by the Southern than by the Northern Hemisphere. 221 



the same time shorter. Consequently it is concluded our hemi- 

 sphere is not cooled to such an extent as the southern, and thus 

 the mean temperature of the winter half year, as well as the in- 

 tensity of the sun's heat, is affected by a change in the sun's 

 distance. 



This circumstance was, so far as I am aware, first noticed by 

 Humboldt in his memoir " On Isothermal Lines and the Distri- 

 bution of Heat over the Globe"*. Upon it M. Adhemar has 

 founded a theory of change of climate, and attributes the great 

 extension of the ice around the south pole to this extra amount 

 of heat lost by radiation in consequence of the seven or eight days 

 of excess in the length of the southern winter over the northern. 

 " The south pole," says Adhemar, " loses in one year more heat 

 than it receives, because the total duration of its nights surpasses 

 that of the days by 168 hours; and the contrary takes place 

 for the north pole. If, for example, we take for unity the mean 

 quantity of heat which the sun sends off in one hour, the heat 

 accumulated at the end of the year at the north pole will be ex- 

 pressed by 168, while the heat lost by the south pole will be 

 equal to 168 times what the radiation lessens it by in one hour, so 

 that at the end of the year the difference in the heat of the two 

 hemispheres will be represented by 336 times what the earth re- 

 ceives from the sun or loses in an hour by radiation" f. 



Adhemar supposes that about 10,000 years hence, when 

 our northern winter will occur in aphelion and the southern in 

 perihelion, the climatical conditions of the two hemispheres 

 will be reversed ; the ice will melt at the south pole, and the 

 northern hemisphere will become enveloped in one continuous 

 mass of ice, leagues in thickness, extending down to temperate 

 regions. 



Although I always regarded this cause of Humboldt's to be 

 utterly inadequate to produce such effects as those attributed 

 to it by Adhemar, still in former papers J I stated it to be a vera 

 causa which ought to produce some sensible effect on climate. 

 On a more careful consideration of the whole subject, I now feel 

 inclined to suspect that the circumstance in question can, accord- 

 ing to theory, produce little or no effect on the climatic condition 

 of our globe. 



The rate at which the earth radiates into space the heat re- 

 ceived from the sun depends upon the temperature of its surface ; 

 and the temperature of its surface (other things being equal) 

 depends upon the rate at which the heat is received. The greater 

 the rate at which the earth receives heat from the sun, the greate 



* Edinb. Phil. Journ. vol. iv. p. 262 (1821). 



t Revolutions de la Mer, p. 37 (second edition). 



J Phil. Mag. S. 4. vol. xxviii. p. 131. Reader, December 2, 1865. 



