18 THE OCEAN. 



that the changes in the atmosphere produced by 

 electricity, which is but another development of the 

 same principle as magnetism, have considerable 

 influence in the production of the variable winds ot 

 temperate regions. Our knowledge of these sub- 

 jects, however, is yet in its infancy; and though iu 

 all ages until the present, navigation has been entirely 

 dependent on the aid of the winds, no laws for their 

 certain prognostication have yet been discovered, and 

 much obscurity, at least in detail, still hangs over 

 their production. But within the tropical regions 

 there are winds which possess great regularity, and 

 may be depended upon with nearly the same precision 

 as the great marine currents already noticed, which 

 indeed they very closely resemble, not only in their 

 direction and their utility, but also in their origin. 

 I refer particularly to the Trade-winds, so named 

 from the facility they afford to commerce, which blow 

 constantly, within the tropics, from the north-east on 

 the north side of the equator, and from the south-east 

 on the south side, the two currents merging near the 

 line into one, which takes an easterly direction. The 

 dividing line, however, is not exactly at the equator, 

 but a little to the north of it. The air in the equato- 

 rial regions becomes strongly heated by the rays of 

 the vertical sun, and rises ; while that from the polar 

 regions moves in to supply its place : thus a northern 

 and a southern current are produced towards the 

 equinoctial. But the earth is revolving from west to 

 east, and the equatorial parts are, as we have before 

 seen, those in which the velocity is greatest : the free 

 air cannot at once acquire this velocity, and is left 



