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



glacier. It is also true that the changes of the sun's heat and 

 the changes of glacier-motion cannot be due to a common cause. 

 And it is admitted that the changes in the glacier-motion must 

 in some way or other be dependent upon the changes in the sun's 

 heat. Further, it is admitted that the changes in the sun's 

 heat are the cause of the changes in glacier-motion ; but it entirely 

 depends upon the meaning which we attach to the term " cause " 

 whether it will be admitted that the sun's heat is the cause of 

 the motion of the glacier. If by cause of the motion of the gla- 

 cier be meant every thing without which the glacier would not 

 descend, then it is admitted that heat is a cause of the motion 

 of the glacier. But if by cause of the motion of the glacier be 

 meant the energy or power that impels the glacier forward (and 

 this is the meaning which Mr. Moseley seems to attach to the 

 term), then we are not compelled logically to admit that heat is 

 the cause of the motion of the glacier ; for it may only be a ne- 

 cessary condition to the operation of the cause, whatever that 

 cause may be, which impels the glacier forward. The absence 

 of a necessary condition will as effectually prevent the occurrence 

 of an effect as the absence of the cause itself. It does not follow 

 that, because a glacier will not move without heat, heat is ne- 

 cessarily the cause of its motion. Gravitation may be the cause, 

 and heat only a condition. 



The fundamental condition in Mr. Moseley's theory of the de- 

 scent of solid bodies on an incline is, not that heat should main- 

 tain these bodies at a high temperature, but that the temperature 

 should vary. The rate of descent is proportionate, not simply 

 to the amount of heat received, but to the extent and frequency 

 of the variations of temperature. As a proof that glaciers are 

 subjected to great variations of temperature, he adduces the fol- 

 lowing : — " All alpine travellers," he says, "from De Saussure to 

 Forbes and Tyndall, have borne testimony to the intensity of the 

 solar radiation on the surfaces of glaciers. ( I scarcely ever,' says 

 Forbes, ' remember to have found the sun more piercing than at 

 the Jardin.' This heat passes abruptly into a state of intense 

 cold when any part of the glacier falls into shadow by an altera- 

 tion of the position of the sun, or even by the passing over it of 

 a cloud " *. 



Mr. Moseley is here narrating simply what the traveller feels, 

 and not what the glacier experiences. The traveller is subjected 

 to great variations of temperature; but there is no proof from 

 this that the glacier experiences any changes of temperature. It 

 is rather because the temperature of the glacier is not affected 

 by the sun's heat that the traveller is so much chilled when the 



* Proceedings of the Bristol Naturalists' Society, vol. iv. p. 37 (new 



series). 



