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44 TRANSMISSION OF PULSES [I. § 21.
drawn back to E, (2), as much to the left of its original position, A, (0), as in (1) it was to the right of it. The air in CD, (1), will, while this is taking place, expand into EF; for, being denser, it will also be at a greater pressure than the air to the right of it. It will, therefore, act on this air in the same way as the piston did on the air in contact with it when moving from A, (0), to C, (1). Hence the air in FG will be condensed, G being the point where the air particles are just beginning to be disturbed at the moment when the piston reaches the position E. Thus the air at D advances to F.
Further, in consequence of the backward motion of the piston, the air in the neighbourhood of (7, (1), moves up to E, (2). Thus the air originally in AB now occupies EF, which is greater than AB. It is therefore less dense than in (0), i.e. is in a