Page:Popular Science Monthly Volume 13.djvu/184

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waves—such pulses as have been described—and the low note by a succession of long waves.

Now, the loudness or softness of a note does not alter its pitch, that is, it does not alter the length of its waves or the rate at which they travel. I can send a wave along the rope either violently or gently,

PSM V13 D184 Sound wave experiment.jpg
Fig. 9.

but with the same tension of the rope we shall find that the length of the waves is the same, provided the period of vibration is the same. Hence, then, the other idea added to the idea of pitch.

There is another point which is worth noting, although it is not needful to refer to it in any great detail, and that is, that we know that sound travels with a certain velocity, and that this rate is subject to certain small variations owing to different causes.

We not only have to deal with amplitude—that is, the departure of the and—parts of the curve from the line AX—and velocity, but we have this most important and very beautiful fact (for fact it is),

PSM V13 D184 Sound waves of different lengths and amplitudes.jpg
Fig. 10.—Sound-Waves of Different Lengths and Amplitudes.

which some will have observed for themselves: If a person in a room in which there is a piano sings a note, the string of a piano tuned to that particular note will respond; and, if he sing another note, then another string will reply, the first string being silent. And if the experimenter were skilled enough to sing one by one all the notes to which the strings of the piano are tuned, all the strings would be set into vibration one by one, note for note. This fact may be explained in this way: a piano-wire, or similar sonorous body, which is constructed to do a certain thing—in this case to sound a particular note—always sounds that note when it is called upon in a proper way to do