73. spindle, as will be seen by fig. 73. Suppose a to be a piece of paper, cut into the form of an inclined plane, and -rolled round the piece of wood d ; its edge would form the spiral line, called the thread of the screw. If the finger be placed between the two threads of a screw, and the screw be turned round once, the finger will be raised upward equal to the distance of the two threads apart. In this manner, the finger is raised up the inclined plane, as it runs round the cylinder 374. The power of the screw is transmitted and employed by means of another screw called the nut, through which it passes. This has a spiral groove running through it, which exactly fits the thread of the screw. 375. If the nut is fixed, the screw itself, on turning it round, advances forward ; but if the screw is fixed, the nut, when turned, advances along the screw. Fig. 74 represents the first kind ^_ ^ of screw, being such as is commonly- used in pressing paper, and other substances. The nut, n % On what principle does the screw act 1 How is it shown that the screw is a modification of the inclined plane ? Explain fig. 74. Which is the screw, and which the nut 1 8* Fig. 74. 90 SCREW. through which the screw passes, answers also for one of the beams of the press. If the screw be turned to the right, il will advance downwards, while the nut stands stih 376. A screw of the second kind is represented by fig. 75. In this, the screw is fixed, while the nut, n, by being turned by the lever, /, from right to left, will advance down the screw. 377. In. practice, the screw is never used as a simple mechani- cal machine; the power being al- ways applied by means of a lever, passing through the head of the screw, as in fig. 74, or into the nut, as in fig. 75. The screw, therefore, acts with the combined power of the inclined plane and the lever, and its force is such as to be limited only by the strength of the materials of which it is made. 378. In investigating the effects of this machine, we must, therefore, take into account both these simple mechanical powers, so that the screw now becomes really a compound engine. 379. In the inclined plane, we have already seen, that the less it is inclined, the more easy is the ascent up it. In applying the same principle to the screw, it is obvious, that the greater the distance of the threads from each other, the more rapid the inclination, and, consequently, the greater must be the power to turn it, under a given weight. On the contrary, if the thread inclines downwards but slightly, il will turn with less power, for the same reason that a man can roll a heavy weight up a plane but little inclined. Therefore, the finer the screw, or the nearer the threads to each other, the greater will be the pressure under a given power. 380. Let us suppose two screws, the one having th Which way must the screw be turned, to make it advance througl the nut 1 How does the screw, fig. 75, differ from fig. 74 1 Is the screy ever used as a simple machine 1 By what other simple power is i moved 1 What two simple mechanical powers are concerned in tb force of the screw*? Why does the nearness of the threads make a dit ference in the force of the screw 1 Suppose one screw, with its thread one inch apart, and another half an inch apart, what will be their dit* ference in force 1 SCREW. 91 threads one inch apart, and the other half an inch apart ; then the force which the first screw will give with the same power at the lever will be only half that given by the second. The second screw must be turned twice as many times round as the first, to go through the same space, but what is lost in velocity is gained in power. At the lever of the firsV two men would raise a given weight to a given height by making one revolution ; while at the lever of the second, one man would raise the same weight to the same height, by making two revolutions. 381. It is apparent that the length of the inclined plane, up which a body moves in one revolution, is the circumfer- ence of the screw, and its height, the interval between the threads. The proportion of its power would therefore be " as the circumference of the screw, to the distance between the threads, so is the weight to the power." 382. By this rule the power of the screw alone can be found ; but as this machine is moved by means of the lever, we must estimate its force by the combined power of both. In this case, the circumference described by the end of the lever employed, is taken, instead of the circumference of the screw itself. The means by which the force of the screw may be found, is therefore by multiplying the circumference which the lever describes by the power. Thus, " the power multiplied by the circumference which it describes, is equal to the weight or resistance, multiplied by the distance between the two contiguous threads} 1 Hence the efficacy of the screw maybe increased, by increasing the length of the lever by which it is turned, or by diminishing the dis- tance between the threads. If, then, we know the length of the lever, the distance between the threads, and the weight to be raised, we can readily calculate the power ; or, the power being given, and the distance of the threads and the length of the lever known, we can estimate the weight the screw will raise. 383. Thus, suppose the length of the lever to be forty inches, the distance of the threads one inch, and the weight 8000 pounds ; required, the power, at the end of the lever, to raise the weight. What is the length of the inclined plane up which a body moves by one revolution of the screw 1 What would be the height to which the same body would move at one revolution 1 How is the force of the screw estimated 1 How may the efficacy of the screw be increased 1 The length of the lever, the distance between the threads, and the weight being known, how can the power be found 1 92 SCREW. 384. The lever being 40 inches, the diameter of the cir- cle, which the end describes, is 80 inches. The circum r er- ence is a little more than three times the diameter, but we will call it just three times. Then, 80X3=240 inches, the circumference of the circle. The distance of the threads is 1 inch, and the weight 8000 pounds. To find the power, multiply the weight by the distance of the threads, and di- vide by the circumference of the circle. Thus, circum. in. weight. power. 240 XI : : 8000 = 33i The power at the end of the lever must therefore be 33| pounds. In practice this power would require to be in- creased about one third, on account of friction. 385. Perpetual Screiv. The force of the screw is some- times employed to turn a wheel, by acting on its teeth. In this case it is called the perpetual screw. 386. Fig. 76 represents such Fig. 76. a machine. It is apparent, that by turning the crank c, the wheel will revolve, for the thread of the screw passes between the cogs of the wheel. By means of an axle, through the centre of this wheel, like the common wheel and axle, this becomes an ex- ceedingly powerful machine, but like all other contrivances for ob- taining great power, its effective motion is exceedingly slow. It has, however, some disadvantages, and particularly the great friction between the thread of the screw and the teeth of the wheel, which prevents it from being generally employed to raise weights. 387. All these Mechanical Powers resolved into three. We have now enumerated and described all the mechanical powers usually denominated simple. They are five in num- ber, namely, the Lever, Wheel and Axle, Pulley, Wedge, Inclined Plane, and Screw. 388. In respect to the principle on which they act, they may be resolved into three simple powers, namely, the lever, the inclined plane, and the pulley; for it has been shown Give an example. What is the screw called when it is employed to turn a wheel"? What is the object of this machine for raising weights 1 How many simple mechanical powers are there 1 and what are they called? How can they be resolved into three simple powers 1 SCREW. 93 that the wheel and axle is only another form of the lever, and that the screw is but a modification of the inclined plane. 389. It is surprising, indeed, that these, simple powers can be so arranged and modified, as to produce the different actions in all that vast variety of intricate machinery which men have invented and constructed. 390. The variety of motions we witness in the little en- gine which makes cards, by being supplied with wire for the teeth, and strips of leather to stick them through, would itself seem to involve more mechanical powers than those enumerated. This engine takes the wire from a reel, bends it into the form of teeth ; cuts it off; makes two holes in the leather for the tooth to pass through ; sticks it through ; then gives it another bend, on the opposite side of the leather ; graduates the spaces between the rows of teeth, and between one tooth and another ; and, at the same time, carries the leather backwards and forwards, before the point where the teeth are introduced, with a motion so exactly correspond- ing with the motions of the parts which make and stick the teeth, as not to produce the difference of a hair's breadth in the distance between them. 391. All this is done without the aid of human hands, any farther than to put the leather in its place, and turn a crank ; or, in some instances, many of these machines are turned at once, by means of three or four dogs, walking on an inclined plane which revolves. 392. Such a machine displays the wonderful ingenuity and perseverance of man, and at first sight would seem to set at nought the idea that the lever and wh^el were the chief simple powers concerned in its motions. But when these motions are examined singly and deliberately, we are soon convinced that the wheel, variously modified, is the principal mechanical power in the whole engine. 393. Use of Machinery. It has already been stated, (332) that notwithstanding the vast deal of time and ingenuity which men have spent on the construction of machinery, and in attempting to multiply their powers, there has, as yet, been none produced, in which the power was not ob- tained at the expense of velocity, or velocity at the expense of power ; and, therefore, no actual force is ever generated fey machinery. What is said of the card making machine 1 What are the chief mechanical powers concerned in its motions 1 Is there any actual foice generated by machinery 7 Can great velocity and great force be pro- d uced by the same machinery 1 W hy not 1 94 HYDROSTATICS. 394. Suppose a man able to raise a weight by means of a compound pulley of ten ropes, which it would take ten men to raise, by one rope, without pulleys. If the weight is to be raised a yard, the ten men by pulling their rope a yard will do the work. But the man with the pulleys must draw his rope ten yards to raise the weight one yard, and in ad- dition to this, he has to overcome the friction of the ten pul- leys, making about one third more actual labour than was employed by the ten men. But notwithstanding these in- conveniences, the use of machinery is of vast importance to the world. 395. On board of a ship, a few men will raise an anchor with a capstan, which it would take ten or twenty times the same number to raise without it, and thus the expense of shipping men expressly for this purpose is saved. 396. One man with a lever, may move a stone which it would take twenty men to move without it, and though it should take him twenty times as long, he would still be the gainer, since it would be more convenient, and less expen- sive for him to do the work himself, than to employ twenty others to do it for him. 397. When men employ the natural elements as a power to overcome resistance by means of machinery, there is a vast saving of animal labour. Thus mills, and" all kinds of engines, which are kept in motion by the power of water, or wind, or steam, save animal labour equal to the power it takes to keep them in motion. HYDROSTATICS. 398. Hydrostatics is the science which treats of the weight, pressure, and equilibrium of water, or other fluids, when in a state of rest. 399. Hydraulics is that part of the science of fluids which treats of water in motion, and the means of raising and conducting it in pipes, or otherwise, for all sorts of purposes. 400. The subject of water at rest, will first claim investi- gation, since the laws which regulate its motion will be best understood by first comprehending those which regulate its pressure. 401. A fluid is a substance whose particles are easily moved among each other, as air and water. Which performs the greatest labour, ten men who lift a weight with their hands, or one man who does the same with ten pulleys 7 Why 1 What is hydrostatics 1 How does hydraulics differ from hydrostatics 1 What is a fluid 1 HYDROSTATICS. 95 402. The air is called an elastic fluid, because it iseasily compressed into a smaller bulk, and returns again to its ori- ginal state when the pressure is removed. Water is called a mw-elastic fluid, because it admits of little diminution of bulk under pressure. 403. The non-elastic fluids, are perhaps more properly called liquids, but both terms are employed to signify water and other bodies possessing its mechanical properties. The term fluid, when applied to the air, has the word elastic be- fore it. 404. One of the most obvious properties of fluids, is the facility with which they yield to the impressions of other bodies, and the rapidity with which they recover their form- er state, when the pressure is removed. The cause of this, is apparently the freedom with which the particles of liquids slide over, or among each other; their cohesive attraction being so slight as to be overcome by the least impression. On this want of cohesion among their particles seem to de- pend the peculiar mechanical properties of these bodies. 405. In solids, there is such a connexion between the particles, that if one part moves, the other part must move also. But in fluids, one portion of the mass may be in mo- tion, while the other is at rest. In solids, the pressure is always downwards, or towards the centre of the earth's gravity ; but in fluids the particles seem to act on each other as wedges, and hence, when confined, the pressure is side- ways, and even upwards, as well as downwards. 406. Water has commonly been called a non-Fig.^77. elastic substance, but it is found that under great pressure its volume is diminished, and hence it is proved to be elastic. The most decisive experi- nents on this subject were made within a few years by Mr. Perkins. 407. The experiments were made by means of a hollow cylinder, fig. 77, which was closed at the bottom, and made water tight at the top, by a cap, screwed on. Through this cap, at a, passed the rod b, which was five sixteenths of an inch in diam- eter. The rod was so nicely fitted to the cap, as also to be water tight Around the rod at c, there was placed a flexible ring, which could be easily push- What is an elastic fluid 1 Why is air called an elastic fluid ? What substances are called liquids 1 What is one of the most obvious pro- }uids 1 On what do the peculiar mechanical properties of perties of liqu fluids depend HYDROSTATICS. ed up or down, but fitted so closely as to remain on any part where it was placed. 408. A cannon of sufficient size to receive this cylinder, which was three inches in diameter, was furnished with a strong cap and forcing pump, and set vertically into the ground. The cannon and cylinder were next filled with water, and the cylinder, with its rod drawn out, and the ring placed down to the cap, as in the figure, was plunged into the cannon. The water in the cannon was then subjected to an immense pressure by means of the forcing pump, af- ter which, on examination of the apparatus, it was found that the ring c, instead of being where it was placed, was eight inches up the rod. The water in the cylinder being compressed into a smaller space, by the pressure of that in the cannon, the rod was driven in, while under pressure, but was forced out again by the expansion of the water, when the pressure was removed. Thus, the ring on the rod would indicate the distance to which it had been forced in, during the greatest pressure. 409. This experiment proved that water, under the pressure of one thousand atmospheres, that is, the weight of 15,000 pounds to the square inch, was reduced in bulk about one part in 24. 410. So slight a degree of elasticity under such immense pressure, is not appreciable under ordinary circumstances, and therefore in practice, or in common experiments on this fluid, water is considered as non-elastic. EQUAL PRESSURE OF WATER. 411. The particles of water, and other fluids, when con- fined, press on the vessel which confines them, in all direc- tions, both upwards, downwards, and sideways. From this property of fluids, together with their weight, or gravity, very unexpected and surprising effects are pro- duced. 412. The effect of this property, which we shall first ex- amine, is, that a quantity of water, however small, will, balance another quantity however large. Such a proposi- In what respect does the pressure of a fluid differ from that of a sond 1 Is water an elastic, or non-elastic fluid 1 Describe fig. 77, and show- now water was found to be elastic? In what proportion does the bulk of water diminish under a pressure of 15,000 pounds to the square, inch 1 In common experiments, is water considered elastic, or nort elastic 1 When water is confined, in what direction does it press 1 HYDROSTATICS. 97 Fig. 78. tion at first thought might seem very improbable. But on examination, we shall find that an experiment with a very simple apparatus will convince any one of its truth. In- deed, we every day see this principle established by actual experiment, as will be seen directly. 413. Fig. 78 represents a common cof- fee-pot, supposed to be filled up to the dot- ted line a, with a decoction of coffee, or any other liquid. The coffee, we know, stands exactly at the same height, both in the body of the pot, and in its spout. Therefore, the small quantity in the spout, balances the large quantity in the pot, or presses with the same force downwards, as that in the body of the pot presses upwards. This is obviously true, other- wise, the large quantity would sink below the dotted line, while that in the spout would rise above it, and run over. 414. The same principle is more strik- Fig. 79. ingly illustrated by fig. 79. c Suppose the cistern a to be capable of holding one hundred gallons, and into its bottom there be fitted the tube b, bent, as seen in the figure, and capable of con- taining one gallon. The top of the cis- tern, and that of the tube, being open, pour water into the tube at c, and it will rise up through the perpendicular bend into the cistern, and if the process be con- tinued, the cistern will be filled by pour- ing water into the tube. Now, it is plain, that the gallon of water in the tube, presses against the hundred gallons in the cistern, with a force equal to the pressure of the hun- dred gallons, otherwise, that in the tube would be forced up- wards higher than that in the cistern, whereas, we find that the surfaces of both stand exactly at the same height. 415. From these experiments we learn, " that the press- ure of a fluid is not in proportion to its quantity, but to its height, and that a large quantity of water in an open ves- sel, presses down with no more force, than a small quantity of the same height." How does the experiment with the coffee-pot show that a small quan- tity of liquid will balance a large one 1 Explain fig. 79, and show how the pressure in the tube is equal to the pressure in the cistern. What conclusion, or general truth, is to be drawn from these experimental HYDROSTATICS. 416. In this respect, the size or shape of a vessel is of no consequence, for if a number of vessels, differing entirely from each other in figure, position, and capacity, have a communication made between them, and one be filled with water, the surface of the fluid, in all, will be at exactly the same elevation. If, therefore, the water stands at an equal, height in all, the pressure in one must be just equal to that in another, and so equal to that in all the others. Fig. 80. 417. To make this obvious, suppose a number of vessels, of different shapes and sizes, as represented by fig. 80, to have a communication between them, by means of a small tube, passing from the one to the other. If, now, one of these vessels be filled with water, or if water be poured into the tube a, all the other vessels will be filled at the same in- stant, up to the line b c. Therefore, the pressure of the water in a, balances that in 1, 2, 3, &c., while the pressure in each of these vessels, is equal to that in the other, and so an equilibrium is produced throughout the whole series. 418. If an ounce of water be poured into the tube a, it will produce a pressure on the contents of all the other ves- sels, equal to the pressure of all the others on the tube ; for, it will force the water in all the other vessels to rise up- wards to an equal height with that in the tube itself. Hence, we must conclude, that the pressure in each vessel, is not only equal to that in any of the others, but also that the pressure in any one, is equal to that in all the others. 419. From this we learn, that the shape or size of a ves- What difference does the shape or size of a vessel make in respect to the pressure of a fluid on its bottom ? Explain fig. 80, and show how the equilibrium is produced. Suppose an ounce of water be pour- ed into the tube a, what will be its effect on the contents of the other vessels 1 What conclusion is to be drawn from pouring the ounce of water into the tube a 1 H S-DROSTATICS. 99 sel has no influence on the pressure of its liquid contents, but that the pressure of water is as its height, whether the quantity be great or small. We learn, also, that in no case will the weight of a quantity of liquid, however large, force another quantity, however small, above the level of its own surface. 420. This is proved by experiment ; for if, from a pond situated on a mountain, water be conveyed in an inch tube to the valley, a hundred feet below, the water will rise just a hundred feet in the tube ; that is, exactly to the level of the surface of the pond. Thus the water in the pond, and that in the tube, press equally against each other, and pro- duce an exact equilibrium. Thus far we have considered the fluid as acting only in vessels with open mouths, and therefore at liberty to seek its balance, or equilibrium, by its own gravity. Its press- ure, we have seen, is in proportion to its height, and not to its bulk. 421. Now, by other experiments, it is ascertained, that the pressure of a liquid is in proportion to its height, and its area at the base. Suppose a vessel, ten feet high, and Fig. 81. two feet in diameter, such as is rep- resented at #, fig. 81, to be filled with water ; there would be a certain amount of pressure, say at c, near the bottom. Let d represent another vessel, of the same diameter at the bottom, but only a foot high, and closed at the top. Now if a small tube, say the fourth of an inch in di- ameter, be inserted into the cover of the vessel d, and this tube be carried to the height of the vessel a, and then the vessel and tube be filled \ with water, the pressure on the bot- toms and sides of both vessels to the same height will be equal, and jets of water starting from d and c, will have ex- actly the same force. What is the reason that a large quantity of water will not force a small quantity above its own level'? Is the force of water in propor- tion to its height, or its quantity 7 How is a small quantity of water shown to press equal to a large quantity by fig. 81 7 Explain the reason why the pressure is as great at d, as at c. 100 HYDROSTATICS. 422. This might at first seem improbable, but to convince ourselves of its truth, we have only to consider, that any im- pression made on one portion of the confined fluid in the vessel d, is instantly communicated to the whole mass. Therefore, the water in the tube b presses with the same force on every other portion of the water in d, as it does on that small portion over which it stands. 423. This principle is illustrated in a very striking man- ner, by the experiment, which has often been made, of burst- ing the strongest wine-cask with a few ounces of water. 424. Suppose a, fig. 82, to be a strong cask, Fig. 82. already filled with water, and suppose the tube b, thirty feet high, to be screwed, water tight, into its head. When water is poured into the tube, so as to fill it gradually, the cask will show increasing signs of pressure, by emitting the water through the pores of the wood, and between the joints ; and, finally, as the tube is filled, the cask will burst asunder. 425. The same apparatus will serve to il- lustrate the upward pressure of water ; for if a small stop-cock be fitted to the upper head, on turning this, when the tube is filled, a jet of water will spirt up with a force, and to a height, that will astonish all who never before saw such an experiment. In theory, the water will spout to the same height with that which gives the pressure, but, in practice, it is found to fall short, in the following proportions : 426. If the tube be twenty feet high, and the orifice for the jet half an inch in diameter, the water will spout nearly nineteen feet. If the tube be fifty feet high, the jet will rise upwards of forty feet, and if a hundred feet, it will rise above eighty feet. It is understood, in every case, that the tubes are to be kept full of water. The height of these jets show the astonishing effects that a small quantity of fluid produces when, pressing from a perpendicular elevation. 427. Hydrostatic Bellows. An instrument called the hy- How is the same principle illustrated by fig. 827 How is the up- ward pressure of water illustrated by the same apparatus 1 Under the pressure of a column of water twenty feet high, what will be tH height of the jet 7 Under a pressure of a hundred feet, how high wilt it rise 1 What is the hydrostatic bellows ? HYDROSTATICS. 101 Fig. 83. drostatic bellows, a.so shows, in a striking manner, the great force of a small quantity of water, pressing in a perpendic- ular direction. 428. This instrument consists of two boards, connected together with strong leather, in the manner of the common bellows. It is then furnished with a tube a, fig. 83, which communicates between the two boards. A person standing on the upper board, may raise himself up by pouring water into the tube. If the tube holds an ounce of water, and has an area equal to a thousandth part of the area of the top of the bellows, one ounce of water in the tube will balance a thousand ounces placed on the bellows. 429. Hydraulic Press. This prop- erty of water was applied by Mr. Bra- mah to the construction of his hy- draulic press. But instead of a high tube of water, which in most cases could not be so readily ob- tained, he substituted a strong forcing pump, and instead of the leather bellows, a metallic pump barrel and piston. 430. This arrangement will Fig. 84. be understood by fig. 84, where the pump barrel, a, b, is rep- resented as divided lengthwise, r in order to show the inside., The piston, c, is fitted so ac- curately to the barrel, as to work up and down water tight ; both barrel and piston being made of iron. The thing to be broken, or pressed, is laid on the flat surface, i, there being above this, a strong frame to meet the pressure, not shown in the figure. The small forcing pump, of which d is the piston, and h, the lever by which it is worked, is also made of iron. 431. Now, suppose the space between the small piston and the large one, at w, to be filled with water, then, on What property of water is this instrument designed to show 1 Ex- plain fi?. 84. Where is the piston 1 Which is the pump barrel, in wnlc* ^ works I In the hydrostatic press, what is the proportion between the press- ure given bv the small piston, and the force exerted on the large one 1 9* ,02 HYDROSTATICS. forcing down the small piston, d, there will be a pressure against the large piston, c, the whole force of which will be in proportion as the aperture in which c works, is great- er than that in which d works. If the piston, d, is half an inch in diameter, and the piston, c, one foot in diameter, then the pressure on c will be 576 times greater than that on d. Therefore, if we suppose the pressure of the small piston to be one ton, the large piston will be forced up against any resistance, with a pressure equal to the weight of 576 tons. It would be easy 'for a single man to give the pressure of a ton at d, by means of the lever, and, therefore, a man, with this engine, would be able to exert a force equal to the weight of near 600 tons. 432. It is evident, that the force to be obtained by this principle, can only be limited by the strength of the mate- rials of which the engine is made. Thus, if a pressure of two tons be given to a piston, the diameter of which is only a quarter of an inch, the force transmitted to the other pis- ton, if three feet in diameter, would be upwards of 40,000 tons ; but such a force is much too great for the strength of any material with which we are acquainted. 433. A small quantity of water, extending to a great ele- vation, would give the pressure above described, it being only for the sake of convenience, that the forcing pump is employed, instead of a column of water. 434. There is no doubt, but in the operations of nature, great effects are sometimes produced among mountains, by a small quantity of water finding its way to a reservoir in the crevices of the rocks far beneath. 435. Sup- Fig. 85. pose in the interior of a mountain, fig. 85, there should be a ypace of ten yards square, and an inch deep, filled with water, and closed up What is the estimated force which a man could give by one of these engines 1 If the pressure of two tons be made on a piston of a quar- ter of an inch in diameter, what will be the force transmitted to the other piston of three feet in diameter 1 ? HYDROSTATICS. 103 on ail sides ; and suppose, that in the course of time, a small fissure, no more than an inch in diameter, should be opened by the water, from the height of two hundred feet above, down to this little reservoir. The consequence might be, that the side of the mountain would burst asunder, for the pressure, under the circumstances supposed, would be equal to the weight of five thousand tons. 436. Pressure on vessels ivith oblique sides. It is obvi- ous that in a vessel, the sides of which are every where per- pendicular to each other, that the pressure on the bottom will be as the height, and that the pressure on the sides will every where be equal at an equal depth of the liquid. 437. But it is not so obvious, that in a vessel having oblique sides, that is, diverging outwards from the bottom, or converging from the bottom towards the top, in what manner the force of pressure will be sustained. 438. Now, the pressure on the bottom of any vessel, no matter what the shape may be, is equal to the height of the fluid, and the area of the bottom. 439. Hence the pressure > Fig. 86. ^ on the bottom of the vessel "~ sloping outwards, fig. 86. will be just equal to what it would be, were the sides perpendicular, and the same would be .the case did the sides slope inwards instead of outwards. 440. In a vessel of this shape, the sides sustain a pressure equal to the perpendicular height of the fluid above any given point. Thus, if the point 1 sustain a pressure of one pound, 2, being twice as far below the surface, will have a pressure equal to two pounds, and so in this proportion with respect to the other eight parts marked on the side of the vessel. 441. On the contrary, did the sides of the vessel slope in- wards instead of outwards, as re- presented by fig. 87, still the same consequences would ensue, that is, the perpendicular height, in both cases, would make -the pressure equal. For although, in the lat- 10 :|| ter case, the perpendicular height What is the pressure of water in the crevices of mountains, and the consequences 1 What is the pressure on the bottom of a vessel contain- ing a fluid equal to"? Suppose the sides of the vessel slope outwards, what effect does this produce on the pressure 1 104 WATER LEVEL. is not above the point pressed upon, still the same effect is produced by the pressure of the fluid in the direction per- pendicular to the plane of the side, and since fluids press equally hi all directions, this pressure is just the same as though it were perpendicularly above the point pressed upon, as in the direction of the dotted lines. 442. To show that this is the case, we will suppose that P, fig. 87, is a particle of the liquid at the same depth below the surface as the division marked 5 on the side of the ves- sel; this particle is evidently pressed downwards by the in- cumbent weight of the column of fluid P, a. But since fluids press equally in all directions, this particle must be pressed upwards and sideways with the same force that it is pressed downwards, and, therefore, must be pressed from P towards the side of the vessel, marked 5, with the same force that it would be if the pressure was perpendicular above that part of the vessel. 443. From all that has been stated, we learn, that if the sides of the vessels, 86 and 87, be equally inclined, though in contrary directions to their bottoms, and the vessels be filled with equal depths of water, the sides being of equal di- mensions, will be pressed equally, though the actual quan tity of fluid in each, be quite different from each other. WATER LEVEL. 444. We have seen, that in whatever situation water is placed, it always tends to seek a level. Thus, if several ves- sels communicating with each other be filled with water, the fluid will be at the same height in all, and the level will be indicated by a straight line drawn through all the ves- sels, as in fig. 80. It is on the principle of this tendency, that the little in strument called the water level is constructed. 445. The form of this Fig. 88. instrument is represented by fig. 88. It consists of a, b, a tube, with its two ends turned at right an- gles, and left open. Into b How is it shown that the pressure of the fluid at 5, is equal to what it would have been had the liquid been perpendicular above that point 1 On what principle is the water-level constructed 1 Describe the. man- ner in which the level with sights is used, and the reason why the floats will ahvays be at the same height 1 WATER LEVEL. 105 one of the ends is poured water or mercury, until the fluid rises a little in the angles of the tube. On the surface of the fluid, at each end, are then placed small floats, carrying up- right frames, across which are drawn small wires or hairs, as seen at c and d. These hairs are called the sights, and are across the line of the tube. 446. It is obvious that this instrument will always indi- cate a level, when the floats are at the same height, in re- spect to each other, and not in respect to their comparative heights in the ends of the tube, for if one end of the instru- ment be held lower than the other, still the floats must al- ways be at the same height. To use this level, therefore, we have only to bring the two sights, so that one will range with the other; and on placing the eye at c, and looking towards d, this is determined in a moment. This level is indispensable in the construction of canals and aqueducts, since the engineer depends entirely on it, to ascertain whether the water can be carried over a given hill or mountain. 447. The common spirit level con- Fig. 89. sists of a glass tube, fig. 89, filled ^ with spirit of wine, excepting a small u ^^m-m^^p i space in which there is left a bubble ' of air. This bubble, when the in- strument is laid on a level surface, will be exactly in the middle of the tube, and therefore to adjust a level, it is only necessary to bring the bubble to this position. The glass tube is enclosed in a brass case, which is cut out on the upper side, so that the bubble may be seen, as represented in the figure. 448. This instrument is employed by builders to leve* their work, and is highly convenient for that purpose, since it is only necessary to lay it on a beam to try its level. 449. Improved Water Level. In this edition we add the figure and description of a more complete Water Level than that seen at fig. 88. What is the use of the level 7 Describe the common spirit level, and the method of using it ? WATER LEVEL. Fig. 90. 106 950. Let A, fig. 90, be a straight glass tube, having two legs, or two other glass tubes, rising from each end at right angles. Let the tube A, and a part of the legs, be filled with mercury, or some other liquid, and on the surfaces, a b, of the liquid, let floats be placed car- rying upright wires, to the ends of which are attached sights at 1,2. These sights are represented by 3, 4, and consist of two fine threads, or hairs, stretched at right angles across a square, and are placed at right angles to the length of the instrument. 451. They are so adjusted that the points where the hairs intersect each other, shall be at equal heights above the floats. This adjustment may be made in the following manner : 452. Let the eye be placed behind one of the sights, look- ing through it at the other, so as to make the points, where the hairs intersect, cover each other, and let some distant object, covered by this point, be observed. Then let the instrument be reversed, and let the points of intersection of the hairs be viewed in the same way, so as to cover each other. If they are observed to cover the same distant object as before, they will be of equal heights above the surfaces of the liquid. But, if the same distant points be not observed in the direction of these points, then one or the other of the sights must be raised or lowered, by an adjustment provided f or that purpose, until the points of intersection be brought to correspond. These points will then be properly adjust- ed, and the line passing through them will be exactly hori- zontal. All points seen in the direction of the sights will be on the level of the instrument. 453. The principles on which this adjustment depends Explain, by fig. 90, how an exact line may be obtained by adjusting the sights 7 SPECIFIC GRAVITY. 107 are easily explained : if the intersections of the hairs be at the same distance from the floats, the line joining those intersections will evidently be parallel to the lines join- ing the surfaces a, b, of the liquid, and will therefore be Wei. But if one of these points be more distant from the floats than the other, the line joining the intersections will point upwards if viewed from the lower sight, and down- wards, if viewed from the higher one. 454. The accuracy of the results of this instrument, will be greatly increased by lengthening the tube A. SPECIFIC GRAVITY. 455. If a tumbler be filled with water to the brim, and an egg, or any other heavy solid, be dropped into it, a quan- tity of the fluid, exactly equal to the size of the egg, or other solid, will be displaced, and will flow over the side of the vessel. Bodies which sink in water, therefore, displace a quantity of the fluid equal to their own bulks. 456. Now, it is found, by experiment, that when any solid substance sinks in water, it loses, while in the fluid, a portion of its weight, just equal to the weight of the bulk of water which it displaces. This is readily made evident bv experiment. 457. Take a piece of Fig. 91. ivory, or any other sub- stance that will sink in water, and weigh it accu- rately in the usual man- ner; then suspend it by a thread, or hair, in the emp- ty cup a, fig. 91, and then balance it, as shown in the figure. Now pour water into the cup, and it will be found that the suspended body will lose a part of its weight, so that a certain number of grains must be taken from the opposite scale, in order to make the scales balance as before the water was poured in. When a solid is weighed in water, why does it lose a part of its weight "? How much less will a cubic inch of any substance weigh in water than in air 1 How is it proved by fig. 91, that a body weighs less in water than in air ? What is the specific gravity of a bodv? How are the specific gravities of solid bodies taken 7 108 *ECIFIC GRAVITY. The number of grains taken from the opposite scale, show the weight of a quantity of water equal to the bulk of the body so suspended. 458. It is on the principle, that bodies weigh less in the water than they do when weighed out of it, or in the air, that water becomes the means of ascertaining their specific gravities, for it is by comparing the weight of a body in the water, with what it weighs out of it, that its specific grav- ity is determined. 459. Thus, suppose a cubic inch of gold weighs 19 ounces, and on being weighed in water, weighs only 18 ounces, or loses a nineteenth part of its weight, it will prove that gold, bulk for bulk, is nineteen times heavier than water, and thus 19 would be the specific gravity of gold. And so if a cube of copper weigh 9 ounces in the air, and only 8 ounces in the water, then copper, bulk for bulk, is 9 times as heavy as water, and therefore has a specific gravity of 9. 460. If the body weigh less, bulk for bulk, than water, it is obvious, that it will not sink in it, and therefore weights must be added to the lighter body, to ascertain how much less it weighs than water. The specific gravity of a body, then, is merely its weight, compared with the same bulk of water ; and water is thus made the standard by which the weights of all other bodies are compared. 461. To take the specific gravity of a solid which sinks in water, firsl weigh the body in the usual manner, and note down the number of grains it weighs. Then, with a hair, or fine thread, suspend it from the bottom of the scale-dish, in a vessel of water, as represented by fig. 91. As it weighs less in water, weights must be added to the side of the scale where the body is suspended, until they exactly balance each other. Next, note down the number of grains so add- ed, and they will show the difference between the weight of the body in air, and in water. It is obvious, that the greater the specific gravity of the body, the less, comparatively, will be this difference, because each body displaces only its own bulk of water, and some bodies of the same bulk, will weigh many times as much as others. 462. For example, we will suppose that a piece of pla- tina, weighing 22 ounces, will displace an ounce of water, Why does a heavy body weigh comparatively less in the water tha a light one ? HYDROMETER. 109 while a piece of silver, weighing- 22 ounces, will displace two ounces of water. The platina, therefore, when sus- pended as above described, will require one ounce to make the scales balance, while the same weight of silver will re- quire two ounces for the same purpose. The platina, there- fore, bulk for bulk, will weigh twice as much as the silver, and will have twice as much specific gravity. Having noted down the difference between the weight of the body in air and in water, as above explained, the specific gravity is found by dividing the weight in air, by the loss in water. The greater the loss, therefore, the less will be the specific gravity, the bulk being the same. Thus, in the above example, 22 ounces of platina was sup- posed to lose one ounce in water, while 22 ounces of silver lost two ounces in water. Now 22, divided by 1, the loss of the platina, is 22; and 22 divided by 2, the loss in the silver, is 11. So that the specific gravity of platina is 22, while that of silver is 11. The specific gravities of these metals are, however, a little less than here estimated. [For other methods of taking specific gravity, see Chemistry.} HYDROMETER. 463. The hydrometer is an instrument, by which the spe- cific gravities of fluids are ascertained, by the depth to which it sinks below their surfaces. Suppose a cubic inch of lead loses, when weighed in water, 253 grains, and when weighed in alcohol, only 209 grains, then, according to the principle already recited, a cubic inch of water actually weighs 253, and a cubic inch of alcohol 209 grains, for when a body is weighed in fluid, it loses just the weight of the fluid it displaces. 464. Water, as we have already seen, (460,) is the stand- ard by which the weights of other bodies are compared, and by ascertaining what a given bulk of any substance weighs in water, and then what it weighs in any other fluid, the comparative weight of water and this fluid will be known. For if, as in the above example, a certain bulk of water weighs 253 grains, and the same bulk of alcohol only 209 Having taken the difference between the weight of a body in air and in water, by what rule is its specific gravity found 1 Give the ex- ample stated, and show how the difference between the specific gravi- ties of platina and silver is ascertained. What is the hydrometer 1 Suppose a cubic inch of any substance weighs 253 grains less in water than in air, what is the actual weight of a cubic inch of water"? 10 1 10 HYDROMETER grains, then alcohol has a specific gravity, nearly one fourth less than water. It is on this principle that the hydrometer is constructed. It is composed of a hollow ball of glass, or metal, with a graduated scale rising from its upper part, and a weight on its under part, which serves to balance it in the fluid. Such an instrument is represented by fig. Fig. 92. 92, of which b is the graduated scale, and a the weight, the hollow ball being between them. 465. To prepare this instrument for use, weights, in grains, or half grains, are put into the little ball a, until the scale is carried down, so that a certain mark on it coincides exactly with the surface of the water. This mark, then, becomes the standard of compari- son between water and any other liquid, in which the hydrometer is placed. If plunged into a fluid lighter than water, it will sink, and consequent! y^the fluid will rise higher on the scale. If the fluid is heavier than water, the scale will rise above the surface in proportion, and thus it is as- certained, in a moment, whether any fluid has a greater or less specific gravity than water. To know precisely how much the fluid varies from the standard, the scale is marked off into degrees, which indi- cate grains by weight, so that it is ascertained, very exactly, how much the specific gravity of one fluid differs from that of another. 466. Water being the standard by which the weights of other substances are compared, it is placed as the unit, or point of comparison, and is therefore 1, 10, 100, or 1000, the ciphers being added whenever there are fractional parts expressing the specific gravity of the body. It is always understood, therefore, that the specific gravity of water is 1, and when it is said a body has a specific gravity of 2, it is only meant, that such a body is, bulk . for bulk, twice as heavy as water. If the substance is lighter than water, it On what principle is the hydrometer founded 1 How is this instru- ment formed 1 How is the hydrometer prepared for use 1 How is it known, by this instrument, whether the fluid is lighter or heavier than water 1 What is the standard by which the weights of other bodies are compared 1 What is the specific gravity of water? When it is said that the specific gravity of a body is 2, or 4, what meaning is intended to be conveyed ? SYPHON. Ill has a specific gravity of 0, with a fractional part. Thus alcohol has a specific gravity of 0,809, that is, 809, water being 1000. * By means of this instrument, it can he told with great ac- curacy, how much water has been added to spirits, for the greater the quantity of water, the higher will the scale rise above the surface. The adulteration of milk with water, can also he readily detected with it, for as new milk has a specific gravity of 1032, water being 1000, a very small quantity of water mix- ed with it would be indicated by the instrument. THE SYPHON. 467. Take a tube, bent like .the letter U, and having filled it with water, place a finger on each end, and in this state plunge one of the ends into a vessel of water, so that the end Fn the water shall be a little the highest, then remove the fingers, and the liquid will flow out, and continue to do so, until the vessel is exhausted. A tube acting in this manner, is called a syphon, and is represented by fig. 93. The reason why the water flows from the end of the tube a, and, consequently, ascends through the other part, is, that there is a greater weight of the fluid from b to a, than from c to Z>, because the perpendicular height from b to a is the greatest. The weight of the water from b to a falling downwards, by its gravity, tends to form a vacuum, or void space, in that leg of the tube; but the pressure of the atmosphere on the water in the vessel, constantly forces the fluid up the other leg of the tube, to fill the void space, and thus the stream is continued as long as any water remains in the vessel. 