106 Rev. C. Trotter. Physical Properties of Ice [Jan. 29, 



rocks will in some places and at some times receive and absorb a 

 large amount of solar radiation, they will also radiate freely into space. 

 The amount of central heat flowing into such a wedge of rock through 

 its comparatively narrow base cannot be large, and there can be little 

 doubt that the mean temperature of the upper portions of such a 

 ridge will be below 0° C. It seems clear that in such a case the iso- 

 thermal surface of 0° C. which follows the common surface of the 

 glacier and its bed, must somewhere leave the surface of the rock and 

 strike across the ridge, so as to be continuous with the isothermal 

 surface at the lower surface of the glacier on the opposite side. In 

 such a case the most probable point for the isothermal to leave the 

 surface of the rock will be about the place where the ridge begins to 

 rise steeply. The winter isothermal of 0° C. within the rock will be 

 below, but probably not far distant from the summer one. 



Now in such a typical system of glaciers as I have been describing 

 there is almost invariably a deep chasm in the neve at about the 

 point at which I have supposed the isothermal to leave the surface 

 of the rock. This chasm, which penetrates obliquely into the neve, 

 and is often nearly continuous for long distances, is familiar in Alpine 

 literature as the " Bergschrund." 



Fig. 3. 



In the annexed diagram, A is the rock of supposed mean tempera- 

 ture greater than 0° C. ; B the rock of supposed mean temperature less 

 than 0° C. ; C the neve; D the "Bergschrund." The dotted lines 

 give the trace upon the plane of the paper of the supposed winter and 

 summer isothermals of 0° C. 



I am not aware that any observations of the motion of the extreme 

 upper portions of a glacier have been made, but I do not think that 

 any one who is familiar with glacier scenery will have much doubt 

 that the "Bergschrund " marks the limit between the moving and the 



