1885.] 



and the Motion of Glaciers. 



105 



and its latent heat of fusion must be used in warming the glacier so 

 long as any stratum is below 0° C. 



The amount of heat which reaches the glacier from the interior- of 

 the earth is no doubt insignificant in amount when compared with 

 that which reaches the upper surface of the glacier, but it is important 

 as necessarily warming and eventually melting the lowest stratum of 

 the glacier. Forbes (" Travels in the Alps," 1843, p. 364) quotes an 

 estimate of M. Elie de Beaumont, according to which the annual 

 flow is sufficient to melt about mm. of ice. This would be 

 sufficient to raise about 1'040 metres of ice 1° C. Considering that 

 the winter's cold cannot possibly penetrate to the bed of the glacier, 

 this is quite sufficient to ensure that the lowest stratum shall be at 

 the temperature of about 0° C. I cannot lay my hand upon any 

 more recent estimates of the flow of central heat from the earth, but 

 I believe that M. de Beaumont's would be regarded as correct at 

 least so far as the order of magnitude is concerned. 



We may say then with confidence that the ice which rests directly 

 upon the bed of the glacier is at the temperature of fusion correspond- 

 ing to the pressure at the spot, i.e., usually very slightly below 0° 0. 

 Neglecting these small variations, and leaving out of consideration 

 for the present the extreme upper portion of the glacier, we may say 

 that the surface of contact of a glacier and its bed is part of an 

 isothermal surface of 0° C. The upper surface of the glacier during 

 a summer's day, when melting is everywhere going on, must also be 

 an isothermal surface of 0° C. At night this upper isothermal will 

 descend a few inches into the ice, and in winter perhaps a few feet. 

 The portions above this upper isothermal will be at temperatures 

 below 0° C. Disregarding as before the small differences of melting- 

 point due to pressure, there can be little doubt that throughout the 

 lower part of the glacier the whole space between these isothermal 

 surfaces will be filled with ice at 0° C. In the upper portions of the 

 neve it is possible that there may be one or more regions in which the 

 temperature is below 0° C. These will be bounded by isothermals of 

 0° C, which in summer will necessarily be closed surfaces, except pos- 

 sibly at the extreme upper end of the glacier ; in winter they may 

 be in some places continuous with the cold region above the main 

 upper isothermal of 0° C. 



We may now consider the state of things near the extreme upper 

 limit of the glacier. 



In a large number of instances the neve basin which forms the 

 upper reservoir of the glacier is bounded by a steep and comparatively 

 narrow rocky ridge, against the lower part of which a relatively 

 shallow mass of neve or snow rests at a high angle ; the other side of 

 the ridge is very frequently the boundary of another glacier. The 

 mean temperature of the air at such elevations is low, and though the 



