INTRODUCTION. 27 
blage of islands which constitute the Eastern <Ar- 
chipelago; it may, however, be recognized in the 
Indian Ocean, and when bent southward by the 
African coast, and confined by the island of Mada- 
gascar, it forms a current of considerable force, 
which rounds the Cape of Good Hope, and merges 
into the Atlantic. Besides these, there are other 
more local currents, which are not so easily ex- 
plained, such as that which constantly flows out 
of the Baltic, and that which flows into the Me- 
diterranean. In each of these cases, while the 
main current occupies the middle of the channel, 
there is a subordinate current on each side close 
to the shore, which sets in the opposite direction. 
As in the case of the tides, it is obvious how 
serviceable these motions of the sea often are in 
aiding navigation, particularly as they are most 
strong and regular in latitudes where calms often 
prevail. 
And this leads us to consider the action of the 
winds upon the sea, which, though affecting only the 
surface, are the most powerful agents in producing 
the irregular motions of this element: By them the 
freighted bark, with her hardy crew, is wafted to the 
wished for haven; and by them the crested billows 
are roused up, which dash her upon the sharp- pointed 
rocks, or swallow her up in fathomless depths, leav- 
ing none to record her destiny. The origin of wind 
has usually been attributed to the rarefaction of the 
air by heat: a stratum of air near the earth being 
heated by the sun’s rays, or by radiation from the 
surface, becomes lighter, and consequently rises to a 
