Wind and Gravitation Theories of Oceanic Circulation. 217 



Bat this "vast movement of antarctic water" necessarily implies 

 a vast counter movement of warm surface-water; so that if 

 there is more polar water in the South Atlantic to produce the 

 cooling effect, there should likewise be more warm water to be 

 cooled. 



According to the wind theory of oceanic circulation the expla- 

 nation of the whole is simple and obvious. It has already been 

 shown that owing to the fact that the south-east trades are 

 stronger than the north-east, and blow constantly over upon the 

 northern hemisphere, the warm surface-water of the South 

 Atlantic is drifted across the equator. It is then carried by 

 the equatorial current into the Gulf of Mexico, and afterwards 

 of course forms a part of the Gulf-stream. 



The North Atlantic, on the other hand, not only does not lose 

 its surface-heat like the equatorial and South Atlantic, but it re- 

 ceives from the Gulf-stream, in the form of warm water, an 

 amount of heat, as we have seen^ equal to one fourth of all the 

 heat which it receives from the sun. The reason why the warm 

 surface-strata are so much thicker in the North Atlantic than 

 in the equatorial regions is perfectly obvious. The surface- 

 water at the equator is swept into the Gulf of Mexico by the 

 trade-winds and the equatorial current as rapidly as it is heated 

 by the sun, so that it has not time to gather to any great depth. 

 But all this warm water is carried by the Gulf-stream into the 

 North Atlantic, where it accumulates. That this great depth 

 of warm water in the North Atlantic, represented in the section, 

 is derived from the Gulf-stream, and not from a direct flow from 

 the equator due to gravitation, is further evident from the fact 

 that temperature-sounding A in latitude 38° N. is made through 

 that immense body of warm water, upwards of 300 fathoms 

 thick, extending from Bermuda to near the Azores, discovered by 

 the ■ Challenger 3 expedition, and justly regarded by Captain 

 Nares as an offshoot of the Gulf-stream. This, in Captain 

 Nares's report, is No. 8 " temperature-sounding," between Ber- 

 muda and the Azores; sounding B is No. 6 u temperature- 

 urve," between Teneriffe and St. Thomas. 



There is an additional reason to the one already stated why 

 the surface-temperature of the South Atlantic should be so much 

 below that of the North. It is perfectly true that whatever 

 amount of water is transferred from the southern hemisphere to 

 the northern must be compensated by an equal amount from 

 the northern to the southern hemisphere ; nevertheless the warm 

 water which is carried off the South Atlantic by the winds is not 

 directly compensated by water from the north, but by that cold 

 antarctic current whose existence is so well known to mariners 

 from the immense masses of ice which it brings from the South- 

 ern Ocean. 



