globules shall, during their descent pass through a stratum of very cold air, they will be completely frozen, and come to the ground as hail, and even (when a multitude of drops are suddenly condensed into large solid masses,) as solid pieces of ice.

The

Explanation of the air-pump in Plate II. Fig. 2. At AA are two brass barrels, each containing a piston with a valve opening upwards, worked by the handle B, having a pinion that works in the teeth of the racks CC. On the wooden frame ED, is a brass plate G, and also a brass tube communicating with the barrels and the cock I. glass vessel K has its rim ground perfectly flat, and rubbed with hog's lard to make it exactly fit the plate G; this vessel is called the receiver, and from it the air is extracted by the action of the pistons, when the cock I is shut, by which alone it could gain admittance.

SECT. IV.

ACOUSTICS.

The

The transition from Pneumatics to Acoustics, is quite natural; for the latter, as the Greek term signifies, instructs us in the nature of hearing and sounds, which are usually conveyed to us by means of the air around us. In the early days of natural knowledge, sounds were supposed to have a real separate existence, like the odorous particles of various substances, and that hearing was excited by the action of the particles of sound upon the ear, in the same way as smelling is excited by the action of odoriferous particles upon the nostrils. Greek philosopher Zeno, however, three centuries before our Saviour, taught that hearing was produced by the air situated between the sounding body and the ear. "The air," said he, " is agitated in every "direction, from the sounding body, and moves from it in waves "which fall upon the ear, in the same manner with the circles pro"duced in water, by a stone thrown into it." This opinion admirably simple and intelligible, has been confirmed and illustrated by the experience of the greatest philosophers, down to the present day. Thus a bell rung under water, gives a sound as distinct as in the open air: and it has been ascertained, that fish have a strong perception of sound, even at the bottom of a deep river; tame carp in the bottom of a pond will rise upon a call or whistle. The production of circular waves in a pond by the impression of a falling stone is an experiment obvious to every eye. In sound we cannot render the operation an object of sight; but when similar effects are produced, we are warranted to suppose the existence of similar causes. A stone slipped gently into water will create no circulating waves; in the same way an impression made gently on the air will excite no sound; but if the impression be sudden and forcible, waves of air will be produced which extending in all directions will excite the sensation of hearing on every ear within the bounds of those circles, the more or less powerfully, as the ear is less or more remote from the centre. The sense of hearing is occasioned by the undulating or waving motion of the air, cul

leeted by the funnel of the outward ear, and conveyed through the auditory passage to a very thin transparent membrane, stretched over the inner end of the passage, like the parchment over the head of a drum. From this circumstance it is called the tympanum or drum of the ear. The waves of air collected by the funnel of the ear, in greater quantity than would fall upon the drum itself if exposed to them, press upon that membrane which, by its motion, acts upon nerves communicating with the brain, when immediately the sense of sound, or hearing, is excited. If from any cause, the air be prevented from reaching the tympanum, or if that membrane be rendered inflexible, so that the motion of the air shall produce no motion in it, or that the internal parts of the organ have lost their activity; in all these cases no impression will be made on the brain, no sound will be perceived, and the person is said to be deaf. It was before said, that a bell rung in water communicates its motion to the water, which does the same to the air upon its surface, and the sound of the bell is heard just as distinctly as if it had been rung in the open air. Again, if a bell be rung in a vessel, from which the air has been exhausted by the air-pump, no sound will be produced: but if it be rung in a vessel into which a greater quantity of air than is naturally the case has been forced, the sound of the bell will be proportionally increased. The progress of the circles of water made by the stone may be measured; but the circles of air travel much faster. By experiment it is found, that sound moves at the rate of 1142 English feet in one second of time, or very nearly 13 English miles in one minute, or 778 miles in one hour. That sound is not produced instantaneously but moves in a certain rate, is obvious to every man's observation. We see a man felling a tree at some distance: we see the fall of the hatchet, but it is again raised in the air to repeat the stroke, before the sound of the first come to our ear. We see the smoke from the fowling-piece across a dozen of fields; but a sensible interval takes place before the report affect our hearing. By the rate of the motion of sound we are enabled to compute the distances of objects. Thus a ship out at sea in the night fires a gun as a signal of distress. The flash is observed by persons on shore, who remark that the report does not reach their ears till just seven seconds after the flash. Multiplying by 7 the number of feet travelled by sound in 1 second, viz. 1142, we have 7994 feet, equal to 1 mile and 296, feet: this therefore is the distance of the ship from the observers at the firing of the gun. By the same rule we may estimate the distance of a thundercloud, if we carefully notice the time elapsed between the flash and the report. Hence we may draw an argument to remove the terror absurdly felt by many persons at the rolling report of the thunder, although the flash of lightning, by which the report was produced in the air, was beheld with little concern. The motion of lightning being incomparably swifter than that of sound, no person who heard the report, could possibly be affected by the flash or discharge. The motion of sound is here given at 1142 feet in one second; but some late observers are inclined, and on good grounds, to diminish that velocity, and to estimate its motion at only 1130 feet in a second.

