DEPTH OF WAVE ACTION 9 



than 100 feet long (i. e., of one advancing at a rate of about 13 knots) 

 would be S3 miles per hour, and anyone knows that even a 10-mile 

 wind is a very decided obstacle to his own advance, if he is walking 

 against it. And even if the wind dies down entirely, still the waves 

 are opposed by the resistance of the air that they must displace in 

 their advance. The rate at which a counterwind will actually flatten 

 the waves down in any given case, and the rate at which the resistance 

 of the air will do the same to the waves running in a calm, depends 

 largely on the shapes of the waves, by which we mean how nearly 

 streamlined they may be, for it is obvious that the counterpush of a 

 headwind will act much more effectively in this respect on a steep wave 

 and on one of irregular contour than on one that is long, low, and 

 more evenly rounded. And the greater the energy of the wave, the 

 longer will the latter survive. The general rule is that storm seas are 

 reduced much more rapidly in height by head winds or by air resistance 

 than old, low swells are. The latter may even survive a series of head 

 winds, if these are gentle, though a stiff head breeze may kill a swell 

 in short order. 



THE DEPTH OF WAVE ACTION 



If an observer crumples a ball of paper or white cloth, wets it, and 

 then drops it overside from a ship lying at anchor when waves of 

 moderate size are running past her, or from a pier under similar 

 circumstances, and if he watches as this marker slowly sinks, he will 

 see that it continues to circle in the vertical plane as just described, 

 with the passage over it of successive crests and troughs, for as long 

 as it remains visible. He may be able to watch it make several such 

 circuits, for it may remain in sight for as long as a minute, if the water 

 is clear. (This demonstration of the orbital motion of the water 

 particles in wind waves, first suggested by Hagen, is cited from Kii'un- 

 mel, 1911, vol. 2, p. 2.) 



This simple experiment is a visual demonstration of the fact, well- 

 established both by experiment and by theory, that the orbits along 

 which the water particles move continue circular down to the greatest 

 depths to which wave action is perceptible, provided only that the 

 water be deeper than this, as is the case over the open ocean generally. 

 If, however, the water is so shoal that wave action extends right down 

 to the bottom, as may be the case near land, the orbits followed by the 

 water particles become elliptical, until the particles next to the bottom 

 simply surge to and fro without any vertical component of motion 

 at all. 



It has long been know T n that the diameters of the orbits (and, con- 

 sequently, wave action) diminishes from the surface downward, 

 although the period occupied by each water particle in circling its 



