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I. § 21.] TRANSMISSION OF PULSES. 45
state of rarefaction. Now, let the piston again advance to LI, (3). The air in FG, being at a greater pressure than that in its front and rear, will expand in both directions, causing a new condensation, LM, to be formed further on, and itself becoming the rarefaction KL, co-operating at the same time with the advancing piston to produce in its own rear the condensation UK. In (4) the piston is again where it was in (2). HK has expanded into the rarefaction NO, KL contracted into the condensation OP, LM expanded into the rarefaction PQ, and a new condensation, QR, been formed in front.
The figure makes it clear that each forward stroke of the piston produces a pulse of condensation, and each backward stroke a pulse of rarefaction; but that, when once formed, these pulses travel onwards independently of any external force. They do so, as we have seen, in virtue of the relation which connects the pressure of the air with its density, that is to say, on the elasticity of the air.
If we suppose our movable piston withdrawn from the tube, and a vibrating tuning-fork held with the extremity of one prong close to the orifice of the tube, the conditions of the problem will not be essentially modified. Each outward swing of the prong will give rise to a condensed, and each inward