GROWTH AND DECAY 7 



heights to which waves actually rise. The explanation most com- 

 monly offered in the older writings is that the gustiness of the wind, 

 pressing upon the surface more strongly in some places and less 

 strongly in others, is responsible by producing depressions and eleva- 

 tions, which then run ahead as waves. And it is certain that this 

 does happen when strong gusts of wind strike the water here and 

 there with what might be termed a plunging motion. This we ob- 

 served when looking out across a flooded meadow, during a recent 

 gale when each of the more violent gusts instantaneously produced a 

 well-marked depression a few inches deep and several yards across, 

 preceded by an equally well-marked elevation several inches high and 

 advancing at a velocity much greater than that of the smaller wavelets. 



It is equally certain, however, that this is not the usual process by 

 which the ripples, that are set up when the surface of the sea is first 

 ruffled, grow into waves, for while gusts of wind are apt to extend 

 over areas at least some yards in extent ( as any one can see who watches 

 a field of grain waving under the wind) and are often to be measured in 

 acres or even larger units, the first tiny ripples are only a few centi- 

 meters long and few millimeters high. The wave pattern is thus far 

 too small to fit the pattern of gustiness. Further, these tiny ripples 

 are at first astonishingly regular in arcs of long radius; that is to say, 

 they are also much too regular to fit the wind pattern. In fact, no 

 fully satisfactory explanation, how the wind does produce waves 

 from ripples, has yet been offered. There is, moreover, a clean-cut 

 difference in physical nature between the one and the other. The 

 smallest ripples are what is known as "capillary" in nature, i. e., they 

 are due to the surface tension of water, not to the force of gravity. 

 They arise instantaneously when a breeze springs up, to die down 

 when the breeze dies; and they advance the more rapidly the smaller 

 they are, whereas the larger gravitational waves advance the more 

 rapidly the longer they are and may continue to run long after the 

 originating force has ceased to act upon them. 



Capillary or "ripple" waves become transformed, somehow, into 

 ordinary gravitational waves when they reach a length of about 0.68 

 inches from crest to crest, and are moving at a velocity of about 0.76 

 feet per second. It has been variously reported that it requires a wind 

 of about one half nautical mile per hour to about 2 nautical miles per 

 hour to generate ripples. A stronger wind alters ripples into gravi- 

 tational waves. Once the alteration has taken place, the waves continue 

 to receive energy from the wind and, hence, to increase in size by the 

 direct push of the wind against the upwind slopes of their crests 

 and by its f rictional pull upon the water. The first of these processes 

 acts only as long as the wind is blowing at a velocity greater than 

 that of the waves, And its efficiency in building up the latter depends 



