off from the water to the air (the movement of the surface water 

 is slowed dc/m) if the surface water moves against the wind. If 

 wind and waves move in the same direction the water particles 

 move in the direction of the mnd drag while at the crest, but 

 against the drag when in the trough (see fig. 2). In thp' absence 

 of a mass transport velocity the particle velocities at the crest 

 and the trough are equal but in opposite directions, so that the 

 effect of the pulling force of the wind at the wave crest is ex- 

 actly balanced by the effect at the wave trough. In the presence 

 of a mass transport velocity , however, the forv/ard motion at the 

 crest is greater than the backward motion in the trough (fig. k) 

 and a net amount of energy is transferred to the water. No satis- 

 factory explanation of the grov.^th of waves can be given without 

 assumjng a transfer of energy due to the wind pulling at the water 

 particles; and this fact is the best argument for the presence of 

 a mass transport velocity in ocean waves. 



Since the pulling force of the wind over the ocean is knovm, 

 the energy transfer from the air to the water by wind drag can be 

 computed with considerable accuracy from the theoretical values 

 for mass transport velocity given on page 9. Ev^n when the wave 

 velocity exceeds the wind velocity, the effect of the wind drag 

 remains nearly the saiae because it depends uoon the difference 

 between wind velocity and particle velocity in the water, and in 

 general the water particles move much more slowly than the wind 

 even when the v/ave fovuL moves much faster. If the wind can not 

 transfer --.nergy to the v/ater by pulling at the water particles, 



20 