468. Intermitting Springs. The action of the syphon depends upon the same principle as the action of the pump, namely, the pressure of the atmosphere, and therefore its ex- planation properly belongs to Pneumatics. It is introduced Alcohol has a specific gravity of 809 ; what, in reference to this, is the specific gravity of water 1 In what manner is a syphon made 1 Explain the reason why the water ascends through one leg of the sy- phoa, and descends through the other. What is an intermittent spring 1 J12 SYPHON. here merely for the purpose of illustrating the phenomena of intermitting springs; a subject which properly belongs to Pneumatics. Some springs, situated on the sides of mountains, flow for a while with great violence, and then cease entirely. After a time, they begin to flow again, and then suddenly stop, as before. These are called intermitting springs. Among ignorant and superstitious people, these strange appearances have been attributed to witchcraft, or the influence of some supernatural power. But an acquaintance with the laws of nature will dissipate such ill founded opinions, by showing that they owe their peculiarities to nothing more than natu- ral syphons, existing in the mountains from whence the water flows. Fig. 94. 469. Fig. 94 is the section of a mountain and spring, showing how the principle of the syphon operates to pro- duce the effect described. Suppose there is a crevice, or hollow in the rock from a to b, and a narrow fissure lead ing from it, in the form of the syphon, b c. The water, from the rills f e, filling the hollow, up to the line a d, it will then discharge itself through the syphon, and continue to run until the water is exhausted down to the leg of the sy- phon b, when it will cease. Then the water from the rills continuing to run until the hollow is again filled up to the same line, the syphon again begins to act, and again dis- charges the contents of the reservoir as before, and thus the spring p, at one moment, flows with great violence, and the next moment ceases entirely. How is the phenomenon of the intermittent spring explained 1 Ex- plain fig. 94, and show the reason why such a spring will flow, and cease to flow, alternately. HYDRAULICS. 113 The hollow, above the line a d, is supposed not to be fill- ed with the water at all, since the syphon begins to ict whenever the fluid rises up to the bend d. During the dry seasons of the year, it is obvious, that such a spring would cease to flow entirely, and would be gin again only when the w^ter from the mountain filled the cavity through the rills. Such springs, although not very common, exist in various parts of the world. Dr. Atwell has described one in the 'Philosophical Transactions, which he examined in Devon- shire, in England. The people in the neighbourhood, as usual, ascribed its actions to some sort of witchery, and ad- vised the doctor, in case it did not ebb and flow readily, when he and his friend were both present, that one of them should retire, and see what the spring would do, when only the other was present. HYDRAULICS. 470. It has been stated, (398,) that Hydrostatics is that branch of Natural Philosophy, which treats of the weight, pressure, and equilibrium of fluids, and that Hydraulics has for its object the investigation of the laws which regulate fluids in motion. If the pupil has learned the principles on which the press- ure and equilibrium of fluids depend, as explained under the former article, he will now be prepared to understand the laws which govern fluids when in motion. Tb*^ pressure of water downwards, is exactly in the same proportion to its height, as is the pressure of solids in the same direction. 471. Suppose a vessel of three inches in diameter has a billft of wood set up in it, so as to touch only the bottom, and suppose the piece of wood to be three feet long, and to \vei (h nine pounds; then the pressure on the bottom of the vessel will be nine pounds. If another billet of wood be set on this, of the same dimensions, it will press on its top with the weight of nine pounds, and the pressure at t^e bot- tom will be 18 pounds, and if another billet be set on vhis. How does the science of Hydrostatics differ from that of Hydrau- lics 1 Does the downward pressure of water differ from the downward pressure of solids, in proportio" 7 How is the downward pressure of water illustrated 1 10* 114 HYDRAULICS. the pressure at the bottom will be 27 pounds, and so on, in this ratio, to any height the column is carried. 472. Now the pressure of fluids is exactly in the same proportion ; and when confined in pipes, may be considered as one short column set on another, each of which increases the pressure of the lowest, in proportion to their number and height. 473. Thus, notwithstanding the lateral press- F L 5 - ure of fluids, their downward pressure is as their height. This fact will be found of importance in the investigation of the principles of certain hydraulic machines, and we have, therefore, en- deavoured to impress it on the mind of the pupil by fig. 95, where it will be seen, that if the pressure of three feet of water be equal to nine pounds on the bottom of the vessel, the pressure of twelve feet will be equal to thirty-six pounds. 474. The quantity of water which will be dis- charged from an orifice of a given size, will be 27 in proportion to the height of the column of water above it, for the discharge will increase in velocity in proportion to the pressure, and the pressure, we have already seen , will be in a fixed ratio to the height. 475. If a vessel, fig. 96, Fig. 96. be filled with water, and three apertures be made in its sides at the points a, b % and c, the fluid will be thrown out in jets, and will fall towards the earth, in the curved lines, a, b, and c. The reason why these curves differ in shape, is, that the fluid is acted on by two forces, namely, the pressure of the water above the jet, which produces its velo- city forward, and the action of gravity, which impels it downward. It therefore obeys the same laws that solids do Without reference to the lateral pressure, in what proportion do fluids press downwards 1 What will be the proportion of a fluid dis- charged from an orifice of a given size 1 Why do the lines described by the jets from the vessel, fig. 96, differ in shape! HYDRAULICS. 116 when projected forward, and falls down in curved lines, the shapes of which depend on their relative velocities. The quantity of water discharged, being in proportion to the pressure, that discharged from each orifice will differ in quantity according to the height of the water above it. 476. It is found, however, that the velocity with which a vessel discharges its contents, does not depend entirely on the pressure, but in part on the kind of orifice through which the liquid flows. It might be expected, for instance, that a tin vessel of a given capacity, with an orifice of say an inch in diameter through its side, would part with its contents sooner than another of the same capacity and orifice, whose side was an inch or two thick, since the friction through the tin might be considered much less than that presented by the other orifice. But it has been found, by experiment, that the tin vessel does not part with its contents so soon as another vessel, of the same height and size of orifice, from which the water flowed through a short pipe. And, on varying the length of these pipes, it is found that the most rapid discharge, other circumstances being equal, is through a pipe, whose length is twice the diameter of its orifice. Such an aperture discharged 82 quarts, in the same time that another vessel of tin, without the pipe, discharged 62 quarts. This surprising difference is accounted for, by supposing that the cross currents, made by the rushing of the water from different directions towards the orifice, mutually inter- fere with each other, by which the whole is broken, and thrown into confusion by the sharp edge of the tin, and hence the water issues in the form of spray, or of a screw, from such an orifice. A short pipe seems to correct this contention among opposing currents, and to smooth the passage of the whole, and hence we may observe, that from such a pipe, the stream is round and well defined. 477. Proportion between the pressure and the velocity of discharge. If a small orifice be made in the side of a ves- What two forces act upon the fluid as it is discharged, and how do these forces produce a curved line 1 Does the velocity with which a fluid is discharged, depend entirely on the pressure 1 What circum- stance, besides pressure, facilitates the discharge of water from an ori- ncel In a tube discharging water with the greatest velocity, what is the proportion between its diameter and its length 1 What is the pro- portion between the quantity of fluid discharged through an orifice of tin, and through a short pipe 1 ? I 10 HYDRAULICS. sel filled with any liquid, the liquid will flow out with a force and velocity, equal to the pressure which the liquid before exerted on that portion of the side of the vessel be- fore the orifice was made. Now, as the pressure of fluids is as their heights, it fol- lows, as above stated, that if several such orifices are made, the lowest will discharge the greatest, while the highest will discharge the least, quantity of the fluid. 478. The velocity of discharge, in the several orifices of such a vessel, will show a remarkable coincidence between the ratio of increase in the quantity of liquid, and the in- creased velocity of a falling body (82.) Thus, if the tall vessel, fig. 97, of equal, Fig. 97. dimensions throughout, be filled with wa- ter, and a small orifice be made at one inch from the top, or below the surface, as at 1 ; and another at 2, 4 inches below this; another at 9 inches, a fourth at 16 inches ; and a fifth at 25 inches ; then the velocities of discharge, from these several orifices, will be in the proportion of 1,2, 3, 4, 5. To express this more obviously, we will place the expressions of the several veloci- ties in the upper line of the following ta- ble, the lower numbers, corresponding, expressing the depths of the several) orifices. Velocity, Depth, 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100 j 479. Thus it appears, that to produce a twofold velocity a fourfold height is necessary. To obtain a threefold V9 locity of discharge, a ninefold height is required, and for ? fourfold velocity, sixteen times the height is necessary, an* 1 so in this proportion, as shown by the table. (See 86.) 480. To apply this law to the motion of falling bodies, it appears that if a body were allowed to fall freely from the surface of the water downwards, being unobstructed by the fluid, it would, on arriving at each of the orifices, have ve- locities proportional to those of the water discharged at the What are the proportions between the velocities of discharge arid th* heights of the orifices, as above explained ? HYDRAULICS. 117 said orifices respectively. Thus, whatever velocity it would have acquired on arriving at 1, the first orifice, it would have doubled that velocity on arriving at 2, the second ori- fice, trebled it on arriving at the third orifice, and so on with respect to the others. 481. In order to establish the remarkable fact, that the velocity with which a liquid spouts from an orifice in a ves- sel, is equal to the velocity which a body would acquire in falling unobstructed from the surface of the liquid to the depth of the orifice, it is only necessary to prove the truth of the principle in any one particular case. 482. Now it is manifestly true, if the orifices be presented downwards, and the column of fluid over it be of small height, then tnis indefinitely small column will drop out of the orifice by the mere effect of its own weight, and, there- fore, with the same velocity as any other falling body ; but as fluids transmit pressure in all directions, the same effect will be produced whatever may be the direction of the ori- fice. Hence, if this principle be true, then the direction and size of the orifice can make no difference in the result, so that the principle, above explained, follows as an incon- trovertible fact. FRICTION BETWEEN SOLIDS AND FLUIDS. 483. The rapidity with which water flows through pipes of the same diameter, is found to depend much on the nature of their internal surfaces. Thus a lead pipe, with a smooth aperture, under the same circumstances, will convey much more water than one of wood, where the surface is rough, or beset with points. In pipes, even where the surface is as smooth as glass, there is still considerable friction, for in all cases, the water is found to pass more rapidly in the middle of the stream than it does on the outside, where it rubs against the sides of the tube. The sudden turns, or angles of a pipe, are also found to be a considerable obstacle to the rapid conveyance of the water, for such angles throw the fluid into eddies or cur- rents, by which its velocity is arrested. In practice, therefore, sudden turns are generally avoid- How is it proved that the velocity of the spouting liquid is equal to that of a falling body 1 Suppose a lead and a glass tube, of the same diameter, which will delivpr the greatest quantity of liquid in the same time 1 Why will a glass tuoe deliver most 7 What is said of the sud- den turnings of a tube in retarding the motion of the fluid 7 118 HYDRAULICS. ea, and where it is necessary that the pipe should change its direction, it is done by means of as large a circle as con- venient. Where it is proposed to convey a certain quantity of water to a considerable distance in pipes, there will be a great disappointment in respect to the quantity actually deli- vered, unless the engineer takes into account the friction, and the turnings of the pipes, and makes large allowances for these circumstances. If the quantity to be actually de- livered ought to fill a two inch pipe, one of three inches will not be too great an allowance, if the water is to be con veyed to any considerable distance. In practice, it will be found that a pipe of two inches in diameter, one hundred feet long, will discharge about five times as much water as one of one inch in diameter of the same length, and under the same pressure. This difference is accounted for, by supposing that both tubes retard the mo- tion of the fluid, by friction, at equal distance from their in- ner surfaces, and consequently, that the effect of this cause is much greater in proportion, in a small tube, than in large one. 484. The effect of friction in retarding the motion of fluids is perpetually illustrated in the flowing of rivers and brooks. On the side of a river, the water, especially where it is shallow, is nearly still, while in the middle of the stream it may run at the rate of five or six miles an hour. For the same reason, the water at the bottoms of rivers is much less rapid than at the surface. This is often proved by the oblique position of floating substances, which in still water would assume a vertical direction. 485. Thus, suppose the stick of wood Fig. 98. e, fig. 98, to be loaded at one end with _ lead, of the same diameter as the wood, ffi so as to make it stand upright in still | water. In the current of a river, where j the lower end nearly reaches the hot- ^ torn, it will incline as in the figure, her j cause the water is more rapid towards { the surface than at the bottom, and hence the tendency of the upper end to move faster than the lower one, gives it an inclination forward. How much more water will a two inch tube of a hundred feet long discharge, than a one inch tube of the same length 1 How is this di Terence accounted for 1 How do rivers show the effect of friction in re- tarding the motion of their water*? Explain fig. 98 HYDRAULICS. 119 MACHINES FOR RAISING WATER. 486. The common pump, though a hydraulic machine, depends on the pressure of the atmosphere for its effect, and therefore its explanation comes properly under the article Pneumatics, where the consequences of atmospheric press- ure will be illustrated. Such machines only, as raise water without the assist- ance of the atmosphere, come properly under the present article. 487. Archimedes 1 Screw. Among these, one of the most curious, as well as ancient machines, is the screw of Archi- medes, and which was invented by that celebrated philoso- pher, two hundred years before the Christian era, and then employed for raising water and draining land in Egypt. Fig. 99. 488. It consists of a large tube, fig. 99, coiled round a shaft of wood to keep it in place, and give it support. Both ends of the tube are open, the lower one being dipped into the water to be raised, and the upper one discharging it in an intermitting stream. The shaft turns on a support at each end, that at the upper end being seen at a, the lower one being hid by the water. As the machine now stands, the lower bend of the screw is filled with water, since it is below the surface c, d. On turning it by the handle, from left to right, that part of the screw now filled with water will nse above the surface c, d, and the water having no place Who is said to have been the inventor of Archimedes' screw 7 Ex- plain this machine, as represented in fig-. 99, and show how the water is elevated by turning it 120 HYDRAULICS. to escape, falls into the next lowest part of the screw at e At the next revolution, that portion which, during- the lasf was at e, will be elevated to g, for the lowest bend will re ceive another supply, which in the mean time will be trans- ferred to e, and thus, by a continuance of this motion, the water is finally elevated to the discharging orifice p. This principle is readily illustrated by winding- a piece of lead tube round a walking stick, and then turning the whole with one end in a dish of water, as shown in the figure. 489. Theory of Archimedes' Screw. By the following cuts and explanations, the manner in which this machine acts will be understood. 490. Suppose Fig. 100. the extremity 1, fig. 100, to be presented up- wards, as in the figure, the screw itself being in- clined as repre- sented. Then, from its peculiar form and position, it is evident, that commencing at 1, the screw will descend until we arrive at a certain point 2 ; in proceeding from 2 to 3 it will ascend. Thus, 2 is a point so situated that the parts of the screw on both sides of it ascend, and therefore if any body, as a ball, were placed in the tube at 2, it could not move in either direction without ascending. Again, the point 3, is so situated, that the tube on each side of it de- scends ; and as we proceed we find another point 4, which, like 2, is so placed, that the tube on both sides of it ascends, and, therefore, a body placed at 4, could not move without ascending. In like manner, there is a series of other points along the lube, from which it either descends or ascends, as is obvious by inspection. 491. Now let us suppose a ball, less in size than the bore of the tube, so as to move freely in it, to be dropped in at 1, As the tube descends from 1 to 2, the ball of course will de- scend down to 2, where it will remain at rest. How may the principle of Archimedes' screw be readily illustrated 1 Explain the manner in which a ball would ascend, fig. 100, by turn- ittg the screw. HYDRAULICS. 121 Next, suppose the ball to be fastened to the tube at i',, and suppose the screw to be turned nearly half round, so that the end 1 shall be turned do\vn\vards, and the point 2 brought nearly to the highest point of the curve 1, 2, 3. 492. This movement of the spiral, it is evident, would change the positions of the ascending and descending parts, as represented by fig. 101. The ball, which we Fig. 101. supposed attached to the iube, is now nearly at the highest point at 2, and if detached will descend down to 3, where it will rest. The point at which 2 was placed in the first position of the screw is marked by b , in the second position. The effect of turning the screw, there- fore, will be to transfer -C the ball from the highest to the lowest point. Another half turn of the screw, will cause the ball to pass over another high point, and descend the declivity down to 5, in fig. 101, where it will again rest. 493. It is unnecessary to explain the steps by which the ball would gain another point of elevation, since it is clear that by continuing the same process of action, and of reason- ing, it would be plain that the ball would be gradually transferred from the lowest to the highest point of the screw. Now all that we have said with respect to the ball, would be equally true of a drop of water in the tube ; and, there- fore, if the extremity of the tube were immersed in water, so that the fluid, by its pressure or weight, be continually forced into the extremity of the screw, it would, by making- it revolve, be gradually carried along the spiral to any height to which it might extend. 494. It will, however, be seen, from the above explana- tion, that the tube must not be so elevated from the point of immersion, that the spirals will not descend from one point to another, in which case it is obvious that the machine What is said concerning the inclination of the tube, in order to in- sure its action 1 II 122 HYDRAULICS. will not act. If the tube be placed in a perpendicular posi- tion, the ball, instead of gaming an increased elevation by turning the screw, would descend to the ground. A certain inclination, therefore, depending on the course of the screw, must be given this machine, in order to ensure its action. 495. Instead of this method, water was Fig. 102. sometimes raised by the ancients, by means of a rope, or bundle of ropes, as shown at fig. 102. This mode illustrates, in a very strik- ing manner, the force of friction between a solid and fluid, for it was by this force alone, that the water was supported and elevated. 496. The large wheel a, is supposed to stand over the well, and b, a smaller wheel, is fixed in the water. The rope is extended between the two wheels, and rises on one side in a perpendicular direc- tion. On turning the wheel by the crank d, the water is brought up by the friction of the rope, and falling into a reservoir at the bottom of the frame which supports the wheel, is discharged at the spout d. It is evident that the motion of the wheel, and conse- quently that of the rope, must be very rapid, in order to raise any considerable quantity of water by this method. But when trie upward velocity of the rope is eight or ten feet per second, a large quantity of water may be elevated to a considerable height by this machine. 497. Barker's Mill. For the different modes of apply- ing water as a power for driving mills, and other useful purposes, we must refer the reader to works on practical mechanics. There is, however, one method of turning ma- chinery by water, invented by Dr. Barker, which is strictly a philosophical, and at the same time a most curious inven- tion, and therefore is properly introduced here. Explain in what manner water is raised by the machine represented by fig. 102. HYDRAULICS. 123 498. This machine is called Fig. 103. Barkers centrifugal mill, and . H_d such parts of it as are necessary to understand the principle on which it acts are represented by fig. 103. The upright cylinder a, is a tube which has a funnel shaped mouth, for the admission of the stream of water from the pipe b. This tube is six or eight inches in diameter, and may be from ten to twenty feet long. The arms n and o, are also tubes communicat- ing freely with the upright one, from the opposite sides of which they proceed. The shaft d, is firmly fastened to the inside of the tube, openings at the same time being left for the water to pass to the arms o and n. The lower part of the tube is solid, and turns on a point resting on a block of stone or iron, c. The arms are closed at their ends, near which are the ori- fices on the sides opposite to tach other, so that the water spouting from them, will fly in opposite directions. The stream from the pipe b, is regulated by a stopcock, so as to keep the tube a constantly full without overflowing. To set this engine in motion, supple the upright tube to be filled with water, and the arms n and 0, to be given, a slight impulse ; the pressure of the water from the perpen- dicular column in the large tube will give the fluid the ve- locity of discharge at the ends of the arms proportionate to its height. The reaction that is produced by the flowing of the water on the points behind the discharging orifice, will continue, and increase the rotatory motion thus begun. After a few revolutions, the machine will receive an addi- tional impulse by the centrifugal force generated in the arms, and in consequence of this, a much more violent and rapid discharge of the water takes place, than would occur by the pressure of that in the upright tube alone. The cen- trifugal force, and the force of the discharge thus acting at the same time, and each increasing the force of the What is fig;. 103 intended to represent 1 Describe this mill. 124 PNEUMATICS. other, this machine revolves with great velocity and pro- portionate power. The friction which it has to overcome, when compared with that of other machines, is very slight, being chiefly at the point c, where the weight of the upright tube and its contents is sustained. By fixing a cog wheel to the shaft at d, motion may be given to any kind of machinery required. 499. Where the quantity of water is small, but its height considerable, this macbine maybe employed to great advan- tage, it being under such circumstances one of the most powerful engines ever invented. PNEUMATICS. 500. The term Pneumatics is derived from the Greek pneuma, which signifies breath, or air. It is that science which investigates the mechanical properties of air, and other elastic fluids. Under the article Hydrostatics, (420,) it was stated that fluids were of two kinds, namely, elastic and non-elastic, and that air and the gases belonged to the first kind, while water and other liquids belonged to the second. 501. The atmosphere which surrounds the earth, and in which we live, and a portion of which we take into our lungs at every breath, is called air, while the artificial pro- ducts which possess the same mechanical properties, are called gases. When, therefore, the word air is used, in what follows, it will be understood to mean the atmosphere which we breathe. 502. Every hollow, crevice, or pore, in solid bodies, not filled with a liquid, or some other substance, appears to be filled with air : thus, a tube of any length, the bore of which is as small as it can be made, if kept open, will be filled with air ; and hence, when it is said that a vessel is filled with air, it is only meant that the vessel is in its ordinary state. Indeed, this fluid finds its way into the most minute pores of all substances, and cannot be expelled and kept out of any vessel, without the assistance of the air-pump, or some other mechanical means. 503. By the elasticity of air, is meant its spring, or the What is pneumatics 1 What is air 1 What is gas 7 What is meant when it is said that a vessel is filled with air"? Is there any difficulty in expelling the air from vessels ? What is meant by the elasticity of air ? PNEUMATICS 125 .he force with which it re-acts, when compressed in a close vessel. It is chiefly in respect to its elasticity and lightness, that the mechanical properties of air differ from those of water, and other liquids. 504. Elastic fluids differ from each other in respect to the permanency of the elastic property. Thus, steam is elastic only while its heat is continued, and on cooling, returns again to the form of water. 505. Some of the gases also, on being strongly compress- ed, lose their elasticity, and take the form of liquids. But air differs from these, in being permanently elastic ; that is, if it be compressed with ever so much force, and retained under compression for any length of time, it does not there- fore lose its elasticity, or disposition to reg-am its former bulk, but always re-acts with a force in proportion to the power by which it is compressed. 506. Thus, if the strong tube, or barrel, fig. 104, be smooth, and equal on the inside, and there be fitted to it the solid piston, or plug a, so as to work up and down air tight, by the handle b, the air in the barrel may be com- pressed into a space a hundred times less than its usual bulk. Indeed, if the vessel be of suf- ficient strength, and the force employed suffi- ciently great, its bulk may be lessened a thou- sand times, or in any proportion, according to the force employed ; and if kept in this state for years, it will regain its former bulk the instant the pressure is removed. Thus, it is a general principle in pneumatics, .hat air is compressible in proportion to the force employed. 507. On the contrary, when the usual pressure of the at- mosphere is removed from a portion of air, it expands and occupies a space larger than before; and it is found by ex- periment, that this expansion is in a ratio, as the removal of the pressure is more or less complete. Air also expands or increases in bulk, when heated. If the stop-cock c, fig. 104, be opened, the piston a may be pushed down with ease, because the air contained in the barrel will be forced out at the aperture. Suppose the pis- How does air differ from steam, and some of the gases, in respect to its elasticity 1 Does air lose its elastic force by being long compressed 1 in what proportion to the force employed is the bulk of air lessened 1 11* _M PNEUMATICS. ton to be pushed down to within an inch of the bottom, and then the stop-cock closed, so that no air can enter below it. Now, on drawing the piston up to the top of the barrel, the inch of air will expand, and fill the whole space, and were this space a thousand times as large, it would still be filled with the expanded air, because the piston removes the press- ure of the external atmosphere from that within the barrel. It follows, therefore, that the space which a given portion of air occupies, depends entirely on circumstances. If it is under pressure, its bulk will be diminished in exact propor- tion ; and as the pressure is removed, it will expand in pro- portion, so as to occupy a thousand, or even a million times as much space as before. 508. Another property which air possesses is weight, or gravity. This property, it is obvious, must be slight, when compared with the weight of other bodies. But that air has a certain degree of gravity in common with other ponderous substances, is proved by direct experiment. Thus, if the air be pumped out of a close vessel, and then the vessel be ex- actly weighed, it will be found to weigh more when the air is again admitted. 509. Pressure of the Atmosphere. It is, however, the weight of the atmosphere which presses on every part of the earth's surface, and in which we live and move, as in an ocean, that here particularly claims our attention. The pressure of the atmosphere may be easi- Fig. 105. ly shown by the tube and piston, fig. 105. Suppose there is an orifice to be opened or closed by the valve b, as the piston a is moved up or down in its barrel. The valve being fast- ened by a hinge on the upper side, on pushing the piston down, it will open by the pressure of the air against it, and the air will make its escape. But when the piston is at the bottom of the bar- rel, on attempting to raise it again, towards the top, the valve is closed by the force of the exter- nal air acting upon it. If, therefore, the piston be drawn up in this state, it must be against the pressure of the atmosphere, the whole weight of In what proportion will a quantity of air increase in bulk as the pressure is removed from it 7 How is thus illustrated by fig. 104 1 On what circumstance, therefore, will the bulk of a given portion of air deoend 1 How is it proved that air has weight ? Explain in what manner the pressure of the atmosphere is shown by fig. 105. AIR PUMP. 127 which, to an extent equal to the diameter of the piston, must be lifted, while there will remain a vacuum or void space below it in the tube. If the piston be only three inches in diameter, it will require the full strength of a man to draw it to the top of the barrel, and when raised, if suddenly let go, it will be forced back again by the weight of the air, and will strike the bottom with great violence. 510. Supposing the surface of a man to be equal to 14 square feet, and allowing the pressure on each square inch to be 15lbs., such a man would sustain a pressure on his whole surface equal to nearly 14 tons. 511. Now, that it is the weight of the atmosphere which presses the piston down, is proved by the fact, that if its di- ameter be enlarged, a greater force, in exact proportion, will be required to raise it. And further, if when the piston is drawn to the top of the tube, a stop-cock, as at fig. 104, be opened, and the air admitted under it, the piston will not be forced down in the least, because then the air will press as much on the under, as on the upper side of the piston. 512. By accurate experiments, an account of which it is not necessary here to detail, it is found that the weight of the atmosphere on every inch square of the surface of the earth is equal to fifteen pounds. If, then, a piston working air tight in a barrel, be drawn up from its bottom, the force employed, besides the friction, will be just equal to that re- quired to lift the same piston, under ordinary circumstances, with a weight laid on it equal to fifteen pounds for every square inch of surface. 513. The number of square inches in the surface of a piston of a foot in diameter, is 113. This being multiplied by the weight of the air on each inch, which being 15 pounds, is equal to 1695 pounds. Thus the air constantly presses on every surface, which is equal to the dimensions of a circle one foot in diameter, with a weight of 1695 pounds. AIR PUMP. 514. The air pump is an engine by which the air can be pumped out of a vessel, or withdrawn from it. The vessel What is the force pressing on the piston, when drawn upward, some- times called ? How is it proved that it is the weight of the atmosphere, instead of suction, which makes the piston rise with difficulty'? What is the pressure of the atmosphere on every square inch of surface on the earth 1 What is the number of square inches in a circle of one foot in diameter 1 What is the weight of the atmosphere on a surface of a foot in diameter 1 What is the air pump *? 128 Alfc PUMP. Fig. 106. so exhausted, is called a receiver, and the space thus left in the vessel, after withdrawing the air, is called a vacuum. The principles on which the air pump is constructed are readily understood, and are the same in all instruments of this kind, though the form of the instrument itself is often considerably modified. 515. The general principles of its construction will be comprehended by an explanation of fig. 106. In this figure, let g be a glass vessel, or receiver, closed at the top, and open at the bottom, standing on a perfectly smooth surface, which is called the plate of the air pump. Through the plate is an aperture, a, which communicates with the inside of the receiver, and the barrel of the pump. The piston rod, p, works air tight through the stuffed collar, c, and the piston also moves air tight through the barrel. At the extremity of the barrel, there is a valve e, which opens outwards, and is closed with a spring. 516. Now suppose the piston to be drawn up to r, it will then leave a free communication between the receiver g, through the orifice a, to the pump barrel in which the pis- ton works. Then if the piston be forced down by its ban die, it will compress the air in the barrel between d and e, and, in consequence, the valve e will be opened, and the air so condensed will be forced out. On drawing the piston up again, the valve will be closed, and the external air not be- ing permitted to enter, a vacuum will be formed in the bar- rel, from e to a little above d. When the piston comes again to c, the air contained in the glass vessel, together with that in the passage between the vessel and the pump barrel, will rush in to fill the vacuum. Thus, there will be less air in the whole space, and consequently in -the receiver, than at first, because all that contained in the barrel is forced out at every stroke of the piston. On repeating the same process, What is the receiver of an air pump 1 What is a vacuum 1 In fig. 106, which is the receiver of the air pump 1 When the piston is pressed down, what quantity of air is thrown out 1 When the piston is drawn up, what is formed in the barrel 7 How is this vacuum again filled with air? AIR PUMP. 129 that is, drawing up and forcing down the piston, the air at each time in the receiver, will become less and less in quan- tity, and, in consequence, more and more rarefied. For it must be understood, that although the air is exhausted at every stroke of the pump, that which remains, by its elas- ticity, expands, and still occupies the whole space. The quantity forced out at each successive stroke is therefore di- minished, until, at last, it no longer has sufficient force be- fore the piston to open the valve, when the exhausting pow- er of the instrument must cease entirely. Now, it will be obvious, that as the exhausting power of the air pump depends on the expansion of the air within it, a perfect vacuum can never be formed by its means, for so long as exhaustion takes place, there must be air to be forced out, and when this becomes so rare as not to force open the valves, then the process must end. 517. A good air pump has two similar pumping barrels to that described, so that the process of exhaustion is per- formed in half the time that it could be performed by one barrel. The barrels, with their Fig 107. pistons, and the usual mode of working them, are represented by fig. 107. The piston rods are furnished with racks, or teeth, and are worked by the toothed wheel a, which is turned back- wards and forwards, by the lever and handle b. The exhaustion pipe, c, leads to the plate on which the receiver stands, as shown in fig. 107. The valves v, n, u, and m, all open upwards. 518. To understand how these pistons act to exhaust the air from the vessel on the plate, through the pipe c, we will suppose, that as the two pistons now stand, the handle b is to be turned towards the left. This will raise the piston A, Is the air pump capable of producing a perfect vacuum 7 Why do common air pumps have more than one barrel and piston 1 How are the pistons of an air pump worked ? 130 CONDENSER. while the valve u will be closed by the pressure of the ex- ternal air acting on it in the open barrel in which it works. There would then be a vacuum formed in this barrel, did not the valve m open, and let in the air coming from the re ceiver, through the pipe c. When the piston, therefore, is at the upper end of the barrel, the space between the piston and the valve m, will be filled with the air from the receiver. Next, suppose the handle to be moved to the right, the pis- ton A will then descend, and compress the air with which the barrel is filled, which, acting against the valve u, forces it open, and thus the air escapes. Thus, it is plain, that every time the piston rises, a portion of air, however rare- fied, enters the barrel, and every time that it descends, this portion escapes, and mixes with the external atmosphere. The action of the other piston is exactly similar to this, only that B rises while A falls, and so the contrary. It will appear, on an inspection of the figure, that the air cannot pass from one barrel to the other, for while A is rising, and the valve m is open, the piston B will be descending, so that the force of the air in the barrel B, will keep the valve n closed. Many interesting and curious experiments, illus- trating the expansibility and pressure of the atmosphere, are shown by this instrument. 519. If a withered apple be placed under the receiver, and the air is exhausted, the apple will swell and become plump, in consequence of the expansion of the air which it contains within the skin. 520. Ether, placed in the same, situation, soon begins to boil without the influence of heat, because its particles, not having the pressure of the atmosphere to force them toge- ther, fly off with so much rapidity as to produce ebul- lition. THE CONDENSER. 521. The operation of the condenser is the reverse of that of the air pump, and is a much more simple machine. The air pump, as we have just seen, will deprive a vessel of its ordinary quantity of air. The condenser, on the contrary, While the piston Ais ascending, which valves will be open, and which closed 1 When the piston A descends, what becomes of the air with which its barrel was filled 1 Why does not the air pass from one barrel to the other, through the valves m and n 1 Why does an apple placed in the exhausted receiver grow plump 1 Why does ether Boil in the same situation ? How does the condenser operate 1 CONDENSER. n'ill double or treble the ordinary quantity of air in a close vessel, according to the force employed. This instrument, fig. 108, consists of a pump Fig. 108. barrel and piston a, a stop-cock Z>, and the vessel c furnished. with a valve opening inwards. The orifice d is to admit the air, when the piston is drawn up to the top of the barrel. 522. To describe its action, let the piston be above d, the orifice being open, and therefore the instrument filled with air, of the same den- sity as the external atmosphere. Then, on forcing the piston down, the air in the pump barrel, below the orifice d, will be compressed, and will rush through the stop-cock b, into the vessel c, where it will be retained, because, on again moving the piston upward, the elasticity of the air will close the valve through which it was forced. On drawing the piston up again, another portion of air will rush in at the orifice d, and on forcing it down, this will also be driven into the vessel c; and this process may be continued as long as sufficient force is applied to move the piston, or there is suf- ficient strength in the vessel to retain the air. When the condensation is finished, the stop-cock b may be turned, to render the confinement of the air more secure. 523. The magazines of air guns are filled in the man- ner above described. The air gun is shaped like other guns, but instead of the force of powder, that of air is em- ployed to project the bullet. For this purpose, a strong hollow ball of copper, with a valve on the inside, is screw- ed to a condenser, and the air is condensed in it, thirty or forty times. This ball or magazine is then taken from the condenser, and screwed to the gun, under the lock. By means of the lock, a communication is opened between the magazine, and the inside of the gun-barrel, on which the spring of the confined air against the leaden bullet is such, as to throw it with nearly the same force as gunpowder. Explain fig. 108, and show in what manner the air is condensed Explain the principle of the air gun. 132 BAROMETER. BAROMETER. Fig. 109. 524. Suppose a, fig. 109, to be a long tube, with the piston b so nicely fitted to its inside, is to work air tight. If the lower end of the lube be dipped into water, and the piston drawn up by pulling at the handle c, the water will follow the piston so closely, as to be in contact with its surface, and apparently to be drawn up by the piston, as though the whole was one solid body. If the tube be thirty-five feet long, the water will continue to follow the piston, until it comes to the height of about thirty- three feet, where it will stop, and if the piston be drawn up still farther, the water will not follow it, but will remain stationary, the space from this height, between the piston and the water, being left a void space, or vacuum. 525. The rising of the water in the above case, which only involves he principle of the common pump, is thought by some to be ? -?jS caused by suction, the piston sucking up the "Y water as it is drawn upward. But according to the common notion attached to this term, there is no rea- son why the water should not continue to rise above the thirty-three feet, or why the power of suction should cease at that point, rather than at any other. Without entering into any discussion on the absurd notions concerning the power of suction, it is sufficient here to state, that it has long since been proved, that the elevation of the water, in the case above described, depends entirely on the weight and pressure of the atmosphere, on that portion of the fluid which is on the outside of the tube. Hence, when the pis- ton is drawn up, under circumstances where the air cannot act on the water around the tube, or pump barrel, no eleva- tion of the fluid will follow. This will be obvious, by the following experiment. Suppose the tube, fig. 109, to stand with its lower end in the water, and the piston a to be drawn upward thirty-five feet, how far will the water follow the piston 1 What will remain in the tube between the piston and the water, after the piston rises higher than thirty-three feet 7 What is commonly supposed to make the water rise in such cases'? Is there any reason why the suction should cease at 33 feet"? What is the true cause of the elevation of the water, when the piston, fig. 109, is drawn up 7 BAROMETER, 133 526. Suppose fig-. 110 to be the sections, or Fig. 110. halves, of two tubes, one within the other, the outer one being made entirely close, so as to ad- mit no air, and the space between the two being also made air tight at the top. Suppose, also, that the inner tube being left open at the lower end, does not reach the bottom of the outer tube, and c thus that an open space be left between the two tubes every where, except at their upper ends, where they are fastened together ; and suppose that there is a valve in the piston, opening up- wards, so as to let the air -which it contains es- cape, but which will close on drawing the piston upwards. Now, let the piston be at a, and in this state pour water through the stop-cock, c, un- til the inner tube is filled up by the piston, and the space between the two tubes filled up to the same point, and then let the stop-cock be closed. If now the piston be drawn up to the top of the tube, the water will not follow it, as in the case first described ; it will only rise a few inches, in consequence of the elasticity of the air above the water, between the tubes, and in the space above the water, there will be formed a vacuum be- tween the water and the piston, in the inner tube. 527. The reason why the result of this experiment dif- fers from that before described, is, that the outer tube pre- vents the pressure of the atmosphere from forcing the water up the inner tube as the piston rises. This may be instantly proved, by opening the stop-cock c, and permitting the air to press upon the water, when it will be found, that as the air rushes in, the water will rise and fill the vacuum, up to the piston. For the same reason, if a common pump be placed in a cistern of water, and the water is frozen over on its surface, so that no air can press upon the fluid, the piston of the pump might be worked in vain, for the water would not, as usual, obey its motion. 528. It follows, as a certain conclusion from such experi- How is it shown by fig. 110, that it is the pressure of the atmos- phere which causes the water to rise in the pump barrel 1 Suppose the jce prevents the atmosphere from pressing on the water in a vessel, can the water be pumped out 1 What conclusion follows from the experi- ments above described 1 12 134 BAROMETER. ments, that when the lower end of a tube is placed in wnter, and the air from within removed by drawing up the piston, that it is the pressure of the atmosphere on the water around the tube, which forces the fluid up to fill the space thus left by the air. It is also proved, that the weight, or pressure of the atmosphere, is equal to the weight of a perpendicular column of water 33 feet high, for it is found (fig. 109) that the pressure of the atmosphere will not raise the water more than 33 feet, though a perfect vacuum be formed to any height above this point. Experiments on other fluids, prove that this is the weight of the atmosphere, for if the end of a tube be dipped in any fluid, and the air be removed from the tube, above the fluid, it will rise to a greater or less height than water, in proportion as its specific gravity is less or greater than that of water. 