CUKEENTS OF THE OCEAN. 29 



return ; for on this principle is based the whole system of currents 

 and counter currents." The differences of temperature between 

 equinoctial and polar countries generate two opposing currents, 

 the upper one proceeding from the Equator to the Poles, the lower 

 one directed from the Poles towards the Equator. On reaching the 

 Equator, the cold current of air from the Poles is warmed and rarefied, 

 and ascends to the upper beds of the atmosphere, whence it is again 

 led to its point of departure ; there it is again cooled, and returns 

 with the lower current towards the tropical regions. But the 

 rotatory movement of the earth modifies the direction of these atmo- 

 spheric 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 atmo- 

 sphere 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 par- 

 take of the diurnal motion, would, in consequence of its own vis inertias, 

 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 atmo- 

 spheric beds, which are gifted with much smaller rapidity of motion 

 towards the Poles, and turn them towards the east in consequence. 

 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 neutralizes their impulse towards the west ; in 



