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I. § 5.] TRANSMISSION OF WA VES. 9
differing from it in horizontal position. Let the dotted line A'B' represent the curve thus formed. As the two outlines AB and A'B' are exactly alike, the joint effect produced by the separate particle-movements on the eye of a spectator is just what it would have been had we pushed the curve AB along horizontally until it came to occupy the position A'B'. In order further to illustrate this point, let us suppose that a hundred men are standing in a line and that the first ten are ordered to kneel down : a spectator who is too far off to distinguish individuals will merely see a broken line like that in the figure below.
Now, suppose that after one second the eleventh man is ordered to kneel and the first to stand; after two seconds the twelfth man to kneel and the second to stand; and so on. There will then continue to be a row of ten kneeling men, but during each second it will be shifted one place along the line. The distant observer will therefore see a depression steadily advancing along the line. The state of things presented to his eye after twenty, sixty and ninety seconds, respectively, is shown in Fig. 2.