50 THE OCEAN. [Book IT. 



simply onward motion of the earth through space. 

 And therefore, in consequence of the difference in 

 the velocity of rotation in diJfferent latitudes and at 

 different depths of the ocean, the current- creat- 

 ing action of vis-inertise brought into play by the 

 axial rotation of the earth is the great paramount 

 force ; and that which is brought into play by the 

 orbital motion of the earth — as also that brought 

 into play by the motion by which the earth is carried 

 along through space with the solar system — simply 

 causes deviations or variations from that which 

 would be the course of the currents of the ocean if 

 under the sole influence of the axial rotation of the 

 earth. 



We have seen that the westward pressure result- 

 ing from the axial rotation of the earth is fixed and 

 unchanging in its action. But as regards any in- 

 fluence which may result from the orbital motion of 

 the earth, such influence will obviously be not only 

 itself a variable mfluence, acting at one season of the 

 year with greater force than at another season ; that 

 is, acting with its greatest force in December, when 

 the speed of the earth in its orbit is fastest, and with 

 its least force in June, when that speed is slowest ; 

 all intermediate gradations of speed mtervening : 

 but also, it will be constantly changing its direction 

 on any given point of the earth's surface, for at 

 one season of the year the northern hemisphere, and 

 at another season the southern, is inclined in the 



