by the Southern than by the Northern Hemisphere. 225 



taincd by the cavtli in winter is as much owing to the excess 

 in the length of the winter nights over those of the summer as to 

 the deficiency of heat received in winter from that received in 

 summer, three days' heat would then in this case be the amount 

 lost by radiation in consequence of this excess in the length of 

 the winter nights. The total length of the winter nights to 

 those, of the summer is, as we have seen, about as 7 to 4. This 

 is a difference of nearly 1200 hours. But the excess of the 

 south polar winter over the north amounts to only about 184 

 hours. Now if 1200 hours give a loss of three days' sun-heat, 184 

 hours will give a loss of scarcely 5^ hours. 



It is no doubt true that the two cases are not exactly analo- 

 gous; but it is obvious that any error which can possibly arise 

 from regarding them as such cannot materially alter the con- 

 clusion to which we have arrived. Supposing the effect were 

 double, or even quadruple, what we have concluded it to be, still 

 it would not amount to a loss of two days' heat, which could 

 certainly have little or no influence on climate. 



But even assuming all the preceding reasoning to be incor- 

 rect, and that the southern hemisphere, in consequence of its 

 longer winter, loses heat to the extravagant extent of 168 hours, 

 supposed by Adhemar, still this could not materially affect cli- 

 mate. The climate is influenced by the mere temperature of the 

 surface of the ground, and not by the quantity of heat or cold 

 that may be stored up under the surface. The climate is deter- 

 mined, so far as the ground is concerned, by the temperature 

 of the surface, and is wholly independent of the temperature 

 which may exist under the surface. Underground temperature 

 can only affect climate through the surface. If the surface 

 could, for example, be kept covered with perpetual snow, we 

 should have a cold and sterile climate, although the tempera- 

 ture of the ground under the snow was actually at the boiling- 

 point. Let the ground to a depth of, say, 40 or 50 feet be de- 

 prived of an amount of heat equal to that received from the sun 

 in 168 hours. This could produce little or no sensible effect on 

 climate ; for, owing to the slow conductivity of the ground for 

 heat, this loss would not sensibly affect the temperature of the 

 surface, as it would take several months for the sun's heat to 

 penetrate to that depth and restore the lost heat. The cold if 

 I may be allowed to use the expression, would come so slowly 

 out to the surface that its effect in lowering the temperature of 

 the surface would scarcely be sensible. And, again, if we sup- 

 pose the 168 hours' heat to be lost by the mere surface of the 

 ground, the effect would certainly be sensible, but it would only 

 be so for a few days. We might in this case have a week's 

 frozen soil, but this would be all. Before the air had time to 



