THE TIDAL WAVE. 295 



ant of all these motions is,_as far as the axial rotation of the 

 globe is concerned, = 0. 



In those actions known as the tides, such compensation, 

 however, does not take place ; for the pressure or pull by 

 which they are produced is always stronger from east to west 

 than from west to east. The currents caused by this pull 

 may ebb and flow in different directions, but their motion pre- 

 dominates in that which is opposed to the earth's rotation. 



The velocity of the currents caused by the tide of the at- 

 mosphere amounts, according to Laplace's calculation, to not 

 more than 75 millimetres in a second, or nearly a geographi- 

 cal mile in twenty-four hours ; it is clear that much more 

 powerful effects produced by the sun's heat would hide this 

 action from observation. The influence of these air-currents, 

 however, on the rotatory effect of the earth is, according to 

 the laws of mechanics, exactly the same as it would be were 

 the atmosphere undisturbed by the sun's radiant heat. 



The combined motions of air and water are to be regarded 

 from the same point of view. If we imagine the influence 

 of the sun and that of the interior of our globe not to exist, 

 the motion of the air and ocean from east to west is still left 

 as an obstacle to the axial rotation of the earth. 



The motion of the waters of the ocean from east to west 

 was long ago verified by observation, and it is certain that 

 the tides are the most effectual of the causes to which this 

 great westerly current is to be referred. 



Besides the tidal wave, the lower air-currents moving in 

 the same direction, the trade-winds of the tropics especially, 

 may be assigned as causes of this general movement of the 

 waters. The westerly direction of the latter, however, is not 

 confined to the region of easterly winds ; it is met with in the 

 region of perpetual calms, where it possesses a velocity of 

 several miles a day ; it is observed far away from the tropics 

 both north and south, in regions where westerly winds pre- 



