INTEEIfAL TEMPERATURES OP ICE-SHEETS. 30B 



waste is probably slower, owing to its low conducting power and high speciilc 

 heat. The point in the interior where we first reach an invariable tempera- 

 ture lies nearer the surface of ice than in other rocks. In addition to these 

 properties which make changes of temperature of the ice mass take place 

 slowly, the glacier has at its command another most important means of 

 maintaining and regulating its temperature. It is known that there is a 

 large amount of surface melting over much or all of the n6vi, and progres- 

 sively more as we approach the distal extremity. A large amount of water 

 is during the day and summer stored up in the snow of the n^v^ and in 

 that contained in crevasses; water is always found in the larger subglacial 

 channels, often also in surface pools and crevasses without outlet beneath 

 into the tunnels, and in internal cavities in the granulated ice near the surface. 

 The moment the temperature at any wet place tends to fall below 32", some 

 of this water is frozen and the temperature maintained. The glacial waters 

 thus serve an important purpose in storing up heat when there is an excess 

 above 32° and in giving it out again when thei'e is a deficiency. Those 

 parts of glaciers at a distance from water must fall in temperature during 

 the cold of night and of the winter, just like other rocks. 



The net result is that the wet parts of the glacier, i. e., all the region 

 of surface melting extending from the distal extremities well up into the 

 ndvd, have the. nearly constant temperature of 32°. In siuumer the isogeo- 

 therm of 32° rises to the top of the glacier in all this region, or rather, the 

 isogeothennal stratum of 32° includes the whole glacier from the bottom to 

 the top. In winter the upper limit of this stratum sinks beneath the sur- 

 face an undetermined and varying distance. 



As we go above the zone of wastage into that of accumulation it 

 becomes imcertain what are the internal temperatures of the snow fields. 

 The addition of new layers of snow is constantly pressing down into the 

 interior of the mass the older layers, many of which would have had a 

 temperature far below zero when covered, and must abstract a great amount 

 of heat from the interior of the n^vd. The heat of summer could not 

 directly penetrate dry granular snow so far as it could clear solid ice. 

 Above the limit of appreciable surface melting it is doubtful if the heat that 

 comes from above can pass in large quantity far down into the snow. 

 Where the snowfall was very great dui-ing the intense cold of winter and 

 at high elevations, it might happen that the heat of summer could not pass 



