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III. §38.] CAUSE OF RESONANCE. 75
moving away from him, helps it along with a vigorous push.
38. The above considerations enable us to explain how a sounding fork can set another fork in unison with itself into vibration through the action of the intervening air. When a continuous musical note is being transmitted, we know that, at any one point we choose to fix upon, the air is undergoing a series of rapid changes, becoming alternately denser, and less dense than it would be but for the passage of the sound. The increase of density is accompanied by an increase of pressure; its diminution by a diminution of pressure [§ 20].
Let A, Fig. 21, be the sounding fork, B that whose vibrations are to be excited by resonance, and let us consider the effect of the alternations of pressure of the air at a on the prong ba. Let the first pulse which arrives at a be one of condensation and let its increased pressure cause the prong ba to swing as far as bc*. The elastic recoil of the fork would
* The extents of vibration shown in this figure are of necessity enormously exaggerated.