PRORMEER AO ket SBA OCHOONS: SIZORE, Are RICA:G 7 7. 
the troughs. This contrast increases as the wave shortens (com- 
pare Figs.1,2,and 5). Thecrests have not a sufficient volume 
of water to fill the troughs, and hence the level of the water 
at rest is lower than the level of the centers of the orbits which 
the surface particles describe. The lifting of the mean position 
of particles above their normal level gives a store of potential 
energy in the wave in addition to the kinetic energy of the 
motion of the water. It may be shown that this energy of 
position is exactly equal in amount to the energy of the water’s 
motion.t. This lifting and this store of potential energy are at 
a maximum when the wave has its greatest steepness (when it 
has the cycloid form). 
The surface layer is thus divided into strips, in one-half of 
which the water is moving forward, while in the other half it is 
moving backward at the same rate. The peculiarity of the case 
lies in the fact that the backward moving strips are broader than 
those moving forward. Fig. 8 shows that the same is true in 
less degree of layers below the surface. Nevertheless, the 
amounts of water moving in the two directions are equal because 
of the greater thickness of the layers in the forward moving 
strips. The contrast of thickness and also of breadth disappears 
with depth. 
MOVEMENTS DURING WIND. 
Effect on size and form.—The immediate effect of wind in the 
direction of wave movement is to accelerate the movement of 
the particles on the crest. It also retards the backward motion 
of those in the trough, but this effect is smaller because these 
particles are largely protected from the wind. The result is 
(1) an increase in the size of the orbits; (2) an increase in the 
differential movement of the particles at the surface; (3) more 
rapid traveling of crest than trough, hence greater steepness in 
front. The first would result in increasing both height and 
length of wave in the same proportion. The second results in 
greater steepness, that is, a shortening in proportion to their 
height. The increased differential movement is accompanied by 
1/bid., p. 132. 
