290 KELVIN 



wind waves, and the great swell of the ocean from one hem- 

 isphere to the other and back again (under the name which 

 I find in the harmonic reduction of tidal observations), 

 proved to take place once a year, and which I can only ex- 

 plain as the result of the sun's heat. 



But while the action of the sun's heat by means of the 

 wind produces ripples and waves of every size, it also pro- 

 duces a heaping-up of the water as illustrated by this dia- 



FIG. 123 Showing the heaping-up of water produced by wind 



gram (FiG. 123). Suppose we have wind blowing across 

 one side of a sheet of water, the wind ruffles the surface, 

 the waves break if the wind is strong, and the result is a 

 strong tangential force exerted by the wind on the surface 

 water. If a ship is sailing over the water there is strong 

 tangential force; thus the water is found going fast to lee- 

 ward for a long distance astern of a great ship sailing with 

 a side wind: and, just as the sails of a ship standing high 

 above the sea give a large area for the wind to act upon, 

 every wave standing up gives a surface, and we have hori- 

 zontal tangential force over the whole surface of a troubled 

 sea. The result is that water is dragged along the surface 

 from one side of the ocean to the other from one side of 

 the Atlantic to the other and is heaped up on the side 

 towards which the wind is blowing. To understand the 

 dynamics of this phenomenon, think of a long straight canal 

 with the wind blowing lengthwise along it. In virtue of 

 the tangential force exerted on the surface of the water by 

 the wind, and which increases with the speed of the wind, 

 the water will become heaped up at one end of the canal, as 

 shown in the diagram (Fie. 123), while the surface water 

 throughout the whole length will be observed moving in the 



