292 POPULAR LECTURES AND ADDRESSES. 



<>f air next the ground) by partially screening 

 the ground from radiation and itself instead 

 radiating heat which it draws from a thicker 

 stratum of still air than could contribute sensibly 

 by conduction to supply radiation from the 

 ground. If these defences suffice to protect 

 vegetation against destructive cold for a summer 

 night over a flat continent in tropical or temperate 

 regions it seems certain that they would suffice 

 to prevent even so much as hoar frost on a small 

 island at the very pole during its whole winter 

 six months night, if it were surrounded by a deep 

 ocean with no land to obstruct free circulation 

 between it and tropical seas. 



Considering, then, the absolute proofs we have 

 in Geology of subsidences and elevations of vast 

 regions of the solid earth, I sec no difficulty in 

 admitting the complete sufficiency of the theory 

 set forth in I, yell's twelfth chapter to account 

 fnr all that is proved of temperate climates in 

 polar regions, and of alternations of glacial and 

 'mild climates in Kuropc, and North America, 

 and New Zealand. And if a few scratched rocks 



