38 THE GULF-STREAM. 



rents to ascend and descend in proportion to the 

 amount of heat they possess over or under the heat of 

 the strata into which they flow. In the tropics the 

 heated surface is cooled by the rising of the cold water 

 from below, whilst the siirface of the colder regions is 

 as constantly warmed by the overflow of heated waters 

 from the tropical seas. Both acting on the air render 

 the torrid zone less hot than it would otherwise be, and 

 the colder zones more temperate. 



It would extend this chapter too far to enter into the 

 details of the several great currents which have been 

 ascertained to traverse the ocean, and cause the cir- 

 culation of its waters to be perfectly maintained. We 

 may, however, mention the great Atlantic whirlpool, set 

 in motion by the celebrated gulf-stream, a strong, 

 steady current of warm water which pours out of the 

 Gulf of Mexico, with a mean velocity of 80 miles per 

 day. The waters of the Mexican Gulf are at least 7 

 higher than those of the Atlantic in similar latitudes; 

 and as they are pressed on from the south by the still 

 hotter waters of the Caribbean Sea which enter the 

 Mexican basin between Cuba and the Campeche shore, 

 they are forcibly expelled through the narrow channel 

 separating Cuba from the reefs and keys of Florida. 

 Through this channel the gulf-stream issues in a rapid, 

 warm, but narrow ocean-river, and flows in a north- 

 easterly direction along the coast-line of the United 

 States, gradually widening as it cools and slackens its 

 speed, under the influence of the cold waters which meet 

 it from the north. 



A portion of these latter, forming the northern coun- 