Sounds of all kinds travel equally quick the report of a gun and the stroke of a hammer; the loudest thunder and the lowest whisper, (as far as it is heard) move all with equal velocity. Smooth and clear

sounds proceed from bodies all of one substance and of an uniform figure: harsh sounds proceed from bodies of a mixed nature and an irregular figure. All substances are, in some measure, conductors of Sound. If you stop one ear, and to the other apply the end of a long rod of timber, at the opposite end of which a watch is applied, the beating of the watch will be distinctly heard through the rod. The same effect will be produced if you stop both ears, and take the end of the rod between the teeth. A gentle scratch made on the end of a long deal plank will be plainly heard, if the ear be applied to the other end, although in the open air it would be quite imperceptible. If between the legs of a pair of tongs you pass a garter or a piece of flannel, and pressing the ends of the garter, by a finger of each hand, into the ears, then raise the tongs from the ground, while another person strikes on the legs of the tongs with a key, the vibratory motion thus excited in the tongs, communicated immediately to the ears, will produce a sensation like the loud ringing of a large church-bell. The earth is also a good conductor of sound: advanced parties of an army, highwaymen, &c. have been warned of the approach of horses, by applying an ear to the ground.

The Echo. When a stone is thrown into a pond, it raises up circles spreading in all directions. If before the impression decay, the waves strike against the margin of the pond, a wall, or any other obstacle, a fresh impulse is given to the water, but in a contrary direction; and the two sets of waves are seen meeting and crossing each other. Just in the same way if sound strike against a wall, the face of a rock, or any other unyielding substance, its waves are turned backwards, and a second sound is produced, at an interval from the first, proportioned to the distance of the original sounding body from the reflecting body. This is called an echo, a Greek name, meaning however merely a sound. It is on the principle of the echo that we account for the prodigious noise excited in a large empty hall, with regular smooth walls. Let one play on the violin in such a space, and the sound will be equal to that produced by a number of such instruments in a common room. But if the instrument be carried into a bed-chamber, with a carpet, window-curtains and hangings let down, these substances being unelastic, do not reflect the sound, and the violin will seem to have lost the best part of its powers. Supposing a room to be constructed in the form of an ellipse or oval, a sound proceeding from a person placed in one focus will be reflected to, and distinctly heard by one standing in the other focus, and distinctly heard no where else. The same etfect is produced likewise in circular domes, as in the whispering-gallery of St. Paul's church in London, where a whisper uttered against the wall, at one side of the dome, is reflected to the opposite side, and there distinctly heard. Hence we see the principle on which are constructed the speaking and the hearing trumpet. When we speak in the open air, the effect on the ear of a distant hearer is produced by only one impulse of the air, moving straight forward: but when we speak through a tube, all the pulses propagated from the mouth, excepting those in the direction of the tube, must strike against its sides, from which being reflected, a great number of impressions must necessarily be reflected to the ear at a distance. Hence it follows that by using a tube or trumpet many more waves of air may be made to strike the ear than without it, and consequently the person at a distance may

hear the voice and words as distinctly, as if he had stood near one speaking with the mouth alone. The hearing trumpet acts in a similar manner, but reversed. In the open air only a few direct impulses of air can touch the ear; and if the membrane be rigid or its sensibility weakened, sounds will be but very feebly, if at all perceived. But the wide open mouth of the trumpet collects a great body of impulses, which being carried forward by repeated reflection from the sides of the instrument, as it contracts in diameter, are at last brought to act so forcibly on the organ of hearing that a sound may now be audible, which, without this aid, would be entirely inaudible. The human voice acts by impressions made on the air by organs in the upper part of the throat and by the application of the tongue, palate, teeth, lips, &c. the sounds thus produced are subjected to an infinite number of modifications, according to the practice of different nations, by which the members of the society are enabled to carry on a mutual interchange of sentiments. The sounds of a violin, a harp, or any other stringed instrument, are equally produced by the rapid impulses communicated to the air by the strings, either when touched with the finger or acted upon by the roughened hairs of the bow; and as a string makes, all its vibrations in the same time, whether they be great or small, the effect in moving the air must be the same, and consequently the sound or note or tone must always be the same, whether the string be touched soft or strong.

