Page:Popular Science Monthly Volume 3.djvu/17

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7
WAVE-ACTION IN NATURE.

Toepler caught the air-waves on the instant, and got a glimpse of their circular, and even their shaded aspect.

We have said that "the difference between noise and music is, that in noise the waves strike the ear irregularly, while in music they are regular, and so rapid as to blend together. Any sound which becomes continuous by rapid periodic strokes is said to be musical. "If a watch, for example, could be caused to tick with sufficient rapidity—say one hundred times a second—the ticks would lose their individuality, and blend to a musical tone. And, if the strokes of a pigeon's wings could be accomplished at the same rate, the progress of the bird through the air would be accompanied by music. In the hummingbird, the necessary rapidity is attained; and, when we pass on from birds to insects, where the vibrations are more rapid, we have a musical note as the ordinary accompaniment of the insect's flight."

Sounds vary in pitch, and the pitch depends upon the rate of vibration. The greater the number of vibrations in a second, the shorter and quicker are the waves, and the higher the tone. It has been determined, in various ways, exactly how many vibrations there are in

Fig. 6.
PSM V03 D017 Constitution of air waves.jpg
Constitution of Air-Waves

each musical note. Savart employed a toothed wheel, which could be set in motion at any desired rate of speed, and which had attached a small recording apparatus that gave the number of revolutions in a second. Fig. 7 represents the mechanism, and the mode of using it. While the wheel is in revolution, a thin visiting-card, or a piece of pasteboard, is held against its toothed edge. The card is bent a little by each tooth, as it goes by, and springs back to its first position as soon as it is released. When the wheel is turned slowly, there is heard only a succession of taps, distinctly separable one from another; but, as the rapidity of the rotations increases, the number of strokes increases also, and they soon unite to form a musical sound, while, exactly as the motion is accelerated, the sound rises in pitch. In this way it is possible to count the number of vibrations in producing every note in the musical scale.