THE CAUSE OF THE GLACIAL PERIOD. 473 



Gulf Stream are projected through the Straits of Florida with 

 a force sufficient to cany them across the Atlantic and to the 

 shores of Iceland and Norway. So far Mr. Croll's explica- 

 tion is certainly very plausible, and seems to proceed from 

 well-known physical principles, and is pretty generally ac- 

 cepted by scientific men. 



This, however, is only the first step in his argument. 

 Another glance at the map will show that if from any cause 

 the relations of the trade-winds should be reversed, so that 

 the northeast trades should predominate over the southeast, 

 and extend some degrees south of the equator, then Cape St. 

 JRoque would intercept the movement caused by the south- 

 east trades, and the warm water from the South Atlantic, 

 which is now forced into the Caribbean Sea, would all of it 

 do what part of it now does, namely, turn to the south, and, 

 after following for a while the southwestern trend of the 

 South American coast, would join the slow-moving whirlpool 

 of the South Atlantic, whose center is on the parallel joining 

 Montevideo and Cape Colony. It will thus appear that, in 

 searching for the cause of the Gulf Stream, we are ultimately 

 compelled to search for the cause of the present preponder- 

 ance of the southeast trades in the Atlantic Ocean. This 

 sends us backward upon a receding series of causes. 



The southeast trades preponderate because the southern 

 hemisphere is cooler than the northern hemisphere, for wind 

 is but the movement of the cooler and therefore heavier at- 

 mosphere of one region toward a partial vacuum produced 

 by a superior degree of heat in another. This conclusion 

 pushes us back one step further to find the cause for the 

 present lower relative temperature of the southern hemi- 

 sphere, and here we strike what is probably a coincidence, 

 but which Mr. Croll and his followers have too readily ac- 

 cepted as a cause. Mr. Croll thinks he finds the cause of 

 the low temperature of the southern hemisphere in the pres- 

 ent prevalence there, in moderate degree, of the astronomical 

 conditions to which he has attributed the production of gla- 

 cial periods. The winters of the southern hemisphere now 



