1846.] DISTENSION AND COMPRESSION ALTERNATE. 155 



ing season, which no words can express, no mathematical sym- 

 bols weave into a demonstration. I can only say that it is 

 easier to believe than to disbelieve ; and that, sooner or later, it 

 will, I doubt not, be generally admitted. 



Considering the crevasses as chiefly superficial in the normal 

 glacier (I mean that of which the inclination is not excessive), 

 it is evident that the formation of the crevasses must depend 

 mainly upon the configuration of the bed. Where the section of 

 the bed parallel to the length of the glacier is convex upwards, 

 there the tension at the surface will cause the crevasses to ex- 

 pand ; when the bed is concave and the surface is being com- 

 pressed, the crevasses tend to close. Hence the surface of the 

 glacier descending, an irregular bed may be alternately in a state 

 of distension and compression, and the crevasses do not tend to 

 widen indefinitely, which would be the case if the whole glacier 

 were distended. This tendency in the crevasses to expand and 

 contract in accordance with their position is beautifully seen in 

 viewing the Mer de Glace from a height, as we have recom- 

 mended. The steep fall opposite Trelaporte shows the expan- 

 sion of the crevasses, but the comparative level opposite the 

 little glacier of Charmoz gives it time to recover its solidity by 

 the general closing of the crevasses under compression. The 

 careful study of such a scene as this gives a more clear insight 

 into the glacier phenomena than any other part of the inquiry, 

 excepting only the measurement of velocities. 



Law of Velocities. To these velocities we now return. 

 The varying velocities in different glaciers, at different seasons, 

 and in different parts of the same season, are all in accordance 

 with the motions of a viscous or plastic body. They depend 

 upon the slope ; being greatest, ceteris paribus, when the slope 

 is greatest ; and upon the climate to which the glacier is ex- 

 posed, being greatest in glaciers which descend into deep val- 

 leys, and least in those which, though very steep (such as that 

 of the Schonhorn described in 6), are placed in so elevated 

 and therefore dry and cold an atmosphere as to afford insuffi- 

 cient water to moisten the snowy mass or neve, and which are 



