Sound how generated.

To assist our conception when we consider the medium which is the vehicle of sound, we find it necessary in philosophy to illustrate the motions of one fluid by the motions of another; of an invisible fluid by a visible one; of air by water: which method of reasoning, as applied to sound, is very ancient, the Stoics having adopted it long ago, as it is observed in the Experiments on Sounds by the Academy Del Cimento *. When a stone falls into a pool of water, the surface is thrown into waves, which spread themselves every way in concentric circles, whose common centre is the point of percussion: and when they strike against a bank, or any other obstacle, they return in the contrary direction to the place from whence they proceeded. And such is the yielding nature of fluids, that when other waves are generated near to the former, and others again near to them, all will perform their courses amongst cach other without interruption: those that are coming back will pass by those that are coming forward, or even through them. Reason

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Reason assures us, that when the fluid of air is struck by any sonorous body, it is thrown into waves similar to those of water, but much finer and sharper. From their impression we have an opportunity of seeing what they are, when a drinking-glass, nearly filled with water, is made to give a sweet ringing sound by rubbing the tip of the finger with a gentle and equable motion along the rim of it. The surface of the water appears fretted and curdled into the finest waves that can be imagined by the undulation of the air. Yet the flame of a candle, as the ingenious Mr. Hales of Dublin has observed in his Doctrina Sonorum*, is not visibly agitated: when placed very near to a sonorous body of the greatest magnitude. It does not appear that in the undulations of sound there is any progressive flux of the particles of air; but that they proceed from a vibratory motion of the parts of air in their proper places : so that sound has nothing in it like the motion of the wind; though, as we shall see hereafter, the motion of the wind can act as a cause of sound.

In this particular the fluids of air and wa

ter agree, that the waves of air in sound are reflected from any obstacle as those of water are, or like the rays of light from a speculum. The sound so reflected is called an echo; and the reflection of a sound is so like the reflection of an object from a glass, that the expression of the Latin poets, who call an echo imago vocis, is equally just and elegant.

When a large circular plot of ground is inclosed with a high wall for the purpose of a fruit-garden, which has been the fashion of late years, a curious echo is produced : for when a person places himself in the centre, and claps his hands together, the sound returns very quick to the ear, and is many times repeated. At noon-day, in a garden of this sort, I could perceive it very distinctly eleven times; and if the experiment were made during the stillness of the night, I suppose it might be heard fifteen times or more : but the returns of the echo are so quick and subtile, that some who have not been apprised of this, have disputed the fact. Here the sound has the like advantage, as when the rays of light fall upon a concave mirror. If the luminous body is in the centre of that sphere of which the mirror is a segment, the rays return to a focus in the luminous point itself:

itself: and this is the case with the pulses of air in the present instance. The space through which the sound flies between every pulse of the echo, is equal to the diameter of the circle; and if it is observed to return ten times in a second over the space of 120 feet, its velocity would then be 1200 feet in a second, which is nearly the velocity of sound as commonly estimated. If the ground is level, the echo is less interrupted; on which account the surface of a calm sea is most favourable to the communication of sound. I have been informed by a gentleman of the navy, who had used the sea many years, and was captain of a man of war, that when he has entered an harbour with a high circular shore, his guns have returned a smarter sound from the land than from the ship's side; insomuch, that he had actually been mistaken, on some occasions, and supposed them to proceed from the fort he was saluting.

Air inadequate to the Phænomena of Sounds.

It seems a question more arduous than is commonly supposed, by what means sound is propagated. Natural philosophy has commonly taught, that air is the vehicle of sound: but if sound goes where no air can convey it, through

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through the most solid bodies, and that with the greatest ease, some other cause beside the air must concur. If this phenomenon is ascribed solely to the pulses of the air, the effect will be superior to the cause. The slightest scratching of a pin's point at one end of the largest and longest piece of timber, is heard very distinctly when the ear is applied to the other end, though it cannot be heard at half the distance when we use the air as the vehicle. This must be owing to the intervention of some cause more moveable and more powerful than air itself. If it is supposed that the particles of the wood, which are in contact with the pin's point, give motion to those that lie next them, and so on, till the vibration reaches to the other extremity; the cause is not adequate. It is therefore much easier to conceive that the effect arises from the vibrations of a medium within the pores, easily agitated, and communicating its pulses to any distance, rather than from the action of the solid parts upon one another. Then will this occult communication of sound be similar in some degree to the passage of the electric ether; which goes with difficulty through the air, but flies instantly through the pores of solid bodies.

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