Chap. XVII.] THE WINDS AND EVAPORATION. 183 



whether it be owing to difference of temperature or 

 to difference of saltness, &c., it is a difference that 

 disturbs equilibrium, and currents are the conse- 

 quence.' ^ 



Though Sir John Herschel argues that the Gulf 

 Stream is caused by the combmed action of the NE. 

 and the SE. Trade Winds, and in the extract on the 

 opposite page Captain Maury argues that it is not 

 caused by the NE. Trade Wind, the latter argument 

 is, nevertheless, applicable against the theory which 

 makes (as Sir John Herschel does) the winds in 

 general the princijaal cause of ocean currents. A 

 general accordance of the movements of the atmo- 

 sphere and ocean — such as an average motion west- 

 wards in the equatorial regions and eastwards in the 

 temperate zones — would naturally result from their 

 being effects of the same cause, and must not be 

 regarded as a proof that they are related as cause 

 and effect. I cannot pretend to do justice to the 

 manner in which the theories of the winds and 

 specific gravity are enforced in the interesting works 

 in which they are respectively maintained. I quote, 

 however, once more from the same article by Sir 

 John Herschel, to show the nature of his objections 

 to the theory which makes differences of specific 

 gravity the principal cause of the currents which 

 exist in the ocean. Allowing that ' Sea-water, by 

 evaporation, acquires additional saltness and density, 

 ' Sec. 406 of the work just quoted. 



