TEE CAUSE OF THE GLACIAL PERIOD. 469 



tion, tliat when the winters occur in aphelion there will be a 

 great increase of snow-fall and a marked lowering of tem- 

 perature during the winter ; and that, during the summers, 

 heat would have less than its average influence in removing 

 the snows of the previous winter. As already remarked, 

 however, it should be noted that Mr. Croll's calculations 

 upon these two points do not rest upon any difference in the 

 estimate of the absolute amount of heat received during 

 these periods. But he assumes that an excessive amount of 

 heat would be lost from the earth by radiation during the long 

 winters in aphelion ; so that the effect from that cause, when 

 accumulated during a number of centuries, would be marked 

 by a noticeable increase in the glacial fields of the hemisphere 

 whose winter was in aphelion, and this would be connected 

 with the decrease of glaciers in the other hemisphere, whose 

 winters were correspondingly short and in perihelion. 



Having got glaciation started in one hemisphere during 

 periods when the winters were in aphelion, Mr. Croll ad- 

 duces an additional cause to help on the refrigeration, in the 

 effect which this cause itself w r ould have upon the winds and 

 the ocean- currents. He estimates that the heat conveyed by 

 the Gulf Stream into the Atlantic Ocean is equal to one 

 fifth of all the heat possessed by the waters of the North 

 Atlantic ; or to the heat received from the sun upon a mill- 

 ion and a half square miles at the equator, or two million 

 square miles in the temperate zone. " The stoppage of the 

 Gulf Stream would deprive the Atlantic of 77,479,650,000,- 

 000,000,000 foot-pounds of energy in the form of heat per. 

 day." The cause of the Gulf Stream, therefore, becomes a 

 most important element in the problem. What is the force 

 driving onward this immense body of warm water, which he 

 estimates " to be equal to that of a stream fifty miles broad, 

 a thousand feet deep, flowing at the rate of four miles an 

 hour," and whose temperature as it emerges from the Straits 

 of Florida averages as high as 65° Fahr., twenty-five degrees 

 of which is eventually parted with to ameliorate the climate 

 of the North Atlantic \ 



