Page:Popular Science Monthly Volume 39.djvu/664

646 represent a flash of lightning, the course of a shell, and a gunshot; but we hesitate to resort to that artifice to represent the flight of an arrow through the air or the movement of a sword to strike. It will be well to make only a moderate use of it in representing water. In fact, these parallel lines do not exist, at least not under that form. Furthermore, lines do not show whether the stream is going to the right or to the left, or vice versa.

The effects accompanying the motion of water are, notwithstanding their extreme variety and apparent complication, subject to unchangeable hydraulic laws which it is possible to fix, with the aid of reason and experiment. Observation, even by itself, in the long run, develops an unconscious apperception in the inhabitant of the banks, whether he be fisherman, boatman, or raftsman. Special acquirements enable him to divine, according to the appearance of the surface, a thousand invisible things that are going on under the water. We do not, of course, intend to explore so vast a domain to the bottom, but to indicate how the subject may be approached, and how art and science are benefited by the investigation of it. What are the typical phenomena of running water, which, to simple sight, give rise to the impression of motion in a definite direction, and which are susceptible of being rendered graphically? Let us begin our experiments by fixing a low dam across an even-bottomed channel. Immediately above the dam the interrupted water will form a swell, on the back of which a system of fine parallel striæ may be observed. According to the depth and speed of the current, a second or several similar swells may be formed, but of lessening dimensions (Fig. 1). These are stationary swells, which we call eddy-waves or ripples.

To simplify the matter, we neglect what goes on below the dam. On the other hand, we inquire what happens when the dam is



high enough to force the formation of a small lake. We should be apt to suppose that the water would pass, gradually diminishing in speed and increasing in depth, following a regular curve, from the condition of motion to that of relative repose. This is not the case. The passage is made suddenly, with a shock. The whole system of ripples and striæ which was before immediately at the head of the dam is transferred to the place where the water of the stream strikes against the comparatively still water of the