646 THE POPULAR SCIENCE MONTHLY. 



represent a flash of lightning, the course of a shell, and a gun- 

 shot ; 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 rep- 

 resenting 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, notwith- 

 standing their extreme variety and apparent complication, sub- 

 ject 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, accord- 

 ing 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 bene- 

 fited by the investigation of it. What are the typical phenomena 

 of running water, which, to simple sight, give rise to the impres- 

 sion 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. Imme- 

 diately above the dam the interrupted water will form a swell, 

 on the back of which a system of fine parallel striae may be ob- 

 served. According to the depth and speed of the current, a sec- 

 ond 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 



Fig. 1. 



high enough to force the formation of a small lake. We should 

 be apt to suppose that the water would pass, gradually diminish- 

 ing 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 strise 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 



