CONDITIONS OF CONTINENTAL ICE. 221 



into ice, were buried under succeeding snows, and so 

 on. The ice that formed the surface a century ago 

 now lies buried below the ice of a hundred years, and 

 a hundred years hence its present position will be 

 occupied by the surface ice of to-day. There is not 

 only a constant motion of the ice from the pole out- 

 wards, but a constant downward motion as layer by 

 layer is successively formed on the surface. 



From what has been proved regarding the small 

 quantity of heat which can be directly transmitted 

 through the ice, it follows that the superficial layers 

 will carry down with them pretty much the same 

 temperature which they possessed at the surface at 

 the time when they were covered up by succeeding 

 snows. Any heat which they can derive from the 

 work of compression, as has been shown, is but trifling. 

 Heat transmitted by conduction could not possibly 

 raise the temperature of the underlying ice above that 

 of the surface ; neither could the heat from direct 

 radiation, nor that derived from melted ice. 



As the temperature of the ice, then, cannot be much 

 above the mean temperature of the surface, which is 

 far below the freezing-point, it follows that the 

 underlying mass must also be below the freezing- 

 point. The very low temperature of the superficial 

 layers is due to the fact that the mean temperature of 

 the air above the surface is far below the freezing- 

 point — a temperature which the icy surface cannot 

 much exceed. The sun during summer may possibly 

 heat the air sometimes above the freezing-point, but it 

 cannot, of course, so raise the temperature of the ice 

 without melting it. 



Again, as solid ice is a better radiator than gaseous 

 air, the surface of the sheet during winter would pro- 

 bably have its temperature lowered by radiation to a 



