302 GLACIAL GRAVELS OF MAINE. 



INTERNAL TEMPERATURES OF ICE-SHEETS. 



Surface rocks and soils experience great changes in temperatures, but 

 as we descend into the earth we pass beyond the influence of the seasons 

 and reach a point of invariable temperature. It has been computed that in 

 temperate zones this point lies at an average depth of about 50 feet, vary- 

 ing greatly according to the local conditions. In far northern countries 

 where there is little snow the earth is permanently frozen after we reach a 

 depth of a few feet. 



Without assuming the causes of the ice epoch we can at least assume 

 practically Arctic conditions as then prevailing over the region overrun by 

 the ice-sheet. It is important to know, if possible, what temperatures pre- 

 vailed within that vast body of snow and ice. Was that 4,000 feet or more 

 of ice a rock which, like other rocks, had beneath its surface a level of 

 invariable temperature? If so, at what depths, and what Avas the tempera- 

 ture? Where did the isogeotherm of 32° lie in winter and in summer? 



The only tests made of the temperature of glaciers have been made 

 near their distal extremities, where both the ice and glacial waters are 

 reported to have a nearly constant temperature of 32°. No observations 

 appear to have been made of the interior temperatures of the nivi, and 

 we can arrive at only ain approximate estimate by reasoning from some 

 known facts. In such an investigation we have to depend chiefly on the 

 following physical properties of water and ice: 



1. Water has a very high specific heat. 



2. Water and ice are poor conductors of molecular heat, especiall}^ ice 

 in the form of snow. 



3. Water freezes without change of temperature at the surface of 

 freezing so long as any water remains unfrozen, the latent heat of liquidity 

 being given up in the act of solidifying. 



4. Ice melts with contraction of volume and without change of tem- 

 ]ierature at the surface of ixielting so long as any portion remains immelted. 



These properties account for the remarkable power the glacier has of 

 regulating its own temperatures. The heat of summer or of the day first 

 raises the mass to 32°, and then the surplus is expended in melting some of 

 the ice, without change of temperature. In winter or at night the surface 

 temperature of dry ice falls like that of other surface rocks, except that the 



