GROWTH AND DECAY 3 



small swells are running. The grasses sway forward as the top of each 

 crest passes over them, then sway backward under the following 

 trough, to rise again under the next crest (fig. 1) . 



If one watches a floating marker in moderately deep water, and if 

 conditions are favorable for estimating how much it rises and falls, 

 and how far it advances and recedes with the passage of each wave, 

 the observer will also see that the length of each of its horizontal jour- 

 neys is about as great as the vertical distance between the point to 

 which it is raised by the crest and that to which it falls in the trough ; 

 this is, of course, equally true of the water particles in which the 









Figure 1. — The movements of beach grass, over which a low swell is running. 

 (From observations at Cohasset, Massachusetts.) 



object is floating, since it is these water particles that carry it to and 

 fro. and up and down. 



The motions, however, of our marker and of the water particles are 

 not simply upward-forward with the passage of the wave crests, and 

 downward-backward with the passage of the troughs, but form a 

 curved path, corresponding to the convex contours of the wave crests, 

 and the concave contours of the wave troughs. Further, it has long 

 been established, both theoretically and by experiments (usually car- 

 ried out by watching or by photographing the tracks of small particles 

 suspended in the water), that the particles actually move along cir- 

 cular orbits, in a vertical plane parallel to the direction in which the 

 wave forms are advancing (fig. 2) ; when the water is so shoal that 

 the proximity of the bottom interferes with the development of the 

 wave, however, the orbits are rendered more or less elliptical (fig. 3). 



It should be emphasized, too, that all the water particles along 

 any given perpendicular are moving in the same direction at any 



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