﻿The Rev. H. Moseley on the Descent of Glaciers. 233 



A third difference between the actual glacier and the imaginary 

 one, to the computation of whose unit of shear the following formulae 

 are applied, is this — that the formulae suppose the daily motion of 

 the surface of the glacier and the daily motion of its side to have 

 been measured at the same place, whereas there exist no measure- 

 ments of the surface motion and the side motion at the same place. 

 The surface motion used has been that of the. Mer de Glace at Les 

 Ponts, and the side motion that of the Glacier du Ge'ant at the 

 Tacul — both from the measurements of Prof. Tyndall. This error 

 again, however, tends to cause the unit of shear, deduced from the 

 case of the artificial glacier, to be greater than that in the actual 

 one ; for the Glacier du Geant moves more slowly than the Mer de 

 Glace. The quantity of ice which actually passes through a section 

 at Les Ponts is therefore greater than it is assumed in the computa- 

 tion to be, whence it follows, as in the last case, that the computed 

 unit of shear is greater than the actual unit of shear. 



To determine the actual value of fi (the unit of shear in the case 

 of ice) the following experiment was made. Two pieces of hard 

 wood, each three inches thick and of the same breadth, but of which 

 one was considerably longer than the other, were placed together, the 

 surfaces of contact being carefully smoothed, and a cylindrical hole, 

 If inch in diameter, was pierced through the two. The longer piece 

 was then screwed down upon a frame which carried a pulley, over 

 which a cord passed to the middle of the shorter piece, which rested 

 on the longer. There were lateral guides to keep the shorter piece 

 from deviating sideways when moved on the longer. The hole in 

 the upper piece being brought so as accurately to coincide with that 

 in the lower, small pieces of ice were thrown in, a few at a time, 

 and driven home by sharp blows of a mallet on a wooden cylinder. 

 By this means a solid cylinder of ice was constructed, accurately fit- 

 ting the hole. Weights were then suspended from the rope, passing 

 over the pulley until the cylinder of ice was sheared across. As by 

 the melting of the ice, during the experiment, the diameter of the 

 cylinder was slightly diminished, it was carefully measured with a 

 pair of callipers. 



1st experiment. — Radius of cylinder * 65 625 in., sheared with 98 lbs. 



2nd experiment. — Radius of cylinder '70312 in., sheared with 

 119 lbs. 



By the first experiment the shear per square inch, or unit of shear, 

 was 72*433 lbs. ; by the second experiment it was 76*6 19 lbs. The 

 main unit of shear of ice, from these two experiments, is therefore 

 75 lbs. 



Now it appears by the preceding calculations, that to descend by 

 its own weight, at the rate at which Prof. Tyndall observed the ice 

 of the Mer de Glace to be descending at the Tacul, the unit of shear- 

 ing force of the ice could not have been more than 1*3193 lb.* 



* By an experiment on the shearing; of putty, similar to that which was made 

 on the shearing of ice, its unit of shear was found to vary from 1 lb. to 3 lbs., 

 according to its degree of hardness. If ice were of the same weight per unit of 



