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 



