Page:Popular Science Monthly Volume 14.djvu/75

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OUND is the sensation peculiar to the ear. This sensation is caused by rapidly-succeeding to-and-fro motions of the air which touches the outside surface of the drum-skin of the ear. These to-and-fro motions may be given to the air by a distant body, like a string of a violin. The string moves to and fro, that is, it vibrates. These vibrations of the string act on the bridge of the violin, which rests on the belly or sounding-board of the instrument. The surface of the sounding-board is thus set trembling, and these tremors, or vibrations, spread through the air in all directions around the instrument, somewhat in the manner that water-waves spread around the place where a stone has been dropped into a quiet pond. These tremors of the air, however, are not sound, but the cause of sound. Sound, as we have said, is a sensation; but, as the cause of this sensation is always vibration, we call those vibrations which give this sensation sonorous vibrations. Thus, if we examine attentively the vibrating string of the violin, we shall see that it looks like a shadowy spindle, showing that the string swings quickly to and fro; but, on closing the ears, the sensation of sound disappears, and there remains to us only the sight of the quick to-and-fro motion which the moment before caused the sound.

Behind the drum-skin of the ear is a jointed chain of three little bones. The one, H in Fig. 4, attached to the drum-skin, is called the hammer; the next. A, is called the anvil; the third, S, has the exact form of a stirrup, and is called the stirrup-bone. This last bone of the chain is attached to an oval membrane, which is a little larger than the foot of the stirrup. This oval membrane closes a hole opening into the cavity forming the inner ear; a cavity tunneled out of the hardest bone of the head, and having a very complex form. The oval hole just spoken of opens into a globular portion of the cavity, known as the vestibule; and from this lead three semicircular canals, SC, and also a cavity, C, of such a marked resemblance to a snail's shell that it is called cochlea, the Latin word for that object. The cavity of the inner ear is filled with a liquid, in which spread out the delicate fibres of the auditory nerve.

Let us consider how this wonderful little instrument acts when sonorous vibrations reach it. Imagine the violin-string vibrating 500 times in one second. The sounding-board also makes 500 vibrations in a second. The air touching the violin is set trembling with 500 tremors a second, and these tremors speed with a velocity of 1,100 feet