8 PHYSICAL NATURE OF WIND WAVES 



not only on how great the difference in velocity is between the two, 

 but also on the shapes of the waves, for the more nearly streamlined 

 these are, relative to the wind, the less hold does the latter have on them. 



The wind, however, exerts its friction, not only on the upwind side 

 of the waves, but also on their troughs. But its effect is different on 

 different parts of a wave, for while it tends to speed up the water 

 particles at the crest, because these are also moving in the same direc- 

 tion as the wind, it tends to slow down those in the trough, since 

 these are moving against the wind. And we should point out that if 

 a strong wind be blowing, the effects of its drag on the wave will be 

 the same, even if the wave form be advancing faster than the wind, 

 because the velocity of the latter always is much greater than the orbital 

 velocities of the water particles of which the wave is composed. 



The wind also exerts a suction on the leeward slopes of the crests, 

 if the waves are traveling more slowly than the wind, much as it does 

 on the leeward side of a sail. And, while the combination of these 

 actions requires complex computation for its exact evaluation, the net 

 result is that waves continue to gain both in height and in length 

 until they reach the maximum heights to which a wind of given 

 strength can lift them (see p. 20) ; or if they have already reached 

 that limiting height, they still continue to gain in length. Meantime, 

 newer and smaller waves are constantly being formed on the older 

 and longer ones, into which they then become incorporated, and so 

 on throughout the period during which the w 7 aves are gaining energy 

 from the wind. Thus each of the higher crests to be seen at any 

 given moment is really a combination of an indeterminate number of 

 smaller waves of successive generations. 



Every seaman knows that after a blow passes, the storm waves 

 that accompany it die down before long, and that a counterwind knocks 

 the sea down very soon indeed. At first sight, this might seem to 

 contradict the rule stated above, that waves of oscillation continue 

 to run, once they are set in motion by the wind. But there is no real 

 contradiction, for it is only when the wave is not opposed by any 

 counterf orce that it conserves its energy and hence its form. And it is 

 obvious that a wave is opposed, not only by the interference of any 

 cross sea that is set in motion when the wind changes, but still more 

 strongly by the counterthrust and counter drag of a wind that springs 

 up against the run of the waves. A strong wind from a new direction 

 may, in fact, flatten the waves with spectacular abruptness. Many 

 times we have seen a tumultuous sea killed in this way within a few 

 hours, as has everyone who has traversed the more stormy parts of 

 the ocean ; nor is there anything astonishing in this. 



The positive difference in velocity, for example, between a counter- 

 wind of 20 miles per hour and the rate of advance of a wave no more 



