252 THE ATMOSPHEEE AND METEOEOLOGY. 



masses of water coining from the two polar seas, corresponds to the 

 equinoctial zone, where the trade-winds of the north-east and south- 

 east meet. Obliged to extend laterally while maintaining themselves 

 under the common level of the maritime reservoir whioh contains 

 them, the tepid waters of the equatorial current flow afterwards 

 towards the north-east parallel to the counter-current of the trade- 

 winds which has risen into the heights of the atmosphere. Under the 

 influence of the same causes, the two oceans of air and water move in 

 the same direction, and their movements are subject to the same 

 oscillations to the north or south during each alternating period of the 

 seasons. In summer, when the Gulf-stream is prolonged far into the 

 northern sea, the double system of the north trade- winds and counter- 

 winds advance several degrees into the temperate zone, while during 

 the winter it flows back again towards the tropic of Cancer, followed 

 by the Gulf-stream, which bends gradually towards the south. The 

 resemblance would be complete if the water were an elastic and com- 

 pressible fluid like air, and were not enclosed in a basin whose borders 

 it could not pass. The difference of the means explains the differ- 

 ence of the currents which the heat of the sun and the terrestrial 

 rotation produce in the ocean and in the atmosphere. 



