﻿The Rev. H. Moseley on the Descent of Glaciers. 229 



sible to obtain that absolute equality of illumination in the instrument 

 which is requisite for a comparison. By the unaided eye one cannot 

 tell which is the brighter half of a paper disk illuminated on one side 

 with a reddish, and on the other with a yellowish light ; but by using 

 the photometer here described the problem becomes practicable. 

 When the contrasts of colour are very strong (when, for instance, 

 one is a bright green and the other scarlet) there is difficulty in 

 estimating the exact point of neutrality ; but this only diminishes 

 the accuracy of the comparison, and does not render it impossible, 

 as it would be according to other systems. 



January 7, 1869. — Lieut, -General Sabine, President, in the 

 Chair. 



The following communication was read : — 



" On the Mechanical Possibility of the Descent of Glaciers by 

 their Weight only." By the Rev. Henry Moseley, M.A., Canon of 

 Bristol, F.R.S., Instit. Imp. Sc. Paris, Corresp. 



All the parts of a glacier do not descend with a common motion ; 

 it moves faster at its surface than deeper down, and at the centre of 

 its surface than at its edges. It does not only come down bodily, 

 but with different motions of its different parts ; so that if a trans- 

 verse section were made through it, the ice would be found to be 

 moving differently at every point of that section. 



This fact*, which appears first to have been made known by M. 

 Rendu, Bishop of Annecy, has since been confirmed by the mea- 

 surements of Agassiz, Forbes, and Tyndall. There is a constant dis- 

 placement of the particles of the ice over one another, and alongside 

 one another, to which is opposed that force of resistance which is 

 known in mechanics as shearing force. 



By the property of ice called regelation, when any surface of ice 

 so sheared is brought into contact with another similar surface, it 

 unites with it, so as to form, of the two, one continuous mass. Thus 

 a slow displacement of shearing, by which different similar sur- 

 faces were continually being brought into the presence and contact 

 of one another, would exhibit all the phenomena of the motion of 

 glacier ice. 



Between this resistance to shearing and the force, whatever it 

 may be, which tends to bring the glacier down, there must be a 

 mechanical relation, so that if the shearing resistance w r ere greater 

 the force would be insufficient to cause the descent. The shearing 



* The remains of the guides lost in 1820, in Dr. Hamel's attempt to ascend 

 Mont Blanc, were found imbedded in the ice of the Glacier des Bossons in 1863. 

 " The men and their things were torn to pieces, and widely separated by many 

 feet. All around them the ice was covered in every direction for twenty or thirty 

 feet with the hair of one knapsack, spread over an area of three or four hundred 

 times greater than that of the knapsack." " This," says Mr. Cowell, from whose 

 paper read before the Alpine Club in April 1864 the above quotation is made, 

 *' is not an isolated example of the scattering that takes place in or on a glacier, 

 for I myself saw on the Theodule Glacier the remains of the Syndic of Val Tom- 

 nanche scattered over a space of several acres." 