529. Mercury, or quicksilver, has a specific gravity of about 13^ times greater than that of water, and mercury is found to rise about 29 inches in a tube under the same circum- stances that water rises 33 feet. Now, 33 feet is 396 inches, which being divided by 29, gives nearly 13, so that mer- cury being 13^ times heavier than water, the water will rise under the same pressure 13| times higher than the mercury. 530. Construction of the Barometer. The barometer is constructed on the principle of atmospheric Fig. 111. pressure, which we have thus endeavoured to explain and illustrate to common compre- hension. This term is compounded of two Greek words, baros, weight, and metron, measure, the instrument being designed to measure the weight of the atmosphere. Its construction is simple, and easily understood, being merely a tube of glass, nearly filled with mercury, with its lower end placed in a dish of the same fluid, and the upper end furnished with a scale, to measure the height of the mercury. 531. Let a, fig. Ill, be such a tube, 34 or fo 35 inches long, closed at one end, and open at the other. To fill the tube, set it upright, How is it proved, that the pressure of the atmosphere is equa to the weight of a column of water, 33 feet high 7 How do experimewts on other fluids show that the pressure of the atmosphere is equal to he weight of a column of water, 33 feet high 7 How high does mercnry rise in an exhausted tube 7 What is the principle oh which the ba- rometer is constructed 7 What does the barometer measure 7 Describe the construction of the barometer, ab repre^nrid bv fig. 111. BAROMETER. 135 and pour the mercury in at the open end, and when it is en- tirely full, place the fore finger forcibly on this end, arid then plunge the tube and finger under the surface of the mercury, before prepared in the cup b. Then withdraw the finger, taking care that in doing this, the end of the tube is not raised above the mercury in the cup. When the finger is removed, the mercury will descend four or five inches, and after several vibrations, up and down, will rest at an elevation of 29 or 30 inches above the surface of that in the cup, as at c. Having fixed a scale to the upper part of the tube, to indicate the rise and fall of the mercury, the ba- rometer would be finished, if intended to remain stationary. It is usual, however, to have the tube enclosed in a mahoga- ny or brass case, to prevent its breaking, and to have the cup closed on the top, and fastened to the tube, so that it can be transported without danger of spilling the mercury. 532. The cup of the portable barometer also differs from that described, for were the mercury enclosed on all sides, in a cup of wood, or brass, the air would be prevented from acting upon it, and therefore the instrument would be use- less. To remedy this defect, and still have the mercury perfectly enclosed, the bottom of the cup is made of leather, which, being elastic, the pressure of the atmosphere acts upon the mercury in the same manner as though it was not enclosed at all. Below the leather bottom, there is a round plate of metal, an inch in diameter, which is fixed on the top of a screw, so that when the instrument is to be trans- ported, by elevating this piece of metal, the mercury is thrown up to the top of the tube, and thus kept from playing backwards and forwards, when the barometer is in motion. 533. A person not acquainted with the principle of the instrument, on seeing the tube turned bottom upwards, will be perplexed to understand why the mercury does not fol- low the common law of gravity, and descend into the cup j were the tube of glass 33 feet high, and filled with water, the lower end being dipped into a tumbler of the same fluid, the wonder would be still greater. But as philosophical facts, one is no more wonderful than the other, and both are readily explained by the principles above illustrated. How is the cup of the portable barometer made, so as to retain the mercury, and still allow the air to press upon it 1 What is the use of the metallic plate and screw, under the bottom of the cup 1 Explain the rea- n>n why the mercury does not fall out of the barometer tube, when its open end is downwards. 136 BAROMETER. 534. It has already been shown, (528,) that it is the pressure of the atmosphere on the fluid around the tube, by which the fluid within it is forced upward, when the pump is exhausted of its air. The pressure of the air, we have also seen, is equal to a column of water 33 feet high, or of a column of mercury 29 inches high. Suppose, then, a tube 33 feet high is filled with water, the air would then be en- tirely excluded, and were one of its ends closed, and the other end dipped in water, the effect would be the same as though both ends were closed, for the water would not escape, unless the air were permitted to rush in and fill up its place. The upper end being closed, the air could gain no access in that direction, and the open end being under water, is equal- ly secure. The quantity of water in which the end of the tube is placed, is not essential, since the pressure of a col- umn of water, an inch in diameter, provided it be 33 feet high, is just equal to a column of air of an inch in diameter, of the whole height of the atmosphere. Hence the water on the outside of the tube serves merely to guard against the entrance of the external air. 535. The same happens to the barometer tube, when fill- ed with mercury. The mercury, in the first place, fills the tube perfectly, and therefore entirely excludes the air, so that when it is inverted in the cup, all the space above 29 inches is left a vacuum. The same effect precisely would be produced, were the tube exhausted of its air, and the open end placed in the cup ; the mercury would run up the tube 29 inches, and then stop, all above that point being left a vacuum. The mercury, therefore, is prevented from falling out of the tube, by the pressure of the atmosphere on that which remains in the cup ; for if this be removed, the air will enter, while the mercury will instantly begin to descend. 536. In the barometer described, the rise and fall of the mercury is indicated by a scale of inches, and tenths of inches, fixed behind the tube ; but it has been found, that very slight variations in the density of ,the atmosphere, are not readily perceived by this method. It being, however, desirable that these minute changes should be rendered more obvious, a contrivance for increasing the scale, called the wheel barometer, was invented. What fills the space above 29 inches, in the barometer tubel In the common barometer, how is the rise and fall of the mercury indicated 7 Why was the wheel barometer invented 1 BAROMETER. 137 537. The whole length of the tube of the Fig. 112. wheel barometer, fig. 112, from c to a, is 34 or 35 inches, and it is filled with mercury, as usual. The mercury rises in the short leg to the point o, where there is a small piece of glass floating on its surface, to which there is attached a silk string, passing over the pulley p. To the axis of the pulley is fixed an index, or hand, and behind this is a graduated circle, as seen in the figure. It is obvious, that a very slight variation in the height of the mercury at 0, will be indicated by a considerable mo- tion of the index, and thus changes in the weight of the atmosphere, hardly perceptible by the common barometer, will become quite apparent by this. 538. The mercury in the barometer tube being sustained by the pressure of the atmo- sphere, and its medium altitude at the surface of the earth being about 29 inches, it might be expected that if the instrument was carried to a height from the earth's surface, the mercury would suffer a proportionate fall, be- cause the pressure must be less at a distance from the earth, than at its surface, and experiment proves this to be the case. When, therefore, this instrument is elevated to any considerable height, the descent of the mercury becomes perceptible. Even when it is carried to the top of a hill, or high tower, there is a sensible depression of the fluid, so that the barometer is employed to measure the height of mountains, and the elevation to which balloons ascend from the surface of the earth. On the top of Mont Blanc, which is about 16,000 feet above the level of the sea, the medium elevation of the mercury in the tube is only 14 inches, while on the surface of the earth, as above stated, it is 29 inches. 539. The medium range of the barometer in several countries, has generally been stated to be about 29 inches. It appears, however, from observations made at Cambridge, Explain fig. 106, and describe the construction of the wheel barome- ter. What is stated to be the medium range of the barometer at the surface of the earth 1 Suppose the instrument is elevated from the earth, what is the effect on the mercury ? How does the barometer in- dicate the heights of mountains 1 What is the medium range of the mercury on Mont Blanc 1 What is stated to be the medium range of the barometer at Cambridge] 12* 138 BAROMETER. in Massachusetts, for the term of 22 years, that its range there was nearly 30 inches. 540. Use of the Barometer. While the barometer stands in the same place, near the level of the sea, the mercury seldom or never falls below 28 inches, or rises above 31 inches, its whole range, while stationary, being- only about 3 inches. These changes in the weight of the atmosphere, indicate corresponding changes in the weather, for it is found, by watching these variations in the height of the mercury, that when it falls, cloudy or falling weather ensues, and that when it rises, fine clear weather may be expected. During the time when the weather is damp and lowering, and tho smoke of chimneys descends towards the ground, the mer- cury remains depressed, indicating that the weight oi the atmosphere, during such weather, is less than it is when the sky is clear. This contradicts the common opinion, that the air is the heaviest, when it contains the greatest quantity of fog and smoke, and that it is the uncommon weight of the atmosphere which presses these vapours towards the ground. A little consideration will show, that in this case the popular belief is erroneous, for not only the barometer, but all the experiments we have detailed on the subject of specific grav- ity, tend to show that the lighter any fluid is, the deeper any substance of a given weight will sink in it. Common ob- servation ought^therefore, to correct the error, for every- body knows that a heavy body will sink in water while a light one will swim, and by the same kind of reasoning ought to consider, that the particles of vapour would de- scend through a light atmosphere, while they would be pressed up into the higher regions, by a heavier air. 541. The principal use of the barometer is on board of .ships, where it is employed to indicate the approach of storms, and thus to give an opportunity of preparing accord- ingly ; and it is found that the mercury suffers a most re- markable depression before the approach of violent winds, or hurricanes. The watchful captain, particularly in south- ern latitudes, is always attentive to this 1 monitor, and when How many inches does a fixed barometer vary in height 7 When the mercury falls, what kind of weather is indicated 1 When the mer- cury rises, what kind of weather may be expected 1 When fog and smoke descend towards the ground, is it a sign of a light or heavy at- mosphere 7 By what analogy is it shown that the air is lightest when filled with vapour 1 Of what use is the barometer, on board of ships 1 When does the mercury suffer the most remarkable depre?sion 1 PUMP. 139 he observes the mercury to sink suddenly, takes his meas- ures without delay to meet the tempest. During a vioient storm, we have seen the wheel barometer sink a hundred degrees in a few hours. But we cannot illustrate the use of this instrument at sea better than to give the following extract from Dr. Arnot, who was himself present at the time. " It was," he says, " in a southern latitude. The sun had just set with a placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch proceeded, when the captain's orders came to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the oldest sailors had not perceived even a threatening in the sky, and were surprised at the extent and hurry of the preparations ; but the required measures were not completed, when a more awful hurricane burst upon them, than the most experienced had ever braved. Nothing could withstand it; the sails, already furled, and closely bound to the yards, were riven into tatters ; even the oare yards and masts were in a great measure disabled ; and at one time the whole rigging had nearly fallen by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and the incessant peals of thunder, that no human voice could be heard, and amidst the general consternation, even the trumpet sounded in vain. On that awful night, but for a little tube of mer- cury, which had given the warning, neither the strength of the noble ship, nor the skill and energies of her commander, could have saved one man to tell the tale." PUMPS. 542. There is a philosophical experiment, of which no one in this country is ignorant. If one end of a straw be introduced into a barrel of cider, and the other end sucked with the mouth, the cider will rise up through the straw, and may be swallowed. The principles which this experiment involve, are exactly the same as those concerned in raising water by the pump. The barrel of cider answers to the well, the straw to the pump log, and the mouth acts as the piston, by which the air is removed. 543. The efficacy of the common pump, in raising water, What remarkable instance is stated, where a ship seemed to be saved l>y the use of the barometer 1 What experiment is stated, as Ulustra- ling the principle of the common pump 1 140 PUMP. Fig. 113. depends upon the principle of atmospheric pressure, wL:h has been fully illustrated under the articles air pump and barometer. 544. These machines are of three kinds, namely, the sucking, common pump, the lifting pump, and the forcing pump. Of these, the common or household pump is the most in use, and for ordi- nary purposes, the most convenient. It consists of'B long tube, or barrel, called the pump log, which reaches from a few feet above the ground to near the bottom of the well. At a, fig. 113, is a valve, opening upwards, called the pump box. When the pump is not in action, this is always shut. The piston b, has an aperture through it, which is closed by a valve, also opening upwards. By the pupil who has learned what has been explained under the articles air pump, and barometer, the action of this machine will be readily understood. 545. Suppose the piston b to be down to a, then on depressing the lever c, a vacuum would be formed between a and b, did not the water in the well rise, in consequence of the pressure of the atmosphere on that around the pump log in the well, and take the place of the air thus removed. Then, on raising the end of the lever, the valve a closes, because the water is forced upon it, in consequence of the descent of the piston, and at the same time the valve in the piston b opens, and the water, which cannot descend, now passes above the valve b. Next, on raising the piston, by again depressing the lever, this por- tion of water is lifted up to b, or a little above it, while an- other portion rushes through the valve a to fill its place- After a few strokes of the lever, the space from the piston b to the spout, is filled with the water, where, on continuing to work the lever, it is discharged in a constant stream. On what does the action of the common pump depend 7 How many kinds of pumps are mentioned 1 Which kind is the common 1 Describe the common pump. Explain how the common pump acts. When the lever is depressed, what takes place in the pump barrel 7 When the lever is elevated, what takes place 1 How far is the water raised by at- mospheric pressure, and now far by lifting ? PUMP. 141 Although, in common language, this is called the suction pump, still it will be observed, that the water is elevated by suction, or, in more philosophical terms, by atmospheric pressure, only above the valve a, after which it is raised by lifting up to the spout. The water, therefore, is pressed into the pump barrel by the atmosphere, and thrown out by lifting. 546. The lifting pump, properly so called, has the piston in the lower end of the barrel, and raises the water through the whole distance, by forcing it upward, without the agency of the atmosphere. 547. In the suction pump, the pressure of the atmosphere will raise the water 33 or 34 feet, and no more, after which it may be lifted to any height required. 548. The forcing pump differs from both these, in hav- ing its piston solid, or without a valve, and also in having a side pipe, through which the water is forced, instead of rising in a perpendicular direction, as in the others. 549. The forcing pump is represented by fig. 114, where a is a solid piston, working air tight in its barrel. The tube c leads from the barrel of the air vessel d. Through the pipe p, the water is thrown into the open air. g is a gauge, by which the pressure of the water in the air vessel is ascertained. Through the pipe i, the water ascends into the barrel, its up- * per end being furnished with a valve opening upwards. 550. To explain the action of this pump, suppose the pis- ton to be down to the bottom of the barrel, and then to be raised upward by the lever I ; the tendency to form a vacuum in the barrel, will bring the water up through the pipe i, Fig. 114. How does the lifting pump differ from the common pump ? How does the forcing pump differ from the common pump* Explain fig. 114, and show in what manner the water is brought up through the pipe i, and afterwards thrown out at the pipe p. 142 FIRE ENGINE. by the pressure of the atmosphere. Then, on depressing the piston, the valve at the bottom of the barrel will be closed, and the water, not finding admittance through the pipe whence it came, will be forced through the pipe c, and opening the valve at its upper end, will enter into the air vessel d, and be discharged through the pipe p, into the open air. , The water is therefore elevated to the piston barrel by the pressure of the atmosphere, and afterwards thrown out by the force of the piston. It is obvious, that by this ar- rangement, the height to which this fluid may be thrown, will depend on the power applied to the lever, and the strength with which the pump is made. The air vessel d contains air in its upper part only, the lower part, as we have already seen, being filled with water. The pipe p, called the discharging pipe, passes down into the water, so that the air cannot escape. The air is there- fore compressed, as the water is forced into the lower part of the vessel, and re-acting upon the fluid by its elasticity, throws it out of the pipe in a continued stream. The con- stant stream which is emitted from the direction pipe of the fire engine, is entirely owing to the compression and elas- ticity of the air in its air vessel. In pumps, without such a vessel, as the water is forced upwards, only while the piston is acting upon it, there must be an interruption of the stream while the piston is ascending, as in the common pump. The air vessel is a remedy for this defect, and is found also to render the labour of drawing the water more easy, be- cause the force with which the air in the vessel acts on the water, is always in addition to that given by the force of the piston. FIRE ENGINE. 551. The fire engine is a modification of the forcing pump. It consists of two such pumps, the pistons of which are moved by a lever with equal arms, the common fulcrum being at c, fig. 115. While the piston a is descending, the Why does not the air escape from the air vessel in this pump ? What effect does the air vessel have on the stream discharged 1 Why does the air vessel render the labour of raising the water more easy 1 FIRE ENGINE. 143 Fig. 115. other piston, b, is ascending. The water is forced by the pressure of the atmosphere, through the common pipe p, and then dividing, ascends into the working barrels of each piston, where the valves, on both sides, prevent its re- turn. By the- alternate de- pression of the pistons, it is then forced into the air box d, ind then by the direction pipe e, is thrown where it is want- ed. This machine acts pre- cisely like the forcing pump, only that its power is doubled, by having two pistons instead of one. 552. There is a beautiful fountain, called the fountain vf Hiero, which acts by the elasticity of the air, and on the Fig. 116. same principle as that already de- scribed. Its construction will be understood by fig. 1 16, but its form may be varied according to the dic- tates of fancy or taste. The boxes a and b, together with the two tubes, are made air tight, arid strong, in proportion to the height it is desired d the fountain should play. 553. To prepare the fountain for action, fill the box a, through the spouting tube, nearly full of water. The tube c, reaching nearly to the top of the box, will prevent the wa- tf>r from passing downwards, while the spouting pipe will prevent the air from escaping upwards, after the vessel is about half filled with wa- ter. Next, shut the stop-cock of the spouting pipe, and pour water into the open vessel d. This will descend into the vessel b, through the tube e, which nearly reaches its bottom, so that Explain fig. 115, and describe the action of the fire engine. What causes the continued stream from the direction pipe of this engine ? How is the fountain of Hiero constructed 1 L44 STEAM ENGINE. after a few inches of water are poured in, air can escape, except by the tube c, up into the vessel a, The air will then be compressed by the weight of the column of water in the tube e, and therefore the force of the water from the jet pipe will be in proportion to the height of this tube. If this tube is 20 or 30 feet high, on turning the stop-cock, a jet of water will spout from the pipe that will amuse and astonish those who have never before seen such an experiment. STEAM ENGINE. 555. Like most other great and useful inventions, the steam engine, from a very simple contrivance, for the pur- pose of' raising water, has been improved at various times, and by a considerable number of persons, until it has been brought to its present state of power and perfection. 556. By most writers, the origin of this invention is at- tributed to the Marquis of Worcester, an Englishman, in about 1663. But as he has left no drawing, nor such a par- ticular description of his machine, as to enable us to. define its mode of action, it is impossible, at the present time, to say how much credit ought to be attributed to this invention. 557. It is certain, that the first engines had neither cylin- ders, piston, nor gearing, by which machinery was made to revolve, these most important parts having been added by succeeding inventors and improvers. 558. Captain Savary's Engine. The first steam engine of which we have any definite description, was that invented by Capt. Thomas Savary, an Englishman, in 1698. By this engine, the water was raised to a certain- height, by means of a vacuum formed by the condensation of steam, and then was forced upward by the direct force of steam from the boiler. 559. It appears that the idea of forming a vacuum by the condensation of steam, was suggested to Capt. Savary by the following circumstances : Having drank a flask of Florence wine at an inn, he threw the empty flask on the fire, and a moment after called for a basin of water to wash his hands. A small quantity of the wine which remained in the flask, began to boil, and On what will the height of the jet from Hiero's fountain depend 1 What was the origin of the steam engine 1 To whom is this inven tion generally attributed 1 Who was the inventor of the first engine of which we have any definite description? What was the origin of Capt. Savary's idea of raising water by a vacuum 1 STEAM ENGINE. 145 steam issued from its mouth. Observing this, it occurred to him to try what effect would be produced by inverting the flask, and plunging its mouth into the cold water of the basin. Putting on a thick glove to defend his hand from the heat, he seized the flask, and the moment he plunged its mouth into the water, the liquid rushed up, and nearly rilled the vessel. 560. Savary states, that this circumstance suggested im- mediately to him the possibility of giving effect to the at- mospheric pressure, by creating a vacuum by the condensa- tion of steam. His plan was to lift the water from the mines to a certain height, in this manner, and to force it to the elevation required by the direct power of the steam. 561. Fig. 117 will show the principle, though not the precise form, of Savary's steam engine. It consists of a boiler, a, for the generation Fig- 117. of steam, which is furnished with a safety valve, b, which opens and lets off the steam, when the pressure would otherwise endanger the burst- ing of the boiler. From the boiler there proceeds the steam pipe, furnished with the stop-cock, c, to the steam, vessel, d. From the bottom of the steam vessel, there c scends the pipe e, called the suction pipe, which dips into the well, or reservoir, from which the water is to be rais- ed. This pipe is furnished with a valve, opening up- wards, at its upper end. From the upper end of the steam vessel rises another pipe, /, called the force pipe, which also has a valve opening up- wards. To this pipe is attached a small cistern, g, furnished with a short pipe, called the condensing pipe, and from which cold water can be drawn, so as to fall upon the steam vessel d. What are the parts of which Savary's engine consisted 1 Describe the process by which water is raised from the well to the steam vessel with this engine. 13 146 STEAM ENGINE. 562. To trace the action of this simple apparatus, suppose the steam vessels and tubes to be filled with atmospheric air, which of course would be the case, while the whole remains cold. But on making a fire under the boiler, steam is gen- erated, which, on turning the stop-cock c, is let into the. steam vessel d, where for a time it is condensed, and falls down in drops on the sides of the vessel. The continued supply of steam will, however, soon heat the vessel, so that no more vapour will be condensed, and its elastic force will open the upper valve, and it will pass off through the pipe /, while, at the same time, and by the same force, the lower valve will be closed. 563. When the steam has driven all the atmospheric air from the vessel d, and the upper pipe, and there remains no- thing in them but the pure vapour of water, suppose the stop-cock c to be turned, so as to stop the further supply of steam, and that at the same time cold water be allowed to run from the condensing cistern g, on the steam vessel d. The steam will thus be condensed into water, leaving the interior of the vessel a vacuum. The pressure of the at- mosphere will close the upper valve, while the same press- ure acting on the water surrounding the tube in the well, will force the fluid up to take the place of the vacuum in the steam vessel d. 564. The height to which water may thus be elevated, we have already seen, is about 33 feet, provided the vacuum be perfect, but Savary was never able to elevate it more than 26 feet by this method. We now suppose that the steam vessel is filled with wa- ter, by the creation of a vacuum, and the pressure of the at- mosphere alone, the direct force of the steam having no agency in the process. But in order to continue the eleva- tion above the level of the steam vessel, the elastic pressure of the steam must be employed. 565. Let us now suppose, therefore, that the vessel d is nearly full of water, and that the stop-cock c is turned, so as to admit the steam from the boiler through the tube to the upper part of the steam vessel, and consequently above the water. At first, the steam will be condensed by the cold surface of the water, but as hot water is lighter than cold, there will soon become a film of heated liquid, by the con- How high did Savary's engine elevate water by atmospheric press- ure? Describe the manner in which the water was elevated above the eleam vessel. STEAM ENGINE. 147 iensation of the steam on the surface of the cold, so that, in a few minutes, no more steam will be condensed. Then the direct force of the steam pressing upon the water, will drive it through the force pipe/ and opening the valve, will de- rate it to the height required. 566. When all the water has been driven out, the con- tinued influx of the steam will heat the vessel until no far- .her condensation will take place, and the vessel will be fill- ed with the pure vapour of water, as before, when the steam, oeing shut off, and the cold water let on, a vacuum will be produced, and another portion of water be elevated to take its place, as already described, and so on continually. This machine, though a mere apology for the complex and effective steam engines of the present day, is neverthe- less highly creditable to the mechanical genius of the in- ventor, considering the low state of science and mechanical knowledge at that time. 567. These engines were chiefly employed in the drain- age of the coal mines, and were sufficiently powerful to elevate the water to the height of about 90 feet, including both the atmospheric pressure, and the direct force of the steam. But the process was exceeding slow ; the quantity of steam wasted in the process was very great, and the quan- tity of fuel consumed immense. Besides these disadvan- tages, the bursting power of the steam, when applied with a force sufficient to elevate a column of water 60 feet high, was such as to require vessels of great strength, and, conse- quently, engines of small capacity only could be employed. In addition to these defects, where the mine was several hundred feet deep, three or four engines must be employed, since each could elevate the water only about 90 feet. It is hardly necessary, therefore, to say, that Savary's engine did not answer the principal object of its design, that of drain- ing the English mines. 568. Newcomers Engine.-?-Th& steam engine which suc- ceeded that of Savary, was invented by Thomas Newcomen, a blacksmith, of Dartmouth, in England. Newcomen's pa- tent was dated 1707, and in it Capt. Savary was united, in consequence of his discovery of the method of forming a vacuum by the condensation of steam, as already de- scribed. What is said of Savary's invention 1 What were the chief objec- tions to Savary's engines 1 Whose steam engine succeeded that of Savary 1 At what time was Ntwcomen's engine invented 1 148 STEAM ENGINE 569. The great object of Newcomen's invention, like that of Savary, was to drain the English mines. To do this, he proposed to connect one arch head of a working beam to a pump rod, while the other arch head should be connected with a piston and rod moving in a cylinder, which piston should be made to descend by the pressure of the atmosphere, in consequence of creating a vacuum under it by the con- densation of steam. When the piston had been made to de- scend in this manner, by which the pump at the other end of the beam was to be worked, the piston was again to be drawn up by the weight of the pump rod, so that this en- gine was moved alternately by means of a vacuum at one end of the beam, and a weight at the other. 570. This was the first proposition which had been made to work a piston by means of steam, or rather by means of a vacuum, created by the condensation of steam, and may be considered as the origin of the present mode of working all steam engines. 571. It is proper to distinguish this as the atmospheric Fig. 118. In what manner was Newcomen's engine worked ? What is said of the originality of this invention 1 ? Why is Newcomen's distinguished by the name of the atmospheric engine 1 STEAM ENGINE. 149 engine, since its movement depended on the pressure of the atmosphere alone. The adjoining cut, fig. 118, and the following description, will show the plan and movement of Newcomen's engine. The boiler a, furnished with a safety valve on the top, has a steam pipe, b, proceeding to the cylinder d. The pis- ton c is of solid metal, and works air tight in the cylinder. The piston is attached by its rod to the arch head of the working beam / To the other arch head is attached he pump rod g, which is connected with its piston in the pump k. This pump descends to the water, to be drawn up by the action of the engine. The small forcing pump h is supplied with water by the pump k, and is designed to raise a portion of the fluid through the condensing pipe i, to the cylinder by which the steam is condensed. This pump, as well as the other, is worked by the action of the working beam. 572. To describe the action of this engine, let us suppose that the piston c is drawn up to the top of the cylinder, by the weight of the pump rod g, as represented in the figure; that the cylinder itself is filled with steam, and that the stop- cock of the steam pipe is turned so that no more steam is admitted. The cylinder was surrounded by another circu- lar vessel, leaving a space between the two, into which the cold water was admitted. Suppose the cold water to be drawn by trie condensing pipe i into this space, and conse- quently the steam to be condensed, leaving a vacuum within the cylinder. The consequence would be, that the pressure of the atmosphere on the piston would instantly force it down to the bottom of the cylinder. This would give ac- tion to the pump k, by which a quantity of water would be drawn up from the well. 573. Now the piston being forced to the bottom of the cyl- inder by the pressure of the atmosphere, unless relieved from that pressure, would not rise again, and therefore a quantity of steam must be admitted under it by the pipe b, so as to balance the pressure on the upper side. When this is effected, the piston is immediately drawn again to the top of the cylinder by the weight of the pump rod, and thus the several parts of the engine become in the precise position ihat they were when our description began ; and in order Describe the several parts of this engine. Describe the action of thU tngine. 13* 150 STEAM EKG1NE. again to depress the piston, a vacuum must once more be produced by the admission of cold water on the cylinder, and so on continually. The power of these engines, although operating by thr* pressure of the atmosphere alone, was much greater than might at first be supposed. 574. The pressure of the atmosphere, when operating on a perfect vacuum, as we have already shown, amounts to 15 pounds on every square inch of surface. The power of this engine therefore depended entirely on the number of square inches which the piston presented to this pressure. 575. Now the number of square inches in a circle may be very nearly found by the following rule : Multiply the number of inches in the diameter by itself: divide the product by 14, and multiply the quotient thus ob- tained by 11, and the result will be the number of square inches in the circle. 576. Thus, a piston having a diameter of only 13 inches, would be pressed down by a weight equal 1980 pounds, or nearly one ton ; and a piston twice this diameter, or 26 inches, would be acted upon by a weight equal 7920 pounds, or nearly four tons. These estimates are, however, too high for practical results, for, after allowing for the friction of the piston, and the imperfection of the vacuum, it was found, in practice, that only about 1 1 pounds of force to the square inch could actually be obtained. 577. Soon after the construction of these engines, an acci- dental circumstance suggested to the inventor a much better method of condensation than the effusion of cold water on the cylinder, which, as we have seen, was that first prac- tised. In order to keep the piston air-tight, it was neces- sary to have a quantity of water on it, which was supplied from a pipe placed over it. On one occasion, a piston was observed to descend several times with unusual rapidity, and this without waiting for the usual supply of condensing water. On examination, it was found that an aperture through the piston admitted the cold water directly to the steam in the cylinder, by which it was instantly condensed. What is said of the power of these engines 1 How may the num- ber of square inches in a circle be found 1 What would be the amount of pressure on a piston of 13 inches in diameter 1 What would be the pressure on a piston of 26 inches in diameter 1 ? How much must be allowed for friction and imperfection of vacuum 1 How did Newco- men discover an improved method of condensing steam 1 STEAM ENGINE. 151 578. On this suggestion, INewcomen abandoned his first method, and by the addition of a pipe, through which a jet of cold water was thrown into the cylinder, condensed the steam instantly, and much more perfectly than could be done even by waiting a long time for the gradual cooling of the cylinder by the old method. This was a highly important improvement, and substantially is the method practised to this day. 579. Newcomen's machine, though so imperfect, when compared with those of the present day, as hardly to deserve the name of a steam engine, was extensively employed in draining the English mines, and for nearly half a century was the only machine moved by the application of steam. And notwithstanding its material and obvious imperfections, still it must be considered as a lasting monument of the com- bining and inventive powers of a man, who appears origi- nally to have had no advantages in life, above what his ex- perience and observations as a blacksmith gave him. 580. Watt's Engine. It does not appear that any con- siderable improvements were made on Newcomen's steam apparatus, until the time when James Watt began his ex- periments and inventions in about 1763. Watt was born at Greenock, in Scotland, and pursued the business of a mathematical instrument maker in London. He was endowed with a mind of the highest order, both as a philosopher and inventor, as will be evinced by the new combinations, improvements, and inventions, which he ap- plied to nearly every part of the apparatus to which steam has been employed as a moving power. 581. Some of his first improvements, or perhaps more properly, inventions, were a pump, for the removal of the air and water, which were accumulated by the condensation of the steam the application of melted wax, or tallow, in- stead of water, to lubricate the piston, and keep it air-tight, and the employment of steam above the piston, to press it down, instead of the atmosphere, as in Newcomen's engine. For the latter purpose, it was necessary to close the top of the cylinder, and allow the piston-rod to play through a steam tight stuffing-box, as is done at the present time in all steam engines. What is said of Newcomen's invention on the whole 1 "When did Watt begin his experiments'? What is said of Watt's capacity? What were among the first improvements of the steam engine 1 What change must be made in Newcomen's cylinder, in order to press down he piston with steam 1 152 STEAM ENGINE. S>*vi* "hj f 582. This improvement is represented by fig. 1 19, where $ is the steam pipe proceeding from the boiler, and oy which steam is admitted to Fig. 119. the cylinder. The piston h works air-tight in the cylin- der g\ the rod of which passes air-tight through the stuffing- box i. The upper valve box a contains a single valve, * which, when open, admits the steam into the cylinder, and also into the pipe which con- nects this with the lower valve box. The lower box contains two valves, b and c ; the valve b, when open, admits the steam to pass from the cylinder above the piston, by the connecting tube to the cylinder below the piston; the valve c, when open, admits the steam to pass from below the cylinder, down into the condenser d. This steam entering the condenser, meets the jet of water through the valve d, where it is condensed. The valve e, opening outwards, permits any steam which is not condensed, together with such atmospheric air as is ac- cumulated, to pass away. The valve a is called the upper steam valve ; b, the lower steam valve ; c, the exhausting valve, and d, the condensing valve. 583. Now let us see in what manner this machine will produce the alternate ascent and descent of the piston. In the first place, all the air which fills the cylinder and tubes must 'be expelled. To do this, the valves a, b, and c, must be opened. The steam will pass through the pipe s t into the upper part of the cylinder, and along the tube down through the valves b and c into the condenser d. After the steam ceases to be condensed by the. cold of the apparatus, it will rush out, mixed with air, through the valve e, which opens outwards. 584. The apparatus is thus filled with steam, and all the What are the ste'uons, names, and uses, of the valves in fig. 119? STEAM ENGINE. 153 valves are now to be closed ; but in a few minutes a vacuum will be formed in the condenser, by the cold surface of that vessel. The apparatus being in this state, let the upper steam valve a, the exhausting valve c, and the condensing valve d, be opened. Steam will thus be admitted through a, to press upon the top of the piston, the steam being prevented from circulating below the piston, by the valve b being closed. But the steam below the piston will rush through the ex- hausting valve c, into the condenser, where a jet of cold water through the condensing valve d, will instantly con- dense it, and thus leave a vacuum below the piston in the cylinder. Into this vacuum the piston is instantly pressed by the action of the steam in the upper part of the cylinder. 585. When the piston has thus been forced to the bottom of the cylinder, let the valves a, c, and d, be closed, and let the lower steam valve b be opened. The effect of this will be, that the further ingress of steam will be stopped, and the further condensation of steam will cease, and thus the steam which is shut within the apparatus, will press equally on all sides, so that the pressure on the upper and under sides of the piston will be equal. Thus there is no force to restrain the piston at the bottom of the cylinder, except its weight, which is more than balanced by the weight of the pump-rod at the other end of the beam, and by the preponderance of which the piston rises, as in the atmospheric engine. 586. When the piston has arrived to the top of the cylin- der, the valves a, c, and d, are again opened, when steam again presses on the top of the piston, while a vacuum is formed below it, into which the piston is driven, as already shown, and so on continually. The valves of this engine were opened and closed by lev- ers, which were worked by the movement of the machine- ry. These, being unnecessary to explain the principle, are not shown in the drawing. 587. Mr. Watt called this his single acting engine, be- cause the steam acted only above the piston, and for the pur- pose of distinguishing it from his double acting engine, in which the piston was moved in both directions, by the force of steam. 588. Double Acting Steam Engine: After the construc- tion of the steam engine above described, Mr. Watt conth> Explain the manner in which this engine acts by means of the fig- ure. Why does Mr. Watt call this his single acting engine ? 154 STEAM ENGINE. ued his improvements and inventions, which resulted in the production of his double acting engine. This consisted in changing 1 the steam alternately from below, to above the pis- ton, and at the same time forming a vacuum alternately in each end of the cylinder, into which the piston was forced. Thus the piston being at the top of the cylinder, steam was introduced from the boiler above it, while the steam in the cylinder below it was condensed. The piston was therefore pressed by the steam above it into a vacuum below. Hav- ing arrived at the bottom of the cylinder, the steam was changed in its direction, and sent below the piston, while a communication was formed between the upper part of the cylinder and the condenser, and thus a vacuum was formed above the piston, into which it was forced by the steam act- ing below it. In this manner was the piston moved by al- ternately substituting steam for a vacuum, and a vacuum for steam, on each side of the piston. 589. Circular motion of machinery by means of steam. The action of the atmospheric engine of Newcomen, and of the improved, or single acting one of Watt, was such as could not be applied to the continued motion of machinery. Their motions were well calculated to raise water from the mines by pumping, and for this purpose they were chiefly employed. Nor could these engines give a perpetual cir- cular, motion, without some changes in their action, and ad- ditions to their machinery. It is obvious, that the extended use of steam in driving machinery, absolutely required such a motion, and it appears that the genius of Watt, soon after his experiments commenced, saw the vast consequences of such an application of this power, and he applied himself to the invention of machinery for this purpose accordingly. 590. In Newcomen's and Watt's first engines, the end of the beam opposite to the piston could only be employed in lifting, since the power was applied only to force the piston downwards. But in the double acting engine, the power of steam was applied to the piston in both directions, and hence the opposite end of the beam had a force down- ward, as well as upward. If, therefore, instead of chains, rods of iron were attached to each arch head of the beam, the one rod connected with the piston, and the other with Describe Watt's double acting steam engine. What is said of the action of Newcomen's and Watt's first engine ? Why were not their motions applicable to machinery 1 Explain the reason why Watt's double acting engine was applicable to the rotation of machinery, while his other engine was not. STEAM ENGINE. 155 machinery to be moved, it is plain that since the end of the beam, connected with the piston, would be pushed up and drawn down with a force equal to the power of the steam applied, the other end of the beam would act with equal force, and thus that a sufficient power might be obtained in both directions. 591. The question with respect to the means by which a continued circular motion might be obtained from the alter- nate motion of the working end of the beam, did not remain long unsettled in the fertile mind of Watt. A crank con- nected with the end of the beam by an inflexible or metalic rod, would convert its up and down motion into one of at least partial, rotation. 592. But still there remained a difficulty to be overcome with respect to the rotation of a crank, for there are two po- sitions in which the vertical motions of the working rod could give it no motion whatever. These are, when the axis of the crank a, fig. 120, Fig. 120. the joint of the crank b, and the working rod, or connector, with the working beam c, are in the same right line as shown in the figure. In this case it is plain, that the vertical action of c could not move the crank in any direc- tion. Again, when the joint b is turned down to d, so as to bring the working rod c, di- 1 rectly over the crank, it will be obvious that the upward or down- ward force of the beam, could not give a any motion what- ever. Hence, in these two positions the engine could have no effect in turning the crank, and, therefore, twice in every revolution, unless some remedy could be found for this defect, the whole machine must cease to act. 593. Now, under Inertia, (21) we have shown that bod- ies, when once put in motion, have a tendency to continue that motion, and will do so, unless stopped by some oppos- Explain the reason why a crank motion alone can not be converted into a continued rotation? In what manner was the crank motion converted into one of perpetual rotation 7 156 STEAM ENGINE. ing force. With respect to circular motion, this subject is sufficiently illustrated by the turning of a coach wheel on its axis when raised from the ground. Every one knows that when a wheel is set in motion, under such circum- stances, it will continue to revolve by its own inertia for some time, without any new impulse. 