402 ANNUAL OP SCIENTIFIC DI&COVERY. 



that the trade winds do not extend beyond the parallels of 25 in the Pacific 

 Ocean, while they occupy a wider range in the Atlantic. 



It might be naturally expected that the unequal temperature and density 

 of the water in the tropical and polar seas should cause a great oceanic cir- 

 culation ; the cold currents pursuing their way towards the equator, at the 

 lowest depths of the watery domain, while warm currents flow back along 

 its surface. But from the limited expansibility of water by heat, the ocean 

 is much less sensitive than air to solar influence; and were its bed perfectly 

 smooth and its depth uniform, the action of temperature in producing 

 northern or southern oceanic movements, would be much inferior to that of 

 the earth's rotation in preventing them. The restraints of centrifugal force 

 are, however, neutralized by the asperities of the ocean's bed; for the water 

 rolling over them partakes of their diurnal velocity ; and accordingly 

 oceanic rivers take their journey towards the poles, in places where the 

 roughness of the sea-bottom, or the presence of coasts or submarine ridges, 

 prevent an eastward deflection. To such circumstances the Gulf Stream is 

 indebted for its existence. A vast body of water impelled due north at the 

 rate of three miles an hour in latitude 30 and unimpeded by friction, 

 should be deflected 45 from the meridian in the course of twenty-four 

 miles ; and the small deflection of the Gulf Stream shows the influence of 

 submerged mountains, in modifying its motion and enabling it to continue 

 its advance to colder climates. 



NOTE. Many facts justify an extension of the theory of Espy. The force and 

 extent of the trade winds must be augmented by the heat arising from the conden- 

 sation of vapor in the " equatorial cloud ring," and the high temperature attending 

 the great rains of Southern Asia, may be regarded as a partial cause at least of the 

 southwest monsoon. Discharges of electricity during such rains, must also con- 

 tribute to produce the same result. The inadequacy of the cause to which mon- 

 soons are usually ascribed, has been, noticed by the most eminent writers who have 

 treated on the subject. 



