322 THEORY OF WINDS. 



It has been long known, that there are three 

 great currents of the aerial ocean, by which it is 

 kept in a state of perpetual circulation : one from 

 the polar regions towards the equator, which is 

 an under current ; another from the equator to 

 the poles, which is an upper current ; and a 

 third, called the great equatorial current, or trade 

 wind, which blows from east to west around the 

 globe for about 30 on each side of the equator ; 

 thus sweeping over the whole extent of the tro- 

 pical regions, which are upwards of 3000 miles 

 in width. 



There is another general wind, which blows 

 from west to east in the middle and higher lati- 

 tudes, about two-thirds of the year in the 

 northern hemisphere ; while in the middle lati- 

 tudes of the southern hemisphere, where there is 

 little or no land, it is said to be nearly as uniform 

 as the trade wind. Dr. Hadley, and after him 

 Dr. Franklin, attributed these general currents 

 to the following causes. " The air between the 

 tropics, being constantly heated and rarefied by 

 the vertical sun, rises, when its place is supplied 

 by air from the higher and polar latitudes, which, 

 coming from parts that had less diurnal motion, 

 and not suddenly acquiring the swifter motion 

 of the equatorial regions, becomes an east wind ; 

 the earth moving from west to east, and slipping 

 under the air*" (See Franklin's Works, vol. iii. 

 p. 236.) 



