Mr. J. Croll on the Cause of the Motion of Glaciers. 165 



sun's rays are cut off. The sun shines down with piercing rays 

 and the traveller is scorched ; the glacier melts on the surface, but 

 it still remains " cold as ice." The sun passes behind a cloud 

 or disappears behind a neighbouring hill; the scorching rays 

 are then withdrawn, and the traveller is now subjected to radia- 

 tion on every side from surfaces at the freezing-point.' 



It is also a necessary condition in Mr. Moseley's theory that 

 the heat should pass easily into and out of the glacier; for unless 

 this were the case sudden changes of temperature could produce 

 little or no effect on the great mass of the glacier. How, then, 

 is it possible that during the heat of summer the temperature of 

 the glacier could vary much ? During that season, in the lower 

 valleys at least, every thing, with the exception of the glacier, is 

 above the freezing-point ; consequently when the glacier goes into 

 the shade there is nothing to lower the ice below the freezing- 

 point ; and as the sun's rays do not raise the temperature of the 

 ice above the freezing-point, the temperature of the glacier must 

 therefore remain unaltered during that season. It therefore 

 follows that, instead of a glacier moving more rapidly during 

 the middle of summer than during the middle of winter, it 

 should, according to Moseley's theory, have no motion whatever 

 during summer. 



The following, written fifteen years ago by Professor Forbes 

 on this very point, is most conclusive: — "But how stands the 

 fact ? Mr. Moseley quotes from De Saussure the following daily 

 ranges of the temperature of the air in the month of July at the 

 Col du Geant and at Chamouni, between which points the gla- 

 cier lies : 



At the Col du Geant . . 4*257 Reaumur. 

 At Chamouni .... 10*092 „ 



And he assumes e the same mean daily variation of temperature 

 to obtain throughout the length ' [and depth ?] ' of the Glacier du 

 Geant which De Saussure observed in July at the Col du Geant.' 

 But between what limits does the temperature of the air oscil- 

 late ? We find, by referring to the third volume of De Saussure' s 

 Travels, that the mean temperature of the coldest hour (4 a.m.) 

 during his stay at the Col du Geant was 33 o, 03 Fahrenheit, and 

 of the warmest (2 p.m.) 42°*61 F. So that even upon that ex- 

 posed ridge, between 2000 and 3000 feet above where the glacier 

 can be properly said to commence, the air does not, on an ave- 

 rage of the month of July, reach the freezing-point at any hour 

 of the night. Consequently the range of temperature attributed 

 to the glacier is between limits absolutely incapable of effecting the 

 expansion of the ice in the smallest degree"*. 



* Phil. Mag. S. 4. vol. x. p. 303. 