SECT. V.

OPTICS.

THE science of optics, that is of sight or vision, is founded on the nature and properties of light, and consists of two principal branches. 1st. Caloptrics, from the Greek catoptron, a speculum or mirror; re lative to light reflected from polished or smooth surfaces. 2d. Dioptrics from dioplesthai to see through; relative to the refraction produced on light in its passage through different media or substances, as air, water, glass, &c.

Light, if it could be defined, is too well known to require any definition. It is by the light of the sun, or by that which proceeds from burning bodies, that we have information of the position and presence of external objects at a distance; or the rays of light proceeding from those bodies, and entering our eyes, produce the sensation of vision. Of the nature of light we have no certain notion; but two principal opinions have divided the learned. The first is that of Des Cartes of France, of Huygens of Holland, and of other eminent philosophers, who conceived universal space to be filled up with a very fine and subtile fluid, which is agitated or put in motion by the sun and burning bodies. This motion consists of vibrations, and undulations, or waves, which extending themselves in all

directions, and reaching the eye, render visible the objects from which these motions are produced. The second theory is proposed by the illustrious Newton and his disciples. According to this theory, light is a real material substance, emanating or proceeding from luminous bodies. It is a subtile fluid, composed of peculiar particles of matter, constantly separating from luminous bodies, which entering the eye excite the sensation of light and of sight, or the perception of the objects from which it originally proceeds, from which it is reflected, or through which it is refracted. This theory deduced from a great number of facts and observations, was established by Newton upon mathematical evidence and demonstration. If then it be admitted that light is a subtile fluid substance, consisting of minute particles, we are naturally led to consider their most obvious properties, such as their velocity, their magnitude, their force, &c. One of the most astonishing properties of light is the velocity with which it moves. The mean distance of our earth from the sun is 95 millions of English miles: that of Jupiter in the midst of his moons is 490 millions. When therefore he and we are both in a line on the same side of the sun, we are nearest together, and 395 millions of miles (the difference between 95 and 490) is the distance between us. Again, when Jupiter and we are directly opposite to one another, on different sides of the sun, the distance between us must be the greatest, or 585 millions of miles (95 and 490): the difference therefore between our greatest and our least distance from Jupiter is 190 millions of miles. Now an eminent astronomer of Denmark, Roemer, observing the eclipses and other appearances of Jupiter's moons or satellites to happen about eight minutes and thirteen seconds sooner than the time calculated, when the earth was between him and the sun; and about eight minutes later than the time calculated, when she was on the opposite side of the sun; making a difference in time of sixteen minutes and a half; he very sagaciously conjectured and proved that this difference could be produced by nothing but the time necessary for the progress of light from Jupiter to the earth, in the opposite points of her course round the sun. The difference between the distances of the earth and Jupiter was shown to be 190 millions of miles; but the sun being only one half of that difference in distance from us, it follows that light travels to us from the sun 95 millions of miles in eight minutes and one-fourth, or about the rate of nearly 200 thousand English miles in one second of time; a velocity of which the human mind can form no distinct conception. A cannon ball is computed to fly at the rate of eight miles in one minute, that is, 64 miles in eight minutes and one-fourth, while light travels 95 millions of miles; light must therefore move with a rapidity towards a million and a half times faster than a cannon ball, which would at its ordinary rate employ above 22 years and a half in passing between the earth and the sun.

From the inconceivable rapidity of light may be inferred the extreme minuteness of its particles. The force with which moving bodies strike others at rest, is in proportion to their mass, or the quantity of matter they contain, multiplied by their velocity: a swift blow with a light hammer will drive a nail into the wood, when a very heavy load merely resting on it would have very little effect. With the prodigious velocity of .light, unless its particles be inconceivably minute, its effects upon the bodies exposed to it would be most destructive. Indeed were the