upon Glacier-motion. 333 



Journal for February, and to ' Nature' for March 24, 1870, 

 where it was pointed out that an ice-plank 6 inches wide and 2 

 inches thick, or thereabout, when supported upon horizontal 

 bearers 6 feet apart, became gradually deflected, even under a 

 temperature constantly below freezing, and was just as rigid in 

 its altered as in its original form. I regarded this experiment 

 as absolutely subversive of the Canon's theory, that the descent 

 of glaciers by their weight alone is a mechanical impossibility, 

 and in the article in l Nature ' expressed myself as follows : — " I 

 shall not now attempt to discuss the nature of the molecular 

 displacements to which the change of form is due. Their occur- 

 rence is indisputable, and whether or not they are dignified by the 

 name of shearing is a mere verbal question of little moment." 

 " If the conclusions drawn from the experiments above described 

 are legitimate, plasticity must be admitted by the side of sliding 

 and fracture and regelation, as one of the constituent elements 

 of the theory of glacier-motion, and a more important place in 

 that theory must be assigned to the views of the late Principal 

 Forbes than has for some years been conceded to them." 



It was not, therefore, without some surprise that I read the 

 following passages in the Canon's recent paper: — "Mr. Mathews, 

 however, and M. Heim after him, as well as Mr. Ball, deny that 

 glacier-ice shears at all. They say that it bends." " The idea 

 present in common to the minds of these gentlemen seems to be 

 that in bending so as to take a set, ice does not shear. In 

 this I venture to think there is a misapprehension. In the 

 bending of a plate of ice every particle, except those at the points 

 of support, is made to move in the direction in which the plate 

 is bent — those particles which are at the point of greatest inflec- 

 tion being made to move furthest, and those nearer to it being 

 always made to move further than those more remote ; so that 

 every particle moves over that which is alongside towards the 

 nearest point of support; and being assumed to have taken a set, 

 it must have sheared over it." 



If the Canon will do me the favour to reperuse the para- 

 graph from the Alpine Journal quoted in support of the lan- 

 guage which he puts into my mouth, he will see that it has an 

 opposite significance to that which he attaches to it. I have 

 hesitated, it is true, to identify the molecular displacements of 

 the ice-plank with the shearing described by Canon Moseley ; but 

 I should be the last to deny that his account of the former pheno- 

 menon is a possible hypothesis. Let us by all means adopt it, 

 for the sake of argument, and see to what it leads us. 



The cross section of the ice-plank is 12 square inches. In 

 order that a slice of the plank may move over the next alongside, 

 it must be subjected to a pressure equal to twelve times the unit 



