x The Hydrosphere 177 



239. Wind- waves. Difference of barometric pressure 

 over a large sheet of water causes a slight change of level 

 and sets up a to-and-fro surge, known as a seiche in the 

 Swiss lakes, without the action of wind. The air, being 

 more mobile, obeys the direct touch of solar energy much 

 more readily and rapidly than water, to which motion is, 

 however, imparted by wind. Part of the water surface 

 yields to the stress of wind striking it obliquely, and is 

 depressed, thereby ridging up the neighbouring portions 

 and originating a wave, the form of which advances as a 

 line of rollers before the wind. OnJyJtheJform advances, 

 for while the particles of water in the crest of a wave are 

 moving rapidly forward, those in the trough move back to 

 almost exactly the same extent. Thus rollers merely lift 

 and lower the vessels that float upon them. Water being 

 an elastic substance continues to swing up and down as a 

 swell after the wind which produced the motion has died 

 away, just as a pendulum continues to swing after the hand 

 setting it in motion is withdrawn. Waves may be trans- 

 mitted from a great distance, and as wind is always blowing 

 somewhere the surface of the ocean is never quite at rest. 

 When a wave enters gradually shallowing water the lower 

 part is retarded by friction, and the upper part sweeps for- 

 ward more rapidly. The wave becomes steeper and shorter, 

 and finally the top curves over in a hollow sheet of clear 

 water, which breaks with a roar into foam and spray, the 

 roller becoming a breaker. Sailors are in the habit of 

 speaking of waves as " mountains high," but this is only a 

 metaphor. The highest wind-waves that have been mea- 

 sured have an amplitude of only 50 feet from trough to 

 crest, and a length of about a quarter of a mile between 

 successive crests. Earthquakes raise waves of much greater 

 height and destructive power than either tide or wind. The 

 wave form travels over the sea at a rate depending on the 

 size of the wave and the depth of the water, the maximum 

 speed being about 80 miles an hour. At the depth of 100 

 fathoms the greatest waves produce a movement too slight, as 

 a rule, to affect anything but the finest mud, and probably wave- 

 motion never penetrates to as great a depth as 5 oo fathoms. 



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