the lightning and the thunder is brief, because the sound does not have far to travel; if the storm is distant, the interval is much longer, corresponding to the greater distance through which the sound travels.  Sound does not move instantaneously, but requires time for its transmission.  The report of a distant cannon is heard after the flash and smoke are seen; the report of a near cannon is heard the instant the flash is seen.

The speed with which sounds travels through the air, or its velocity, was first measured by noting the interval (54.6 seconds) which elapsed between the flash of a cannon and the sound of the report.  The distance of the cannon from the observer was measured and found to be 61,045 feet, and by dividing this distance by the number of seconds, we find that the distance traveled by sound in one second is approximately 1118 feet.

High notes and low notes, soft notes and shrill notes, all travel at the same rate.  If bass notes traveled faster or slower than soprano notes, or if the delicate tones of the violin traveled faster or slower than the tones of a drum, music would be practically impossible, because at a distance from the source of sound the various tones which should be in unison would be out of time—­some arriving late, some early.

255.  Sound Waves.  Practically everyone knows that a hammock hung with long ropes swings or vibrates more slowly than one hung with short ropes, and that a stone suspended by a long string swings more slowly than one suspended by a short string.  No two rocking chairs vibrate in the same way unless they are exactly alike in shape, size, and material.  An object when disturbed vibrates in a manner peculiar to itself, the vibration being slow, as in the case of the long-roped swing, or quick, as in the case of the short-roped swing.  The time required for a single swing or vibration is called the period of the body, and everything that can vibrate has a characteristic period.  Size and shape determine to a large degree the period of a body; for example, a short, thick tuning fork vibrates more rapidly than a tall slender fork.

[Illustration:  FIG. 171.—­The two hammocks swing differently.]

Some tuning forks when struck vibrate so rapidly that the prongs move back and forth more than 5000 times per second, while other tuning forks vibrate so slowly that the vibrations do not exceed 50 per second.  In either case the distance through which the prongs move is very small and the period is very short, so that the eye can seldom detect the movement itself.  That the prongs are in motion, however, is seen by the action of a pith ball when brought in contact with the prongs (see Section 250).

[Illustration:  FIG. 172.—­The pitch given out by a fork depends upon its shape.]

The disturbance created by a vibrating body is called a wave.

256.  Waves.  While the disturbance which travels out from a sounding body is commonly called a wave, it is by no means like the type of wave best known to us, namely, the water wave.