226 DISCUSSIONS IN CLIMATOLOGY. 



temperature of the air gives a comparatively low 

 temperature to the surface. Suppose our globe to 

 be enveloped for some thousands of years with a 

 covering at the uniform temperature of say 100°; 

 and suppose, further, that 5000° should represent the 

 temperature of the earth's mass ; then, in such a 

 case, there would be a gradual decrease of tempera- 

 ture from 5000° at the centre to 100° at the surface. 

 Let us suppose now the warm covering is removed, 

 and replaced by one at —100°. In the course of some 

 thousands of years there will be a gradual decrease of 

 temperature from 5000° at the centre, as before, to 

 — 100° at the surface. Internal heat limits the tem- 

 perature at the centre, but external heat limits in 

 every case the temperature at the surface. 



To maintain, as Sir Wyville Thomson does, that 32° 

 is the temperature of the floor on which the Antarctic 

 ice-sheet rests, is virtually to beg the whole question 

 at issue. It is the temperature of the ice that deter- 

 mines that of the floor on which it rests, and not the 

 latter that determines the former. 



What the temperature of the ground under the 

 Antarctic ice-sheet may be is a question which at 

 present we have no means of ascertaining with cer- 

 tainty; we know only that it must be far below the 

 freezing-point, for the ice resting on it is considerably 

 under that point. 



Although the temperature of the ice must impose a 

 limit to the thickness of the sheet, underground tem- 

 perature cannot do so, for the temperature of the ice 

 is not determined by underground heat. 



But supposing we knew the temperature of the 

 Antarctic ice, yet this knowledge would not enable 

 us to determine with certainty the limit imposed by 

 temperature on the thickness of the ice. For, except- 



