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46 FREELY EXPANDING SOUNDWAVE. [I. § 22.
swing to a rarefied pulse, and thus, during every complete vibration of the fork one sonorous wave, consisting of a pulse of condensation and a pulse of rarefaction, will be started on its journey along the tube.
22. We have examined the transmission of Sound along a column of air contained in a tube of uniform bore. A more important case is that in which a sound originated at an assigned point spreads out from it freely in all directions. Here we must conceive a series of spherical shells, alternately of condensed and of rarefied air, one inside the other, and having the point of origination of the sound as their common centre. All the shells must be supposed to expand uniformly like an elastic globular balloon constantly inflated with more and more gas. Of course no shell in any two consecutive positions consists of exactly the same materials. The condensations and rarefactions spread progressively: the air-particles merely vibrating about their positions of original rest. The great difference between this case and that last considered lies in the fact, that, as the spherical shells of condensation and rarefaction spread, it is necessary, in order to keep up the wave-motion, to throw larger and larger surfaces of air into vibration; whereas within the tube the transverse section remained the same throughout. Hence, as the same amount of