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I. § 18.] CONDENSATION AND RAREFACTION. 35
tion has occurred. On the other hand, when a portion of the filament is lengthened, a smaller quantity is made to occupy the space before occupied by a larger quantity. Here the matter is more loosely packed, more rare, than it was, i. e. a process of rarefaction has taken place.
Let us now suppose the particles of the filament to be thrown into successive vibrations in the manner already so fully explained. Alternate states of condensation and rarefaction will then travel along the filament. It will be convenient to call these states ' pulses'—of condensation or rarefaction as the case may be. A pulse of condensation and a pulse of rarefaction together make up a complete wave.
18. The degree of condensation, or rarefaction, existing at any given point of a wave has been shown to depend on the mode in which the particles of the filament vibrate. It is therefore desirable to have some simple method, appealing directly to the eye, of exhibiting the law of any assigned mode of vibration. We may arrive at such a method by the following considerations.
When a line of particles vibrate longitudinally, they give rise to alternate pulses of condensation and rarefaction; when ' transversely,' they produce
alternate protuberances on opposite sides of the line