CAUSE OF MILD POLAR CLIMATES. 151 



impelling force of the winds. Take, for example, the 

 case of the Gulf Stream. If the amount of heat 

 conveyed from intertropical regions into the North 

 Atlantic, by means of difference of density resulting 

 from difference of temperature, were equal to that 

 conveyed by the Gulf Stream, it would follow, as has 

 been proved, * that the Atlantic would be far warmer 

 in temperate and Arctic than in intertropical regions. 

 Taking the annual quantity of heat received from the 

 sun per unit surface at the equator as 1000, the 

 quantities received by the three zones would be 

 respectively as follows : — 



Equator 1000 



Torrid zone 975 



Temperate zone . . . 757 



Frigid zone 454 



Assume, then, that as much heat is conveyed from 

 intertropical regions into the Atlantic and Arctic seas 

 by this circulation from difference of specific gravity 

 as by the Gulf Stream, and assume also that one half 

 of the total heat conveyed by the two systems of 

 circulation goes to warm the Arctic Ocean, and the 

 other half remains in temperate regions, the following 

 would then be the relative quantities of heat possessed 

 by the three zones : — 



Atlantic in torrid zone . . . 671 

 „ in temperate zone . 940 

 „ in frigid zone . . . 766 



There is a still more formidable objection to the 

 theory. It has been demonstrated, from the 

 temperature - soundings made by the ' Challenger ' 

 Expedition,-)- that the general surface of the North 



* 'Climate and Time,' Chap. XI. ; "Phil. Mag.," March, 1874. 

 f 'Climate and Time,' pp. 220-225; " Phil Mag.," September and 

 December, 1875; "Nature," November 25, 1875. 