594. This principle Watt applied to continue the motion of the crank. A large heavy iron wheel was fixed to the axis of the crank, which wheel being put in motion by the machinery, had the effect to turn the crank beyond the po- sition in which we have shown the working rod had no power to move it, and thus enabled the working rod to con- tinue the rotation. 595. Such a wheel, called the fly wheel, or balance wheel, is represented attached to the crank in fig. 120, arid is now universally employed in all steam engines used in driving machinery. 596. Governor, or Regulator. In the application of steam to machinery for various purposes, a steady or equal motion is highly important ; and although the fly wheel, just described, had the effect to equalize the motion of the engine when the power and the resistance were the same, yet when the steam was increased, or the resistance dimin- ished or increased, there was no longer a uniform velocity in the working part of the engine. In order to remedy this defect, Mr. Watt applied to his engines an apparatus called a governor, and by which the quantity of steam admitted to the cylinder was so regulated as to keep the velocity of the engine nearly the same at all times. 597. Of all the contrivances for regulating the motion of machinery, this is said to be the most effectual. It will be readily understood by the following description of fig. 121. It consists of two heavy iron Fig. 121. balls />, attached to the ex- tremities of the two rods b, e. These rods play on a joint at e, passing through a mor- tise in the vertical stem d, d. At / these pieces are united, by joints to the twoj( short rods f, h, which, at their, upper ends, are again Give a general description of the Governor, by means of the figure. STEAM ENGINE. 157 connected by joints at h, to a ring which slides upon the vertical stem d d. Now it will be apparent that when these balls are thrown outward, the lower links connected atf t will be made to diverge, in consequence of which the up- per links will be drawn down the ring with which they are connected at h. With this ring at i is connected a lever having its axis at g, and to the other extremity of which, at k, is fastened a vertical piece, which is connected by a joint to the valve v. To the lower part of the vertical spindle d, is attached a grooved wheel w, around which a strap passes, which is connected with the axis of the fly wheel. 598. Now when it so happens that the quantity of steam is too great, the motion of the fly wheel will give a pro- portionate velocity to the spindle d, d, by means of the strap around w, and by which the balls, by their centrifugal force, will be widely separated ; in consequence of which the ring h will be drawn down. This will elevate the arm of the lever 7c, and by which the end i, of the short lever, connected with the valve v, in the steam pipe, will be raised, and thus the valve turned so as to diminish the quantity of steam ad- mitted to the piston. When the motion of the engine is slow, a contrary effect will be produced, and the valve turn- ed so that more steam will be admitted to the engine. 599. Low and High pressure Engines. After having given a description of Watt's double acting engine, it will hardly be necessary to describe those of the present day, since though they have some additional apparatus, still the principle of action is the same in both, and it is this, rather than details, with which it is our object to make the student acquainted. 600. To comprehend the working of the piston, which is usually hid from the eye of the observer, it is only neces- sary to remember, that in the upper valve box there are two valves, called the upper steam valve, and the upper exhaust- ing valve ; and that in the lower steam box, or bottom of the cylinder, there are also two valves, called the lower steam valve, and the lower exhausting valve. 601. Now suppose the piston to be at the top of the cylin- der, the cylinder below it being filled with steam, which has just pressed the piston up. Then let the upper steam What is the difference between Watt's double acting engine and those of the present day 1 What are the valves called in the upper, and what in the lower valve box 1 When the piston is at the toj. of the cylinder, what valves are opened 1 14 158 STEAM ENGINE. valve, and the lower exhausting valve be opened, the otin-i two being closed; the steam which fills the cylinder below the piston, will thus be allowed to pass through the ex- hausting valve into the condenser, and a vacuum will be form- ed below the piston. At the same time, the upper steam, valve being open, steam will be admitted above the piston to press it down into the vacuum, which has been formed below. On the arrival of the piston to the bottom of the cylinder, the upper steam valve, and the lower exhausting valve are closed, and the lower steam valve, and upper ex- hausting valve are opened, on which the steam above the piston is condensed, while steam is admitted below the piston to press it into the vacuum thus formed, and so on continu- ally. 602. The upper steam valve, and lower exhausting valve, are opened at the same time ; the same being the case with the lower steam valve, and upper exhausting valve. 603. The above is a description of the movement of what is known under the name of the low pressure engine, in which the steam is condensed, and a vacuum formed, alter- nately, above and below the piston. To this engine there must be attached a cold water pump and cistern, for the condensation of the steam; an air pump for the removal of the air and condensed water, and a condenser, into which a jet of cold water is thrown to condense the steam. 604. In the high pressure engines, the piston is pressed up and down by the force of the steam alone, and without the assistance of a vacuum. The additional power of steam required for this purpose is very considerable, being equal to the entire pressure of the atmosphere on the surface of the piston. We have already had occasion to show that on a piston of 13 inches in diameter, the pressure of the atmo- sphere amounts to nearly two tons. 605. Now in the low pressure engine, in which a vacuum is formed on one side of the piston, the force of steam re- quired to move it is diminished by the amount of atmo spheric pressure equal to the size of the piston. 606. But in the high pressure engine, the piston works in both directions against the weight of the atmosphere, and hence requires an additional power of steam equal to the weight of the atmosphere on the piston. When at the bottom, whr*t valves are opened 7 What constitutes a low pressure engine ? How much more force of steam is required in high than in low pressure engines? ACOUSTICS. 159 607. These engines are, however, much more simple and cheap than the low pressure, since the condenser, cold water pump, air pump, and cold water cistern, are dispensed with , nothing more being necessary than the boiler, cylinder, pis- ton, and valves. Hence for rail-roads, and all locomotive purposes, the high pressure engines are, and must be used. 608. With respect to engines used on board of steam- boats, the low pressure are universally employed by the English, and it is well known, that few accidents from the bursting of machinery have ever happened in that country. In most of their boats two engines are used, each of which turns a crank, and thus the necessity of a fly wheel is avoided. In this country high pressure engines are in common use for boats, though they are not universally employed. In some, two engines are worked, and the fly wheel dispensed with, as in England. 609. The great number of accidents which have happen- ed in this country, whether on board of low or high press- ure boats, must be attributed, in a great measure, to the eagerness of our countrymen to be transported from place to place with the greatest possible speed, all thoughts of safety being absorbed in this passion. It is, however, true, from the very nature of the case, that there is far greater danger from the bursting of the machinery in the high, than in the low pressure engines, since not only the cylinder, but the boiler and steam pipes, must sustain a much higher pressure in order to gain the same speed, other circumstances being equal. ACOUSTICS. 610. Acoustics is that branch of natural philosophy which treats of the origin, propagation, and effects of sound. 6 11-. When a sonorous, or sounding body is struck, it is thrown into a tremulous, or vibrating motion. This mo- tion is communicated to the air which surrounds us, and by ihe air is conveyed to our ear drums, which also undergo a vibratory motion, and this last motion, throwing the audi- tory nerves into action, we thereby gain the sensation of sound. What parts are dispensed with in high pressure engines'? What is acoustics 1 When a sonorous body is struck within hearing, in what manner do we gain from it the sensation of sound ? 160 ACOUSTICS. 612. If any sounding body, of considerable size, is sus- pended in the air and struck, this tremulous motion is dis- tinctly visible to the eye, and while the eye perceives its mo- tion, the ear perceives the sound. 613. That sound is conveyed to the ear by the motion which the sounding body communicates to the air, is proved by an interesting experiment with the air pump. Among philosophical instruments, there is a small bell, the hammer of which is moved by a spring connected with clock-work, and which is made expressly for this experiment. If this instrument be wound up, and placed under the re- ceiver of an air pump, the sound of the bell may at first be heard to a considerable distance, but as the air is exhausted, it becomes less and less audible, until no longer to be heard, the strokes of the hammer, though seen by the eye, produ- cing no effect upon the ear. Upon allowing the air to re- turn gradually, a faint sound is at first heard, which be- comes louder and louder, until as much air is admitted as was withdrawn. 614. On the contrary, when the air is more dense than ordinary, or when a greater quantity is contained in a ves- sel, than in the same space in the open air, the effect of sound on the ear is increased. This is illustrated by the use of the diving bell. The diving bell is a large vessel, open at the bottom, un- der which men descend to the beds of rivers, for the pur- pose of obtaining articles from the wrecks of vessels. When this machine is sunk to any considerable depth, the water above, by its pressure, condenses the air under it with great force. In this situation, a whisper is as loud as a common voice in the open air, and an ordinary voice becomes pain ful to the ear. 615. Again, on the tops of high mountains, where the pressure, or density, of the air is much less than on the sur face of the earth, the report of a pistol is heard only a few rods, and the human voice is so weak as to be inaudible at ordinary distances. Thus, the atmosphere which surrounds us, is the medium by which sounds are conveyed to our ears, and to its vibra- How is it proved that sound is conveyed to the ear by the medium of the air 1 When the air is more dense than ordinary how does it af- fect sound 1 What is said of the effects of sound on the tops of high mountains 1 ACOUSTICS. 161 tions we are indebted for the sense of hearing, as well as to al. we enjoy from the charms of music. 616. The atmosphere, though the most common, is not. however, the only, or the best conductor of sound. Solid bodies conduct sound better than elastic fluids. Hence, if a person lay his ear on a long stick of timber, the scratch of a pin may be heard from the other end, which could not be perceived through the air. 617. The earth conducts loud rumbling sounds made below its surface to great distances. Thus, it is said, that in countries where the volcanoes exist, the rumbling noise which generally precedes an eruption, is heard first by the beasts of the field, because their ears are commonly near the ground, and that by their agitation and alarm, they give warning of its approach to the inhabitants. The Indians of our country will discover the approach of horses or men, by laying their ears on the ground, when they are at such distances as not to be heard in any other manner. 618. Sound is propagated through the air at the rate of 1142 feet in a second of time. When compared with the velocity of light, it therefore moves but slowly. Any one may be convinced of this by watching the discharge of cannon at a distance The flash is seen apparently at the instant the gunner touches fire to the powder; the whizzing of the ball, if the ear is in its direction, is next heard, and lastly, the report. Solid substances convey sounds with greater velocity than air, as is proved by the following experiment, lately made at Paris, by M. Riot. 619. At the extremity of a cylindrical tube, upwards of 3000 feet long, a ring of metai was placed, of the same diameter as the aperture of the tube; and in the centre of this ring, in the mouth of the tube, was suspended a clock bell and hammer. The hammer was made to strike the ring and the bell at the same instant, so that the sound of the ring would be transmitted to the remote end of the tube, ihrough the conducting power of the tube itself, while the sound of the bell would be transmitted through the medium Which are the best conductors of sound, solid or elastic substances 1 What is said of the earth as a conductor of sounds 1 How is it said that the Indians discover the approach of horses 1 How fast does sound pass through the air"? Which convey sounds with the greatest velocity, solid substances or air 1 14* 162 ACOUSTICS. of the air inclosed in the tube. The ear being then placed at the remote end of the tube, the sound of the ring, trans- mitted by the metal of the tube, was first heard distinctly, and after a short interval had elapsed, the sound of the bell, transmitted by the air in the tube, was heard. The result of several experiments was, that the metal conducted the sound at the rate of about 11,865 feet per second, which is about ten and a half times the velocity with which it is con- ducted by the air. 620. Sound moves forward in straight lines, and in this respect follows the same laws as moving bodies, and light. It also follows the same laws in being reflected, or thrown back, when it strikes a solid, or reflecting surface. 621. Echo. If the surface be smooth, and of considera- ble dimensions, the sound will be reflected, and an echo will be heard ; but if the surface is very irregular, soft, or small, no such effect will be produced. In order to hear the echo, the ear must be placed in a certain direction, in respect to the point where the sound is produced, and the reflecting surface. If a sound be produced at a, fig. 122, and strike the plain surface b, it will be reflected back in the same line, and the echo will be heard at c or a. That is, the angle under which it approaches the re- flecting surface, and that under which it leaves it, will be equal. 622. Whether the sound strikes the re- flecting surface at right angles, or oblique- ly, the angle of approach, and the angle of reflection, will always be the same, and equal. Fig. 122. b This is illustrated by fig. 123, where suppose a pistol to be fired at a, while the reflecting sur- face is at c ; then the echo will be heard at b, he angles 2 andl being equal to each other. Pig. 123. Describe the experiment, proving that sound is conducted by u metsu with greater velocity than by the air. In what lines does sound movel From what kind of surface is sound reflected, so as to produce an echo 1 Explain fig. 122. Explain fig. 123, and show in what direction sound approaches and leaves a reflecting surface. ACOUSTICS. 16* 623. If a sound be emitted between two reflecting sur- faces, paiT.llel to each other, it will reverberate, or be an- swered backwards and forwards several times. Thus, if the sound be made at a, fig. Fig. 124. 124, it will not only rebound back again to a, but will also be reflected from the points c and d, and were such reflecting surfaces placed at every point around a circle from a, the ound would be thrown back from them all, at the same instant, and would meet again at the point a. We shall see, under the article Optics,/ that light observes exactly the same law in respect to its reflection from plane suriaces, and that the angle at which it strikes, is called the angle of incidence, and that under which it leaves the reflecting surface, is call- ed the angle of reflection. The same terms are employed in respect to sound. 624. In a circle, as mentioned above, sound is reflected from every plane surface placed around it, and hence, if the sound is emitted from the centre of a circle, this centre will be the point at which the echo will be most distinct. Suppose the ear to be placed at the point a, fig. 125, in the centre of a circle ; and let a sound be produced at the same point, then it will move along the line a e, and be reflected from the plane surface, back on the same d line to a ; and this will take place from all the plane surfaces placed around the circumference of a circle ; and as all these surfaces are at the same distance from the centre, so the reflected sound will arrive at the point &, at the same instant; and the echo will be loud, in proportion to the number and perfection of these reflecting surfaces. 625. It is apparent that the auditor, in this case, must be placed in the centre from which the sound proceeds, to re- Fig. 125. e What is the angle under which sound strikes a reflecting surface called'? What is the angle under which it leaves a reflecting sur- face called'? Is there any difference in the quantity of these two aj ^lesl Suppose a pistol to be fired in the centre of a circular room wnere would be the echo? Explain fig. 124, and give the reason. 164 ACOUSTICS* Fig. 126. ceive the greatest effect. But if the shape of the room be oval, or elliptical, the sound may be made in one part, and the echo will be heard in another part, because the ellipse has two points, called foci, at one of which, the sound being- produced, it will be concentrated in the other. Suppose a sound to be produced at a, fig. 126, it will be reflected from the sides of the room, the angles of incidence being equal to those of reflection, and will be concentrated at b. Hence a hearer standing at b, will be affected by the united rays of sound from different parts of the room, so that a whisper at #, will become audi- ble at b, when it would not be heard in any other part of the room. Were the sides of the room lined with a pol- ished metal, the rays of light or heat would be concentrated in the same manner. The reason of this will be understood, when we consider, that an ear, placed at c, will receive only one ray of the sound proceeding from a, while if placed at b, it will receive the rays from all parts of the room. Such a room, whether constructed by design or accident, would be a whispering gallery. 626. On a smooth surface, the rays, or pulses of sound, will pass with less impediment than on a rough one. For this reason, persons can talk to each other on tho opposite sides of a river, when they could not be understood to the same distance over the land. The report of a cannon, at sea, when the water is smooth, may be heard at a great distance, but if the sea is rough, even without wind, the sound will be broken, and will reach only half as far. 627. Musical Instruments. The strings of musical in- struments are elastic cords, which being fixed at each end, produce sounds by vibrating in the middle. The string of a violin, or piano, when pulled to one side by its middle, and let go, vibrates backwards and forwards, Suppose a sound to be produced in one of the foci of an ellipse, where then might it be distinctly heard? Explain fig. 126, and give the reason. Why is it that persons can converse on the opposite sides of a river, when they could not hear each other at the same distance over the land ? How do the strings of musical instruments produce sounds 1 ACOUSTICS. 165 like a pendulum, and striking rapidly against the air, pro- duces tones, which are grave, or acute, according to its ten- sion, size, or length. 628. The manner in which such a string vibrates, is shown by fig. 127. If pulled from e to a, it will not stop again at e, but in passing from a to e, it will gain a momentum, which will carry it to c, and in returning, its momentum will again carry it to d, and so on, backwards and forwards, like a pendulum, until its tension, and the re- sistance of the air, will finally bring it to rest. The grave, or sharp tones of the same string, depend on its different degrees of tension; hence, if a string be struck, and while vibrating, its tension be increased, its tone will be changed from a lower to a higher pitch. 629. Strings of the same length are made to vibrate slow, or quick, and consequently to produce a variety of sounds, oy making some larger than others, and giving them dif- ferent degrees of tension. The violin and bass viol are fa- miliar examples of this. The low, or bass strings, are cov- ered with metallic wire, in order to make their magnitude and weight prevent their vibrations from being too rapid, and thus they are made to give deep or grave tones. The o&er strings are diminished in thickness, and increased in tension, so as to make them produce a greater number of vibrations in a given time, and thus their tones become sharp, or acute, in proportion. 63G. Unvler certain circumstances, a long string will di- vide AS^'I nito halves, thirds, or quarters, without depress- ing: any part of it, and thus give several harmonious tones at the same time. The fairy tones of the jEolian harp are produced in this manner. This instrument consists of a simple box of wood, with four or five strings, two or three feet long, fastened at each end. These are tuned in unison, so that when made Explain fig. 127. On what do the grave or acute tones of the same string depend 1 Why are the bass strings of instruments covered with metallic wire 7 Why is there a variety of tones in the JSolian harp, since all the strings are tuned in unison 7 166 WIND. to vibrate with force, they produce the same tones. But when suspended in a gentle breeze, each string, according to the manner or force in which it receives the blast, either sounds, as <* whole, or is divided into several parts, as above described. " The result of which," says Dr. Arnot, " is the production of the most pleasing combination, and succession of sounds, that the ear ever listened to, or fancy perhaps conceived. After a pause, this fairy harp is often heard be- ginning with a low and solemn note, like the base of dis- tant music in the sky ; the sound then swells as if approach- ing, and other tones break forth, mingling with the first, and with each other." 631. The manner in which a string vibrates in parts, will be understood by fig. 128. Fig. 128. Suppose the whole length of the string to be from a to b, and that it is fixed at these two points. The portion from b to c, vibrates as though it was fixed at c, and its tone dif- fers from those of the other parts of the string. The same happens from c to d, and from d to a. While a string is thus vibrating, if a small piece of paper be laid on the part c, or d, it will remain, but if placed on any other part of the string, it will be shaken off. WIND. 632. Wind is nothing more than air in motion. The use of a fan, in warm weather, only serves to move the air, and thus to make a little breeze about the person using it. 633. As a natural phenomenon, that motion of the air which we call wind, is produced in consequence of there being a greater degree of heat in one place than in another. The air thus heated, rises upward, while that which sur- rounds this, moves forward to restore equilibrium. The truth of this is illustrated by the fact, that during the burning of a house in a calm night, the motion of the air toward? the place where it is thus rarefied, makes the wind blow from every point towards the flame. Explain fig. 128, showing the manner in which strings vibrate in parts. Wha' is wind 7 As a natural phenomenon, how is wind pro- duced, or, what is the cause of wind 1 How is this illustrated 7 WIND. 167 634. In islands, situated in hot climates, this principle is charmingly illustrated. The land, during the day time, be- ing under the rays of a tropical sun, becomes heated in a greater degree than the surrounding ocean, and, consequent- ly, there rises from the land a stream of warm air, during the day, while the cooler air from the surface of the water, moving forward to supply this partial vacancy, produces a cool breeze setting inland on all sides of the island. This constitutes the sea breeze, which is so delightful to the in- habitants of those hot countries, and without which men could hardly exist in some of the most luxuriant islands be- tween the tropics. During the night, the motion of the air is reversed, be- cause the earth being heated superficially, soon cools when the sun is absent, while the water being warmed several feet below its surface, retains its heat longer. Consequently, towards morning, the earth becomes colder than the water, and the air sinking down upon it, seeks an equilibrium, by flowing outwards, like rays from a centre, and thus the land breeze is produced. The wind then continues to blow from the land until the equilibrium is restored, or until the morning sun makes the land of the same temperature as the water, when for a time there will be a dead calm. Then again the land becoming warmer than the water, the sea breeze returns as before, and thus the inhabitants of those sultry climates are con- stantly refreshed during the summer season, with alternate land and sea breezes. 635. At the equator, which is a part of the earth con- tinually under the heat of a burning sun, the air is expand- ed, ana 1 ascends upwards, so as to produce currents from the north and south, which move forward to supply the place of the heated air as it rises. These two currents, coming from latitudes where the daily motion of the earth is less than at the equator, do not obtain its full rate of motion, and therefore, when they approach the equator, do not move so fast eastward as that portion of the earth, by the difference between the equator's velocity, and that of the latitudes from which they come. This wind therefore falls behind the earth in her diurnal motion, and, consequently, has a rela- In the islands of hot climates, why does the wind blow inland du- ring the day, and off the land during the night'? What are these breezes called 1 What is said of the ascent of heated air at the equa- tor? What is the consequence on the air towards the north and south? 168 WIND. live motion towards the west. This constant breeze towards the west is called the trade wind, because a large portion of the commerce of nations comes within its influence. 636. While the air in the lower regions of the atmosphere is thus constantly flowing from the north and south towards the equator, and forming the trade winds between the trop- ics, the .heated air from these regions as perpetually rises, and forms a counter current through the higher regions, to- wards the north and south from the tropics, thus restoring the equilibrium. 637. This counter motion of the air in the upper and low- er regions is illustrated by a very simple experiment. Open a door a few inches, leading into a heated room, and hold a lighted candle at the top of the passage ; the current of air. as indicated by the direction of the flame, will be out of the room. Then set the candle on the floor, and it will show that the current is there into the room. Thus, while the heated air rises and passes out of the room, that which is colder flows in, along the floor, to take its place. This explains the reason why our feet are apt to suffer with the cold, in a room moderately heated, while the other parts of the body are comfortable. It also explains why those who sit in the gallery of a church are sufficiently warm, while those who sit below may be sh'vering with the cold. 638. From such facts, showing the tendency of heated air to ascend, while that which is colder moves forward to supply its place, it is easy to account for the reason why th^ wind blows perpetually from the north and south towards? the tropics; for, the air being heated, as stated above, it as- cends, and then flows north and south towards the po lo s, until, growing cold, it sinks down, and again flows towards the equator. 639. Perhaps these opposite motions of the two currents will be better understood by the sketch, figure 129. Suppose a b c to represent a portion of the earth's sur- face, a being towards the north pole, >c towards the south pole, and b the equator. The currents of air are supposec to pass in the direction of the arrows. The wind, therefore from a to b would blow, on the surface of the earth, from How are the trade winds formed 1 While the air in the lower re- gions flows from the north and south towards the equator, in what di- rection does it flow in higher regions 1 How is this counter current ( r Unvcr and upper regions illustrated by a simple experiment 1 OPTICS. 169 north to south, while from e to a, the upper current would pass from south to north, until it came to a, when it would change its direction towards the south. The currents in the southern hemisphere being governed by the same laws, would assume similar directions. OPTICS. 640. Optics is that science which treats of vision, and the properties and phenomena of light. The term optics is derived from a Greek word, which signifies seeing. This science involves some of the most elegant and im- portant branches of natural philosophy. It presents us with experiments which are attractive by their beauty, and which astonish us by their novelty ; and, at the same time, it inves- tigates the principles of some of the most useful among the articles of common life. 641. There are two opinions concerning the nature of light. Some maintain that it is composed of material parti- cles, which are constantly thrown off from the luminous body ; while others suppose that it is a fluid diffused through all nature, and that the luminous, or burning body, occa- sions waves or undulations in this fluid, by which the light is propagated in the same manner as sound is conveyed through the air. The most probable opinion, however, is, that light is composed of exceedingly minute particles of matter. But whatever may be the nature or cause of light, it has certain general properties or effects which we can investigate. Thus, by experiments, we can determine the laws by which it is governed in its passage through differ- What common fact does this experiment illustrate 1 Define Optics 1 What is said of the elegance and importance of this science'? Wha; are the two opinions concerning the nature of light 1 What is the most probable opinion 1 15 170 OPTICS. ent transparent substances, and also those by which it is governed when it strikes a substance through which it can- not pass. We can likewise test its nature to a certain de- gree, by decomposing or dividing it into its elementary parts, as the chemist decomposes any substance he wishes to analyze. 642. To understand the science of optics, it is necessary to define several terms, which, although some of them may be in common use, have a technical meaning, when applied to this science. a. Light is that principle, or substance, which enables us to see any body from which it proceeds. If a luminous substance, as a burning candle, be carried into a dark room, the objects in the room become visible, because they reflect the light of the candle to our eyes. b. Luminous bodies are such as emit light from their own substance. The sun, fire, and phosphorus, are luminous bodies. The moon, and the other planets, are not luminous, since they borrow their light from the sun. c. Transparent bodies are such as permit the rays of light to pass freely through them. Air and some of the gasses are perfectly transparent, since they transmit light without being visible themselves. Glass and water are also considered transparent, but they are not perfectly so, since they are themselves visible, and therefore do not suffer the light to pass through them without interruption. d. Translucent bodies are such as permit the light to pass, but not in sufficient quantity to render objects distinct, when seen through them. e. Opaque is the reverse of transparent. Any body which permits none of the rays of light to pass through it, is opaque. / Illuminated, enlightened. Any thing is illuminated when the light shines upon it, so as to make it visible. Every object exposed to the sun is illuminated. A lamp illuminates a room, and every thing in it. g. A Ray is a single line of light, as it comes from a lu- minous body. What is light 1 What is a luminous body 1 What is a transpa- rent body 7 Are glass and water perfectly transparent 1 How is it proved that air is perfectly transparent 1 What are translucent bod- ies 1 What are opaque bodies 1 What is meant by illumir\ated ? What is a ray of light 1 OPTICS. 17] n, A Beam of light is a body of parallel rays. i. A Pencil of light is a body of diverging or converging rays. k. Divergent rays, are such as come from a point, and continually separate wider apart, as they proceed. /. Convergent rays, are those which approach each other, so as to meet at a common point. m. Luminous bodies emit rays, or pencils of light, in every direction, so that the space through which they are visible is filled with them at every possible point. 643. Thus, the sun illuminates every point of space, within the whole solar system. A light, as that of a light house, which can be seen from the distance of ten miles in one direction, fills every point in a circuit of ten miles from it, with light. Were this not the case, the light from it could not be seen from every point within that circumfer- ence. 644. The rays of light move forward in straight lines from the luminous body, and are never turned out of their course except by some obstacle. Let a, fig. Fig. 130. 130, be a beam of light from the sun passing through a small orifice in the window shutter b. The sun cannot be seen through the crooked tube e, because the beam passing in a straight line, strikes the side of the tube, and therefore does not pass through it. 645. All the illuminated bodies, whether natural or arti- ficial, throw off light in every direction of the same color as themselves, though the light with which they are illumi- nated is white or without colour. This fact is obvious to all who are endowed with sight. Thus, the light proceeding from grass is green, while that proceeding from a rose is red, and so of every other colour. What is a beam 1 What a pencil 1 What are divergent rays 7 What are convergent rays'? In what direction do luminous bodies emit light 1 How is it proved that a luminous body fills every point within a certain distance with light 1 Why cannot a beam of light be seen through a bent tube"? What is the colour of the light which dif- ferent bodies throw ""* " of the other rays'? 172 OPTICS. We shall be convinced, in another place, that the light with which things are illuminated, is really composed of several colors, and that bodies reflect only the rays of their own colors, while they absorb all the other rays. 646. Light moves with the amazing rapidity of about 95 millions of miles in 8 minutes, since it is proved by certain astronomical observations, that the light of the SUD comes to the earth in that time. This velocity is so great, that to any distance at which an artificial light can be seen, it seems to be transmitted instantaneously. If a ton of gunpowder were exploded on the top of a mountain, where its light could be seen a hundred miles, no perceptible difference would be observed in the time of its appearance on the spot, and at the distance of a hundred miles. REFRACTION OF LIGHT. 647. Although a ray of light will always pass in a straight line, when not interrupted, yet when it passes ob- liquely from one transparent body into another, of a differ- ent density, it leaves its linear direction, and is bent, or re- fracted, more or less, out of its former course. This change in the direction of light, seems to arise from a certain pow- er, or quality, which transparent bodies possess in different degrees ; for some substances bend the rays of light much more obliquely than others. The manner in which the rays of A Fig. 131. light are refracted, may be readily understood by fig. 131. Let a be a ray of the sun's light, proceeding obliquely towards the sur- face of the water c, d, and let e be the point which it would strike, if moving only through the air. Now, instead of passing through the water in the line a, e, it will be bent or re- fracted, on entering the water, from ,0 to n, and having passed through the fluid it is again refracted in a contrary What is the rate of velocity with which light moves'? Can we perceive any difference in the time which it takes an artificial light to j)ass to us from a great or small distance 7 What is meant by the re- fraction of light 1 ? Do all transparent bodies refract light equally 1 Ex- plain fig. 131, and show how the ray is refracted in passing into anc out of the water. OPTICS. 173 direction on passing out of the water, and then proceeds onward in a straight line as before. 648. The refraction of water is beautifully proved by the following simple experiment. Place an empty cup, fig. 132, with a shilling on the bottom, in such a position, that the side of the cup will just hide the piece of money from the eye. Then let another per-^x Fl S- son fill the cup with v/ater,^ keeping the eye in the same position as before. As the water is poured in, the shil- ling will become visible, ap- pearing to rise with the wa- ter. The effect of the water is to bend the ray of light coming from the shilling, so as to make it meet the eye e below the point where it otherwise would. Thus the eye could not see the shilling in the direction of c, since the line of vision is towards a, and c is hidden by the side of the cup. But the refraction of the water bends the ray down wards, producing the same effect as though the object had been raised upwards, and hence it becomes visible. 649. The transparent body through which the light passes is called the medium, and it is found in all cases, " that where a ray of light passes obliquely from one medium into another of a different density, it is refracted, or turned out of its former course" This is illustrated in the above pxamples, the water being a more dense medium than air. The refraction takes place at the surface of the medium, and the ray is refracted in its passage out of the refracting substance as well as into it. 650. If the ray, after having passed through the water, then strikes upon a still more dense medium, as a pane of glass, it will again be refracted. It is understood, that in all cases the ray must fall upon the refracting medium ob- liquely, in order to be refracted, for if it proceeds from one medium to another perpendicularly to their surfaces, it will pass straight through them all, and no refraction will take place. Explain fig. 132, and state the reason why the shilling seems to be raised up by pouring in the water. What is a medium 1 In what direction must a ray of light pass towards the medium to be refracted 1 Will a ray falling: perpendicularly on a medium be refracted? 15 174 OPTICS. Thus, in fig. 133, let a represent air, b Pig- 133. water, and c a piece of glass. The ray d, striking each medium in a perpendicular di- rection, passes through them all in a straight line. The oblique ray passes through the air in the direction of c, but meeting the water, is refracted in the direction of o ; then falling upon the glass, it is again refracted in the direction of p, nearly parallel with the perpendicular line d. 651. In all cases where the ray passes out of a rarer into a denser medium, it is re- fracted towards a perpendicular line, raised from the surface of the denser medium, and P so, when it passes out of a denser, into a rarer medium, it is refracted from the same perpendicular. Let the medium b, fig. 134, be glass, and the medium c t water. The ray a, as it falls upon the medium b, is refract- ed towards the perpendicular line e, d; Fig. 134. but when it enters the water, whose re- fractive power is less than that of glass, it is not bent so near the perpendicular as before, and hence it is refracted from, instead of towards, the perpendicular line, and approaches the original direc- tion of the ray a, g, when passing through the air. The cause of refraction appears to be the power of attraction, which the denser medium exerts on the passing ray ; and in all cases the at- tracting force acts in the direction of a perpendicular to the refracting surface. 652. The refraction of the rays of light, as they fall upon the surface of the water, is the reason why a straight rod, with one end in the water, and the other end rising above it, appears to be broken, or bent, and also to be shortened. Suppose the rod a, fig. 135, to be set with one half of its length below the surface of the water, and the other half above it. The eye being placed in an oblique direction, Explain fig. 133, and show how the ray e is refracted. When the ray passes out of a rarer into a denser medium, in what direction is it refracted? When it passes out of a denser into a rarer medium, in what direction is the refraction 7 Explain this by fig. 134. What i* the cause of refraction 7 What is the reason that a rod, with one end in the water, appears distorted and snorter than it really is 7 OPTICS. 175 will see the lower end apparently at the point o, while the real termination of the rod would be at n: Fig. 135. the refraction will therefore make the rod appear shorter by the distance from o to n, or one fourth shorter than the part be- low the water really is. The reason why. the rod appears distorted, or broken, is, that we judge of the direction of the part which is under the water, by that which! is above it, and the refraction of the rays coming from below the surface of the water, give them a different direction, when compared with those coming from that part of the rod which is above it. Hence, when the whole rod is below the water, no such distorted appearance is observed, because then all the rays are refracted equally. For the reason just explained, persons are often deceived in respect to the depth of water, the refraction making it appear much more shallow than it really is; and there is no doubt but the most serious accidents have often happen- ed to those who have gone into the water under such decep- tion ; for a pond which is really six feet deep, will appear to the eye only a little more than four feet deep. REFLECTION OF LIGHT. 653. If a boy throws his ball against the side of a house swiftly, and in a perpendicular direction, it will bound back nearly in the line in which it was thrown, and he will be able to catch it with his hands ; but if the ball be thrown oblique- ly to the right, or left, it will bound away from the side of the house in the same relative direction in which it was thrown. The reflection of light, so far as re- Fig. 136. gards the line of approach, and the line c of leaving a reflecting surface, is gov- erned by the same law. Thus, if a sun beam, fig. 136, passing through a small aperture in the window shutter a, be permitted to fall upon the plane mirror, or looking glass, c, d, at right angles, it will be reflected back at right angles with the mirror, and therefore will pass back again in exactly the same direction in which it approached. Why does the water in a pond appear less deep than it really is ? Suppose a sun beam fall upon a plane mirror, at right angles with its surface, in what direction will it be reflected? 176 MIRRORS. 654. But if the ray strikes the mirror in an oblique di rection, it will also be thrown off in an Fig. 137. oblique direction, opposite to that in which it was thrown. Let a ray pass towards a mirror in the line a, c, fig. 137, it will be reflected off in the direction of c, d, making the an- c gles 1 and 2 exactly equal. The ray a, c, is called the inci&tnt ray, and the ray c, d, the reflected ray j and it is found, in all cases, that whatever angle the ray of incidence makes with the reflecting sur- face, or with a perpendicular line drawn from p- j^g the reflecting surface, exactly the same angle is made by the reflected ray. 655. From these facts, arise the general law in optics, that the angle of reflection is equal to the angle of incidence. The ray a, c, fig. 138, is the ray of inci-^j dence, and that from c to d, is the ray of re- flection. The angles which a, c, make with the perpendicular line, and with the plane of the mirror, is exactly equal to those made by c, d, with the same perpendicular, and the same plane surface. MIRRORS. 656. Mirrors are of three kinds, namely, plane, convex, and concave. They are made of polished metal, or of glass covered on the back with an amalgam of tin and quicksilver. The common looking glass is a plane mirror, and con- sists of a plate of ground glass so highly polished as to per- mit the rays of light to pass through it with little interrup- tion. On the back of this plate is placed the reflecting sur- face, which consists of a mixture of tin and mercury. The glass plate, therefore, only answers the purpose of sustain- ing the metallic surface in its place, of admitting the rays Suppose the ray falls obliquely on its surface, in what direction will it then be reflected 1 What is an incident ray of light ? What is a reflected ray of light ? What general law in optics results from ob- servations on the incident and reflected rays ? How many kinds of mirrors are there 1 What kind of mirror is the common looking glass I Of what use is the glass plate in the construction of this mirror 7 MIRRORS. 177 of light to and from it, and of preventing its surface from tarnishing, by excluding the air. Could the metallic surface, however, be retained in its place, and not exposed to the air, without the glass plate, these mirrors would be much more perfect than they are, since, in practice, glass cannot be made so perfect as to transmit all the rays of light which fall on its surface. 657. When applied to the plane mirror, the angles of in- cidence and of reflection are equal, as already stated ; and it therefore follows, that when the rays of light fall upon it obliquely in one direction, they are thrown off under the same angle in the opposite direction. This is the reason why the images of objects can be seen when the objects themselves are not visible. Suppose the mirror a b, fig. 139, to Fig. 139. be placed on the side of a room, and a lamp to be set in another room, but so situated, as that its light would shine upon the glass. The lamp itself could not be seen by the eye placed at e, be- cause the partition d is between them ; but its image would be visible at e, be- cause the angle of the incident ray, coming from the light, and that of the reflected ray which reaches the eye, are equal. 658. An image from a plane mir- ror appears to be just as far behind the mirror as the object is before it, so that when a person approaches this mirror, his image seems to come forward to meet him ; and when he withdraws from it, his image appears to be moving back- ward at the same rate. For the same reason, the different parts of the same object will appear to extend as far behind the mirror, as they are before it. If, for instance, one end of a rod, two feet long, be made to touch the surface of such a mirror, this end of the rod, and its image, will seem nearly to touch each other, there being only the thickness of the glass between them ; while the other end of the rod, and the other end of its image, will appear to be equally distant from the point of contact. Explain fig. 139, and show how the image of an object can be seen in a plane mirror, when the real object is invisible. "The image of an object appears just as far behind a plane mirror, as the object is before it; explain fig. 140, and show why this is the case. 178 MIRRORS. The reason of this is explained on the principle, that the angle of incidence and that of reflection is equal. Suppose the arrow a, to he the object reflected by the mirror d c, fig. 140; the inci- Fig. 140. dent rays a, flowing from the end of the arrow, being thrown back by reflection, will meet the eye in the same state of di- vergence that they would do >( if they proceeded to the same distance behind the mirror, that the eye is before it, as at o. Therefore, by the same law, the reflected rays, where they meet the eye at e, appear to di- verge from a point A, just as far behind the mirror, as a is before it, and consequently the end of the arrow most re- mote from the glass, will appear to be at A, or the point where the approaching rays would meet, were they contin- ued onward behind the glass. The rays flowing from every other part of the arrow follow the same law; and thus every part of the image seems to be at the same distance behind the mirror, that the object really is before it. 659. In a plane mirror, a person may see his whole im- age, when the mirror is only half as long as himself; let him stand at any distance from it whatever. This is also explained by the law, that the angles of in- cidence and reflection are equal. If the mirror be elevated, so that the ray of light from the eye falls perpendicularly upon the mirror, this ray will be thrown back by reflection in the same direction, so that the incident and reflected ray by which the image of the eyes and face are formed, will be nearly parallel, while the ray flowing from his feet will fall on the mirror obliquely, and will be reflected as ob- liquely in the contrary direction, and so of all the other rays by which the image of the different parts of the person is formed. Thus, suppose the mirror c e, fig. 141, to be just half as long as the arrow placed before it, and suppose the eye to be placed at a. Then the ray a e, proceeding from the eye at What must be the comparative length of a plane mirror, in which a person may see his whole image*? In what part of the image, fig. 141, are the incidental and reflected rays nearly parallel? MIRRORS. 