﻿The Rev. H. Moseley on the Descent of Glaciers. 231 



descend with a uniform motion if it descended by its weight only, 

 because the forces acting upon it would be uniformly distributed and 

 constant forces*. The conditions of the descent of any one portion 

 of it would therefore be the same as those of any other equal and 

 similar portion. The portion, the conditions of whose descent it 

 is sought in this paper to determine, is that which has descended 

 through any given transverse section in a day ; or, rather, it is one 

 half this mass of ice, for the glacier is supposed to be divided by a 

 vertical plane, passing through the central line of its surface, it being 

 evident that the conditions of the descent of the two halves are the 

 same. The measurements which have been made of the velocities 

 of the surface-ice at different distances from the sides make it pro- 

 bable that the differences of the spaces described in a given time 

 would be nearly proportional to the distances from the edge in a 

 uniform channelf, and the similar measurements made on the velo- 

 cities at different depths on the sides that, under the same circum- 

 stances, the increments of velocity would be as the distances from the 

 bottom. This law, which observation indicates as to the surface 

 and the sides, is supposed to obtain throughout the mass of the 

 glaciers. Any deviation from it, possible under the circumstances, 

 will hereafter be shown to be such as would not sensibly affect the 

 result 



The trapezoidal mass of ice thus passing through a transverse 

 section in a day is conceived to be divided by an infinite number of 

 equidistant vertical planes, parallel to the central line, or axis of the 

 glacier, and also by an infinite number of other equidistant planes 

 parallel to the bed of the glacier. It is thus cut into rectangular 

 prisms or strips Iving side by side and above one another. II any 

 one of these strips be supposed to be prolonged through the whole 

 length of the glacier, every part of it will be moving with the same 

 velocity, and it will be continually shearing over two of the similar 

 adjacent strips, and being sheared over by two others. The position 

 of each of these elementary prisms in the transverse section of the 

 glacier is determined by rectangular coordinates ; and in terms of 

 these, its length, included in the trapezoid. The work of its weight, 

 while it passes through the transverse section into its actual position, 

 is then determined, and the work of its shear, and the work of its 



* It is supposed that the weight is only just sufficient to cause the descent. 



t Prof. Tyndall measured the velocity of the surface of the Mer de Glace at a 

 series of points in the same straight line across it at a place called Les Pouts. 

 The distances of these points in feet along the line up to the point of greatest 

 velocity are set off to a scale in fig. 1 ; and the space in feet through which each 

 point would pass in thirty-six days, if its velocity continued uniformly the same, 

 is shown bv a corresponding line at right angles to the other. The extremities 

 of these last lines are joined. It will be seen that the line joining them is for 

 some distance nearly straight ; if it were exactly so, the law stated in the text 

 would, in respect to this ice, be absolutely true. Fig. 2 shows in the same manner 

 the spaces described in thirty-six days by points at different depths on the side 

 of the Glacier du Geant, as measured by Prof. Tyndall at the Tacul. See Phil. 

 Trans. Royal Society, vol. cxlix. part 1, pp. 265, 266. [The figures referred to 

 in this note accompany the MS. of the paper.] 



