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30 LONGITUDINAL VIBRATIONS. [I. § 15.
stalk of C has just swung back into its erect position. The ears about B are closer to, and those about C further apart from, each other than is the case with those on which the wind has not yet acted.
After this illustration, it will be easy to conceive a kind of wave-motion in which there is no longer any movement transverse to the direction in which the wave is advancing.
15. Let a series of particles, originally at rest in equidistant positions along a straight line, as in that at the head of Fig. 14, be executing equal periodic vibrations in that line, in such a manner that each is the same fixed amount further back in its path than is its neighbour to the left, and therefore exactly as much more forward than is its neighbour to the right.
(0) shows the condition of the row of particles at the moment when that on the extreme left is beginning its swing from left to right, which, in accordance with the direction of the wave's advance as indicated by an arrow in the figure, we may call its forward swing. The equidistant vertical straight lines fix the extent of vibration for each separate particle. The constant amount of relative backwardness or 'retardation/ as it is technically called, between successive particles is, as in Figs. 5