o give short,
quick shocks to the air; to produce music a body must not only impart
short, quick shocks to the air, but must impart these shocks with
unerring regularity and strength. A flickering light irritates the
eye; a flickering sound or noise irritates the ear; both are painful
because of the sudden and abrupt changes in effect which they cause,
the former on the eye, the latter on the ear.

The only thing essential for the production of a musical sound is that
the waves which reach the ear shall be rapid and regular; it is
immaterial how these waves are produced. If a toothed wheel is mounted
and slowly rotated, and a stiff card is held against the teeth of the
wheel, a distinct tap is heard every time the card strikes the wheel.
But if the wheel is rotated rapidly, the ear ceases to hear the
various taps and recognizes a deep continuous musical tone. The
blending of the individual taps, occurring at regular intervals, has
produced a sustained musical tone. A similar result is obtained if a
card is drawn slowly and then rapidly over the teeth of a comb.

[Illustration: FIG. 176.--A rotating disk.]

That musical tones are due to a succession of regularly timed impulses
is shown most clearly by means of a rotating disk on which are cut two
sets of holes, the outer set equally spaced, and the inner set
unequally spaced (Fig. 176).

If, while the disk is rotating rapidly, a tube is held over the
outside row and air is blown through the tube, a sustained musical
tone will be heard. If, however, the tube is held, during the rotation
of the disk, over the inner row of unequally spaced holes, the musical
tone disappears, and a series of noises take its place. In the first
case, the separate puffs of air followed each other regularly and
blended into one tone; in the second case, the separate puffs of air
followed each other at uncertain and irregular intervals and the
result was noise.

Sound possesses a musical quality only when the waves or pulses follow
each other at absolutely regular intervals.

262. The Effect of the Rapidity of Motion on the Musical Tone
Produced. If the disk is rotated so slowly that less than about 16
puffs are produced in one second, only separate puffs are heard, and a
musical tone is lacking; if, on the other hand, the disk is rotated in
such a way that 16 puffs or more are produced in one second, the
separate puffs will blend together to produce a continuous musical
note of very low