CURRENTS OF THE OCEAN. • 2/ 



rotatory movement of the earth modifies the direction of these 

 atmospheric currents. The movement by which it is earned from 

 west to east being almost nothing at the Poles, but inconceivably 

 rapid under the Equator, it follows that the cold air, in proportion as 

 it advances towards the Tropics, ought to incline a little towards the 

 west. This is just what takes place with these counter currents. 

 The north-east trade winds, which prevail in the northern hemisphere, 

 move in a sort of spiral curve, turning to the west as they rush from 

 the Poles to the Equator, and in the opposite direction as they move 

 from the Equator towards the Poles : the immediate cause of this 

 motion being the rotation of the earth on its axis. "The earth," 

 says Dr. Maury, " moves from west to east. Now, if we imagine a 

 particle of atmosphere at the North Pole, where it is at rest, to be 

 put in motion in a straight line towards the Equator, we can easily 

 see how this particle of air, coming from the very axis of diurnal 

 rotation, where it did not partake of the diurnal motion, would, in 

 consequence of its own vis ifiertice, find as it travelled south that the 

 earth was slipping from under it, as it were, and it would appear to 

 be coming from the north-east and going towards the south-west ; in 

 other words, it would be a north-east wind." 



In the same manner, the upper currents of air, which proceed 

 towards the Poles with equatorial rapidity, ought to outstrip the 

 atmospheric beds, which are gifted with much smaller rapidity of 

 motion towards the Poles, and turn them towards the east in conse- 

 quence. These are the south-west and north-west counter trade- 

 winds, which, passing above the north and south-east trades, often 

 sweep the surface of the sea in the latitudes of the temperate zone. 

 The two trades are separated by a belt more or less broad, where the 

 friction experienced at the surface of the sea neutralises their impulse 

 towards the west ; in general, the current of air there is an ascending 

 current. This belt, which does not exactly correspond with the 

 Equator, is called the Zone of Calms, where atmospheric tempests 

 frequently occur, and the winds make the entire tour of the compass, 

 which has acquired for them the name of tornadoes. 



The trade-winds, whose movement towards the west is retarded 

 by the friction which the waves of the ocean oppose to them, com- 

 municate to these waves, by a sort of reaction, a tendency towards 

 the west, or, to speak more exactly, towards the south-west in the 

 northern hemisphere, and towards the north-west in the opposite 

 hemisphere. The currents on the surface of the water which result 

 from this reaction, reunite under the Equator, and form the grajid 

 equinoctial current which impels the waters of the east towards the 



