Professor Forbes on the Leading Phenomena of Glaciers, 223 



the water freezes rarely, and to an insignificant extent ; and se- 

 condly, from the motion of the glacier in its different parts, 

 and at different times, being at variance with what must have 

 held true upon the theory in question. 



1. The water included in a glacier is rarely in a freezing 

 condition. I need not now repeat the arguments which I 

 have adduced to shew, that, upon every principle of the doc- 

 trine of heat, especially the doctrine of latent heat, it is 

 impossible that the transient cold of the night should in any 

 circumstances produce more than a superficial and most im- 

 perfect congelation, — that to suppose any thing else, would be 

 to suppose in a glacier an indefinite supply of cold,* contrary 

 to first principles, and to direct observations with the ther- 

 mometer on the temperature of the ice, which has been found 

 by M. Agassiz himself to be constantly, and at all depths, with- 

 in a fi'action of a degree of 32°. But besides this, the most 

 direct observation shews, that the nocturnal congelation, which 

 is so visible at the surface, drying up the streamlets of water, 

 and glazing the ice with a slippery crust, extends to but the 

 most trifling depth into the mass of the glacier. This is so evi- 

 dent, upon consideration, that when fairly placed before him, 

 M. de Charpentier has been obliged to abandon the idea that 

 the diurnal variations of temperature produce any effect. In 

 truth, there is positive evidence that no internal congelation 

 takes place during the summer season, when the motion is 

 most rapid, and when, therefore, the cause of motion must be 

 most energetic. Of this I will give one striking example. 



Towards the end of September 1842, when a premature win- 

 ter had covered the Mer de Glace with snow, and lowered the 

 temperature of the air to 20° Fahrenheit, I had occasion to 

 make an expedition over nearly its whole extent, in the direction 

 of the Glacier de Lechaud, in order to observe the marks which 

 had been placed in that direction, and to determine the motion 

 of the higher parts of the ice. The excursion promised to be far 

 from agreeable. The sky was lowering when we started from 



* This argument has been well put by M. Elie de Beaumont, with his ac- 

 customed clearness. 