179 a, and falling perpen- Fig. 141. dicularly on the glass at c, wilL be reflected back to the eye in the same line, and this part of the image will ap- pear at b, in the same line, and at the same distance behind the glass, that the arrow is before it. But the ray flowing from the lower extremity of the arrow, will fall on the mirror obliquely, as at e, and will be reflected under the same angle to the eye, and therefore the extremity of the image, appearing in the direction of the reflected ray, will be seen at d. The rays flowing from the other parts of the arrow, will observe the same law, and thus the whole image is seen distinctly, and in the same position as the object. To render this still more obvious, suppose the mirror to be removed, and another arrow to be placed in the position where its image appears, behind the mirror, of the same length as the one before it. Then the eye, being in the same position as represented in the figure, would see the different parts of the real arrow in the same direction that it before saw the image. Thus, the ray flowing from the upper extremity of the arrow, would meet the eye in the direction of b c, while the ray, coming from the lower extremity, would fall on it in the direction of e d. 660. CONVEX MIRROR. A convex mirror is a part of a sphere, or globe, reflecting from the outside. Suppose fig. 1 42 to be a sphere, then the part from a to o, would be a section of the sphere. Any part of a hollow ball of glass, Fig. 142. c Why does the image of the lower part of the arrow appear at d ? Suppose the mirror, fig. 141, to be removed, and an arrow of the same length to be placed where the image appeared, would the direction of the rays from the arrow be the same that they were from the image ? W hat is a convex mirror ? 180 MIRRORS. Fig. 143. %vith an amalgam of tin and quicksilver spread on the in- side, or any part of a metallic globe polished on the outside, would form a convex mirror. The axis of a convex mirror, is a line, as c b, passing through its centre. 661. Rays of light are said to diverge, when they proceed from the same point, and constantly cede from each other, as from the point a, fig. 143. Rays of light are said to converg&^-when. they approach each other in such u direction as finally to meet at a point, as at b, fig. 143. The image formed by a plane mirror, as we have al- ready seen, is of the same size as the object, but the image reflected from the convex mirror is always smaller than cne object. The law which governs the passage of light with respect to the angles of incidence and reflection, to and from the convex mirror, is the same as already stated, for the plane mirror. 662. From the surface of a plane mirror, parallel rays are reflected parallel ; but the convex mirror causes parallel rays falling on its surface to diverge, by reflection. To make this understood, Fig. 144. let 1, 2, 3, fig. 144, be parallel rays, falling on the surface of the convex reflector, of which a would be the centre, were the reflector a whole sphere. The ray 2 is perpendicular to the surface of the mirror, for when continued in the same direction, it strikes the axis, or centre of the circle a. The two rays, 1 and 3, being parallel to this, all three would fall on a plane mirror in a perpendi- cular direction, and conse- quently would be reflected in the lines of their incidence. What is the axis of a convex mirror 1 What are diverging rays 7 What are converging rays 1 What law governs the passage of light from and to the convex mirror ? Are parallel rays falling on a con. vex mirror, reflected parallel? Explain fig. 144. MIRRORS. 1P1 But the obliquity of the convex surface, it is obvious, will lender the direction of the rays 1 and 3, oblique to that sur- face, for the same reason that 2 is perpendicular to that part of the circle on which it falls. Rays falling on any part of this mirror, in a direction which, if continued through the circumference, would strike the centre, are perpendicu- lar to the side where they fall. Thus, the dotted lines, c a, and d a, are perpendicular to the surface, as well as 2. Now the reflection of the ray 2, will be back in the line of its incidence, but the rays 1 and 3, falling obliquely, are reflected under the same angles at which thoy fall, and there- fore their lines of reflection will be as far without the per- pendicular lines c a, and d a, as the lines of their incident rays, 1 and 3, are within them, and consequently they will diverge in the direction of e and o ; and since we always see the image in the direction of the reflected ray, an object placed at 1, would appear behind the surface of the mirror at n, or in the direction of the line o n. 663. Perhaps the subject of the convex mirror will be better understood, by considering its surface to be formed of a number of plane surfaces, indefinitely small. In this case, each point from which a ray is reflected, would act in the same manner as a plane mirror, and the whole, in the man- ner of a number of minute mirrors inclined from each other. Suppose a and b, fig. 145, to be the points on a convex mir- ror, from which the two parallel rays, c and d, are reflected. Now, from the surface of a plane mir- ror, the reflected rays would be parallel, whenever the incident ones are so, because each will fall upon the surface under the same angles. But it is obvious, in the present case, that these rays fall upon the surfaces, a> and b, under different angles, as respects the surfaces, c approaching in a more oblique direction than d ; consequently c is reflected more otliquelv than d, and the two reflected rays, instead of being parallel, ns before, diverge in the direction of n and o. How is the action of the convex mirror illustrated by a number of plane mirrors 7 16 182 MIRRORS. 664. Again, the two con- . Fig. 146. verging rays a and b, fig. 146, without the interposition of the reflecting surfaces, ^ would meet at c } but because the angles of reflection are equal to those of incidence, and because the surfaces of reflection a r e inclined to each other, these -ays are reflected less converge! t, and instead of meeting at the same dis- tance before the mirror that c is behind it, are sent off in the direction of e, at which point they meet. 665. " Thus parallel rays falling on a convex mirror, are rendered diverging by reflection ; converging rays are made less convergent, or parallel, and diverging rays more divergent." The effect of the convex mirror, therefore, is to disperse the rays of light in all directions ; and it is proper here to remind the pupil, that although the rays of light are repre- sented on paper by single lines, there are in fact probably millions of rays, proceeding from every point of all visible bodies. Only a comparatively small number of these rays, it is true, can enter the eye, for it is only by those which proceed in straight lines from the different parts of the ob- ject, and enter the pupil, that the sense of vision is ex- sited. Now, to conceive how exceedingly small must be the proportion of light thrown off, from any visible object which enters the eye, we must consider that the same object re- flects rays in every other direction, as well as in that in which it is seen. Thus, the gilded ball on the steeple of a church may be seen by millions of persons at the same time, who stand upon the ground ; and were millions more raised above these, it would be visible to all. . When, therefore, it is said, that the convex mirror dis- Explain fig. 146 What effect does the convex mirror have upon parallel rays by reflection 1 What is its effect on converging rays 1 What is its eff^t on diverging rays 1 Do the rays of light proceed only from ther xtremities of objects, as represented in figures, or from all their parts 1 Do all the rays of light proceeding from an object en- ter the eye, or only a few of them ? MIRRORS. 183 pei es the rays of light which fall upon it from any ob- ject, and when the direction of these reflected rays are- shown only by single lines, it must be remembered, that each line represents pencils of rays, and that the light not only flows from the parts of the object thus designated, but from all the other parts. Were this not the case, the object would be visible only at certain points. 666. The images of objects reflected from the convex mirror, appear curved, because their different parts are not equally distant from its surface. If the object a be placed Fig. 147. obliquely before the convex mirror, fig. 147, then the con- verging rays from its two ex- tremities falling obliquely on its surface, would, were they prolonged through the mir- ror, meet at the point c, hind it. But instead of be- ing thus continued, they are thrown back by the mirror, in less convergent lines, which meet the eye at c, it being, ns we have seen, one of the properties of this mirror, to re- flect converging rays less convergent than before. The image being always seen in the direction from which '.he rays approach the eye, it appears behind the mirror at d. If the eye be kept in the same position, and the object, a, be moved further from the mirror, its image will appear smaller, in a proportion inversely to the distance to which it is removed. Consequently, by the same law, the two ends of a straight object will appear smaller than its mid- dle, because they are further from the reflecting surface of the mirror. Thus, the images of straight objects, held be- fore o convex mirror, appear curved, and for the same rea- son, the features of the face appear out of proportion, the nose being too large, and the cheeks too small, or narrow. The reason why the image appears less than the object is, that the convex surface of the mirror has the property, as What would be the consequence, if the rays of light proceeded only from the parts of an object shown in diagrams 7 Why do the images of objects "effected from convex mirrors appear curved 1 Why do the features o f the face appear out of proportion, by this mirror f Why ^oes an -. s a section. If the whole circle, Fig. 161. iig. 161, be considered *he circumference of a sphere, of which the pla- no-convex lens, b a, is a section, then the focus of parallel rays, or the pr cipal focus, will be at the opposite side of the sphere, or at c. 686. The focal dis- tance of a double convex lens, is the radius, or half the diam- eter of the sphere of which it is a part. Hence the plano- convex lens, being one half of the double convex lens, the latter has about twice the refractive power of the former ; for the rays suffer the same degree of refraction in passing out of the one convex surface, that they do in passing into the other. The shape of the dou- Fig. 162. ble convex lens, d c, fig. 162, is that of two plano- convex lenses, placed with their plane surfaces / in contact, and conse-/ quently the focal distance! of this lens is nearly the\ centre of the sphere of ' which one of its surfaces is a part. If parallel rays fall on a convex lens, it is evident that the ray only, which penetrates the axis and passes towards the centre of the sphere, will pro- On what do the focal distances of convex lenses depend 7 What is the focal distance of any plano-convex lens"? What is the focal dis- tance of the double convex lens 7 What is the shape of the double convex lens 7 196 LENSES ceed without refraction, as shown in the above figures. All the others will be refracted so as to meet the perpendicular ray at a greater or less distance, depending on the convexity of the lens. 687. If diverging rays fall on the surface of the same lens, they will, by refraction, be rendered less divergent, parallel or convergent, according to the degrees of their divergency, and the convexity of the surface of the lens. Thus, the diverg- Fig. 163. ing rays 1, 2, &c. fig. 163, are re- fracted by the lens a o, in a degree just sufficient to render them parallel, and therefore would pass off in right lines, indefinitely, or without ever forming a focus. 688. It is obvious by tne same law, that were the rays less divergent, or were the surface of the lens more convex, the rays in fig. 163 would become convergent, instead of parallel, because the same refractive power which would render divergent rays parallel, would make parallel rays convergent, and converging rays still more convergent. Thus the pencils of converging rays, Fig. 164. fig. 164, are rendered still more conver- gent by their passage through the lens, and are therefore brought to a focus nearer the lens, in proportion to their previous convergency. 689. The eye glasses of spectacles for old people are double convex lenses, more or less spherical, according to the age of the person, or the magnifying power required. The common burning glasses, which are used for light- ing cigars, and sometimes for kindling fires, are also convex lenses. Their effect is to concentrate to a focus, or point, the heat of the sun which falls on their whole surface; and How are divergent rays affected by passing through a convex lens 1 What is its effect on parallel rays 1 What is its effect on converging rays ? What kind of lenses are spectacle glasses for old people 1 LENSES, 197 hence the intensity 01" their effects is in proportion to the extent of their surfaces, and their focal lengths. One of the largest burning glasses ever constructed, was made by Mr. Parker, of London. It was three feet in diam- eter, with a focal distance of three feet nine inches. But in order to increase its power still more, he employed ano- ther lens about a foot in diameter, to bring its rays to a smaller focal point. This apparatus gave a most intense degree of heat, when the sun was clear, so that 20 grains of gold were melted by it in 4 seconds, and ten grains of platina, the most infusible of all metals, in 3 seconds. 690. It has been explained, that the reason why the con- vex mirror diminishes the images of objects is, that the rays which come to the eye from the extreme parts of the object are rendered less convergent by reflection, from the convex surface, and that, in consequence, the angle of vision is made more acute. Now, the refractive power of the convex lens has exactly the contrary effect, since by converging the rays flowing from the extremities of an object, the visual angle is rendered more obtuse, and therefore all objects seen through it appeal- magnified. Suppose the object a, fig. 165, appears to the naked eye of the length represented in the drawing. Now, as the rays coming from each end of the object, form, by their convergence at the eye, the visual angle, or the angle under which the object is seen, and we call objects large or small, in proportion as this angle is obtuse or acute, if there- fore the object a be withdrawn further from the eye, it is apparent that the rays o, 0, proceeding from its extremities, will enter the eye under a more acute angle, and therefore, that the object will appear diminished in proportion. This is the reason why things at a distanc.e appear smaller than when near us. When near, the visual angle is increased, and when at a distance, it is diminished. What is said to be the diameter of Mr. Parker's great convex lens 7 What is the focal distance of this Jens'? What is said of its heating power! What is the visual angle! Why does the same object, wnen at a distance, appear smaller than when near! 17* 198 LENSES. 691. The effect of the convex lens is Fig. 166. to increase the visual angle, by bending the rays of light coming from the object, so as to make them meet at the eye more obtusely; and hence it has the same ef- fect, in respect to the visual angle, as bringing the object nearer the eye. This is shown by fig. 166, where it is obvious, that did the rays flowing from the extrem- ities of the arrow meet the eye without refraction, the visual angle would be less, and therefore the object would appear shorter. Another effect of the convex lens, is to enable us to see objects nearer the eye, than with- out it, as will be explained under the article vision. Now, as the rays of light flow from all parts of a visible object of whatever shape, so the breadth, as well as the length, is increased by the convex lens, and thus the whole object appears magnified. 692. CONCAVE LENS. The effect of the concave lens is directly opposite to that of the convex. In other terms, by a concave lens, parallel rays are rendered diverging, con- verging rays have their convergency diminished, and di- verging rays have their divergency increased, according to the concavity of the lens. These glasses, therefore, exhibit things smaller than they really are, for by diminishing the convergence of the rays coming from the extreme points of an object, the visual an- gle is rendered more acute, and hence the object appears diminished by this lens, for the opposite reason that it is increased by the convex lens. This will be made plain by the two following diagrams. Suppose the object a b, fig. Fi g- * 6 ~' 167, to be placed at such a dis- tance from the eye, as to give the rays flowing from it, the degrees of convergence repre- sented in the figure, and sup- pose that the rays enter the eye under such an angle as to make the object appear two feet in lenp-th. What is the effect of the convex lens on the visual angle? "Why does an object appear larger through the convex lens than otherwise ? What is the effect of the concave lens 1 What effect does this lens have upon parallel, diverging, and converging rays ? Why do objects ap- pear smaller through this glass than they do to the naked eye ? 199 Now, the length of the same Fig. 168. object, seen through the concave lens, fig. 168 will appear dimin- ished, because the rays coming from it are bent outwards, or made less convergent by refrac- tion, as foen in the figure, and conseqrmtly the visual angle is more riute than when the same object is seen by the naked eye. Its length, therefore, will appear less, in proportion as the rays are rendered less convergent. The spectacle glasses of short-sighted people are concave leases, by which the images of objects are formed further J ack in the eye than otherwise, as will be explained under he next article. VISION. 693. In the application of the principles of optics to the explanation of natural phenomena, it is necessary to give a lescription of the most perfect of all optical instruments, he eye. 694. Fig. 169 is a Fig. 169. vertical section of the numan eye. Its form is nearly globular, with a slight projection or elongation in front. It consists of four coats, or membranes; name- ly, the sclerotic, the cornea, the cAoroid, and the retina. It has two fluids confined within these membranes, called the aqueous, and the vitreous hum- ours, and one lens, called the crystalline. The sclerotic coat is the outer and strongest membrane, and its anterior part is well known as the white of the eye. This coat is marked in the figure a, a, a, a. It is joined to the cornea, Explain figures 1C7 and 168, and show the reason why the same ob- ject appears smaller through 168. What defect in the eye requires con- cave lenses 1 What is the most perfect of all optical instruments'? What is the form of the human eye 1 How many coats, or membranes, lias the eye! What are they called.'? How many fluids has the eye, and what are they called "? What is the lens of the eye called 7 What coat forms the white of the eye 1 200 VISION. b, b t which is tne transparent membrane in front of the eye, through which we see. The choroid coat is a thin, deli- cate membrane, which lines the sclerotic coat on the inside. On the inside of this lies the retina, d, d, d, d, which is the innermost coat of all, and is an expansion, or continuation, of the optic nerve o. This expansion of the optic nerve is the immediate seat of vision. The iris, 0, 0, is seen through the cornea, and is a thin membrane, or curtain, of different colours in different persons, and therefore gives colour to the eyes. In black eyed persons it is black, in blue eyed persons it is blue, &c. Through the iris, is a circular open- ing, called the pupil, which expands or enlarges when the light is faint, and contracts when it is too strong. The space between the iris and the cornea is called the anterior chamber of the eye, and is filled with the aqueous humour, so called from its resemblance to water. Behind the pupil and iris is situated the crystalline lens t, which is a firm and per- fectly transparent body, through which the rays of light pass from the pupil to the retina. Behind the lens is situa- ted the posterior chamber of the eye, which is filled with the vitreous humour, v, v. This humour occupies much the largest portion of the whole eye, and on it depends the shape and permanency of the organ. 695. From the above description of the eye, it will be easy to trace the progress of the rays of light through its several parts, and to explain in what manner vision is per- formed. In doing this, we must keep in mind that the rays of light proceed from every part and point of a visible object, as heretofore stated, and that it is necessary only for a few of the rays, when compared with the whole number, to enter the eye, in order to make the object visible. Thus, the object a b. fig. 170, being placed in the light, sends forth pencils of rays in all possible direc- Describe where the several coats and humours are situated. "What s the iris 1 What is the retina 1 Where is the sense of vision ? What s the design of fig. 170 1 What is said concerning the small number of the rays which enter the eye from a visible object 1 Explain the de. sign of fig. 170. VISION. which will in any posi- dons, some of strike the eye tion where it is visible. These pencils of rays not only flow from the points designated in the figure, but in the same manner from every other point on the sur- face of a visible object. To render an object visible, therefore, it is only neces- sary that the eye should col- lect and concentrate a suffi- cient number of these rays on the retina, to form its image there, and from this image the sensation of vision is ex- cited. 696. From the luminous body Z, fig. 171, the pencils of r avs flow in all directions, but it is only by those which en- Fig. 171. ter the pupil, that we gain any knowledge of its existence ; and even these would convey to the mind no distinct idea of the object, unless they were refracted by the hu- mours of the eye, for did these rays proceed in their natural state of divergence to the retina, the image there formed would be too extensive, and consequently too feeble to give a distinct sensation of the object. It is, therefore, by the refracting power of the aqueous humour, and of the crystalline lens, that the pencils of rays are so concentrated as to form a perfect picture of the object on the retina. We have already seen, that when the rays of light are made to cross each other by reflection from the concave mir- Why would not the rays of light give a distinct idea of the object, without refraction by the humovus of the eye 7 202 VISION. ror, the image of the object is inverted ; the same happens when the rays are made to cross each other by refraction through a convex lens. * This, indeed, must be a necessary consequence of the intersection of the rays: for, as light proceeds in straight lines, those rays which come from the lower part of an object, on crossing those which come from its upper part, will represent this part of the picture on the upper half of the retina, and, for the same reason, the upper part of the object will be painted on the lower part of the retina. 697. Now, all objects are represented on the retina in an inverted position ; that is, what we call the upper end of a vertical object, is the lower end of its picture on the retina, and so the contrary. This is readily pioved by taking the eye of an ox, ana cutting away the sclerotic coat, so as to make it transparent on the back part, next the vitreous humour. If now a piece of white paper be placed on this part of the eye, the images of objects will appear figured on the paper in an inverted position. The same effect will be produced on looking at things through an eye thus prepared; they will appear in- verted. The actual position of the vertical object a, fig. 172, 39 painted on the retina, is therefore such as is represented by the figure. The rays from its up- per extremi- ty, coming in divergent lines, are con- verged by the o ] crystalline lens, and fall on the retina at o ; while those from its lower extremity, by the same law, fall on the retina at c. 698. In order that vision may be perfect, it is necessary that the images of objects should be formed precisely on the retina, and consequently, if the refractive power of the eye be too small, or too great, the image will not fall ex Explain how it is that the images of objects are inverted ina. What experiment proves that the images of objects a on the ret- are inverted on the retina 1 'Explain fig. 172. Suppose the refractive power of the eye is too great, or too little, why will vision be imperfect ? VISION. 203 actly on the seat of vision, but will be formed either before, or tend to form behind it. In both cases, perhaps, an out- line of the object may be visible, but it will be confused and indistinct. 699. If the cornea is too convex, or prominent, the imag*e will be formed before it reaches the retina, for the same rea- son, that of two lenses, that which is most convex will have the least focal distance. Such is the defect in the eyes of persons who are short sighted, and hence the necessity of their bringing objects as near the eye as possible, so as to make the rays converge at the greatest distance behind the crystalline lens. The effect of uncommon convexity in the cornea on the rays of light, is shown at fig. 173, where it will be ob- Fig. 173. served that the image, instead of being formed on the retina r, is suspended in the vitreous humour, in consequence of there being too great a refractive power in the eye. It is hardly necessary to say, that in this case, vision must be very imperfectly performed. This defect of sight is remedied by spectacles, the glasses of which are concave lenses. Such glasses, by rendering the rays of light less convergent, before they reach the eye, counteract the too great convergent power of the cornea and lens, and thus throw the image on the retina. 70Q. If, on the contrarv, the humours of the eye, in con- sequence of age, or cuv other cause, have become less in quantity than ordinary, tne eyeball will not be sufficiently distended, and the cornea will become too flat, or not suffi- ciently convex, to make the rays of light meet at the proper place, and the image will therefore tend to be formed be- If the cornea is too convex, where will the image be formed 1 How is the sight improved, when the cornea is too convex 1 How do such lenses act to improve the sight 1 Where do the rays tend to meet when tne cornea is not sufficiently convex 1 204 VISION. yond the retina, instead of before it, as in the other case. Hence, aged people, who labour under this defect of vision, cannot see distinctly at ordinary distances, but are obliged to remove the object as far from the eye as possible, so as to make its refractive power bring the image within the seat of vision. The defect arising from this cause is represented by fig- ure 174, where it will be observed that the image is formed Fis:. 174. behind the retina, showing that the convexity of the cornea is not sufficient to bring the image within the seat of dis- tinct vision. This imperfection of sight is common to aged persons, and is corrected in a greater or less degree by double convex lenses, such as the common spectacle glasses. Such glasses, by causing the rays of light to converge, be- fore they meet the eye, assist the refractive power of the crystalline lens, and thus bring the focus, or image, within the sphere of vision. 701. It has been considered difficult to account for the reason why we see objects erect, when they are painted on the retina inverted, and many learned theories have been written to explain- this fact. But it is most probable that this is owing to habit, and that the image, at the bottom of the eye, has no relation to the terms above and below, but to the position of our bodies, and other things which surround us. The term perpendicular, and the idea which it con- veys to the mind, is merely relative ; but when applied to an object supported by the earth, and extending towards the skies, we call the body erect, because it coincides with the position of our own bodies, and we see it erect for the same reason. Had we been taught to read by turning our books upside down, what we now call the upper part of the book How is vision assisted when the eye wants convexity 1 How clo convex lenses help the sight of aged people 1 Why do we see things erect, when the images are inverted on the retina 1 VISION. 205 would have been its under part, and that reading would have been as easy in that position as in any other, is plain from the fact that printers read their types, when set up, as rea- dily as they do its impressions on paper. 702. Angle of Vision. The angle under which the rays of light, coming from the extremities of an object, cross each other at the eye, bears a proportion directly to the length, and inversely to the distance of the object. Suppose the object a b, fig. 175, to be four feet long, and -o be placed ten feet from the eye, then the rays flowing from its extremities, would intersect each other at the eye, Fig. 175. under a given angle, which will always be the same when the object is at the same distance. If the object be gradu- ally moved towards the eye, to the place c d, then the angle will be gradually increased in quantity, and the object will appear larger, since its image on the retina will be increas- ed in length in the proportion as the lines i i are wider apart than o o. On the contrary, were a b removed to a greater dis- tance from the first position, it is obvious that the angle would be diminished in proportion. The lines thus proceeding from the extremities of an ob- iect, and representing the rays of light, form an angle at the eye, which is called the visual angle, or the angle under which things are seen. These lines a n b, therefore, brm one visual angle, and the lines end another visual mgle. We see from this investigation, that the apparent magni- ude of objects depending on the angles of vision, will vary tccording to their distances from the eye, and that these magnitudes diminish in a proportion inversely as their dis- What is the visual angle'? How may the visual angle of the same object be increased or diminished 1 When do objects of different mag litudes form the same visual angle 1 18 206 VISION. tances increase. We learn, also, from the same principles, that objects of different magnitudes may be so placed, with respect to the eye, as to give the same visual angle, and thus to make their apparent magnitudes equal. Thus the three arrows, a, e, and m, though differing so much in length, are all seen under the same visual angle. 703. In the apparent magnitude of objects seen through a lens, or when their images reach the eye by reflection from a mirror, our senses are chiefly, if not entirely, guided by the angle of vision. In forming our judgment of the sizes of distant objects, whose magnitudes were before unknown, we are also guided more or less by the visual angle, though in this case we do not depend entirely on the sense of vision. Thus, if we see two balloons floating in the air, one of which is larger than the other, we judge of their comparative mag- nitudes by the difference in their visual angles, and of their real magnitudes by the same angles, and the distance \ve suppose them to be from us. But when the object is near us, and seen with the naked eye, we then judge of the magnitude by our experience, and not entirely by the visual angle. Thus, the three arrows, a, e, m, fig. 175, all of them make the same angle on the eye, and yet we know, by further examination, that they are all of different lengths. And so the two arrows a b, and c d, though seen under different visual angles, w r ill appear of the same size, because experience has taught us that this difference depends only on the comparative distance of the two objects. 704. As the visual angle diminishes inversely in propor- tion as the distance of the object increases, so when the dis- tance is so great as to make the angle too minute to be per- ceptible to the eye, then the object becomes invisible. Thus, when we watch an eagle, flying from us, theano-le of vision is gradually diminished, until the rays proceeding from the bird form an image on the retina too small to excite sensa- tion, and then we say the ea^le has flown out of sight. The same principle holds with respect to objects which are near the eye, but are too small to form an image on the retina which is perceptible to the senses. Such objects, to Explain fig. 175. Under what circumstances is our sense of vision guided entirely by the visual angle *? How do we judge of the mag- nitudes of distant objects ? How do we judge of the comparative size o* objects near us 1 When does a retreating object become invisible to the eye 1 VISION. 207 ue naked eye, are of course invisible, but when the Tisual a*gle is enlarged, by means of a convex lens, they become visible; that is, their images on the retina excite sensation. 705. The actual size of an image on the retina, capable of exciting sensation, and consequently of producing vision, may be too small for us to appreciate by any of our other senses ; for when we consider how much smaller the image must be than the object, and that a human hair can be dis- tinguished by the naked eye at the distance of twenty or thirty feet, we must suppose that the retina is endowed with the most delicate sensibility, to be excited by a cause so mi- nute. It has been estimated that the image of a man, on the retina, seen at the distance of a mile, is not more than the five thousandth part of an inch in length. 706. On the contrary, if the object be brought too near the eye, its image becomes confused and indistinct, because the rays flowing from it, fall on the crystalline lens in a state too divergent to be refracted to a focus on the retina. This will be apparent Fig. 176. by fig. 176, where we suppose that the object a, is brought within an inch or two of the eye, and that the rays proceeding from it enter the pupil so ob- liquely as not to be re- fracted by the lens, so as to form a distinct image. Could we see objects distinctly at the shortest distance, we should be able to examine things that are now invisible, since the visual angle would then be increased, and conse- quently the image on the retina enlarged, in proportion as objects were brought near the eye. This is proved by intercepting the most divergent rays ; in which case an object may be brought near the eye, and will then appear greatly magnified. Make a small orifice, as a pin-hole, through a piece of dark coloured paper, and then look through the orifice at small objects, such as the How does a convex lens act to make us see objects which are invisi- ble without it 1 What is said of the actual size of an image on the ret- ina 1 Why are objects indistinct, when brought too near the eye r i Suppose objects could be seen distinctly within an inch or two of the eye, how would their dimensions be affected 1 How is it proved that objects placed near the eye are magnified 1 How does a small orifice enable us to see an object distinctly near the eyel 208 MICROSCOPE. letters of a printed book. The letters will appear much magnified. The rays, in this case, are refracted to a focus, on the retina, because the small orifice prevents those which are most divergent from entering the eye, so that notwith- standing the nearness of the object, the rays which form the image are nearly parallel. OPTICAL INSTRUMENTS. 707. Single Microscope. The principle of the single microscope, or convex lens, will be readily understood, if the pupil will remember what has been said on the refrac- tion of lenses, in connexion with the facts just stated. For, the reason why objects appear magnified through a convex lens, is not only because the visual angle is increased, but because when brought near the eye, the diverging rays from the object are rendered parallel by the lens, and are thus thrown into a condition to be brought to a focus in the pro- per place by the humours. Let a, fig. Fig. 177. 177, be the dis- tance at which an object can be seen dis- tinctly, and b, the distance at which the same object is seen through the lens, and suppose th,e dis- tance of a t from the eye, be twice that of b. Then, because the object is at half the distance that it was before, it will appear twice as large ; and had it been seen one third, one fourth, or one tenth its former distance, it would have been magnified three, four, or ten times, and consequently its sur face would be increased 9, 16, or 100 times. 708. The most powerful single microscopes are made of minute globules of glass, which are formed by melting the ends of a few threads of spun glass in a candle. Small globules of water placed in an orifice through a piece of tin, or other thin substance, will also make very powerful microscopes. In these minute lenses, the focal distance is only a tenth or twelfth part of an inch from the lens, and Why does a convex lens make an object distinct when near the eye"? Explain fig. 177. How are the most powerful single microscopes made? MICROSCOPE. 209 therefore the eye, as well as the object to be magnified, must be brought very near the instrument. 709. The Compound Microscope consists of two convex lenses, by one of which the image is formed within the tube of the instrument, and by the other this image is magnified, as seen by the eye ; so that by this instrument the object it- self is not seen, as with the single microscope, but we see only its magnified image. The small lens placed near the object, and by which its image is formed within the tube, is called the object glass, while the larger one, through which the image is seen, is called the eye glass. This arrangement is represented at fig. 178. The object a is placed a little beyond the focus of the object glass b, by which an inverted and enlarged image of it is formed within the instrument at c. This image is seen through the eye Fig. 178. glass d, by which it is again magnified, and it is at last figured on the retina in its original position. These glasses are set in a case of brass, the object glass being made to take out, go that others of different magnify- ing powers may be used, as occasion requires. 710. The Solar Microscope consists of two lenses, one of which is called the condenser, because it is employed to concentrate the rays of the sun, in order to illuminate more strongly the object to be magnified. The other is a double convex lens, of considerable magnifying power, by which the image is formed. In addition to these lenses, there is a plain mirror, or piece of common looking glass, which can How many lenses foitn the compound microscope 1 Which is the ob- ject and which the eye glass 1 Is the object seen with this instrument, or only its image 1 Explain fig. 178, and show where the image is formed in this tube. How many lenses has the solar microscope? Why is one of the lenses of the solar microscope called the condenser 1 Describe the uses of the two lenses ana tr:e reflector. 18* 210 MICROSCOPE. be moved in any direction, and which reflects the rays of the sun on the condenser. The object a, fig. 179, being placed nearly in the focus of the condenser b, is strongly illuminated, in consequence of the rays of the sun being thrown on b, by the mirror c. The object is not placed exactly in the focus of the conden- ser, because, in most cases, it would be soon destroyed by its heat, and because the focal point would illuminate only a small extent of surface, but may be exactly in the focus of the small lens d, by which no such accident can happen. The lines o 0, represent the incident rays of the sun, which are reflected on the condenser. When the solar microscope is used, the room is darkened, the only light admitted being that which is thrown on the object by, the condenser, which light passing through the small lens, gives the magnified shadow e, of the small object fi, on the wall of the room, or on a screen. The tube con- taining the two lenses is passed through the window of the room, the reflector remaining outside. In the ordinary use of this instrument, the object itself is not seen, but only its shadow on the screen, and it is not de- signed for the examination of opaque objects. 711. When the small lens of the solar microscope is of great magnifying power, it presents some of the most striking and curious of optical phenomena. The shadows of mites from cheese, or figs, appear nearly two feet in length, presenting an appearance exceedingly formidable and disgusting ; and the insects from common vinegar ap- pear eight or ten feet long, and in perpetual motion, resem- bling so many huge serpents. Is the object, or only the shadow, seen by this instrument ? TELESCOPE. 211 TELESCOPE. 712. The Telescope is an optical instrument, employed to view distant bodies, and, in effect, to bring them nearer the eye, by increasing the apparent angles under which such objects are seen. These instruments are of two kinds, namely, refracting and reflecting telescopes. In the first kind, the image of the object is seen with the eye directed towards it; in the sec- i ond kind, the image is seen by reflection from a mirror, while the back is towards the object, or by a double reflec- tion, with the face towards the object. The telescope is the most important of all optical instru- ments, since it unfolds the wonders of other worlds, and gives us the means of calculating the distances of the heav- enly bodies, and of explaining their phenomena for astro- nomical and nautical purposes. The principle of the telescope will be readily compre- hended after what has been said concerning the compound microscope, for the two instruments differ chiefly in respect to the place of the object lens, that of the microscope having a short, while that of the telescope has a long, focal distance. 713. REFRACTING TELESCOPE. The most simple re- fracting telescope consists of a tube, containing two convex lenses, the one having a long, and the other a short, focal distance. (The focal distance of a double convex lens, it will be remembered, is nearly the centre of a sphere, of which it is a part.) These two lenses are placed in the tube, at a distance from each other equal to the sum of their two focal distances. Fig. 180. Thus, if the focus of the object glass a, fig. 180, be eight inches, and that of the eye glass b, two inches, then the dis- What is a telescope 1 How many kinds of telescopes are mention- ed 1 What is the difference between them 7 In what respect does the refracting telescope differ from the compound microscope 1 How is the most simple refracting telescope formed 1 Wh ; ch is the object, and which the eye lens, in fig. 180? What is the rule by which the dis- tance of the two glasses apart is found ? 212 TELESCOPE. tance of the sums of the foci will be ten inches, and, there* fore, the two lenses must be placed ten inches apart ; and the same rule is observed, whatever may be the focal lengths of any two lenses. Now, to understand the effect of this arrangement, sup- pose the rays of light, c d, coming from a distant object, as a star, to fall on the object glass a, in parallel lines, and to be refracted by the lens to a focus at e, where the image of the star will be represented. This image is then magnified by the eye glass b, and thus, in effect, is brought near the eye. 714. All that is effected by the telescope, therefore, is to form an image of a distant object, by means of the object lens, and then to assist the eye in viewing this image as nearly as possible by the eye lens. It is, however, necessary here to state, that by the last figure, the principle only of the telescope is intended to be explained, for in the common instrument, with only two glasses, the image appears to the eye inverted. The reason of this will be aeeri by the next figure, where the direction of the rays of light will show the position of the image. Fig. 181. Supposes, fig. 181, to be a distinct object, from which pencils of rays flow from every point toward the object lens b. The image of a, in consequence of the refraction of the rays by the object lens, is inverted at c, which is the fo- cus of the eye glass d, and through which the image is then seen, still inverted. 715. The inversion of the object is of little consequence when the instrument is employed for astronomical purposes, for since the forms of the heavenly bodies are spherical, their positions, in this respect, do not affect their general appearance. But for terrestrial purposes, this is manifestly a great defect, and therefore those constructed for such pur- How do the two glasses act, to bring an object near the eyel Ex- plain fig. 181, and show how the object comes to be inverted by the two lenses 1 Hw is the inversion of the object corrected 1 213 poses, as ship, or spy glasses, have two additional lenses, by means of which, the images are made to appear in the same position as the objects. These are called double tele, scopes. Fig. 182. 4**- Such a telescope is represented at fig. 182, and consists of an object glass a, and three eye glasses, b, c, and d. The eye glasses are placed at equal distances from each other, so that the focus of one may meet that of the other, and thus the image formed by the object lens, will be transmitted through the other three lenses, to the eye. The rays coming from the object o, cross each other at the focus of the object lens, and thus form an inverted image at / This image be- ing also in the focus of the first eye glass, b, the rays having- passed through this glass become parallel, for, we have seen, in another place, that diverging rays are rendered par- allel by refraction through a convex lens. The rays, there- fore, pass parallel to the next lens c, by which they are made to converge, and cross each other, and thus the image is inverted, and made to assume the original position of the object o. Lastly, this image, being in the focus of the eye glass d, is seen in the natural position, or in that of the ob- ject. The apparent magnitude of the object is not changed by these two additional glasses, but depends, as in fig. 182, on the magnifying power of the eye and object lenses; the two glasses being added merely for the purpose of making the image appear erect. 7l6. It is found that an eye glass of very high magnify- ing power cannot be employed in the refracting telescope, because it disperses the rays of light, so that the image be :omes indistinct. Many experiments were formerly made Explain fig. 182, and show why the two additional lenses make the image of the object erect. Does the addition of these two lenses make *ny difference with the apparent magnitude of the object 1 Wry can- not a highly magnifying eye glass be used in the telescope"? 