2 PHYSICAL NATURE OF WIND WAVES 



(he tiniest ripples that stir the surface here and there, when a breeze 

 first springs up, to the fiercest of storm seas; or in a flat calm, the 

 glassy surface may heave itself upward at intervals in the long, smooth 

 ridges of a swell that comes from afar. 



Owing to the extreme mobility of water, to the fact that truly wind- 

 less areas seldom reach far, and to the rapidity with which even a light 

 breeze sets up a series of undulations, it is rare indeed that waves of 

 some sort are not running out at sea anywhere, though they may be so 

 low as to escape notice. And it is unusual to encounter a truly plane 

 sea surface of more than a few hundred yards in extent or for more 

 than a brief period of time, even in coastal waters. 



The natural impression of anyone viewing waves for the first time — 

 or even after viewing them for years, unless he has paid attention — 

 might well be that the mass of water composing each successive crest 

 was moving bodily ahead across the surface of the sea. But it does not 

 require much study to convince the observer that such is not the case. 

 If he watches the movements of any floating object, such as a piece of 

 wood or a seine cork, when waves are running, he will see that his 

 marker does not drift along continuously as it would if the water in 

 which it floats were constantly advancing, but that it moves ahead only 

 a short distance as it is lifted by each crest, to recede again as it de- 

 scends into each successive trough at so nearly the same velocity at 

 which it had advanced that it returns almost to its original position 

 (but see below, p. 6, for further discussion of this last point). On 

 the other hand, no argument is needed to convince one that the wave 

 forms do progress, even if the water particles composing them do not 

 do so to any appreciable extent. 



In short, two distinct types of motion are combined in the advance 

 of a wave. The one, mirrored by the movements of the floating cork, 

 is the oscillatory motion of the water particles of which the waves are 

 composed; the other is the undulatory advance of the wave form. 

 Waves in their advance indeed recall, though they do not truly par- 

 allel, the "waves" that one can see running across a field of grain or 

 tall grass on a windy day, when the tips of the grasses are carried 

 ahead with each gust of wind but then return to their original posi- 

 tions, just as any bit of flotsam nearly does on the surface of the sea. 

 And it is fortunate that this is the case, as has often been pointed out; 

 so rapidly do waves often run that, if the enormous masses of water of 

 which storm waves are composed advanced bodily across the sea, the 

 ocean would not be navigable. 



A convincing demonstration of the forward and backward move- 

 ments of the water particles, with the passage of a wave, can often be 

 obtained if one looks down upon a field of submerged beach grass in an 

 estuarine situation at high tide when the sea surface is unrippled and 



