58 THE WATER 



winds which drive the surface water along. These 

 terms, however, are not always accurately used. 



There is yet a third type of current, which either 

 upwells from below or sinks from above, its movement 

 being determined by the relations which its tempera- 

 ture and salinity bear to those of the adjacent water. 



Surface Currents. 



If we begin with the surface movements of the 

 ATLANTIC OCEAN as being better known than the 

 others, and simpler in so far as it is not studded with 

 islands like the Pacific, nor subject to monsoonal dis- 

 turbance like the Indian the most prominent is the Gulf 

 Stream. The North Equatorial Drift and the north- 

 westerly arm of the South Equatorial Drift (Chart V., 

 p. 60), hurried along in the first instance by their respec- 

 tive trade winds, strike ultimately on the Windward 

 Islands ; while a part of their water skirts the outside 

 of the West Indian chain, in a north-westerly direction, 

 much passes into the Caribbean Sea, and thence, in 

 a highly heated condition, through the Straits of 

 Yucatan into the Gulf of Mexico at anything from 

 15 to 70 knots a day. The water thus driven into the 

 gulf, supplemented by the great volume of the Missis- 

 sippi, forms a " hydrostatic " reservoir, the level of 

 which is something like 40 inches above mean sea-level 

 at Sandy Hook. Having practically only one outlet, 

 the Straits of Florida, it rushes through them with 

 a mean annual temperature of over 80 F., and at a 

 rate of 20 to 100 knots a day, passes over the Blake 

 plateau which it sweeps clean, and debouches into 

 the main ocean about 30 N., where it is joined by 

 that branch which skirted the outside of the West 

 Indian Islands. Here it is diverted, chiefly by preva- 



