INTRODUCTION. 99 
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 certaiu 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. 
