Mild Polar Climates, 273 



manent ice in those regions, is a conclusion which, I think, 

 can hardly be doubted. It is with the greatest deference that 

 I venture to differ from so eminent a physicist ; but I am 

 unable to believe that such a transference of water from in- 

 tertropical and temperate regions could be effected by the 

 agency to which he attributes it. Certainly the amount of 

 heat conveyed by means of a circulation resulting from dif- 

 ference of specific gravity, produced by difference of tempera- 

 ture, must be trifling when compared with that of ocean- 

 currents produced by the 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 dif- 

 ference 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 respec- 

 tively as follows : — 



Equator 1000 



Torrid zone 975 



Temperate zone. ... 757 



Frigid zone ..... 454 



Assume, then, that as much heat is conveyed from inter- 

 tropical regions into the Atlantic and Arctic seas by this cir- 

 culation from difference of specific gravity as by the Gulf- 

 stream, and assume also that one half of the total heat con- 

 veyed 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 l Challenger ' Expedition f , that the general surface of 

 the North Atlantic must, in order to produce equilibrium, 

 stand at a higher level than at the equator : in other words, 



* ' Climate and Time/ Chap. xi. ; Phil. Mag., March 1874. 

 t ' Climate and Time/ pp. 220-225 ; Phil. Mag-., September and Decem- 

 ber 1875 ; ' Nature/ November 25th, 1875. 



