166 Mr. J. Croll on the Cause of the Motion of Glaciers, 



Again, during winter, as Mr. Ball remarks, the glacier is com- 

 pletely covered with snow and thus protected both from the in- 

 fluence of cold and of heat, so that there can be nothing either to 

 raise the temperature of the ice above the freezing-point, or to 

 bring it below that point ; and consequently the glacier ought to 

 remain immoveable during that season also. 



"There can be no doubt, therefore," Mr. Moseley states, 

 " that the rays of the sun, which in those alpine regions are of 

 such remarkable intensity, find their way into the depths of the 

 glacier. They are a power, and there is no such thing as the 

 loss of power. The mechanical work which is their equivalent, 

 and into which they are converted when received into the sub- 

 stance of a solid body, accumulates and stores itself up in the ice 

 under the form of what we call elastic force or tendency to di- 

 late, until it becomes sufficient to produce actual dilatation of 

 the ice in the direction in which the resistance is weakest, and 

 by its withdrawal to produce contraction. From this expansion 

 and contraction follows of necessity the descent of the glacier"*. 

 When the temperature of the ice is below the freezing-point, the 

 rays which are absorbed will, no doubt, produce dilatation ; but 

 during summer, when the ice is not below the freezing-point, no 

 dilatation can possibly take place. All physicists, so far as I am 

 aware, agree that the rays that are then absorbed go to melt the 

 ice and not to expand it. But to this Mr. Moseley replies as 

 follows : — " To this there is the obvious answer that radiant heat 

 does find its way into ice as a matter of common observation, 

 and that it does not melt it except at its surface. Blocks of ice 

 may be seen in the windows of ice-shops with the sun shining 

 full upon them, and melting nowhere but on their surfaces. 

 And the experiment of the ice-lens shows that heat may stream 

 through ice in abundance (of which a portion is necessarily 

 stopped in the passage) without melting it, except on its sur- 

 face." But what evidence has Mr. Moseley to conclude that if 

 there is no melting of the ice in the interior of the lens there is 

 a portion of the rays " necessarily stopped " in the interior ? It 

 will not do to assume a point so much opposed to all that we 

 know of the physical properties of ice as this really is. Has 

 Mr. Moseley, after accurately determining the amount of work 

 performed in melting the ice of his lens during any given time, 

 found it to fall short of the amount of work which ought to have 

 been performed by the heat absorbed during that given time ? 

 If he has done this in a manner that can be relied upon, then he 

 has some warrant to conclude that there is a portion of the rays 

 stopped which goes to perform work different from that of melting 



* Proceedings of the Bristol Naturalists' Society, vol. iv. p. 39 (new 

 series). 



