GBEAT CURRElNTS OF THE OCEAN. 7 



surface of water, will put it in motion and to some extent 

 produce a current, is so evident, that it must be and indeed is 

 admitted : but it is said that currents produced by this cause 

 are superficial, whilst the tropical currents are of great depth. 

 The depth of the current, however, may depend on the velocity 

 with which the wind blows, the constancy of its action, and 

 the extent of water on which it acts. When the wind first 

 presses on the water, it appears to act on the surface alone ; 

 but when that surface is put in motion, the upper water, 

 while in motion, presses on that which is lower, and carries it 

 also forward in a horizontal direction ; and this pressure 

 of the water while in motion is propagated to greater depths, 

 so long as the pressure of the wind on the surface is 

 continued. For the wind, moving as it does with greater 

 velocity than the water, exerts its force in every successive 

 instant of time, like gravity in the descent of bodies, and that 

 force is added to all the previous effect that had been pro- 

 duced. And over a wide ocean, there is no reason to be 

 assigned that the pressure of wind, acting constantly on the 

 surface of water, should not give motion to that water even at 

 great depths. 



The tropical trade wind of the Pacific Ocean, in which 

 exists one of the great oceanic currents that have been named, 

 is first found moving slowly near to the Galapagos islands, in 

 say about 90 degrees of west longitude, where it produces but 

 a slight eiFect on the water of the ocean : but it continues 

 blowing westward, and generally with increasing velocity, over 

 not less than say 120 degrees of longitude, or 7,200 geogra- 

 phical miles; there is therefore, over this ocean, sufficient 

 space to permit the action of the comparatively rapid wind on 

 the surface of the water to press that water forward with 

 increasing rapidity, and to greater and greater depths, and 

 the current thus created, by the wind alone, may, it is con- 

 tended, be found to extend to great depths. 



Another wind of the tropical regions, — the trade wind of 



