INTRODUCTION. 09 



dians of the magnetic poles, that of the American 

 cutting the West Indies, and that of the Siberian 

 the China Sea, are peculiarly liable to tempests ; 

 the hurricanes of the former, and the typhoons of 

 the latter, being well known.* It is pretty certain, 

 also, 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 

 of temperate regions. ' Our knowledge of these sub- 

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

 in 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 dis- 

 covered, and much obscurity, at least in detail, still 

 hangs over their production. But within the tro- 

 pical 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 re- 

 semble, 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 tro- 

 pics, 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, how- 

 ever, is not exactly at the equator, but a little to the 

 north of it. The air in the equatorial regions be- 

 comes strongly heated by the rays of the vertical sun, 



* See Reid on Storms. 



