92 Mr. H. H. Howorth on 



the ice at Mon tan vert, cannot possibly have its melting point 

 lowered by pressure more than a small fraction of a degree." 

 Tyndall then goes on to urge that his experiments in the 

 winter on the surface ice of the Mer de Glace shewed con- 

 siderable movement at — 5° centigrade, when it would require 

 66j atmospheres of pressure to melt it, equivalent to a column 

 of ice 26,680 feet, or to a height two and a half times that of 

 Mont-Blanc above the Montanvert, whose portentous summit 

 must have been connected with it by a continuous glacier, 

 with its bed absolutely smooth to secure the requisite pres- 

 sure {Glaciers of the Alps, 342 — 345). 



Lastly, it seems to me. Professor Thomson's theory 

 fails to meet the difficulty that the motion of a glacier 

 is differential, that its centre moves faster than its sides, and 

 its surface more than its base ; that is, that the motion is 

 greatest where the pressure, and therefore the postulated 

 liquidation, is least. 



Let us now turn to Dr. Croll's theory, which, like all his 

 theories, has the advantage of being ingenious and of en- 

 deavouring frankly to meet the conditions of the problem. 

 In a paper he published in 1870 he took for granted that 

 Mr. Moseley's experiments on the shearing of ice, to which 

 reference will be made presently, were conclusive against 

 the notion that ice is a plastic substance. His words are, 

 " Unless some very serious error could be pointed out 

 in the mathematical part of his investigation, it would 

 be hopeless to overturn his general conclusion as regards 

 the received theory of the cause of the descent of glaciers, 

 by searching for errors in the experimental data on which 

 the conclusion rests" {Phil. Mag. XL. 154). 



He, on the other hand, saw clearly that none of the theories 

 already described would account for the differential motion of 

 glaciers as proved by numerous experiments, and that unless 

 this difficulty could be explained it was useless to produce 

 a theory, however ingenious. He accordingly appealed to a 



