314 THE LIFE OF JAMES D. FORBES. [CHAP 



possible by the increased number of these longitudinal 

 dislocations. 



Forbes held the contrary opinion, namely, that the 

 forces by which this differential motion was produced, 

 found in the molecular structure of the ice, conditions 

 which enabled them to do their work without such tearing 

 or * shearing/ He started for Chamounix, and proceeded 

 to ascertain the relative change of position produced by 

 the motion of the glacier, in a considerable number of 

 points fixed across an uncrevassed portion of the Mer de 

 Glace, at right angles to its axis. Such a spot he suc- 

 ceeded, after some trouble, in finding between ' 1'Angle ' 

 and the Trelaporte, below the little glacier of Charmoz. 

 On a space of extremely flat and compact ice about 

 seventy yards in width, and several hundred feet in 

 length, he fixed a series of marks, thirty feet apart, between 

 three of which he subsequently placed pegs dividing that 

 space into forty-five parts, measuring two feet each. 



1 You will probably be surprised/ he writes to Pro- 

 fessor Jameson, ' when I state, that in seventeen days, 

 that part of the glacier ninety feet nearer the centre 

 than the theodolite, had moved past the theodolite 

 by a space of twenty-six inches, and the interme- 

 diate spaces in proportion. When I was reluctantly 

 compelled to cease my observations on the forty-five 

 marks, they had, in the course of six days, formed a 

 beautiful curve slightly convex toward the valley ; and 

 as the vertical wire of the theodolite ranged over 

 them, their deviations from a perfect curve were slight 

 and irregular, nor was there any great dislocation 

 to be observed in their whole extent ; proving the 

 general continuity of the yielding by which each was 

 pushed in advance of its neighbour. . . . All this, 

 viewed in prospective with the theodolite, left no 

 remaining doubt as to the plasticity of the glacier on 

 a large scale/ 1 



But this was not the only result of three weeks which 



1 Eighth Letter on Glaciers, August 30, 1844; 'Philosophical 

 Transactions,' 1846, p. 137. 



