[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).
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