214 TELESCOPE. with a view to obviate this difficulty, and among these it was found that increasing the focal distance of the object .ens, was the most efficacious. But this was attended with great inconvenience, and expense, on account of the length of tube which this mode required. These experiments were, however, discontinued, and the refracting telescope itself chiefly laid aside for astronomical purposes, in consequence of the discovery of the reflecting telescope. 717. REFLECTING TELESCOPE. The common reflecting telescope consists of a large tube, containing two concave re- flecting mirrors, of different sizes, and two eye glasses. The object is first reflected from the large mirror to the small one, and from the small one, through the two eye glasses, where it is then seen. 718. In comparing the advantages of the two instru- ments, it need only be stated, that the refracting telescope, with a focal length of a thousand feet, if it could be used, would not magnify distinctly more than a thousand times, while a reflecting telescope, only eight or nine feet long, will magnify with distinctness twelve hundred times. Fig. 183. r --P d e 719. The principle and construction of the reflecting tele- jcope will be understood by fig. 183. Suppose the object o to be at such a distance, that the rays of light from it pass in parallel lines, p p, to the great reflector r r. This reflector being concave, the rays are converged by reflection, and cross each other at a, by which the image is inverted. The rays then pass to the small mirror, b t which being also con- cave, they are thrown back in nearly parallel lines, and having passed the aperture in the centre of the great mirror, fall on the plano-convex lens e. By this lens they are re- What is the most efficacious means of increasing the power of the refracting telescope 1 How many lenses and mirrors form the reflect- ing telescope 1 What are the advantages of the reflecting over the re- fracting telescope? Explain fig. 183, and show the course of the rays from the object to the eye. TELESCOPE. 215 fracted to a focus, and cross each other between e and d, and thus the image is again inverted, and brought to its original position, or in the position of the object. The rays then, passing the second eye glass, form the image of the object on the retina. The large mirror in this instrument is fixed, but the small one moves backwards and forwards, by means of a screw, so as to adjust the image to the eyes of different persons. Both mirrors are made of a composition, consisting of sev- eral metals melted together. 720. One great advantage which the reflecting telescope possesses over the refracting, appears to b^ that it admits of an eye glass of shorter focal distance, and, consequently, of greater magnifying power. The convex object glass of the refracting instrument, does not form a perfect image of the object, since some of the rays are dispersed, and others co- loured by refraction. This difficulty does not occur in the reflected image from the metallic mirror of the reflecting telescope, and consequently it may be distinctly seen, when more highly magnified. The instrument just described is called " Gregory's tele- scope" because some parts of the arrangement were invent- ed by Dr. Gregory. 721. In the telescope made by Dr. Herschel, the object' s reflected by a mirror, as in that of Dr. Gregory. But the second, or small reflector, is not employed, the image being- seen through a convex lens, placed so as to magnify the linage of the large mirror, so that the observer stands with his back towards the object. The magnifying power of this instrument is the same as that of Dr. Gregory's, but the image appears brighter, be- cause there is no second reflection ; for every reflection ren- ders the image fiinter, since no mirror is so perfect as to throw back all the rays which fall upon its surface. 722. In Dr. Herschel's grand telescope, the largest ever constructed, the reflector was 48 inches in diameter, and had a focal distance of 40 feet. This reflector was three and a half inches thick, and weighed 2000 pounds. Now, since the focus of a concave mirror is at the distance of one Why is the small mirror in this instrument made to move by means of a screw] What is the advantage of the reflecting telescope in re- spect to the eye glass 1 Why is the telescope with two reflectors called Gregory's telescope 1 How does this instrument differ from Dr. Her- schel's telescope 7 What was the focal distance and diameter of the mirror in Dr. Herschel's great telescope? CAMERA QBSCURA. half the semi-diameter of the sphere, of which it is a section DJ. Herschel's reflector having a focal distance of 40 feev, formed a part of a sphere of 160 feet in diameter. This great instrument was begun in 1785, and finished four years afterwards. The frame by which this wonder to all astronomers was supported, having decayed, it was taken down in 1822, and another of 20 feet focus, with a reflector of 18 inches in diameter, erected in its place, by Herschel's son. The largest Herschel's telescope now in existence is that of Greenwich observatory, in England. This has a con- cave reflector of 15 inches in diameter, with a focal length of 25 feet, and was erected in 1820. 723. CAMERA OBSCURA. Camera obscura strictly signi- fies a darkened chamber, because the room must be dark- ened, in order to observe its effects. To witness the phenomena of this instrument, let a room be closed in every direction, so as to exclude the light. Then from an aperture, say of an inch in diameter, admit a single beam of light, and the images of external things, such as trees, and houses, and persons walking the streets, will be seen inverted on the wall opposite to where the light is admit- ted, or on a screen of white paper, placed before the aperture. 724. The reason why the image is inverted, will be ob- vious, when it is remembered that the rays proceeding from the extremities of the object must converge in order to pass through the small aperture ; and as the rays of light always proceed in straight lines, they must cross each other at the point of admission, as expjained under the article Vision. Thus, the Fig. 184. pencil a, fig. 184, coming from the up- per part of the tower, and proceeding straight, will represent the image of that part at b, while the lower part Where is the largest Herschel's telescope now in existence ? What is the diameter and focal distance of the reflector of this telescope'? Describe the phenomena of the camera obscura. Why is the image formed by the camera obscura inverted 1 MAGIC LANTERN. 217 Fig. 185. c, for the same reason will be represented at d. If a con- vex lens, with a short tube, be placed in the aperture through which the light passes into the room, the images of things will be much more perfect, and their colours more brilliant. 725. This instrument is sometimes employed by paint- ers, in order to obtain an exact delineation of a landscape, an outline of the image being ea- sily taken with a pencil, when the image is thrown on a sheet of paper. There are several modifica- tions of this machine, and among them the revolving ca- mera obscura is the most in- teresting. It consists of a small house, fig. 185, with a plane reflect-*? or, a b, and a convex lens, c b, placed at its top. The reflect- or is fixed at an angle of 45 degrees with the horizon, so as to reflect the rays of light perpendicularly downwards, and is made to revolve quite around, in either direction, by pulling a string. Now suppose the small house to be placed in the open air, with the mirror, a b, turned towards the east, then the rays of light flowing from the objects in that direction, will strike the mirror in the direction of the lines o, and be re- flected down through the convex lens c b, to the table e e, where they will form in miniature a most perfect and beau- tiful picture of the landscape in that direction. Then, by making the reflector revolve, another portion of the land- scape may be seen, and thus the objects, in all directions, can be viewed at k without changing the place of the in- strument. 726. MAGIC LANTERN. The Magic Lantern is a mi- croscope, on the same principle as the solar microscope. But instead of being used to magnify natural objects, it is t-ommonly employed for amusement, by the casting shadows How may an outline of the image formed by the camera obscura be taken? Describe the revolving camera obscura. What is the magic lantern 1 For what purpose is this instrument employed? 19 218 CHROMATICS. of small transparent paintings done on glass, upon a screen placed at a proper distance. Fig. 186. o n Let a candle c, fig. 186, be placed on the inside of a box, or tube, so th^t its light may pass through the plano-convex lens n, and strongly illuminate the object o. This object is generally a small transparent painting on a slip of glass, which slides through an opening in the tube. In order to show the figures in the erect position, these paintings are in- verted, since their shadows are again inverted by the refrac- tion of the convex lens m. In some of these instruments, there is a concave mirror, d y by which the object, o, is more strongly illuminated than it would be by the lamp alone. The object is magnified by the double convex lens, m, which is moveable in the tube by a screw, so that its focus can be adjusted to the required dis- tance. Lastly, there is a screen of white cloth, placed at the proper distance, on which the image, or shadow of the picture, is seen greatly magnified. The pictures being* of various colours, and so transparent, that the light of the lamp shines through them, the shadows are also of various colours, and thus soldiers and horsemen are represented in their proper costume. CHROMATICS, OR THE PHILOSOPHY OF COLOURS. 727. We have thus far considered light as a simple sub- stance, and have supposed that all its parts were equally re fracted, in its passage through the several lenses described. But it will now be shown that light is a compound body, and that each of its rays, which to us appear white, is corn- Describe the construction and effect of the magic lantern. CHROMATICS. 219 posed of several colours, and that each colour suffers a dif- ferent degree of refraction, when the rays of light pass through a piece of glass, of a certain shape. 728. The discovery, that light is a compound substance, and that it may be decomposed, or separated into parts, was made by Sir Isaac Newton. If a ray, proceeding from the sun, be admitted into a darkened chamber, through an aperture in the window shut- ter, and allowed to pass through a triangular shaped piece of glass, called a prism, the light will be decomposed, and- instead of a spot of white light, there will be seen, on the opposite wall, a most brilliant display of colours, including all those which are seen in the rainbow. Fig. 187. Suppose s, fig. 187, to be a ray from the sun, admitted through the window shutter a, in such a direction as to fall on the floor at c, where it would form a round, white spot. Now, on interposing the prism p, the ray will be refracted, and at the same time decomposed, and will form on the screen m, n, an oblong figure, containing seven colours, which will be situated in respect to each other, as named in the figure. It may be observed, that of all the colours, the red is least refracted, or is thrown the smallest distance from the direc tion of the original sun beam, and that the violet is most re fracted, or bent out of that direction. The oblong image containing the coloured rays, is called the solar or prismatic spectrum. 729. That the rays of the sun are composed of the seven Who made the discovery, that light is a compound substance"? In what manner, and by what means, is light decomposed? What are the prismatic colours, and how do they succeed each other in the spec- trum ? Which colour is refracted most, and which least *? 220 CHROMATICS. colours above named, is sufficiently evident by the fact, that such a ray is divided into these several colours by passing- through the prism, but in addition to this proof, it is found by experiment, that if these several colours be blended or mixed together, white will be the result. This may be done by mixing together seven powders, whose colours represent the prismatic colours, and whose quantities are to each other, as the spaces occupied by each colour in the spectrum. When this is done, it will be found that the resulting colour will be a grayish white. A still more satisfactory proof that these seven colours form white, when united, is obtained by causing the solar spectrum to pass through a lens, by which they are brought to a focus, when it is found that the focus will be the same colour as il would be from the original rays of the sun. 730. From the oblong shape of the solar spectrum, we learn that each of the coloured rays is refracted in a differ- ent degree by passing through the same medium, and con- sequently that each ray has a refractive power of its own. Thus, from the red to the violet, each ray, in succession, is refracted more than the other. 731. The prism is not the only instrument by which light can be decomposed. A soap bubble blown up in the sun will display most of the prismatic colours. This is ac- counted for by supposing that the sides of the bubble vary in thickness, and that the rays of light are decomposed by these variations. The unequal surface of mother of pearl, and many other shells, send forth coloured rays on the same principle. 732. Two surfaces of polished glass, when pressed to- gether, will also decompose the light. Rings of coloured light will be observed around the point of contact between the two surfaces, and their number may be increased CT di- minished by the degrees of pressure. Two pieces of com- mon looking glass, pressed together with the fingers, will display most of the prismatic colours. 733. A variety of substances, when thrown into the form of the triangular prism, will decompose the rays of light, When the several prismatic colours are blended, what colour is the result ? When the solar spectrum is made to pass through a lens, what is the colour of the focus 1 How do we learn that each coloured ray has a refractive power of its own ? By what other means besides the prism, can the rays of light be decomposed'? How may light be de- composed by two pieces of glass 1 Of what substances may prisms be formed, besides glass T RAINBOW. 221 is well as a prism of glass. A very common instrument for this purpose is made by putting together three pieces of plate glass, in form of a prism. The ends may be made of wood, and the edges cemented with putty, so as to make the whole water tight. When this is filled with water, and held before a sun beam, the solar spectrum will be formed, displaying the same colours, and in the same order, as that above described. 734. In making experiments with prisms, filled with dif- ferent kinds of liquids, it has been found that one liquid will make the spectrum J mger than another ; that is, the red and violet rays, which form the extremes of the spectrum, will be thrown farther apart by one fluid, than by another. For example, if the prism be filled with oil of cassia, the spec- trum formed by it, will be more than twice as long as that formed by a prism of solid glass. The oil of cassia is there- fore said to disperse the rays of light more than glass, and hence to have a greater dispersive power. 735. THE RAINBOW. The rainbow was a phenomenon, for which the ancients were entirely unable to account ; but after the discovery that light is a compound principle, and that its colours may be separated by various substances, the solution of this phenomenon became easy. Sir Isaac Newton, after his great discovery of the com- pound nature of light, and the different refrangibility of the coloured rays, was able to explain the rainbow on optical principles. 736. If a glass globe be suspended in a room, where the rays of the sun can fall upon it, the light will be decom- posed, or separated into several coloured rays, in the same manner as is done by the prism. A well defined spectrum will not, however, be formed by the globe, because its shape is such as to disperse some of the rays, and converge others; but the eye, by taking different positions in respect to the globe, will observe the various prismatic colours. Trans- parent bodies, such as glass and water, reflect the rays of light from both their surfaces, but chiefly from the second surface. That is, if a plate of naked glass be placed so as to reflect the image of the sun, or of a lamp, to the eye, the What is said of some liquids making the spectrum larger than oth- ers 1 What is said of oil of cassia, in this respect 1 What discovery oreceded the explanation of the rainbow 1 Who first explained the rainbow on optical principles 7 Why do*:s not a glass globe form a well defined spectrum '1 From which surface do transparent bodies chiefly reflect the light ? 19* 222 RAINBOW. most distinct image will come from the second surface, 01 that most distant from the eye. The great brilliancy of the diamond is owing to this cause. It will be understood di- rectly, how this principle applies to the explanation of the jainbow. Suppose the circle a b c, fig. 188, to represent a globe, or a drop of rain, for each drop of rain, as it falls through the air, is a small Fig. 188. globe of water. Suppose, also, that the sun is at s, and the eye of the spectator at e. Now, it has already been stated, that from a single globe, the whole solar spectrum is not seen in the same position, but that the different colours are seen from different places. Suppose, then, that a ray of light from the sun s, on entering the globe at a, is separated into its primary colours, and at the same time the red ray, which is the least refrangible, is refracted in the line from a to b. From the second, or inner surface of the globe, it would be reflected to c, the angle of reflection being equal to that of incidence. On passing out of the globe, its re- fraction at c, would be just equal to the refraction of the in- cident ray at a, and therefore the red ray would fall on the eye at e. All the other coloured rays would follow the same law, but because the angles of incidence and those of reflection are equal, and because the colored rays are separa- ted from each other by unequal refraction, it is obvious, that if the red ray entered the eye at e, none of the other coloured rays could be seen from the same point. 737. From this it is evident, that if the eye of the spec- tator is moved to another position, he will not see the red ray coming from the same drop of rain, but only the blue, and if to another position, the green, and so of all the others. Explain fig. 188, and show the different refractions, and the reflection concerned in forming the rainbow. In the case supposed, why will only the red ray meet the eye! Suppose a person looking at a rain- bow moves his eye, will he see the same colours from the same drop of rain ? RAINBOW. 223 But m a shower of rain, there are drops at all heights and distances, and though they perpetually change their places, in respect to the sun and the eye, as they fall, still there will be many which will be in such a position as to reflect the red rays to the eye, and as many more to reflect the yellow rays, and so of all the other colours. This will be Fig. 189 made obvious by fig. 189, where, to avoid confu- sion, we will sup- pose that only three drops of rain, and, con- sequently, only three colours, are to be seen. The numbers 1, 2, 3, are the rays of the sun, proceeding to the drops a, b, c, and from which these rays are reflect- ed to the eye, ma- king different angles with the horizontal line A, because one coloured ray is refracted more than another. Now, suppose the red ray only reaches the eye from the drop a, the green from the drop b, and the violet from the drop c, then the spectator would see a minute rainbow of three colours. But during a shower of rain, all the drops which are in the po- sition of a, in respect to the eye, would send forth red rays, and no other, while those in the position of b, would emit green rays, and no other, and those in the position of c, vio- let rays, and so of all the other prismatic colours. Each circle of colours, of which the rainbow is formed, is there- fore composed of reflections from a vast number of differ- ent drops of rain, and the reason why these colours are dis- tinct to our senses, is, that we see only one colour from a single drop, with the eye in the same position. It follows, then, that if we change our position, while looking at a Explain fig. 189, and show why we see different colours from differ- ent drops of rain. Do several persons see the same rainbow at the same time 1 224 RAINBOW, rainbow, we still see a bow, but not the same as before, and hence, if there are many spectators, they will all see a differ- ent rainbow, though it appears to be the same. 738. There are often seen two rainbows, the one formed as above described, and the other, which is fainter, appear- ing on the outside, or above this. The secondary bow, as this last is called, always has its order of colours the reverse of the primary one. Thus, the colours of the primary bow, beginning with its upper, or outermost portion, are red, orange, yellow, &c., the lowest, or innermost portion, being violet; while the secondary bow, beginning with the same corresponding part, is coloured violet, indigo, &c., the low- est, or innermost circle, being red. 739. In the primary bo\v, we have seen, that the coloured rays arrive at the eye after two refractions, and one reflec- tion. In the secondary bo\v, the rays reach the eye after two refractions, and two reflections, and the order of the colours is reversed, because, in this case, the rays of light enter the lower part of the drop, instead of the upper part, as in the primary bow. The reason why the colours are fainter in the secondary than in the primary bow is, because a part of the light is lost or dispersed, at each reflection, and there being two reflections, by which this bow is form- ed, instead of one, as in the primary, the difference in bril- liancy is very obvious. 740. The direction of a single ray, showing how the secondary bow is formed, will be seen at fig. 190. The ray r, from the Fig. 190. sun, enters the drop of water at a, and is re- fracted to c, then re- flected to b, then again reflected to d, where it suffers an- other re- fraction, and lastly, passes to the eye of the Spectator at e. Explain the reason of this. How are the colours of the primary and secondary bows arranged in respect to each other 1 How many refractions and reflections produce the secondary bow 1 Why is the secondary bow less brilliant than the primary ? COLOURS. 225 The rainbow, being the consequence of the refracted and reflected rays of the sun, is never seen, except when the sun and the spectator are in similar directions, in respect to the shower. It assumes the form of a semicircle, because it is only at certain angles that the refracted rays are visible to the eye. 741. Of the colours of things. The light of the sun, we have seen, may be separated into seven primary rays, each of which has a colour of its own, and which is different from that of the others. In the objects which surround us, both natural and artificial, we observe a great variety of colours, which differ from those composing the solar spectrum, and hence one might be led to believe that both nature and art afford colours different from those afforded by the decomposition of the solar rays. But it must be remembered, that the solar spectrum contains only the primary colours of nature, and that by mixing these colours in various proportions with each other, an indefinite variety of tints, all differing from their primaries, may be obtained. 742. It appears that the colours of all bodies depend on some peculiar property of their surfaces, in consequence of which, they absorb some of the coloured rays, and reflect the others. Had the surfaces of all bodies the property of re- flecting the same ray only, all nature would display the monotony of a single colour, and our senses would never have known the charms of that variety which we now behold. 743. All bodies appear of the colour of that ray, or of a tint depending on the several rays which it reflects, while all the other rays are absorbed, or, in other terms, are not reflected. Black and white, therefore, in a philosophical sense, cannot be considered as colours, since the first arises from the absorption of all the rays, and the reflection of none, and the last is produced by the reflection of all the rays, and the absorption of none. But in all colours, or shades of colour, the rays only are reflected, of which the colour is composed. Thus, the colour of grass, and the leaves of plants, is green, because the surfaces of these substances reflect only the green rays, and absorb all the others. For Why are the colours of things different from those of the solar spec- trum 1 On what do the colours of bodies depend 1 Suppose all bodies reflected the same ray, what would be the consequence, in regard to colour 1 Why are not black, and white, considered as colours 1 Why is the colour of grass green"? 226 COLOURS. the same reason, the rose is red, the violet blue, and so of all coloured substances, every one throwing out the ray of its own colour, and absorbing all the others. 744. To account for such a variety of colours as we see in different bodies, it is supposed that all substances, when made sufficiently thin, are transparent, and consequently, that they transmit through their surfaces, or absorb, certain rays of light, while other rays are thrown back, or reflected, as above described. Gold, for example, may be beat so thin as to transmit some of the rays of light, and the same is true of several of the other metals, which are capable of being ham- mered into thin leaves. It is therefore most probable, that all the metals, could they be made sufficiently thin, would permit the rays of light to pass through them. Most, if not quite all mineral substances, though in the mass they may seem quite opaque, admit the light through their edges, when broken, and almost every kind of wood, when made no thinner than writing paper, becomes translucent. Thus we may safe- ly conclude, that every substance with which we are ac- quainted, will admit the rays of light, when made sufficiently thin. 745. Transparent colourless substances, whether solid or fluid, such as glass, water, or mica, reflect and transmit light of the same colour ; that is, the light seen through these bodies, and reflected from their surfaces, is white, This is true of all transparent substances under ordinary circum- stances; but if their thickness be diminished to a certain extent, these substances will both reflect and transmit coloured light of various hues, according to their thickness. Thus, the thin plates of mica, which are left on the fingers, after handling that substance, will reflect prismatic rays of various colours. 746. There is a degree of tenuity, at which transparent substances cease to reflect any of the coloured rays, but absorb, or transmit them all, in which case they become black. This may be proved by various experiments. If a soap bubble be closely observed, it will be seen that at first, the thickness is sufficient to reflect the prismatic rays from How is the variety of colours accounted for, by considering all bodies transparent ? What is said of the reflection of coloured light by transparent substances 1 What substance is mentioned, as illustrating this fact ? When is it said that transparent substances become black 7 How is it proved that fluids of extreme tenuity absorb all the rays and reflect none ? COLOURS. 227 all its parts, but as it grows thinner, and just before it bursts, there may be seen a spot on its top, which turns black, thus transmitting all the rays at that part, and re- flecting none. The same phenomenon is exhibited, when a film of air, or water, is pressed between two plates of glass. At the point of contact, or where the two plates press each other with the greatest force, there will be a black spot, while around this there may be seen a system of coloured rings. From such experiments, Sir Isaac Newton concluded, that air, when below the thickness of half a millionth of an inch, ceases to reflect light ; and also that water, when below the thickness of three eighths of a millionth of an inch, ceases to reflect light. But that both air and water, Avhen their thickness is in a certain degree above these limits, reflect all the coloured rays of the spectrum. 747. Now all solid bodies are more or less porous, having among their particles either void spaces, or spaces filled with some foreign matter, differing in density from the body itself, such as air or water. Even gold is not perfectly com- pact, since water can be forced through its pores. It is most probable, then, that the parts of the same body, differ- ing in density, either reflect, or transmit the rays of light, according to the size or arrangement of their particles ; and in proof of this, it is found that some bodies transmit the rays of one colour, and reflect that of another. Thus, the colour which passes through a leaf of gold is green, while that which it reflects is yellow. 748. From a great variety of experiments on this sub- ject, Sir Isaac Newton concludes that the transparent parts of bodies, according to the sizes of their transparent pores, reflect rays of one colour, and transmit those of another, for the same reason that thin plates, or minute particles of air, water, and some other substances, reflect certain rays, and absorb, or transmit others, and that this is the cause of all their colours. 749. In confirmation of the truth of this theory, it may be observed, that many substances, otherwise opaque, become transparent, by filling their pores with some transparent fluid. "What is the conclusion of Sir Isaac Newton, concerning the tenuity at which water and air ceases to reflect light 1 What is said of the porous nature of the solid bodies ? 228 ASTRONOMY. Thus, the stone called Hydrophane, is perfectly opaque when dry, but becomes transparent when dipped in uater; and common writing paper becomes translucent, after it has absorbed a quantity of oil. The transparency, in these cases, may be accounted for, by the different refractive powers which the water and oil possess, from the stone or paper, and in consequence of which the light is enabled to pass among their particles by refraction. ASTRONOMY. 750. Astronomy is that science which treats of the mo tions and appearances of the heavenly bodies ; accounts for the phenomena which these bodies exhibit to us ; and explains the laws by which their motions, or apparent motions, are regulated. Astronomy is divided into Descriptive, Physical, and Practical. Descriptive astronomy demonstrates the magnitudes, dis- tances, and densities of the heavenly bodies, and explains the phenomena dependant on their motions, such as the change of seasons, and the vicissitudes of day and night. Physical astronomy explains the theory of planetary motion, and the laws by which this motion is regulated and sustained. Practical astronomy details the description and use ot :is tronomical instruments, and develops the nature and appli- cation of astronomical calculations. The heavenly bodies are divided into three distinct classes, or systems, namely, the solar system, consisting of the sun, moon, and planets, the system of the fixed stars, and the system of the comets. THE SOLAR SYSTEM. 751. The Solar System consists of the sun, and twenty- nine other bodies, which revolve around him at various dis- tances, and in various periods of time. The bodies which revolve around the sun as a centre, are What is astronomy ? How is astronomy divided 1 What does des- criptive astronomy teach 1 What is the object of physical astronomy 1 What is practical astronomy 1 How are the heavenly bodies divided 1 Of what does the solar system consist 1 What are the bodies called, which revolve around the sun as a centre 7 ASTRONOMY, 229 called primary planets. Thus, the Earth, Venus, and Mars, are primary planets. Those which revolve around the pri- mary planets, are called secondary planets, moons, or satel- lites. Our moon is a secondary planet or satellite. The primary planets revolve around the sun in the fol- lowing order, and complete their revolutions in the follow- ing times, computed in our days and years. Beginning with that nearest to the sun, Mercury performs his revolu- tion in 87 days and 23 hours ; Venus, in 224 days, 17 hours ; the Earth, attended by the moon, in 365 days, 6 hours j Mars, in one year, 322 days ; Ceres, in 4 years, 7 months, and 10 days; Pallas, in 4 years, 7 months, and 10 days; Juno, in 4 years and 128 days ; Vesta, in 3 years, 66 days, and 4 hours; Jupiter, in 11 years, 315 days, and 15 hours; Saturn, in 29 years, 161 days, and 19 hours ; Herschel, in 83 years, 342 days, and 4 hours. 752. A year consists of the time which it takes a planet to perform one complete revolution through its orbit, or to pass once around the sun. Our earth performs this revolu- tion in 365 days, and therefore this is the period of our year. Mercury completes her revolution in 88 days, and therefore her year is no longer than 88 of our days. But the planet Herschel is situated at such a distance from the sun, that his revolution is not completed in less than about 84 of our years. The other planets complete their revolutions in va- rious periods of time, between these ; so that the time of these periods is generally in proportion to the distance of each planet from the sun. Ceres, Pallas, Juno, and Vesta, are the smallest of all the planets, and are called Asteroids. Besides the above enumerated primary planets, our sys- tem contains eighteen secondary planets, or moons. Of these, our Earth has one moon, Jupiter four, Saturn seven, and Herschel six. None of these moons, except our own, and one or two of Saturn's, can bs seen without a telescope. The seven other planets, so far as has been discovered, are entirely without moons. 753. All the planets move around the sun from west to What are those called, which revolve around these primaries as a centre 1 In what order are the several planets situated, in respect to the sun 1 How long does it take each planet to make its revolution around the sun 1 What is a year 1 What planets are called asteroids? How many moons does our system contain 1 Which of the planets are at- tended by moons, and how many has each 1 In what direction do the planets move around the sun? 30 230 ASTRONOMY. east, and in the same direction do the moons revolve around their primaries, with the exception of those of Herschel, which appear to revolve in a contrary direction. 754. The paths in which the planets move round the sun, and in which the moons move round their primaries, are called their orbits. These orbits are not exactly circular, as they are commonly represented on paper, but are elliptical, or oval, so that all the planets are nearer the sun, when in one part of their orbits, than when in another. In addition to their annual revolutions, some of the plan- ets are known to have diurnal, or daily revolutions, like our earth. The periods of these daily revolutions have been ascertained, in several of the planets, by spots on their sur- faces. But where no such mark is discernible, it cannot be ascertained whether the planet has a daily revolution or not, though this has been found to be the case in every instance where spots are seen, and, therefore, there is little doubt but all have a daily, as well as a yearly motion. 755. The axis of a planet is an imaginary line passing through its centre, and about which its diurnal revolution is performed. The poles of the planets are the extremities of this axis. 756. The orbits of Mercury and Venus are within that of the earth, and consequently they are called inferior plan- ets. The orbits of all the other planets are without, or ex- terior to that of the earth, and these are called superior planets. That the orbits of Mercury and Venus are within that of the earth, is evident from the circumstance, that they are never seen in opposition to the sun, that is, they never ap- pear in the west, when the jsun is in the east. On the con- trary, the orbits of all the other planets are proved to be out- side of the earth's, since these planets are sometimes seen in opposition to the sun. This will be understood by fig. 191, where suppose s to be the sun, m the orbit of Mercury or Venus, e the orbit of the earth, and^' that of Jupiter. Now, it is evident, that if What is the orbit of a planet ? What revolutions have the planets, besides their yearly revolutions'? Have all ,the planets diurnal revo- lutions'? How is it known that the planets have daily revolutions'? What is the axis of a planet 1 What is the pole of a planet 1 Which are the superior, and which the inferior planets 1 How is it proved that the inferior planets are within (he earth's orbit, and the superior ones without it 1 ASTRONOMY. 231 a spectator be placed any Fig. 191. where in the earth's or- oit, as at e, he may some- times see Jupiter in op- position to the sun, as at j, because then the spec- tator would be between Jupiter and the sun. But the orbit of Venus, being surrounded by that of the earth, she never can come in opposition to the sun, or in that part of the heavens opposite to him, as seen by us, because our earth never passes between her and the sun. 757. It has already been stated, that the orbits of the planets are elliptical, and that, consequently, these bodies are sometimes nearer the sun than at others. An ellipse, or oval, has two foci, and the sun, instead of being in the common centre, is always in the lower foci of their orbits. The orbit of a planet is represented by fig. 192, where a, d, b t e, is an ellipse, with its two foci, s and 0, the sun be- ing in the focus s, which is called^the lower focus. When the earth, or any other planet, revolv- ing around the sun, is in that part of its orbit near- est the sun, as at a, it is said to be in its perihelion ; and when in that part which is at the greatest distance from the sun, as at b, it is said to be in its aphelion. The line s, d, is the mean, or average distance of a planet's orbit from the sun. 758. ECLIPTIC. The planes of the orbits of all the planets pass through the centre of the sun. The plane of an orbit is an imaginary surface, passing from one extremity, or side of the orbit, to the other. If the rim of a drum Explain fig. 191, and show why the inferior planets never can be in opposition to the sun. What are the shapes of the planetary orbits? What is meant by perihelion 1 What is the plane of an orbit 1 232 ASTRONOMY. head be considered the orbit, its plane would be the parch ment extended across it, on which the drum is beaten. Let us suppose the earth's orbit to be such a plane, cut- ting the sun through his centre, and extending out on every side to the starry heavens ; the great circle so made, would mark the line of the ecliptic, or the sun's apparent path through the heavens. This circle is called the sun's apparent path, because the revolution of the earth gives the sun the appearance of pass- ing through it. It is called the ecliptic, because eclipses happen when the moon is in, or near, this apparent path. 759. ZODIAC. The Zodiac is an imaginary belt, or broad circle, extending quite around the heavens. The ecliptic divides the zodiac into two equal parts, the zodiac ex- tending 8 degrees on each side of the ecliptic, and therefore is 16 degrees wide. The zodiac is divided into 12 equal parts, called the signs of the zodiac. 760. The sun appears every year to pass around the grear circle of the ecliptic, and consequently, through the 12 con- stellations, or signs of the zodiac. But it will be seen, IP another place, that the sun, in respect to the earth, stand? still, and that his apparent yearly course through the heav ens is caused by the annual revolution of the earth around its orbit. Fig. 193. To understand the cause of this deception, let us suppose that s, fig. 193, is the sun, a b, a part of the circle of the ecliptic, and c d, a part of the earth's orbit. Now, if a spectator be placed at c, he will see the sun in that part of the eclip- tic marked by b, but when the earth moves in her annual revolution to d, the spectator will see the sun in that part of the heavens marked by a; so that the motion of the earth in one direction, will give the sun an apparent motion in the con- trary direction. Explain what is meant by the ecliptic. Why is the ecliptic called the sun's apparent path 1 What is the zodiac 1 How does the ecliptic divide the zodiac? How far does the zodiac extend on each side of the ecliptic ? Explain fig. 193, and show why the sun seems to pass through the ecliptic, when the earth only revolves around the sun. ASTRONOMY. 233 761. A sign, or constellation, is a collection of fixed stars, and, as \ve have already seen, the sun appears to move through the twelve signs of the zodiac every year. Now, the sun's place in the heavens, or zodiac, is found by his ap- parent conjunction, or nearness to any particular star in the constellation. Suppose a spectator at c, observes the sun to be nearly in a line with the star at b, then the sun would be near a particular star in a certain constellation. When the earth moves to d, the sun's place would assume another direction, and he would seem to have moved into another constellation, and near the star a. 762. Each of the 12 signs of the zodiac is divided into 30 smaller parts, called degrees ; each degree into 60 equal parts, called minutes, and each minute into 60 parts, called seconds. The division of the zodiac into signs, is of very ancient date, each sign having also received the name of some ani- mal, or thing, which the constellation, forming that sign, was supposed to resemble. It is hardly necessary to say, that this is chiefly the result of imagination, since the fig- ures made by the places of the stars, never mark the out- lines of the figures of animals, or other things. This is, however, found to be the most convenient method of finding any particular star at this day, for among astronomers, any star, in each constellation, may be designated by describing the part of the animal in which it is situated. Thus, by knowing how many stars belong to the constellation Leo, or the Lion, we readily know what star is meant by that which is situated on the Lion's ear or tail. 763. The names of the 12 signs of the zodiac are, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sa- gittarius, Capricorn, Aquarius, and Pisces. The common names, or meaning of these words, in the same order, are, the Ram, the Bull, the Twins, the Crab, the Lion, the Vir- gin, the Scales, the Scorpion, the Archer, the Goat, the Waterer, and the Fishes. What is a constellation, or sign"? How is the sun's apparent place in the heavens found 1 Into how many parts are the signs of the zo- diac divided, and what are these parts called 1 Is there any resem- blance between the places of the stars, and the figures of the animals after which they are called 1 Explain why this is a convenient method of finding any particular Ptar in a sign 1 What are the names of the twelve si^ns^ 20* 234 ASTRONOMY. The twelve signs of the zodiac, together with the sun, and the earth revolving around him, are represented at fig Fig. 194. 194. When the earth is at A, the sun will appear to be just entering the sign Aries, hecause then, when seen from the earth, he ranges towards certain stars at the beginning of that constellation. When the earth is at C, the sun will appear in the opposite part of the heavens, and therefore in the beginning of Libra. The middle line, dividing the cir- cle of the zodiac into equal parts, is the line of the ecliptic. 764. DENSITY OF THE PLANETS. Astronomers have no means of ascertaining whether the planets are composed of the same kind of matter as our earth, or whether their sur- faces are clothed with vegetables and forests, or not. They have, however, been able to ascertain the densities of se- veral of them^ by observations on their mutual attraction. Explain why the sun will be in the beginning of Aries, when the earth is at A. fig. 191* How has the density of the planets been as- certained 1 ASTRONOMY. 235 By density, is meant compactness, or the quantity of matter in a given space. When two bodies are of equal bulk, that which weighs most, has the greatest density. It was shown, while treating of the properties of bodies, that substances attract each other in proportion to the quantities of matter they contain. If, therefore, we know the dimensions of several bodies, and can ascertain the proportion in which they attract each other, their quantities of matter, or densi- ties, are easily found. 765. Thus, when the planets pass each other in their circuits through the heavens, they are often drawn a little out of the lines of their orbits by mutual attraction. As bodies attract in proportion to their quantities of matter, it is obvious that the small planets, if of the same density, will suffer greater disturbance from this cause, than the large ones. But suppose two planets, of the same dimen- sions, pass each other, and it is found that one of them is attracted twice as far out of its orbit as the other, then, by the known laws of gravity, it would be inferred, that one of them contained twice the quantity of matter that the other did, and therefore that the density of the one was twice that of the other. By calculations of this kind, it has been found, that the density of the sun is but a little greater than that of water, while Mercury is more than nine times as dense as water, having a specific gravity nearly equal to that of lead. The earth has a density about five times greater than that of the sun, and a little less than half that of Mercury. The densi- ties of the other planets seem to diminish in proportion as their distances from the sun increase, the density of Saturn, one of the most remote of planets, being only about one third that of water. THE SUN. 766. The sun is the centre of the solar system, and the great dispenser of heat and light to all the planets. Around the sun all the planets revolve, as around a common centre, he being the largest body in our system, and, so far as we know, the largest in the universe. What is meant by density ? In what proportion do bodies attract each other? How are the deputies of the planets ascertained 1 What is the density of the sun, of Mercury, and of the earth 1 In what pro- portions do the densities of the planets appear to diminish 1 Where is the place of the sun, in the solar system 1 236 ASTRONOMY. 767. The distance of the sun from the earth is 95 mil- lions of miles, and his diameter is estimated at 88,000 miles. Our globe, when compared with the magnitude of the sun, is a mere point, for his bulk is about thirteen hundred thousand times greater than that of the earth. Were the sun's centre placed in the centre of the moon's orbit, his circumference would reach two hundred thousand miles beyond her orbit in every direction, thus filling the whole space between us and the rnoon, and extending nearly as far beyond her as she is from us. A traveller, who should go at the rate of 90 miles a day, would perform a journey of nearly 33,000 miles in a year, and yet it would take such a traveller more than 80 years to go round the circumference of the sun. A body of such mighty dimensions, hanging on nothing, it is certain, must have emanated from an Al- mighty power. 768. The sun appears to move around the earth every 24 hours, rising in the east, and setting in the west. This mo- tion, as \vill be proved in another place, is only apparent, and arises from the diurnal revolution of the earth. 769. The sun, although he does not, like the planets, re- volve in an orbit, is, however, not without motion, having a revolution around his own axis, once in 25 days and 10 hours. Both the fact that he has such a motion, and the time in which it is performed, have been ascertained by the spots on his surface. If a spot is seen, on a revolving body, in a certain direction', it is obvious, that when the same spot is again seen, in the same direction, that the body has made one revolution. By such spots the diurnal revolutions of the planets, as well as the sun, have been determined. 