78 TERRESTRIAL ADAPTATIONS. 



Here, then, is a kind of circulation of heat ; 

 and the quantity and rapidity of this circulation, 

 determine the quantity of heat in the solid part 

 of the earth, and in each portion of it ; and 

 through this, the mean temperature belonging to 

 each point on its surface. 



If the earth conducted heat more rapidly than 

 it does, the inequalities of temperature would be 

 more quickly balanced, and the temperature of 

 the ground in different parts of the globe of the 

 earth, (below the reach of annual and diurnal 

 variations) would differ less than it does. If the 

 surface radiated more rapidly than it does, the 

 flow of heat from the polar regions would increase, 

 and the temperature of the interior of the globe 

 would find a lower level ; the differences of tem- 

 perature in different latitudes would increase, but 

 the mean temperature of the globe would diminish. 



There is nothing which, so far as we can per- 

 ceive, determines necessarily, either the conduct- 

 ing or the radiating power of the earth to its 

 present value. The measures of such powers, in 

 different substances, differ very widely. If the 

 earth were a globe of pure iron, it would conduct 

 heat, probably, twenty times as well as it does ; if 

 its surface were polished iron, it would only radiate 

 one-sixth as much as it does. Changes in the 

 amount of the conduction and radiation far less th an 

 these, would, probably, subvert the whole thermal 

 constitution of the earth, and make it uninhabitable 

 by any of its present vegetable or animal tenants. 



