ii 



PHYSICAL NATUKK OF WIND WAVES 



its velocity would be about '2,1 knots (p. 35), but the velocities of its 

 water particles would be only about '2 knots. And in the case of long, 

 low swells, the orbital velocities may be as little as 1/100 to 1/200 as 

 great as the velocity of the wave. Consequently, the effects of the 

 alternating following and opposing motions due to the orbital veloc- 

 ities of water particles that a ship encounters are so small as to be 

 negligible; then too, it is only for a very brief instant when the water 

 part icles are at the top of each of their orbits (at the crest of the wave) , 

 or at the bottom (in the trough of the wave), that their movement is 

 purely horizontal. 



We should also note, in passing, that the passage of a wave also 

 involves some actual progress of the water particles in the same di- 

 rection, as has long been appreciated. In the case of small and not 

 very steep waves, this "heave of the sea," as it is sometimes called, is 

 so small that it is not of practical importance in navigation, though it 

 may be for drifting objects; it is somewhat larger for very large waves, 

 especially if these are very steep. (See table 1.) 



Table 1. — Velocity, in knots, of mass transport at the surf ace for waves of various 

 heights, periods, and lengths 



[Derived from the basic formula u' = ( -j)C, where u' is the velocity of mass transport, H the height of 

 the wave, L its length, and C its velocity] 



This mass transport, caused by the orbital motions of the water 

 particles with the passage of waves, has no direct connection with the 

 drifts, or currents, that are set in motion by the frictional drag of the 

 wind across the water, though it often is in the same general direction. 



THE GROWTH AND DECAY OF WAVES 



Whoever watches a passing "cat's paw" of wind as it ruffles the glassy 

 surface on a calm day sees the first stage in the process of wave forma- 

 tion by the wind. But while it is obvious enough why the wind blow- 

 ing across the surface of the sea should start a mass movement of water 

 in the same general direction, i. e., should set up a wind current 

 it is not so apparent why the wind should first transform the previ- 

 ously level surface of the sea into a series of minute undulations 

 and then build up these tiny crests and troughs to the very considerable 



