100 Mr. H. H. Howorth on 



we can experiment, we find that the sun's rays acting on a 

 mass of ice melt its surface layers, and they flow away by 

 gravity, or remain as tightly enclosed by the nether ice 

 where gravity cannot work as water in a well-puddled 

 reservoir does. But Dr. Croll ignores this every-day melting 

 of ice altogether, and introduces us to a transcendental 

 method. 



This completes the roll of the various theories which 

 have been forthcoming to account for the motion of glaciers 

 independently of gravity, and it must be admitted that they 

 all fail to reconcile themselves to the ordinary laws of physics, 

 and fail, therefore, to secure for themselves a place among 

 the postulates of empirical science. Let us now turn to the 

 theories which in various ways invoke gravity as the chief 

 motor in glacier motion. 



We will begin with the earliest of these theories, namely,, 

 the sliding hypothesis. 



It was first scientifically stated by G. S. Gruner, in a 

 work published at Berne in 1760, entitled Beschreibung 

 des Eisgehirges der Schweizerlandes. In this he refers to 

 the fact of stones on the back of the glacier at Grindelwald 

 having been seen to move gradually down, so that a stone 

 had been noticed to advance 50 paces in six years, and 

 urges that the whole mass of ice which is embowered in the 

 valley moves down en masse by its own weight. This 

 movement, he argues, is assisted by the greater humidity of 

 the nether surface of the glacier. 



Deluc the elder somewhat modified Gruner's position. 

 He argued that great caverns and hollows are formed under- 

 neath glaciers which are thus supported on a kind of ice 

 pillars, when they give way the ice mass gives way, and its 

 tendency is to move down the slope on which it rests. "This," 

 he says, "causes the march down of great masses of ice called 

 glaciers. They do not slide down en masse^ but piecemeal, 

 the pieces or fields of ice being separated by crevasses 



