146 POPULAR LECTURES AND ADDRESSES. 



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 (Fig. 19), while the surface water 

 throughout the whole length will be observed 

 moving in the direction of the wind say in the 

 direction of the two arrows near to the surface of 

 the water above and below it. But to re-establish 

 the disturbed hydrostatic equilibrium, the water 

 so heaped up will tend to flow back to the end 

 from which it has been displaced, and as the wind 

 prevents this taking place by a surface current, 

 there will be set up a return current along the 

 bottom of the canal, in a direction opposite to 

 that of the wind, as indicated by the lowermost 