770. Spots on the sun seem first to have been observed in the year 1611, since which time they have constantly at tracted attention, and have been the subject of investigation among astronomers. These spots change their appear- ance as the sun revolves on his axis, and become greater or less, to an observer on the earth, as they are turned to, or from him ; they also change in respect to real magnitude and number : one spot, seen by Dr. Herschel, was estimated What is the distance of the sun frorn the, earth 1 What is the di- ameter of the sun "? Suppose the centre of the sun and that of the moon's orbit to be coincident, how far would the sun extend beyond the moon's orbit 1 How is it proved that the sun has a motion around nis own axis! How often does the sun revolve] When were spots of the sun first observed 1 ASTRONOMY. 237 to be more than six times the size of our earth, being 50,000 miles in diameter. Sometimes forty or fifty spots may he seen at the same time, and sometimes only one. They are often so large as to be seen with the naked eye ; this was the case in 1816. 771. In respect to the nature and design of these spots, almost every astronomer has formed a different theory. Some have supposed them to be solid opaque masses of scoriae, floating in the liquid fire of the sun ; others, as satellites, revolving round him, and hiding his light from us; others, as immense masses, which have fallen on his disc, and which are dark coloured, because they have not yet become sufficiently heated. In two instances, these spots have been seen to burst into several parts, and the parts to fly in several directions, like a piece of ice thrown upon the ground. Others have supposed that these dark spots were the body of the sun, which became visible in conse- quence of openings through the fiery matter, with which he is surrounded. Dr. Herschel, from many observations with his great telescope, concludes, that the shining matter of the sun consists of a mass of phosphoric clouds, and that the spots on his surface are owing to disturbances in the equili- brium of this luminous matter, by which openings are made through it. There are, however, objections to this theory, as indeed there are to all the others, and at present it can only be said, that no satisfactory explanation of the cause of these spots has been given. 772. That the sun, at the same time that he is the great source of heat and light to all the solar worlds, may yet be capable of supporting animal life, has been the favourite doctrine of several able astronomers. Dr. Wilson first sug- gested that this might be the case, and Dr. Herschel, with his telescope, made observations which confirmed him in this opinion. The latter astronomer supposed that the func- tions of the sun, as the dispenser of light and heat, might be performed by a luminous, or phosphoric atmosphere, sur- rounding him at many hundred miles distance, while his solid nucleus might be fitted for the habitations of millions of reasonable beings. This doctrine is, however, rejected by most writers on the subject at the present day. What has been the difference in the number of spots observed 7 What was the size of the spot seen by Dr. Herschel 1 What has been advanced concerning the nature of these spots 1 Have they been ac- counted for satisfactorily "? What is said concerning the sun s being a habitable globe? ASTRONOMY. MERCURY. 773. Mercury, the planet nearest the sun, is about 3000 miles in diameter, and revolves around him, at the distance of 37 millions of miles. The period of his annual revolu- tion is 87 days, and he turns on his axis once in about 24 hours. The nearness of this planet to the sun, and the short time his fully illuminated disc is turned towards the earth, has prevented astronomers from making many observations on him. No signs of an atmosphere have been observed in this planet. The sun's heat at Mercury is about seven times greater than it is on the earth, so that water, if nature fol- lows the same laws there that she does here, cannot exist at Mercury, except in the state of steam. The nearness of this planet to the sun, prevents his being often seen. He may, however, sometimes be observed just before the rising, and a little after the setting of the sun. When seen after sunset, he appears a brilliant, twinkling star, showing a white light, which, however, is much ob- scured by the glare of twilight. When seen in the morn- ing, before the rising of the sun, his light is also obscured by the sun's rays. Mercury sometimes crosses the disc of the sun, or comes between the earth and that luminary, so as to appear like a small dark spot passing over the sun's face. This is called the transit of Mercury. VENUS. 774. Venus is the other planet, whose orbit is within that of the earth. Her diameter is about 8600 miles, being somewhat larger than the earth. Her revolution around the sun is performed in 224 days, at the distance of 68 millions of miles from him. She turns on her axis once in 23 hours, so that her day is a little shorter than ours. 775. Venus, as seen from the earth, is the most brilliant of all the primary planets, and is better known than any What is the diameter of Mercury, and what are his periods of annual and diurnal revolution 1 How great is the sun's heat at Me *- cury 1 At what times is Mercury to be seen 1 What is a transit of Mercury 1 Where is the orbit of Venus, in respect to that of the earth 1 What is the time of Venus' revolution round the sun 1 How often does she turn on her axis 1 ASTRONOMY. 239 nocturnal luminary except the moon. When seen through a telescope, she exhibits the phases or horned appearance of the moon, and her face is sometimes variegated with dark spots. Venus may often be seen in the day time, even when she is in the vicinity of the blazing light of the sun. A luminous appearance around this planet, seen at certain times, proves that she has an atmosphere. Some of her mountains are several times more elevated than any on our globe, being from 10 to 22 miles high. Venus sometimes makes a transit across the sun's disc, in the same manner as Mercury, already described. The transits of Venus oc- cur only at distant periods from each other. The last transit was in 1769, and the next will not happen until 1874. These transits have been observed by astronomers with the greatest care and accuracy, since it is by observations on them that the true distances of the earth and planets from the sun are determined. 776. When Venus is in that part of her orbit which gives her the appearance of being west of the sun, she rises before him, and is then called the morning star ; and when she appears east of the sun, she is behind him in her course, and is then called the evening star. These periods do not agree, either with the yearly revolution of the earth, or of Venus, for she is alternately 290 days the morning star, and 290 days the evening star. The reason of this is, that the earth and Venus move round the sun in the same direction, and hence her relative motion, in respect to the earth, is much slower than her absolute motion in her orbit. If the earth had no yearly motion, Venus would be the morning star one half of the year, and the evening star the other half. THE EARTH. 777. The next planet in our system, nearest the sun, is the Earth. Her diameter is 7912 miles. This planet re- volves around him in 365 days, 5 hours, and 48 minutes; and at the distance of 95 millions of miles. It turns round its own axis once in 24 hours, making a day and a night. The Earth's revolution around the sun is called its annual, or yearly motion, because it is performed in a year ; while What is said of the height of the mountains in Venus? On what account are the transits of Venus observed with great care 1 When is Venus the morning, and wh^n the evening star 7 ? How long is Venus the morning, and how long the evening star ] How long does it take the earth to revolve round the. sun 1 240 ASTRONOMY. the revolution around its own axis, is called the diurnal 01 daily motion, because it takes place every day. The figurt of the earth, with the phenomena connected with, her motion, will be explained in another place. THE MOON. 778. The Moon, next to the sun, is, to us, the most bril- liant and interesting of all the celestial bodies. Being the nearest to us of any of the heavenly orbs, and apparently designed for our use, she has been observed with great at- tention, and many of the phenomena which she presents, are therefore better understood and explained, than those of the other planets. While the earth revolves round the sun in a year, it is attended by the Moon, which makes a revolution round the earth once in 27 days, 7 hours, and 43 minutes. The dis- tance of the Moon from the earth is 240,000 miles, and her diameter about 2000 miles. Her surface, when seen through a telescope, appears diversified with hills, mountains, valleys, rocks, and plains, presenting a most interesting and curious aspect : but the explanation of these phenomena are reserved for another section. MARS. 779. The next planet in the solar system, is Mars, his orbit surrounding that of the earth. The diameter of this planet is upwards of 4000 miles, being about half that of the earth. The revolution of Mars around the sun is per- formed in nearly 687 days, or in somewhat less than two of our years, and he turns on his axis once in 24 hours and 40 minutes. His mean distance from the sun is 144 millions of miles, so that he moves in his orbit at the rate of about 55,000 miles in an hour. The days and nights, at this planet, and the different seasons of the year, bear a consider- able resemblance to those of the earth. The density of Mars is less than that of the earth, being only three times that of water. What is meant by the earth's annual revolution, and what by her diurnal revolution 1 Why are the phenomena of the moon better ex- plained than those of the other planets 7 In what time is a revolution of the moon about the earth performed ? What is the distance of the moon from the earth 1 What is the diameter of Mars 1 How much longer is a year at Mars than our yenr 1 What is his rate of motion in his orbit 1 ASTRONOMY. 241 Mars reflects a dull red light, by which he may be dis- tinguished from the other planets. His appearance through the telescope is remarkable for the great number and variety of spots which his surface presents. Mars has an atmosphere of great density and extent, as is proved by the dim appearance of the fixed stars, when seen through it. When any of the stars are seen nearly in a line with this planet, they give a faint, obscure light, and the nearer they approach the line of his disc, the fainter is their light, until the star is entirely obscured from the sight. This planet sometimes appears much larger to us than at others, and this is readily accounted for by his greater or less distance. At his nearest approach to the earth, hir, distance is only 50 millions of miles, while his greatest dis tance is 240 millions of miles ; making a difference in his distance of 190 millions of miles, or the diameter of the earth's orbit. The sun's heat at this planet is less than half that which we enjoy. To the inhabitants of Mars, our planet appears alternately as the morning and evening star, as Venus does to us. VESTA, JUNO, PALLAS, AND CERES 780. These planets were unknown until recently, and are therefore sometimes called the new planets. It has been mentioned, that they are also called Asteroids. 78 1. The orbit of Vesta is next in the solar system to that of Mars. This planet was discovered by Dr. Olbers, of Bremen, in 1807. The light of Vesta is of a pure white, and in a clear night she may be seen with the naked eye, appearing about the size of a star of the 5th or 6th magni- tude. Her revolution round the sun is performed in 3 years and 66 days, at the distance of 223 millions of miles from him. 782. Juno was discovered by Mr. Harding, of Bremen, in 1804. Her mean distance from the sun is 253 millions of miles. Her orbit is more elliptical than that of any other planet, and, in consequence, she is sometimes 127 millions of miles nearer the sun than at others. This planet com- What is his appearance through the telescope 1 How is it proved that Mars has an atmosphere of great density 1 Why does Mara sometimes appear to us larger than at others 7 How great is the sun's heat at Mars 1 Which are the new planets, or asteroids ? When was Vesta discovered 1 What is the period of Vesta's annual, revolution 1 When was Juno discovered 1 What is her distance from the sunl 21 242 ASTRONOMY. pletes its annual revolution in 4 years and about 4 months, and revolves round its axis in 27 hours. Its diameter is 1400 miles. 783. Pallas was also discovered by Dr. Olbers, in 1802. Its distance from the sun is 226 millions of miles, and its periodic revolution round him, is performed in 4 years and 7 months. 784. Ceres was discovered in 1801, by Piazzi, of Paler- mo. This planet performs her revolution in the same time as Pallas, being 4 years and 7 months. Her distance from the sun 260 millions of miles. According to Dr. Herschel, this planet is ^nly about 160 miles in diameter. JUPITER. 785. Jupiter is 89,000 miles in diameter, and performs his annual revolution once in about 1 1 years, at the distance of 490 millions of miles from the sun. This is the largest planet in the solar system, being about 1400 times larger than the earth. His diurnal revolution is performed in nine hours and fifty-five minutes, giving his surface, at the equator, a motion of 28,000 miles per hour. This motion is about twenty times more rapid than that of our earth at the equator. 786. Jupiter, next to Venus, is the most brilliant of the planets, though the light and heat of the sun on him is near- ly 25 times less than on the earth. This planet is distinguished from all the others, by an ap- pearance resembling bands, which extend across his disc Fig. 175. What is the period of her revolution, and what her diameter 1 What is said of Pallas and Ceres 1 What is the diameter of Jupiter 1 What is his distance from the sun! What is the period of Jupiter's diurnal revolution ? What is the sun's heat and light at Jupiter, when compared with that of the earth 1 For what is Jupiter particularly dis- tinguished? ASTRONOMY. 243 These are termed belts, and are variable, both in respect to number and appearance. Sometimes seven or eight are seen, several of which extend quite across his face, while others appear broken, or interrupted. These bands, or belts, when the planet is observed through a telescope, appear as represented in fig. 195. This ap- pearance is much the most common, the belts running quite across the face of the planet in parallel lines. Sometimes, however, his aspect is quite different from this, for in 1780, Dr. Herschel saw the whole disc of Jupiter covered with small curved lines, each of which appeared broken, or in- terrupted, the whole having a parallel direction across his disc, as in fig. 196. Fig. 196. Different opinions have been advanced by astronomers re- specting the cause of these appearances. By some they have been regarded as clouds, or as openings in the atmosphere of the planet, while others imagine that they are the marks of great natural changes, or revolutions, which are perpet- ually agitating the surface of that planet. It is, however, most probable, that these appearances are produced by the agency of some cause, of which we, on this little earth, must always be entirely ignorant. 787. Jupiter has four satellites, or moons, two of which are sometimes seen with the naked eye. They move round, and attend him in his yearly revolution, as the moon does our earth. They complete their revolutions at different pe- riods, the shortest of which is less than two days, and the longest seventeen days. Is the appearance of Jupiter's belts always the same, or do they change? What is said of the cause of Jupiter's belted appearance? How many moons has Jupiter, and what are the periods of their rev- olutions 1 244 ASTRONOMY. These satellites often fall into the shadow of their pri- mary, in consequence of which they are eclipsed, as seen from the earth. The eclipses of Jupiter's moons have been observed with great care by astronomers, because they have been the means of determining the exact longitude of places, and the velocity with which light moves through space. How longitude is determined by these eclipses, cannot be explained or understood at this place, hut the method by which they become the means of ascertaining the velocity of light, may be readily comprehended. An eclipse of one of these satellites appears, by calculation, to take place six- teen minutes sooner, when the earth is in that part of hei orbit nearest to Jupiter, than it does when the earth is in that part of her orbit at the greatest distance from him. Hence, light is found to be sixteen minutes in crossing the earth's orbit, and as the sun is in the centre of this orbit, 01 nearly so, it must take about 8 minutes for the light to come from him to us. Light, therefore, passes at the velocity of 95 millions of miles, our distance from the sun, in about 8 minutes, which is nearly 200 thousand miles in a second. SATURN. 788. The planet Saturn revolves round the sun in a pe- riod of about 30 of our years, and at the distance from hiir of 900 millions of miles. His diameter is 79,000 miles, making his bulk nearly nine hundred times greater than that of the earth, but notxvith standing this vast size, he re- volves on his axis once in about ten hours. Saturn, there- fore, performs upwards of 25,000 diurnal revolutions in one> of his years, and hence his year consists of more than 25,006 days; a period of time equal to more than 10,000 of our days. On account of the remote distance of Saturn from the sun, he receives only about a 90th part of the heat and light which we enjoy on the earth. But to compensate, in some degree, for this vast distance from the sun, Saturn has seven moons, which revolve round him at different distances, and at various periods, from 1 to 80 days. What occasions the eclipses of Jupiter's moons 1 Of what use are these eclipses to astronomers 7 How is the velocity of light ascertain- ed by the eclipses of Jupiter's satellites 1 What is the time of Saturn's periodic revolution round the sun 1 What is his distance from the sun 1 What his diameter"? What is the period of his diurnal revolution? How many days make a year at Saturn ? How many moons has Saturn 1 ASTRONOMY. 245 789. Saturn is distinguished from the other planets by his ring, as Jupiter is by his belt. When this planet is viewed through a telescope, he appears surrounded by an immense luminous circle, which is represented by fig. 197. There are indeed two luminous circles, or rings, one within the other, with a dark space between them, so that they do not appear to touch each other. Neither does the inner ring touch Fig. 197. the body of the planet, there be- ing, by estima- tion, about the distance of thirty thousand miles between them. The external circumference of the outer ring is 640,000 miles, and its breadth from the outer to the inner circumference, 7,200 miles, or nearly the diameter of our earth. The dark space, between the two rings, or the interval between the inner and the outer ring, is 2,800 miles. This immense appendage revolves round the sun with the planet, performs daily revolutions with it, and, accord- ing to Dr. Herschel, is a solid substance, equal in density to the body of the planet itself. 790. The design of Saturn's ring, an appendage so vast, and so different from any thing presented by the other plan- ets, has always been a matter of speculation and inquiry among astronomers. One of its most obvious uses appears to be that of reflecting the light of the sun on the body of the planet, and possibly it may reflect the heat also, so as in some degree to soften the rigour of so inhospitable a climate. 791. As this planet revolves around the sun, one of its sides is illuminated during one half of the year, and the other side during the other half; so that, as Saturn's year is equal to thirty of our years, one of his sides will be en- lightened and darkened, alternately, every fifteen years, as the poles of our earth are alternately in the light and dark every year. Fig. 198 represents Saturn as seen by an eye, placed at How is Saturn particularly distinguished from all the other planets ? What distance is there between the body of Saturn and his inner ring? What distance is there between his inner and outer ring 1 What is the circumference of the outer ring 7 How long is one of Saturn's sides alternately in the light and dark? 21* ASTRONOMY. right angles to the plane of his ring. When seen from the earth, his position is al- Fig. 198. ways oblique, as repre-j sented by fig. 198. The inner white circle,! represents the body of the planet, enlightened by the sun. The dark circle next! to this, is the unenlighten- ed space between the body I of the planet and the in- ner ring, being the dark expanse of the heavens beyond the planet. The two white circles are the rings of the planet, with the dark space between' them, which also is the dark expanse of the heavens. HERSCHEL. 792. In consequence of some inequalities in the motions of Jupiter and Saturn, in their orbits, several astronomers had suspected that there existed another planet beyond the orbit of Saturn, by whose attractive influence these irregu- larities were produced. The conjecture was confirmed by Dr. Herschel, in 1781, who in that year discovered the planet, which is now generally known by the name of its discoverer, though called by him Georgium sidus. The orbit of Herschel is beyond that of Saturn, and at the dis- tance of 1800 millions of miles from the sun. To the naked eye this planet appears like a star of the sixth mag- nitude, being, with the exception of some of the comets, the most remote body, so far as is known, in the solar system. 793. Herschel completes his revolution round the sun in nearly 84 of our years, moving in his orbit at the rate of 15,000 miles in an hour. His diameter is 35,000 miles : so that his bulk is about eighty times that of the earth. The light and heat of the sun at Herschel, is about 360 times less than it is at the earth, and yet it has been found, by cal- In what position is Saturn represented by fig. 198 1 What circum- stance led to the discovery of Herschel 1 In what year, and by whom, was Herschel discovered 1 What is the distance of Herschel from the sun? In what period is his revolution round the sun performed 1 What is the diameter of Herschel ? What is the quantity of light and heat at Herschel, when compared with that of the earth ? ASTRONOMY. 24T eulation, that this light is equal to 248 of our full moons, a striking proof of the inconceivable quantity of light emitted by the sun. This planet has six satellites, which revolve round him at various distances, and in different times. The period of some of these have been ascertained, while those of the others remain unknown. Fig. 199. HerscfKl 794. Relative situations of the Planets. Having now given a short account of each planet composing the solar system, the relative situation of their several orbits, with the exception of those of the Asteroids, are shown by fig. 199. In the figure, the orbits are marked by the signs of each planet, of which the first, or that nearest the sun, is Mer- cury, the next Venus, the third the Earth, the fourth Mars; then come those of the Asteroids, then Jupiter, then Saturn, and lastly Herschel. 248 ASTRONOMY. 795. Comparative dimensions of the Planets. The com- parative dimensions of the planets are delineated at fig. 200. Fiff. 200. MOTIONS OF THE PLANETS. 796. It is said, that when Sir Isaac Newton was near de- monstrating the great truth, that gravity is the cause which keeps the heavenly bodies in their orbits, he became so agi- tated with the thoughts of the magnitude and consequences of his discovery, as to be unable to proceed with his demon- strations, and desired his friend to finish what the intensity of his feelings would not allow him to complete. We have seen, in a former part of this work, that all un- disturbed motion is straight forward, and that a body pro- jected into open space, would continue, perpetually, to move in a right line, unless retarded or drawn out of this course by some external cause. 797. To account for the motions of the planets in their orbits, we will suppose that the earth, at the time of its cre- ation, was thrown by the hand of the Creator into open space, the sun having been before created and fixed in his present place. 798. Under Compound Motion, it has been shown, that when a body is acted on by two forces perpendicular to each other, its motion will be in a diagonal line between the di- rection of the two forces. But we will again here suppose that a ball be moving in the line m x, fig. 201, with a given force, and that Suppose a body to be acted on by two forces perpendicular to each other, in what direction will it move? ASTRONOMY. 249 Fig. 201. another force half as great should strike it in the direc- tion of w, the ball would then describe the diagonal of a parallelogram, whose length would be just equal to twice its breadth, and the line of the ball would be straight, because it would obey the impulse and direction of these two forces only. Fig. 202. Now let a, fig. 202, represent the earth, and S the sun ; and suppose the earth to be moving forward, in the line from a to b, and to have arrived at a, with a ve- locity sufficient, in a given time, and without disturbance, to have car- ried it to b. But at the point a, the sun, S, acts upon the earth with his attractive power, and with a force which would draw it to c, in the same space of time that it would otherwise have gone to b. Then the earth, instead of passing to b, in a straight line, would be drawn down to d, the diagonal of the parallel- ogram a, b, d, c. The line of direction, in fig. 201, is straight, because the body moved obeys only the direction of the two forces, but it is curved from a to d, fig. 202, in consequence of the continued force of the sun's attraction, which produces a constant deviation from a right line. When the earth arrives at d, still retaining its projectile or centrifugal force, its line of direction would be towards n, but while it would pass along to n without disturbance, the attracting force of the sun is again sufficient to bring it to e, in a straight line, so that, in obedience to the two impulses, it again describes the curve to o. 799. It must be remembered, in order to account for the circular motions of the planets, that the attractive force of the sun is not exerted at once, or by a single impulse, as is Why does the ball, fig. 201, move in a straight linel Why does the earth, fig. 202, move in a curved line 7 Explain fig. 202, and show how the two forces act to produce a circular line of motion 1 250 ASTRONOMY. the case with the cross forces, producing a straight line, nut that this force is imparted by degrees, and is constant. It therefore acts equally on the earth, in all parts of the course from a to d, and from d to o. From 0, the earth having the same impulses as before, it moves in the same curved or cir- cular direction, and thus its motion is continued perpetually. 800. The tendency of the earth to move forward in a straight line, is called the centrifugal force, and the attrac- tion of the sun, by which it is drawn downwards, or towards a centre, is called its centripetal force, and it is by these two forces that the planets are made to perform their constant revolutions around the sun. 801. In the above explanation, it has been supposed that the sun's attraction, which constitutes the earth's gravity, was at all times equal, or that the earth was at an equal distance from the sun, in all parts of its orbit. But, as heretofore ex- plained, the orbits of all the planets are elliptical, the sun oeing placed in the lower focus of the eclipse. The sun's Fig. 203. attraction is, therefore, stronger in some parts of their orbits than in others, and for this rea- son their velocities are greater at some periods of their revolutions than at others. To make this under- stood, suppose, as before, that the centrifugal and centripetal forces so bal- ance each other, that the earth moves round the circular orbit a e b, fig. ^^^ 203, until it comes to the point e ; and at this point, let us suppose, that the gravitating force is too strong for the force of projection, so that the earth, instead of continuing its former direction towards b, is attract- ed by the sun s, in the curve e c. When at c, the line of the earth's projectile force, instead of tending to carry it farther from the sun, as would be the case, were it revolving in a cir- What is the projectile force of the earth called? What is the attract ive force of the sun, which draws the earth towards him, called! Ex- plain fig. 203, and show the reason why the velocity is increased from c to d, and why it is not retarded from d to g 1 ASTRONOMY. 251 cular orbit, now tends to draw it still nearer to him, so that at this point, it is impelled by both forces towards the sun. From <;, therefore, the force of gravity increasing in proportion as the square of the distance between the sun and earth dimin- ishes, the velocity of the earth will be uniformly accelerated, until it arrives at the point nearest the sun, d. At this part of us orbit, the earth will have gained, by its increased velocity, so much centrifugal force, as to give it a tendency to over- come the sun's attraction, and to fly off in the line d o. But the sun's attraction being also increased by the near approach of the earth, the earth is retained in its orbit, notwithstand- ing its increased centrifugal force, and it therefore passes through the opposite part of its orbit, from d to g, at the same distance from him that it approached. As the earth passes from the sun, the force of gravity tends continually to retard its motion, as it did to increase it while approach- ing him. But the velocity it had acquired in approaching the sun, gives it the same rate of motion from d to g, that it had from c to d. From g, the earth's motion is uniformly retarded, until it again arrives at e, the point from which it commenced, and from whence it describes the same orbit, by virtue of the same forces, as before. The earth, therefore, in its journey round the sun, moves at very unequal velocities, sometimes being retarded, and then again accelerated, by the sun's attraction. 802. It is an interesting circumstance, respecting the Fig. 204. motions of the planets, that if the contents of their or- bits be divided into une- qual triangles, the acute angles of which centre at the sun, with the line of the orbit for their bases, the centre of the planet will pass through each of these bases in equal times. This will be understood by fig. 204, the elliptical circle being supposed to be the earth's orbit, with the sun, s, in one of the foci. Now the spaces 1, 2, 3, &c. though of different What is meant by a planet's passing through equal spaces in equal times ? 252 ASTRONOMY. shapes, are of the same dimensions, or contain the same juantity of surface. The earth, we have already seen, in its journey round the sun, describes an ellipse, and moves more rapidly in one part of its orhit than in another. But whatever may be its actual velocity, its comparative motion is through equal areas in equal times. Thus its centre passes from E to C, and from C to A, in the same period of time, and so of all the other divisions marked in the figure. If the figure, therefore, be considered the plane of the earth's orbit, divided in 12 equal areas, answering to the 12 months of the year, the earth will pass through the same areas in every month, but the spaces through which it passes will be increased, during every month, for one half the year, and diminished, during every month, for the other half. 803. The reason why the planets, when they approach near the sun, do < not fall to him, in consequence of his in- creased attraction, and why they do not fly off into open space, when they recede to the greatest distance from him, may be thus explained. 804. Taking the earth as an example, we have shown that when in the part of her orbit nearest the sun, her velo- city is greatly increased by his attraction, and that conse- quently the earth's centrifugal force is increased in propor- tion. As an illustration of this, we know that a thread which will sustain an ounce ball, when whirled round in the air, at the rate of 50 revolutions in a minute, would be broken, were these revolutions increased to the number of 60 or 70 in a minute, and that the ball would then fly off jn a straight line. This shows that when the motion of a re- volving body is increased, its centrifugal force is also in- creased. Now, the velocity of the earth increases in an inverse proportion, as its distance from the sun diminishes. and in proportion to the increase of velocity is its centrifugal force increased ; so that, in any other part of its orbit, except when nearest the sun, this increase of velocity would carry the earth away from its centre of attraction. But this in- crease of the earth's velocity is caused by its near approach vO the sun, and consequently the sun's attraction is increased, as well as the earth's velocity. In other terms, when the How is it shown, that if the motion of a revolving body is increas- ed, its projectile force is also increased 1 By what force is the earth's ve- Jocitv increased, as it approaches the sun 1 When the earth is nearest the sun, why does it not fall to him 7 When the earth's centrifugal forct is greatest, what prevents its flying to the sun 1 EARTH. 253 centrifugal force is increased, the centripetal force is in- creased in proportion, and thus, while the centrifugal force prevents the earth from falling to the sun, the centripetal force prevents it from moving off in a straight line. 805. When the earth is in that part of its orbit most distant from the sun, its projectile velocity being retarded by the counter force of the sun's attraction, becomes greatly diminished, and then the centripetal force becomes stronger than the centrifugal, and the earth is again brought back by the sun's attraction, as before, and in this manner its motion goes on without ceasing. It is supposed, as the planets move through spaces void of resistance, that their centrifugal forces remain the same as when they first emanated from the hand of the Creator, and that this force, without the influence of the sun's attraction, would carry them forward into infinite space. THE EARTH. 806. It is almost universally believed, at the present day, that the apparent daily motion of the heavenly bodies from east to west, is caused by the real motion of the earth from west to east, and yet there are comparatively few who have examined the evidence on which this belief is founded. For this reason, we will here state the most obvious, and to a common observer, the most convincing proofs of the earth's revolution. These are, first, the inconceivable velocity of the heavenly bodies, and particularly the fixed stars around the earth, if she stands still. Second, the fact, that all as- tronomers of the present age agree that every phenomenon which the heavens present, can be best accounted for, by supposing the earth to revolve. Third, the analogy to be drawn from many of the other planets, which are known to revolve on their axis ; and fourth, the different lengths of days and nights at the different planets, for did the sun re- volve about the solar system, the days and nights at many of the planets must be of similar lengths. 807. The distance of the sun from the earth being 95 millions of miles, the diameter of the earth's orbit is twice its distance from the sun, and, therefore, 190 millions of miles. Now, the diameter of the earth's orbit, when seen from the nearest fixed star, is a mere point, and were the What are the most obvious and convincing proofs that the earth re- volves on its axis 1 Were the earth's orbit a solid mass, could it be seen by us, at the distance of the fixed stars 1 22 ' 254 EARTH, orbit a solid mass of opaque matter, it could not be seen, with such eyes as ours, from such a distance. This is known by the fact, that these stars appear no larger to us, even when our sight is assisted by the best telescopes, when the earth is in that part of her orbit nearest them, than when at the greatest distance, or in the opposite part of her orbit. The approach, therefore, of 190 millions of miles towards the fixed stars, is so small a part of their whole distance from us, that it makes no perceptible difference in their ap- pearance. Now, if the earth does not turn on her axis once in 24 hours, these fixed stars must revolve around the earth at this amazing distance once in 24 hours. If the sun passes around the earth in 24 hours, he must travel at the rate of nearly 400,000 miles in a minute ; but the fixed stars are at least 400,000 times as far beyond the sun, as the sun is from us, and, therefore, if they revolve around the earth, must go at the rate of 400,000 times 400,000 miles, that is, at the rate of 160,000,000,000, or 160 billions of miles in a minute ; a velocity of which we can have no more concep- tion than of infinity or eternity. 808. In respect to the analogy to be drawn from the known revolutions of the other planets, and the different lengths of days and nights among them, it is sufficient to state, that to the inhabitants of Jupiter, the heavens appear to make a revolution in about 10 hours, while to those of Venus, they appear to revolve once in 23 hours, and to the inhabitants of the other planets a similar difference seems to take place, depending on the periods of their diurnal re- volutions. Now, there is no more reason to suppose that the heavens revolve round us, than there is to suppose that the'y revolve around any of the other planets, since the same apparent revolution is common to them all; and as we know that the other planets, at least many of them, turn on their axis, and as all the phenomena presented by the earth, can be accounted for by such a revolution, it is folly to conclude otherwise. Suppose the earth stood still, how fast must the sun move to go round it in 24 hours 1 At what rate must the fixed stars move to go round the earth in 24 hours 1 If the heavens appear to revolve every 10 hours at Jupiter, and every 24 hours at the earth, how can this dif- ference be accounted for, if they revolve at all 1 Is there any more reason to believe that the sun revolves round the earth, than round any of the other planets 1 How can all the phenomena of the heavens be accounted for, if they do not revolve 1 EARTH. 255 Circles and Divisions of the JEarth. 809. It will be necessary for the pupil to retain in his memory the names and directions of the following lines, or circles, by which the earth is divided into parts. These lines it must be understood, are entirely imaginary, there being no such divisions marked by nature on the earth's surface. They are, however, so necessary, that no accurate descrip- tion of the earth, or of its position with respect to the hea venly bodies, can be conveyed without them. The earth, whose diameter is 7912 miles, is represented by the globe, or sphere, fig. 205. The straight line passing thro' its cen- tre, and about which jj it turns, is called its axis, and the two ex- tremities of the axis are the poles of the earth, A being the north pole, and B the south pole. The line C D, crossing the axis, passes quite round the earth, and divides it into two equal parts. This is called the equinoctial line, or the equator. That part of the earth, situated north of this line, is called the northern, hemisphere, and that part south of it, the southern hemi- sphere. The small circles E F, and G H, surrounding or including the poles, are called the polar circles. That sur- rounding the north pole is called the arctic circle, and that sur- rounding the south, the antarctic circle. Between these cir- cles, there is, on each side of the equator, another circle, which marks the extent of the tropics towards the north and south, from the equator. That to the north of the equator, I K, is called the tropic of Cancer, and that to the south, L M, the tropic of Capricorn. The circle L K, extending What is the axis of the earth 7 What are the poles of the earth ? What is the equator 1 Where are the northern and southern hemis- pheres 1 What are the polar circles 1 Which is the arctic, and which the antarctic circle 7 Where is the tropic of Cancer and where the tropic of Capricorn 1 256 EARTH. obliquely across the two tropics, and crossing the axis of the earth, and the equator at their point of intersection, is called the ecliptic. This circle, as already explained, belongs rather to the heavens than the earth, being an imaginary extension of the plane of the earth's orbit in every direction towards the stars. The line in the figure, shows the com- parative position or direction of the ecliptic in respect to tht, equator, and the axis of the earth. The lines crossing those already described, and meeting at the poles of the earth, are called meridian lines, or mid- day lines, for when the sun is on the meridian of a place, it is the middle of the day at that place, and as these lines ex- tend from north to south, the sun shines on the whole length of each, at the same time, so that it is 12 o'clock, at the same time, on every place situated on the same meridian. The spaces on the earth, between the lines extending from east to west, are called zones. That which lies between the tropics, from M to K, and from I to L, is called the torrid zone, because it comprehends the hottest portion of the earth. The spaces which extend from the tropics, north and south, to the polar circles, are called temperate zones, because the climates are temperate, and neither scorched with heat, like the tropics, nor chilled with cold, like the frigid zones. That lying north of the tropic of Cancer, is called the north temperate zone, and that south of the tropic of Capricorn, the south temperate zone. The spaces included within the polar circles, are called the frigid zones. The lines which divide the globe into two equal parts, are called the great circles ; these are the ecliptic and the equator. Those dividing the earth into smaller parts are called the lesser circles ; these are the lines dividing the tropics from the temperate zones, and the temperate zones from the frigid zones, &c. 810. Horizon. The horizon is distinguished into the. sensible and rational. The sensible horizon is that portion of the surface of the earth which bounds our vision, or the circle around us, where the sky seems to meet the earth. When the sun rises, he appears above the sensible horizon, and when he sets, he sinks below it. The rational horizon Wha>is the ecliptic 7 What are the meridian lines'? Or ^.lav part of the earth is the torrid zone 1 How are the north ar/ '.outt temperate zones bounded? Where are the frigid zones *? "WKca ar/ the great, and which the lesser circles of the earth 1 How Is the sensi ble horizon distinguished from the rational 7 EARTH. '457 is ad imaginary line passing through the centre of the earth, and dividing it into two equal parts. 811. Direction of the Ecliptic. The ecliptic, (758) we have already seen, is divided into 360 equal parts, called degrees. All circles, however large or small, are divided into degrees, minutes, and seconds, in the same manner as the ecliptic. 812. The axis of the ecliptic is an imaginary line pass- ing through its centre and perpendicular to its plane. The extremities of this perpendicular line, are called the poles of the ecliptic. If the ecliptic, or great plane of the earth's orbit, be con- sidered on the horizon, or parallel with it, and the line of the earth's axis be inclined to the axis of this plane, or the axis of the ecliptic, at an angle of 23| degrees, it will repre- sent the relative positions of the orbit, and the axis of the earth. These positions are, however, merely relative, for if the position of the earth's axis be represented perpendicu- lar to the equator, as A B, fig. 205, then the ecliptic will cross this plane obliquely, as in that figure. But when the earth's orbit is considered as having no inclination, its axis, of course, will have an inclination, to the axis of the ecliptic, of 23 degrees. As the tfrbits of all the other planets are inclined to the ecliptic, perhaps it is the most natural and convenient method to consider this as a horizontal plane, with the equator in- clined to it, instead of considering the equator on the plane of the horizon, as is sometimes done. 813. Inclination of the Earth 1 s axis. The inclination of the earth's axis to the axis of its orbit never varies, but always makes an angle with it of 23 degrees, as it moves round the sun. The axis of the earth is therefore always parallel with itself. That is, if a line be drawn through the centre of the earth, in the direction of its axis, and ex- tended north and south, beyond the earth's diameter, the line so produced will always be parallel to the same line, or any number of lines, so drawn, when the earth is in different parts of its orbit. How are circles divided 1 What is the axis of the ecliptic ? What are the poles of the ecliptic 7 How many degrees is the axis of the earth mclined to that of the ecliptic 1 What is said concerning the relative positions of the earth's axis and the plane of the ecliptic? Are the jrbits of the other planets parallel to the earth's orbit, or inclined to itl What is meant by the earth's axis being parallel to itself 1 23* 258 814. Suppose a rod to be fixed V.j the flat surface of a table, and so inclined as to make e. protector, unless it reaches the moist earth, or ends in water connected, with the earth. Conductors of cop- per may be three rourths of an inch in diameter, but those of iron should be 5tt least an inch in diameter. In large build- ings, complete imrtection requires many lightning rods, or that they should be elevated 10 a height above the building in proportion to the smailness of their numbers, for modern experiments have proved that a rod only protects a circle around it, the radius of which is equal to twice its length above the building. At what times does the atmosphere contain most electricity 7 How are the different electrical states of the atmosphere ascertained 1 Who first discovered that electricity and lightning are the same 7 What phenomena are mentioned which belong in common to electricity and lightning 1 How may buildings be protected from the effects of lightning 1 Which is the best conductor, iron or copper 1 What circumstances are neces- sary, that the rod may be relied on as a protector 7 316 MAGNETISM. 934. Some fishes have the power of giving electrical shocks, the effects of which are the same as those obtained by the friction of an electric. The best known of these are the Torpedo, the Gymnotus electricus, and the Silurus elec- tricus. 935. The torpedo, when touched with both hands at the same time, the one hand on the under, and the other on the upper surface, will give a shock like that of the Leyden vial ; which shows that the upper and under surfaces of the electric organs are in the positive and negative state, like the inner and outer surraces of the electrical jar. 936. The gymnotus electricus, or electrical eel, possesses all the electrical powers of the torpedo, but in a much higher degree. When small fish are placed in the water with this animal, they are generally stunned, and sometimes killed, by his electrical shock, after which he eats them if hungry. The strongest shock of the gymnotus will pass a short dis- tance through the air, or across the surface of an electric, from one conductor to another, and then there can be per- ceived a small but vivid spark of electrical fire ; particularly if the experiment be made in the dark. MAGNETISM. 937. The native Magnet, or Loadstone, is an ore of iron, which is found in various parts of the world. Its colour is iron black ; its specific gravity from 4 to 5, and it is some- times found in crystals. This substance, without any pre- paration, attracts iron and steel, and when suspended by a string, will turn one of its sides towards the north, and another towards the south. 938. It appears that an examination of the properties of this species of iron ore, led to the important discovery of the magnetic needle, and subsequently laid the foundation for the science of Magnetism ; though at the present day magnets are made without this article. 939. The whole science of magnetism is founded on the fact, that pieces of iron or steel, after being treated in a certain manner, and then suspended, will constantly turn one of their ends towards the north, and consequently the other towards What animals have the power of giving electrical shocks 1 Is this electricity supposed to differ from that obtained by art 1 How must the hands be applied, to take the electrical shock of these animals? What is the native magnet, or loadstone"? What are the properties of the loadstone ? On what is the whole subject of magnetism founded 1 MAGNETISM. 