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76 CAUSE OF RESONANCE. [III. §38.
alone cause the prong to spring back to an equally inclined position on the other side of ba. Since, however, the period of vibration is by supposition the same for the two forks, a pulse of rarefaction, entailing diminished pressure, will arrive at a just as the prong is passing through its erect position 6a. The prong will therefore swing a little further to the left than it would otherwise have done, say to bd, making the angle abd slightly larger than the angle ahc. The prong then starts to the right and is, while passing through the position ba, again helped on by the arrival there of a new pulse of condensation. Thus the alternate condensations and rarefactions of the Sound-waves play towards the fork exactly the parts sustained by the two boys in our illustration of the swing. Accordingly, these air-impulses are applied under precisely the conditions which we saw to be most favourable to the rapid development of vibratory motion. Their large number makes up for the feebleness of each separate impulse. Accordingly resonance is produced more slowly between unison-forks of low than between those of high pitch. I find that, with two forks making 256 vibrations per second, about one second is requisite to bring out an audible resonance ; while with another pair, making 1,920 vibrations per second, I can with difficulty damp the first fork sufficiently