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through hard substances. Light passes through glass and diamonds, and sound in a similar manner passes through iron and other metals, as well as through wood. It may be also remarked, that, other circumstances being the same, the harder or more dense the substance, sound penetrates it the more easily, and in this respect it bears a strong resemblance to heat.

When the ear is placed close to one end of a log of wood, however long it may be, and the other end struck or a watch applied to it, we are sensible of the sound of the watch or of the stroke, though it be too slight to be otherwise perceptible. When a piece of metal is applied to the bones of the head or to the teeth and struck, we also feel an indistinct sensation of sound, and this is felt even by those who are deaf to sounds conveyed through the air. The blind and deaf boy Mitchel, found his greatest pleasure in striking his teeth with a key, and was highly displeased when he was given a piece of wood as a substitute.

CONDUCTORS OF SOUND.

Wool, down, and feathers, are consequently bad conductors of heat, and also bad conductors of sound; though perhaps, there is no substance whatever through which sound will not pass more or less. The sound of the grand cataract of Niagara

is found to be sensibly diminished when the ground is covered with snow, which, according to our principle, is a non-conductor of sound.

Air, when very much rarefied, as in the exhausted receiver of an air-pump, will scarcely transmit sound at all; and it has hence been hastily concluded, that air is the only medium of sounds. It would, we think, have been more philosophical to have inferred that every substance whatever when rarefied, if that were possible, as much as the air in the exhausted receiver, would transmit sound with equal indistinctness.

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The clearness of sound, indeed, seems to depend very much on the density of the vehicle by which it is conveyed. In the cool of the evening, and still more as the night advances, when the air is no longer rarefied by the sun, we hear sounds, those for example of distant waters, which in the day are altogether imperceptible. The circumstance of sounds being more distinctly transmitted in cool air, has not escaped the observation of our poets.

-Who the melodies of morn can tell,

The wild brook babbling down the mountain's side;
The lowing herd, the sheepfold's simple bell,

The pipe of early shepherd dim descried
In the low valley. Echoing far and wide;
The clamorous horn high o'er the cliffs above,

The hollow murmur of the ocean tide,

The sky-lark's song, the linnet's lay of love,
And the full choir that wakes the universal grove.

The cottage curs at early travellers bark,

Crown'd with her pail the tripping milk-maid sings, The whistling ploughman stalks a field and park, Down the rough slope the ponderous waggon rings. Beattie's Minstrel.

We have often observed a similar clearnesss of sounds in frosty weather. For example, we think we can hear the sound of a distant carriage much more distinctly in a frosty day than in the heat of It is evidently the same principle which causes iron to transmit sounds well, and feathers to transmit them imperfectly.

summer.

WATER A MEDIUM OF SOUNDS.

Many experiments have been made on the capability of water to transmit sound. The Abbé Nollet, among others, took much pains to decide the question. By practice he acquired such ma nagement of himself under water, that he could hear the sound of the human voice, and even re

cognise airs of music. two stones which he held in his hands, his ears were shocked almost beyond bearing, and he even felt a sensation on all the surface of his body, like that produced when a piece of metal held in the teeth is struck by another piece of metal. He

When he struck together

bserved also that the more sonorous bodies, when truck under water, gave a less vivid impression han others less sonorous. These experiments vere successfully repeated by the late Dr. Monro, of Edinburgh.

SONOROUS SUBSTANCES.

All bodies are not equally fitted for producing sound. Those which have the greatest degree of elasticity appear to be the most sonorous. It is ɔwing, indeed, to the great expansible force and elasticity of the air, that gunpowder and the electric flash, by rending it and forming a vacuum, occasion the loud sounds which often strike us with terror. The cracking of a waggoner's whip affords a good illustration of the sound of thunder or any other explosion. The sudden jerk of the end of the whip-cord displaces a portion of air, and forms an empty space into which the adjacent air violently rushes. The air which formed the several sides of this empty space, thus collapsing with a shock, produces the sound.

The changes which take place among the minute particles of bodies, in consequence of the vibrations from which sounds arise, are remarkably different in metals, in wood, and in musical strings. This can be illustrated in the case of metals, by repeating the experiments of Dr. Chladni

in its turn displaces that portion of air which is next to it or beyond it on every side, above and below, before and behind, on the right and on the left. This displaced portion of air displaces again what is beyond it, and so on, in a manner similar to the circles of water which arise from throwing a stone into a pond. In the case of sound, however, the waves are not in superficial circles, but in spheres like the coats of an onion.

The air, besides, is elastic, or has the quality of springing back to its first position, like Indian rubber when stretched out and let go. Therefore the waves of sound are not regularly progressive, like those of water, but vibrate or tremble forwards and backwards as a musical string is seen to do when it is struck. The first wave, accordingly, when it strikes on the air around it, drives this air forwards while it is itself driven backwards.

This shows that the motion of sound is also very different from that of wind, and is scarcely, if at all, perceptible to sight or touch; for it is well known that sounds which would shatter windows to pieces, will not move a feather nor the flame of a candle, so different is the motion from wind. Though, however, sound is not usually felt by touch, there are instances in which it appears to have been thus perceptible. Kersting, who lost both his sight and hearing after manhood, had his sense of touch so

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