This elevation would again subside into a hollow, and the process would be continued until the water-motion was brought to rest by friction, or by the gradual dispersion of the original energy. The process by which a wave is started on the surface of water, as a consequence of these two qualities of resistance to being made unlevel and persistence in motion, is beautifully shown by the study of waves made by throwing stones into a pond. The events which give rise to the expanding wave are, however, over so quickly that they can only be studied by the aid of instantaneous photography. The most interesting work on this subject is that of Professor A. M. Worthington, who has photographed, by the exceedingly brief light of an electric spark, the various stages of the events which happen when a drop of water or a stone falls into water.[5] These photographs show us all that happens when the falling object touches the water, and the manner in which it gives rise to the wave or ripple which results. Some of Professor Worthington’s results for a drop of water falling into milk are reproduced in the appended diagrams. In the first place (Fig. 8) the drop is seen just entering the water. As it plunges down, it leaves behind it a cavity, or, as it may be called, a hole in the water (see Fig. 9). [Illustration: Time after contact = ·0262 sec. FIG. 8.] This hole, at a certain stage, begins to fill up. The water rushes in on all sides, and the impetus carries up the inrushing water so that it builds up a tall pillar of water in the place where an instant ago there was a hole (see Fig. 10). No one could anticipate such an extraordinary effect; but the instantaneous photographs, taken by the light of an electric spark, which reveal it, cannot but be truthful. The next stage is that this pillar of water breaks up, and falls back again on the surface. Hence the water, at the place where the drop plunges into it, is subjected to two violent impulses—a downward, succeeded by an up-lifting, force. The effect of this is exactly analogous to that of giving a blow to the interconnected string of balls in the model shown in Fig. 7—it propagates a wave. In Fig. 10 is illustrated the next stage, in which this outward-moving initial wave-crest is shown. [Illustration: Time after contact = ·0391 sec. FIG. 9.] [Illustration: Time after contact = ·101 sec. FIG. 10.] So much for the events revealed by the flash-light of an electric spark; but succeeding these there is a long train of interesting wave-making performance which can be watched with the eye, or the stages photographed with a hand camera. This wave production is best seen when a large stone is thrown into calm water in a lake or pond.