THE REVOLUTIONS OP THE CRUST OF THE EARTH. 341 



equator the principal motor of the liquid envelope of the globe. Now, 

 if the difference between these two temperatures was moderated, the 

 intensity and direction of the currents would be modified in propor- 

 tion. In fact, the austral hemisphere is the colder, and from the Ant- 

 arctic pole proceed the principal oceanic currents. The grand current 

 of cold water which passes into the Pacific ocean is divided into two. 

 One arm passes by the Asiatic archipelago and joins a second current 

 of cold water, which expands itself in the Indian Ocean. These two 

 currents united, then pass to the west, around the Cape of Good Hope, 

 and join a third austral current which passes along the western coast of 

 Africa, and finally turns toward America, forming what is called the 

 equatorial current of the Atlantic. Near Cape Saint Roque this current 

 is divided into two; the principal branch enters the Gulf of Mexico and 

 forms, when it emerges, the current of warm water known as the Gulf- 

 Stream. The second branch passes along the coast of Brazil and is lost 

 in the south. 



A modification of the superficial heat does not act directly upon the 

 intensity and direction of the oceanic currents, but rather upon the aerial 

 currents wbich favor them. As there is a close relation between these 

 two phenomena, an examination of their production will allow us per- 

 haps to understand the influence of the cosmic causes upon the climates 

 of the globe. 



The cause of the aerial currents lies in the unequal density of the 

 upper strata and the lower strata heated by contact with the seas, and 

 still more by contact with the continents warmed by solar heat. The 

 lower strata, rarified by the heat, rise in the atmosphere, while the 

 ambient and cooler strata replace them, to be elevated in their turn 

 when they are rarified by the heat. This tendency of atmospheric strata 

 toward the line of the greatest heat continues to the poles, the cold of 

 which tempers the tropical climates, while the ascending air produces 

 currents inverse to the lower currents, and which go to heat the poles, 

 transporting to them at the same time the humidity proceeding from 

 the enormous evaporation of the torrid zone. But as the two hemis- 

 pheres are unequally cold, the air is denser at a less distance in the hem- 

 isphere which is in a glacial state; consequently the currents must be 

 more intense during the greater part of the year. This is in fact the 

 case. Now the trade-winds of the southeast blow with greater force 

 than those of the northeast, and the consequence is that the former 

 extend even to the fifteenth degree of north latitude, while those of the 

 northeast are very rarely felt south of the equator, and almost never 

 beyond the sixth degree of south latitude. The equator of terrestrial 

 heat depends entirely upon these aerial currents, for if we glance at an 

 isothermal chart, we will see that the equator of heat passes for three- 

 quarters of the terrestrial equator to the north of this line, and for one- 

 quarter to the south. This shows in a sufficient manner that the solar 

 heat, accumulated at the south of the equator, is carried by the winds 