317 .he south. The same property has been more recently- proved to belong to the metals mckel and cobalt, though with much less intensity. 940. The poles of a magnet are those parts which possess the greatest power, or in which the magnetic virtue seems to be concentrated. One of the poles points north, and the other south. The magnetic meridian is a vertical circle in the heavens, which intersects the horizon at the points to which the magnetic needle, when at rest, directs itself. 941. The axis of a magnet, is a right line which passes from one of its poles to the other. 942. The equator of a magnet, is a line perpendicular to its axis, and is at the centre between the two poles. 943. The leading properties of the magnet are 'the fol- lowing. It attracts iron and steel, and when suspended so as to move freely, it arranges itself so as to point north and south : this is called the polarity of the magnet. When the south, pole of one magnet is presented to the north pole of another, they will attract each other : this is called magnetic attraction. But if the two north or two south poles be brought together, they will repel each other, and this is called magnetic repulsion. When a magnet is left to move freely, it does not lie in a horizontal direction, but one pole inclines downwards, and consequently the other is elevated above the line of the horizon. This is called the dipping, or inclination of the magnetic needle. Any magnet is ca- pable of communicating its own properties to iron or steel, and this, again, will impart its magnetic virtue to another piece of steel, and so on indefinitely. 944. If a piece of iron or steel be brought near one of the poles of a magnet, they will attract each other, and if suffered to come into contact, will adhere so as to require force to separate them. This attraction is mutual ; for the iron attracts the magnet with the same force that the mag- net attracts the iron. This may be proved, by placing the iron and magnet on pieces of wood floating on water, when they will be seen to approach each other mutually. What other metals besides iron possess the magnetic property 1 What are the poles of a magnet 1 What is the axis of a magnet 1 What is the equator of a magnet 1 What is meant by the polarity of a mag- net 7 When do two magnets attract, and when repel each other"? What is understood by the dipping of the magnetic needle 1 How is it nroved that the. iron attracts the magnet with the same force that the magnet attracts the iron 1 27* 318 MAGNETISM. 945. The force of magnetic attraction varies with the dip- tance in the same ratio as the force of gravity j the attract- ing force being inversely as the square of the distance be- tween the magnet and the iron. 946. The magnetic force is not sensibly affected by the interposition of any substance except those containing iron, or steel. Thus, if two magnets, or a magnet and piece of iron, attract each other with a certain force, this force will be the same, if a plate of glass, wood, or paper, be placed be- tween them. Neither will the force be altered, by placing the two attracting bodies under water, or in the exhausted receiver of an air pump. This proves that the magnetic in- fluence passes equally well through air, glass, wood, paper, water, and a vacuum, 947. Heat weakens the attractive power of the magnet, and a white heat entirely destroys it. Electricity will change the poles of the magnetic needle, and the explosion of a small quantity of gun-powder on one of the poles, will have the same effect. 948. The attractive power of the magnet may be increased by permitting a piece of steel to adhere to it, and then sus- pending to the steel a little additional weight every day, for it will sustain, to a certain limit, a little more weight on one day than it would on the day before. 949. Small natural magnets will sustain more than large ones in proportion to their weight. It is rare to find a na- tural magnet, weighing 20 or 30 grains, which will lift mose than thirty or forty times its own weight. But a minute piece of natural magnet, worn by Sir Isaac Newton, in a ring, which weighed only three grains, is said to have been capable of lifting 746 grains, or nearly 250 times its own weight. 950. The magnetic property may be communicated from the loadstone, or artificial magnet, in the following manner, it being understood that the north pole of one of the mag- nets employed, must always be drawn towards the south pole of the new magnet, and that the south pole of the other mag- net employed, is to be drawn in the contrary direction. The How does the force of magnetic attraction vary with the distance ? Does the magnetic force vary with the interposition of any substance between the attracting bodies 1 What is the effect of heat on the mag- net 7 What is the effect of electricity, or the explosion of g-un -powder on it 1 How may the power of a magnet be increased 1 What is sai j concerning the comparative powers of great and small magnets 7 MAGNETISM. 319 north poles of magnetic bars are usually marked with a line across them, so as to distinguish this end from the other. 951. Place two mag- Fig. 234. netic bars, a and b, fig. 234, so that the north end of one may be near- est the south end of the other, and at such a dis- tance that the ends of the steel bar to be touched, may rest upon them. Having thus arranged them, as shown in the figure, take the two magnetic bars, d and , and apply the south end of e, and the north end of d, to the middle of the bar c, elevating their ends as seen in the figure. Next separate the bars e and d, by drawing them in oppo- site directions along the surface of c, still preserving the ele- vation of their ends ; then removing the bars d and e to the distance of a foot or more from the bar c, bring their north and south poles into contact, and then having again placed them on the middle of c, draw them in contrary directions, as before. The same process must be repeated many times on each side of the bar c, when it will be found to have ac quired a strong and permanent magnetism. 952. If a bar of iron be placed, for a long period of time, in a north and south direction, or in a perpendicular posi- tion, it will often acquire a strong magnetic power. Old tongs, pokers, and fire shovels, almost always possess more or less magnetic virtue, and the same is found to be the case with the iron window bars of ancient houses, whenever they have happened to be placed in the direction of the magnetic line. 953. A magnetic needle, such as is employed in the mari- ner's and surveyor's compass, may be made by fixing a piece of steel on a board, and then drawing two magnets from the centre towards each end, as directed at fig. 234. Some magnetic needles in time lose their virtue, and require again to be magnetized. This may be done by placing the needle, still suspended on its pivot, between the opposite poles of two magnetic bars. While it is receiving the magnet- ism, it wilf be agitated, moving backwards and forwards, as Explain fig. 234, and describe the mode of making a magnet. In what positions do bars of iron become magnetic spontaneously 1 How may a needle be magnetized without removing it from its pivot 1 320 MAGNETISM. though it were animated, but when it has become perfectly magnetized, it will remain quiescent. 954. The dip, or inclination of the magnetic needle, is its deviation from its horizontal position, as already mentioned. A piece of steel, or a needle, which will rest on its centre, in a direction p .rallel to the horizon, before it is magnet- ized, will afterwards incline one of its ends towards the earth. This property of the magnetic needle was discovered by a compass maker, who, having finished his needles be- fore they were magnetized, found that immediately after- wards, their north ends inclined towards the earth, so that he was obliged to add small weights to their south poles, in order to make them balance, as before. 955. The dip of the magnetic needle is measured by a graduated circle, placed in the vertical position, with the needle suspended by its side. Its inclination from a hori- zontal line marked across the face of this circle, is the mea- sure of its dip. The eircle, as usual, is divided into 360 de- grees, and these into minutes and seconds. 956. The dip of the needle does not vary materially at the same place, but differs in different latitudes, increasing as it is carried towards the north, and diminishing as it is carried towards the south. At London, the dip for many years has varied little from 72 degrees. In the latitude of 80 degrees north, the dip, according to the observations of Capt. Parry, was 88 degrees. 957. Although, in general terms, the magnetic needle is said to point north and south, yet this is very seldom strictly true, there being a variation in its direction, which differs in degree at different times and places. This is called the va- riation, or declination, of the magnetic needle. 958. This variation is determined at sea, by observing the different points of the compass at which the sun rises, o*- sets, and comparing them with the true points of the sun's rising or setting, according to astronomical tables. By such observations it has been ascertained that the magnetic needle is continually declining alternately to the east or west from due north, and that this variation differs in different parts of the How was the dip of the magnetic needle first discovered 1 In what manner is the dip measured 1 What circumstance increases or dimi- nishes the dip of the needle 1 What is meant by the declination of the magnetic needle 1 How is this variation determined 7 What has been ascertained concerning the variation of the needle at different time*. and places 1 GALVANISM. world at the same time, and at the same place at different times. 959. In 1580, the needle at London pointed 11 degrees 15 minutes east of north, and in 1657 it pointed due north and south, so that it moved during that time at the mean rate of about 9 minutes of a degree in each year, towards the north. Since 1657, according to observations made in Eng- land, it has declined gradually towards the west, so that in 1803, its variation west of north was 24 degrees. 960. At Hartford, Connecticut, in latitude about 41, it ap- pears from a record of its variations, that since the year 1824, the magnetic needle has been declining towards the west, at the mean rate of 3 minutes of a degree annually, and that on the 20th of July, 1829, the variation was 6 de- grees 3 minutes west of the true meridian. 961. The cause of this annual variation has not been demonstrated, though according to the experiment of Mr. Canton, it has been ascertained that there are slight varia- tions during the different months of the year, which seem to depend on the degrees of heat and cold. 962. The directive power of the magnet is of vast im- portance to the world, since by this power, mariners are enabled to conduct their vessels through the widest oceans, in any given direction, and by it travellers can find their way across deserts which would otherwise be impassable. GALVANISM. 963. The design of this epitome of the principles of Gal- vanism, is to prepare the pupil to understand the subject of Electro-Magnetism, which, on account of several recent pro- positions to apply this power to the movement of machinery, has become one of the exciting scientific subjects of the day. We shall therefore leave the student to learn the history and progress of Galvanism from other treatises, and come at once to the principles of the science. 964. When two metals, one of which is more easily ox idated than the other, are placed in acidulated water, and the two metals are made to touch each other, or a metallic communication is made between them, there is excited an electrical or galvanic current, which passes from the metal most easily oxidated, through the water, to the other metal, What conditions are necessary to excite the galvanic action 1 From which metal does the galvanism proceed 1 322 GALVANISM. and from the other metal through the water around 10 die first metal again, and so in a perpetual circuit 965. If we take, for example, a slip of zinc, and another of copper, and place them in a cup of diluted sulphuric acid, fig. 235, their upper ends in contact, and above the water, and their lower ends separated, then Fig. 235. there will be constituted a galvanic circle, of the simplest form, consisting of three elements, zinc, acid, copper. The galvanic influence being excited by the acid, will pass from the zinc, Z, the metal most easily oxidated, through the acid, to the copper, C, and from the copper to the zinc again, and so on continually, until one or the other of the elements is destroyed, or ceases to act. 966. The same effect will be produced, if, instead of allow- ing the metallic plates to come in contact, a communication between them be made by means of wires, as shown by fig. 236. In this case, as well _ Fig. 236. as in the former, the electri- city proceeds from the zinc, Z, which is the positive side, to the copper, C, being con- ducted by the wires in the i direction shown by the ar- 1 rows. 1 967. The completion of the circuit by means of wires, enables us to make experi- ments on different substances by passing the galvanic in- fluence through them, this being the method employed to exhibit the effects of galvanic batteries, and by which the most intense heat may be produced. COMPOUND GALVANIC CIRCLES. 968. In the above instances we have only illustrations of what is termed a simple galvanic circle, the different ele- ments being all required to elicit the electrical influence. When these elements are repeated, and a series is formed, Describe the circuit. What is the effect if wires be employed in- stead of allowing the two metals to touch 1 What is a compound gal- vanic circle 1 GALVANISM. 32? ronsisting of zinc, copper, acid; zinc, copper, acid, there is constituted what is termed a compound galvanic circle. It is by this method that large quantities of electricity are ob- tained, and which then becomes a highly important chemical agent, and by which experiments of great brilliancy and in- terest are performed. 969. The pile of Volta was one of the earliest means by which a compound galvanic series was exhibited. This consisted of a great number of silver or copper coins, and thin pieces of zinc of the same dimensions, together with circular pieces of card, wet with an acid, piled, one series above the other, in the manner shown by fig. 237. 970. The student should be informed that it makes no difference what the metals are which form the galvanic series, provided one be more easily oxidated, or dissolved in an acid, than the other, and that the Fig. 237. most oxidable one always forms the positive side. Thus, copper is negative when placed with zinc, but becomes positive with silver. 971. The three substances com- A posing the pile, zinc, silver, wet card,! and marked Z, S, W, succeed each Pi other in the same order throughout the series, and its power is equal to a single circle, multiplied by the num- ber of times the series is repeated. TROUGH BATTERY. 972. The galvanic pile is readily constructed, and an- swers for small experiments, but when large quantities of electricity are required, other means are resorted to, and among these, what is termed the trough battery is the most convenient and efficacious. 973. The zinc and copper plates are fastened to a slip of mahogany wood, and are united in pairs by a piece of metal soldered to each. Each pair is so placed as to enclose a partition of the trough between them, each cell containing a plate of zinc connected with the copper plate of the succeed- ing cell, and a plate of copper joined with the zinc plate of the preceding cell. How is the pile of Volta constructed 7 What qualities are requisite in the two metals in order to yield the galvanic influence 1 Describa the trough battery. 324 ELECTRO-MAGNETISM. 974. This arrange- Fig. 238. ment will be under- stood by figure 238, where the plates P are connected in the order described, and below them the trough T, to contain the acid into which the plates are to be plunged. 975. The trough re made of wood, with partitions of glass, or what is better, of Wedge wood's ware. Each trough contains eight or ten cells, which being filled with diluted acid, the plates are suspended and let down into them by means of a pulley. The advantage of this method is, that the plates can be elevated at any moment, and are easily kept clean from rust, without which the galvanic ac- tion becomes feeble. 976. A great variety of other forms of metallic combina- tions have boen devised to exhibit the galvanic action, but the same elements, namely, two metals and an acid, are required in all, nor do the results differ from those above described. The several kinds of galvanic machines already described, are therefore considered sufficient for the objects of this epitome. ELECTRO-MAGNETISM. 977. Long before the discovery of galvanism, it was sus- pected by those who had made the subjects of magnetism and electricity objects of experiment and research, that there ex- isted an affinity or connection between them. In the year 1774, one oflhe philosophical societies of Germany pro- posed as the subject of a prize dissertation, the question, " Is there a real and physical analogy between the electric and magnetic forces?" The question was, however, then an- swered in the negative ; but naturalists still appear to have kept the same subject in view, and by the observation of What are the advantages of the trough battery 1 What is said of the suspicion of analogy between electricity and magnetism before the dis- covery of galvanism 1 ELECTRO-MAGNETISM. 325 new facts, the existence of such an analogy was from time to time affirmed by various philosophers. 978. The aurora borealis, which has long been supposed to be an electrical phenomenon, was observed to influence the magnetic needle ; and lightning, well known to be nothing more than an electrical movement, was known in many instances to have destroyed or reversed the polarity of trie compass. 979. An instance of this kind, which might have led to very disastrous consequences, is related of a ship in the midst of the Atlantic, which being struck with lightning, had the polarity of all her compasses reversed. This being unknown, the ship was directed as usual by the compass, until the ensuing evening, when the stars showed that her direction was in the exact opposite course from what was intended, and then it was that the phenomenon in question was first suspected. 980. These discoveries of course led philosophers to try the effects of powerful electrical batteries on pieces of steel, and although polarity was often induced in this manner, yet the results were far from being uniform, and the experi- ments on this subject seem in a measure to have ceased, when the discovery of the galvanic influence opened a new field of inquiry, and gave such an impulse to the labours, investigations, and experiments of philosophers throughout Europe, as perhaps no other subject had ever done. 981. It was, however, more than twenty years from the time of Galvani's discovery, before the science of Electro- Mao-netism was developed, the first having taken place in 1791, while the experiments of M. Oersted, the real disco- verer of Electro-Magnetism, were made in 1819. 982. M. Oersted was Professor of Natural Philosophy, and Secretary to the Royal Society of Copenhagen. His experiments, and others on the subject in question, are de- tailed at considerable length, and illustrated by many draw- ings, but we shall here -only give such an abstract as to make the subject clearly understood. 983. The two poles of the battery, fig. 255, are connected by means of a copper wire of a yard or two in length, the two parts being supported on a table in a north and south direction, for some of the experiments, but in others the di- Is there any connection between the aurora borealis and the magnetic needle 1 What is said to have been the effect of lightning on the -ompasses of a ship at sea? What is the uniting wire 1 326 ELECTRO-MAGNETISM. rection must be changed, as will be seen. This wire, it will be remembered, is called the uniting wire. 984. Being thus prepared, and the galvanic battery in action, take a magnetic needle six or eight inches long, pro- perly balanced on its pivot, and having detached the wire from one of the poles, place the magnetic needle under the wire, but parallel with it, and having waited a moment foi the vibrations to cease, attach the uniting wire to the pole. The instant this is done, and the galvanic circuit completed, the needle will deviate from its north and south position, turning towards the east or west, according to the direction in which the galvanic current flows. If the current flows from the north, or the end of the wire along which it passes to the south is connected with the positive side of the battery, then the north pole of the needle will turn towards the east; but if the direction of the current is changed, the same pole will turn in the opposite direction. 985. If the uniting wire is placed under the needle, in- stead of over it, as in the above experiment, the contrary ef feet will be produced, and the north pole will deviate to- wards the west. 986. These deviations will be understood by the follow- ing figures. In fig. 239, N presents the north, and S the south pole of the magnetic nee- Fig. 239. die, and p the positive and n the P > iv_ negative ends of the uniting ^ E wire. The galvanic current, ..-" v:: .'; -.. therefore, flows from p towards u n, or, the wire being parallel with the needle, from the north towards the south, as shown by the direction of the arrow in the figure. Now the uniting wire being above the needle, the pole N, which is towards the positive side of the battery, will de- viate towards the east, and the needle will assume the diree tion N' S'. On the contrary, when the uniting wire is carried below tht. needle, the galvanic current being in the same direction ;i? before, as shown by fig. 240, then the same, or north pole, will deviate towards the west, or in the contrary direction from the former, and the needle will assume the position N' S'. If the needle is stationary, and the current flows from the north, what way will the needle ktrn 1 Explain fig. 239. ELECTRO-MAGNETISM. J!^S 987. When the uniting wire Fig. 240. is situated in the same horizon- ^ tal plane with the needle, and is parallel 'to it, no movement , takes place towards the east or west ; but the needle dips, or the end towards the positive end of the wire is depressed, when the wire is on the east side, and ele- vated when it is on the west side. Thus, if the uniting wire p n, Fig. 241. fig. 241, is placed on the, east :cr* f s' side of the needle N S, and paral- ^ \ ..^ n lei to, and on a level with it, ' * ' -.. s, : >7 = then the north pole, N, being H. towards the positive end of the wire, will be elevated, and the needle will assume the position of the dotted needle N' S'. But if the wire be changed to the western side, other circumstances being the same, then the north pole will be depressed, and the needle will take the direction of the dotted line N" S". 988. If the uniting wire, instead of being parallel to the needle, be placed at right angles with it, that is, in the direc- tion of east and west, and the needle brought near, whether above or below the wire, then the pole is depressed when the positive current is from the west, and elevated when it is from the east. Thus, the pole S, fig. 242, is elevated, the current of positive electricity being from p to n, that is, across the nee- dle from the east towards the west. If the direction of the positive current is changed, and made to flow from n to p the other circumstances being .the same, the south pole of the needle will be de- pressed. 989. When the uniting wire, instead of being placed in a horizontal position as in the last experiment, is placed ver- Explain figures 240, 241, and 242. *^^ ^S d*> 328 ELECTRO-MAGNETISM. tically, either to the north Fig. 243. or south of the needle, and near its pole, as shown by fig. 243, then if the lower extremity of the wire re- ceives the positive current, as from p to n, the needle will turn its pole towards the west. If now the wire be made to cross the needle at a point about half way between the pole and the middle, the same pole will deviate towards the east. If the positive current be made to flow from the upper end of the wire, all these phenomena will be reversed. LAWS OF ELECTRO-MAGNETIC ACTION. 990. An examination of the facts which may be drawn from an attentive consideration of the above experiments, are sufficient to show that the magnetic force which emanates from the conducting wire, is different in its operation from any other force in nature, with which philosophers had been acquainted. 991. This force does not act in a direction parallel to that of the current which passes along the wire, " but its action produces motion in a circular direction around the wire, that is, in a direction at right angles to the radius, or in the di- rection of the tangent to a circle described round the wire- in a plane perpendicular to it." 992. In consequence of this circular current, which seems to emanate from the regular polar currents of the battery, the magnetic needle is made to assume the positions indi- cated by the figures above described, and the effect of which is, to change the direction of the needle from the magnetic meridian, moving it through the section of a circle in a di- rection depending on the relative position of the wire and the course of the electric fluid. And we shall see hereafter that there is a variety of methods by which this force can be applied to produce a continued circular motion. CIRCULAR MOTION OF THE ELECTRO-MAGNETIC FLUID. 993. We have already stated that the action of this fluid produces motion in a circular direction. Thus, if we sup- Explain figure 243. Does the magnetic force of galvanism differ from any force before known, or not 1 In what direction does this force act, as it passes along the wire 1 ELECTRO-MAGNETISM. 329 pose the conducting wire to be placed in a vertical situation, as shown by fig. 244, and p ra, the current of positive electricity, to be descending through it, from p to n, and if through the point c in the wire the plane N N be taken, perpendi- cular to p n, that is in the present case a hori- zontal plane, then if any number of circles be described in that plane, having c for their common centre, the ac- tion of the current in the wire upon the north pole of the magnet, will be to move it in a direction corresponding to the motion of the hands of a watch, having the dial towards the positive pole of the battery. The arrows show the di- rection of the current's motion in the figure. 994. When the direction of the electrical current is re- versed, the wire still having its vertical position, the direc- tion of the circular action is also reversed, and the motion is that of the hands of the watch moving backwards. As the magnetic needle cannot perform entire revolutions when it is crossed by the conducting wire, it becomes neces- sary, in order to show clearly that such a circulation as we have supposed actually exists, to describe more clearly than we have yet done, the means of demonstrating such an ac- tion, and the corresponding motion. 995. Now the metals being conductors of the electric fluid, if we employ one through the substance of which the mag- netic needle can move, we shall have an opportunity of know- ing whether the fluid has the circular action in question, for then the needle will have liberty to move in the direction of the electrical current. 996. For this purpose mercury is well adapted, being a good conductor of electricity, and at the same timeo fluid as to allow a solid to circulate in it, or on its surface, with Explain by fig. 244 in what direction the electro-magnetic fluid moves. Why is mercury v/eTi adapted to show the circular action of the gal- vanic fluid 1 28* 330 ELECTRO-MAGNETISM. considerable facility. This, ^therefore, is the substance em- ployed in these experiments. MEANS OF PRODUCING ELECTRO-MAGNETIC ROTATIONS 997. The continued revolution of one of the poles of a mag-net round a vertical conducting Wire, may be produced in the following manner : The small glass cup, fig. 245, of which the right hand cut is a section, is pierced Fig. 245. c at the bottom for the ad- mission of the crooked piece of copper wire d, which is made to commu- nicate with one of the poles of a galvanic battery. To the end of this wire, which projects within the cup, is attached by means of a fine thread, the end of the magnet a. The string must be of such length as ' to allow the upper end of the magnet to reach nearly the top of the cup. The vertical wire c is the positive pole of the battery. 998. Having made these preparations, fill the cup so full of mercury as only to allow a small portion of the upper end of the magnet to float above the surface, as shown in the figure. Then, by means of a little frame, or otherwise, fix the copper wire of the positive pole in the centre of the mercury, letting it dip a little below the surface, and on con- necting the negative pole with the wire d, the magnet will revolve round the copper wire, and continue to do so as long as the connection between the two poles of the battery and the mercury remains unbroken. 999. To insure close contact between the poles of the bat- tery and the mercury, the ends of the wires where they dip into the mercury are amalgamated, which is done by means of a little nitrate of mercury, or by rubbing them, being of copper, with the metal itself. Explain fig. 245, and show how the pole of a magnet may be made to move in a circle. In these experiments, why are the ends of the con- ducting wires amalgamated 1 ELECTRO-MAGNETISM. 331 REVOLUTION OF THE CONDUCTING WIRE ROUND THE POLE OF THE MAGNET. 1000. In the above example the wire is fixed, while the electrical current gives motion to the magnet. But this or- der may be reversed, and the wire made to revolve, while the magnet is stationary. 1001. The apparatus for this purpose is represented by fig. 246, and consists of a shallow glass cup, with a tubu- lar stem to hold the Fig. 246. mercury. In the stem, as seen in the section on the right, there is a small copper socket, which is fixed there by being fastened to a cop- per plate below, which plate is cemented to the glass so that no mer- cury can escape. This plate is tinned and amalgamated on the under side, and stands on another plate, the upper side of which is also tinned and amal- gamated, and between these the conducting wire passes, so as to insure a perfect metallic continuity between the poles of the battery. A strong cylindrical magnet is placed in the copper socket, with its upper end so high as to reach a little above the mercury when the cup is filled. The wire connected with the pole of the battery, which dips into the mercury, is suspended by means of loops, as seen in the figures. 1002. When the apparatus is thus arranged, and a com- munication made through it, between the poles of the bai- tery, the wire will revolve round the magnet with great ra- pidity. 1003. A more simple apparatus, answering a similar pur- pose, and in which the wire revolves very rapidly, with a very small voltaic power, is represented by fig. 247. 1004. It consists of a piece of glass tube, g g, the lower end of which is closed by a cork, through which a small piece of soft iron wire, m, is passed, so as to project above and below. Explain fig. 246. 332 ELECTRQ-31 A.GNET ISM . A little mercury is then poured in so as to Fig. 247. make a channel between the wire and the glass tube. The upper orifice of the tube is also closed by a cork, through which a piece of copper wire, b, passes,andterminates in a loop. Another piece of wire, c, is suspended from this by a loop, the end of which dips into the mercurv, and is amalgamated. 1005. In this arrangement, a temporary magnet is formed of the soft iron wire, m a, by the electrical fluid, and around which the o\ moveable wire, c, revolves rapidly, changing its direction, as usual, when the direction of the current is changed. REVOLUTION OF A MAGNET ROUND ITS OWN AXIS. 1006. After it was found that a conducting wire might be made to revolve round a mag- net, and a magnet round a conducting wire, many attempts were made to obtain the rota- tion of a magnet and of a conductor around their own axes. The rotation of a magnet on its axis may be accomplished by means of galvanism, by the following method : 1007. The cylindrical magnet, a, fig. 248, terminates at its lower ex- tremity in a sharp point, which rests in a conical cavity of agate, so as much as possible to avoid friction. The vessel, the section of which is here shown, may be of glass or wood. The upper end of The mag- net is supported in the perpendicular position by a thin slip of wood, pass- ing across the upper part of the ves- sel, and having an aperture through it, of proper size. 1008. A piece of quill is fitted on the upper end of the magnet, and rising a little above it, forms a cup to hold a globule of mercury. Into this mercury is inserted the lower end of the wire c, which has a cup on Explain figures 247 and x?48. ELECTRO-MAGNETISM. 333 the top, containing mercury for the usual purpose. The end of the wire c must be amalgamated, as also the termination of the poles of the battery, which dip into the cups c and d. A copper wire of considerable size pierces the bottom of the vessel, and ends in the cup d, like the other, containing mer- cury, in order to make the contact perfect. The vessel being now filled with mercury nearly up to a, so as to cover about one half the magnet, and the ends of the galvanic poles inserted into the cups c and d, the magnet be- gins to revolve, and continues to do so as long as the con- nection is unbroken. 1009. In order to produce the rotation of a magnet, it is necessary that the electrical influence, in every instance, should act only on one of the poles at the same time, because the direction of the current on the two ends are contrary to each other, and therefore the two forces would be neutral- ized, and no motion be produced. In the above experiment, the electrical current, having passed the upper half of the magnet, is diffused in the mer- cury in which the lower half is Fig. 249. buried, and thus produces no sensible effect upon it. 1010. Another method of pro- ducing the rotation of a mag- net, is represented by fig. 249. In this, a is a cylindrical mag- net pointed at both ends, and run- ning in an agate cup, which is fixed on a stem rising from the bottom of the stand. Its upper point runs in a little cavity in the end of the thumb screw b, which passes through the cap of the frame-work of the apparatus. Near the middle of the magnet, this frame, which is of wood, supports a shelf, on which rests the circu- lar cistern of mercury, c, the mag-1 net passing freely through the centre of both. A cistern of mercury, d, also surrounds the To produce the rotation of a magnet, on what part must the galvan* ism act 1 Why 1 Explain fig. 249, and show the course of the elec- trical fluid from one CUD to the other. 334 ELECTRO-MAGNETISM. A ower point of the magnet, and in the centre of which is placed the agate cup. A piece of copper wire projecting into the interior of these cisterns, terminates in a cup holding mercury, for the purpose of effecting a communica- tion with the galvanic battery, in the usual manner. A small wire of copper, pointed, and amalgamated at the lower end, is fastened to the magnet, and made to dip into each of the cisterns of mercury, as seen in the figure. 1011. In this arrangement, the lower half of the magnet only, forms a part of the galvanic circle, the fluid passing in at one cup and-out at the other hy the following routine, which is apparent by the figure. Suppose the positive wire is placed in the upper cup, then the circuit will be from the cup along the wire to the mercury in the cistern, and from the mercury through the bent wire to the magnet down the magnet through the lower bent wire to the mercury, and thence to the cup, and the negative pole of the battery. When the galvanic current is thus established, the rota- tion of the magnet begins, and if the points of the axis are delicate, and the friction small, it will revolve rapidly. VIBRATORY AND ROTATORY MOTIONS PRODUCED BY MEANS OF HORSE-SHOE MAGNETS. 1012. By the use of these magnets, both the magnetic poles conspire to give the motion. The influence of the two poles being in contrary directions, and so near each other that the wire or wheel placed between them are within the magnetic currents of both, the effect appears to be, to form a current at right angles to the vibrating wire. The wire be- comes magnetic by the galvanic power, every time it touches the mercury between the poles of the magnet, and conse- quently is thrown backwards or forwards by the magnetic current, according to its direction; hence, if the poles of the battery are charged so as to carry the electricity in a con- trary direction through the apparatus, the impulse on the wire or wheel will also be changed to the opposite direc- tion. If the poles of the magnet be changed, by turning it over, the same effect will be produced, and the wheel will revolve in a contrary direction from what it did before. 1013. Thus, if the magnet be laid in the direction of north and south, with the poles towards the north, the north pole be^ng on the east side, and the positive electricity be How may the direction of the vibrating wire be changed ? ELECTRO-MAGNETISM. 335 sent through the vibrating wire, upwards, then the vibrating force will be towards the north ; but if either the poles of the magnet or those of the battery be changed, the wire will be thrown towards the south. VIBRATION OF A WIRE. 1014. A conducting copper wire, w> fig. 250, is suspend- ed by a loop from a hook of the same metal, w T hich passes Fig. 250. through an arm of metal or wood, as seen in the cut. The upper end of the hook terminates in the cup P, to contain mer- cury. The lower end of the copper wire just touches the mercury, Q,, contained in a lit- tle trough about an inch long 1 , formed in the wood on which the horse-shoe magnet, M, is laid, the mercury being equally distant from the two poles. The cup, N, has a stem of wire., Avhich passes through the wood of the platform into the mercury, this end of the wire being tinned, or amalgamated, so as to form a perfect contact. 1015. Having thus prepared the apparatus, put a little mercury into the cups P and N, and then form the galvanic circuit by placing the poles of the battery in the two cups, and if every thing* is as it should be, the wire will begin to vibrate, being thrown with considerable force either towards M or Q,, according to the position of the magnetic poles, or the direction of the cur- rent, as already explained. In either case it is thrown out of the mercury, and the galvanic circuit being thus broken, the effect ceases until the wire falls back again by its own weight, and touches the mercury, when the current being again perfected, the same influence is repeated, and the wire is again thrown away from the mercury, and thus the vibra- tory motion becomes constant. This forms an easy and beautiful electro-magnetic experi- ment, and may be made by any one of common ingenuity, Explain fig. 250, and describe the course of the electric fluid from one cup to the other 336 ELECTRO-MAGNETISM. who possesses a galvanic battery, even of small power, and a good horse-shoe magnet. 1016. The platform may be nothing more than a piece of pine board eight inches long and six wide, with two sticks of the same wood, forming a standard and arm for suspend- ing the vibrating wire. The cups may be made of percus- sion caps, exploded, and soldered to the ends of pieces of cop- per bell wire. 1017. The wire must be nicely adjusted with respect to the mercury, for if it strikes too deep, or is too far from the surface, no vibrations will take place. It ought to come so near the mercury as to produce a spark of electrical fire, as it passes the surface, at every vibration, in which case it may be known that the whole apparatus is well arranged. The vibrating wire must be pointed and amalgamated, and may be of any length, from a few inches to a foot or two. ROTATION OF A WHEEL. 1018. The same force which throws the wire away from the mercury, will cause the rotation of a spur-wheel. For this purpose the conducting wire, Fig. 251. instead of being suspended as in the former experiment, must be fixed firmly to the arm, as shown by fig. 251. A support for the axis of the wheel may be made by soldering a short piece to the side of the conducting wire, so as to make the form of a fork, the lo\ve ends of which must be flattened with a hammer, and pierced with fine orifices, to re- ceive the ends of the axis. 1019. The apparatus for a revolving wheel is in every re- spect like that already described for the vibrating wire, except in that above noticed. The wheel may be made of brass or copper, but must be thin and light, and so suspended as to move freely and easily. The points of the notches must be amal- How must the points of the vibrating wire be adjusted in order to act"? Explain fig. 251. In what manner may the points of the spur wheel be amalgamated 1 ELECTRO-MAGNETISM. 337 gamated, which is done m a few minutes, by placing the wheel on a flat surface, and rubbing them with mercury by means of a cork. A little diluted acid from the galvanic battery will facilitate the process. The wheel may be from half an inch to several inches in diameter. A cent ham- mered thin, which may be done by heating it two or three times during the process, and then made perfectly round, and its diameter cut into notches with a file, will answer every purpose. 1 020. This affords a striking and novel experiment ; for when every thing is properly adjusted, the wheel instantly begins to revolve by touching with one of the wires of the battery the mercury in the cup P or N. When the poles of the magnet, or those of the battery, are changed, the wheel instantly revolves in a contrary direction from what it did before. 1021. It is, however, not absolutely necessary to divide the wheel into notches, or rays, in order to make it revolve, though the motion is more rapid, and the experiment suc- ceeds much better by doing so. REVOLUTION OF TWO WHEELS. 1022. If two wheels be arranged as represented by fig 252, they will both re- ^ i Fig. 252. volve by the same elec- trical current. Eachpj horse-shoe magnet has its trough of mercury. The magnets have been omitted in the drawing, but are to be placed precisely as in the last figure. The electrical communication is to be made through the cups of mercury, P and N, and its course is as follows: From the cup it passes into the mercury j from the mercury through the radii to the axis of the wheel, and along the axis to the other wheel, down which it passes to the mercury, and so to the other cups, and to the opposite pole of the battery. The poles of the magnets for this experiment, must be opposed to each other. ELECTRO-MAGNETIC INDUCTION. 1023. Experiment proves that the passage of the gal- vanic current through a copper wire renders iron magnetic Explain fig. 252, and show how two wheels may be made to revolve by the same current. 23 338 ELECTRO-MAGNETISM. when in the vicinity of the current. This is called mag netic induction. 1 024. The apparatus for this purpose is represented by fig. 253, and consists of _ Fig. 253. a copper wire coiled, by winding it around a piece of wood. The turns of the wire should be close together for actual experiment, they being parted in the figure to show the place of the iron to be magnetized. The best method is, to place the coiled wire, which is called an electrical helix, in a glass tube, the two ends of the wire of course projecting. Then placing the body to be magnetized within the folds, send the gal- vanic influence through the whole, by placing the poles of the battery in the cups. 1025. Steel thus becomes permanently magnetic, the poles, however, changing as often as the fluid is sent through it in a contrary direction. A piece of watch-spring placed in the helix, and then suspended, will exhibit polarity, but if its position be reversed in the helix, and the current again sent through it, the north pole will become south. If one blade of a knife be put into one end of the helix, it will re- pel the north pole of a magnetic needle, and attract the south ; and if the other blade be placed in the opposite end of the helix, it will attract the north pole, and repel the south, of the needle. 1026. Temporary Magnets. Temporary magnets, of al- most any power, may be made by winding a thick piece of soft iron with many coils of insulated copper wire. The best form of a magnet for this purpose is that of a horse-shoe, and which may be made in a few minutes by heating and bending a piece of cylinder iron, an inch or two in diameter, into this form. 1027. The copper wire (bell wire) may be insulated by winding it with cotton thread. If this cannot be procured, common bonnet wire will do, though *t makes less powerful magnets than copper. What is meant by magnetic induction 1 Explain fig. 253. What is this figure called 1 Does any substance become permanently magnetic by the action of the electrical helix 1 How may the poles of a magnet be changed by the helix 1 How may temporary magnets be made ? ELECTRO-MAGNETISM. 339 1028. The coils of wire may begin near one pole of the magnet and terminate near the other, as represented by fig. 254, or the wire may Fig. 254. consist of shorter pieces wound over each other, on any part of the mag- net. In either case, the ends of the wire, where several pieces are used, must be soldered to two strips of tinned sheet copper, for the com-p bined positive and nega- tive poles of the wires. To form the magnet, these pieces of copper are made to communi- cate with the poles of the battery, by means of cups containing mercury, as shown in the figure, or by any other method. 1029. The effect is surprising, for on completing the cir- cuit with a piece of iron an inch in diameter, in the proper form, and properly wound, a man will find it difficult to pull off the armature from the poles ; but on displacing one of the galvanic poles, the attraction ceases instantly, and the man, if not careful, will fall backwards, taking the armature with him. Magnets have been constructed in this manner, which would suspend two or three thousand pounds. 1030. GALVANIC BATTERY. One of the most con- venient forms of a galvanic battery for the experiments above Described, is represented by fig. 255. It consists of two concentric cylinders, of sheet copper, soldered to the same bottom. The diameter of the outer cylinder may be six inches, and the inner one four and a half inches. The height may be a foot or more. Between these cylinders of copper is placed one of zinc, but so as not to touch them nor the bottom. This is best done by tying three or four pieces of pine lengthwise to the zinc cylinder, letting them project half an inch below the bottom. By this ar- rangement the zinc can be taken from the acid, or plunged For what purpose are the ends of the wires to be soldered to pieces of copper 7 340 ELECTRO-MAGWETISM. Fig. 255. into it, at any moment. Another cylinder of zinc within the smaller one of copper may be added, to Increase the power, when a single one is found in- sufficient. This must have a metallic connection with the other zinc cylinder. 1031. The cups PN are the positive and negative sides of the battery. The best way of forming this part of the appara- tus is to solder a slip of tinned copper to the inside of the cop- per cylinder, and another to the zinc, as shown in the plate. The outer ends of these may readily be formed into cups by cutting the copper slip one third in two on each side, then turning this part at right angles with the other, and rolling what were the outer edges together, and soldering them. Such a battery is ample for all the experiments we have described. 1032. A cheap and convenient liquid for the battery con- sists of water saturated with common salt, with a little sul- phuric acid, say an ounce or two to a quart. Describe the battery fig. 255. Which is the positive, and which the negative metal 1 j>C (S4C? THE UNIVERSITY OF CALIFORNIA LIBRARY L