372 Prof. Bischoff on the Temperature of 



coming slippery by the melting of the ice, and thus allowing the 

 glacier to slide down the sloping declivity, is a well known fact. 

 Whether this advance be greater than the loss sustained by the 

 melting away of the ice, depends not only on the warmth of the 

 summer, but also on the greater or less quantity of avalanches, 

 which fall during the winter on the upper part of the glacier. 

 And this latter condition again depends not only on the quanti- 

 ty of snow which has fallen in the winter, but also on the direc- 

 tion which the avalanches take. But this direction may be mo- 

 dified by various local circumstances, by the change in the slope 

 of the side of the mountains, caused by the falling in of large 

 masses of rock, &c. Escher further observes, that,in warmer years, 

 the side edges of the more elevated glacier, are often melted to a 

 greater extent, and that that part of the glacier which is situated 

 at the entrance of a cross valley, is thus more easily carried for- 

 ward, and is forced in greater masses into the valleys below. 

 Thus it may often happen, that some arms of a glacier increase 

 in length for several successive years, whilst others, situated 

 quite near them, become shorter.* It must be remarked, that 

 the glaciers in elevated valleys extend down both sides of the 

 mountain, and thus two or more arms or branches are formed, 

 which meet together on the top of the ridge. 



If the lower part of a glacier, by being kept slippery by the 

 melting of the ice from its under surface, or higher up, by the 

 melting of the upper surface, and the penetrating of the water 

 to the under surface, slip down, the upper part, which only con- 

 sists of loose snow, without any cohesion, will follow like an 

 avalanche, and thus come into a region, when the snow will be 

 converted into ice. 



Besides the increase and decrease of the glaciers, caused by 

 the above mentioned circumstances, it is a pretty general opinion 



V. Charpenlier in Gilb. Annal. vol. Ixiii. p. 388, and Biselx in the same, 

 p. 192. Kiihn (Hopfner's Magazine, vol. i. p. 129), observed an advance of 

 t'ae lower Grindelwald glacier, accompanied with a violent subterranean noise, 

 when he was on the Eismeer, by which several very wide fissures were sud- 

 denly cloied, and the water which was in them forced up to a considerable 

 height. 



" Numerous examples of this are to be found in the Biblioth. Univ. xiv. 

 285. 



