CURRENTS OF THE OCEAN. 27 
rotatory movement of the earth modifies the direction of these 
atmospheric currents. The movement by which it is carried 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 af 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 ws znertie, 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 worth and south-east trades, often 
sweep the surface of the sea in the latitudes of the temperate zone. 
The two ¢vades 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 grand 
equinoctial current which impels the waters of the east towards the 
