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ON THE INTERFERENCE OF SOUND, AND ON 'BEATS'.
74. In § 71 we examined the principle on which the general problem of the composition of vibrations is solved. We now approach certain very important particular cases of that problem, which it will be worth while to solve both independently and as instances of the method repeatedly applied in § 72.
Suppose that a particle of air is vibrating between the extreme positions A and B while convey-
ing a sustained simple tone produced by a tuning-fork, or stopped flue-pipe. Now let a second instrument of the same kind be caused to emit a tone exactly in unison with the first. We will assume that, when the waves of the second tone reach the particle, it is just on the point of starting from A towards B. Two extreme cases are now possible, depending on the movement which the particle would have executed in virtue of the later-impressed vibration alone.