CONDITIONS OF CONTINENTAL ICE. 213 



filling the Baltic and the North Sea, during the Glacial 

 Epoch must have been over 3000 feet thick at least. * 



The Temperature of the Antarctic Ice. 



In examining the physical reasons which have been 

 advanced for the limit assigned to the thickness of the 

 Antarctic ice-cap, we must first consider the probable 

 temperature of the ice ; for not only does the thickness 

 of the sheet depend, as we shall see, to a considerable 

 extent on the temperature of the ice, but misappre- 

 hensions on this point will tend to vitiate all our 

 reasoning on the subject. 



There are but three directions from which the ice-cap 

 can receive an appreciable amount of heat, viz., (1) the 

 air above ; (2) the earth beneath ; and (3) the work of 

 compression. Other sources can yield little, if any 

 at all. For instance, the amount carried inward 

 horizontally from the outer edge of the cap by 

 conduction must be infinitesimal, and indeed can never 

 affect the interior, as the ice moves outward more 

 rapidly than the heat can possibly travel inward. 



Heat derived from Beneath. — We shall begin with 

 the consideration of the heat derived by the bottom of 

 the sheet from the earth's crust. The researches of Sir 

 William Thomson enable us to determine with a 

 tolerable degree of certainty the amount received from 

 this source. He tells us that through every square 

 metre of the earth's surface 220 metre-tons, or 1,613,700 

 foot-pounds, of underground heat pass upwards 

 annually. Through every square foot, therefore, there 

 must come 149,600 foot-pounds. This amount is 

 sufficient to melt a layer of ice, already at the melting- 

 point, one-fifth of an inch in thickness. But under- 



* 'Climate and Time,' Chap, xxvii. 



