1844.] PLASTICITY OF ICE ASCERTAINED BY OBSERVATION. 65 



their daily progress was similarly noted. I have now before 

 me the registers and also the graphical projections of the actual 

 places of this portion of the curve of flexure of the ice, cleared 

 of the errors arising from the movement of the theodolite, which 

 was itself placed upon the ice, which error was independently 

 determined. You will probably be surprised when I state, that 

 in seventeen days, the part of the glacier 90 feet nearer the 

 centre than the theodolite, had moved past the theodolite by a 

 space of 26 inches, and the intermediate spaces in proportion. 

 When I was reluctantly compelled to cease my observations on 

 the 45 marks, they had, in the course of six days, formed a 

 beautiful curve slightly convex towards the valley ; and as the 

 vertical wire of the theodolite ranged over them, their devia- 

 tions 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. During these six 

 days the 45th mark had shifted 10 inches ; and besides this 

 obliquity of the line of pins (= 31' 46"), they had a convexity 

 whose versed sine was about an inch. All this, viewed in 

 prospective with the theodolite, left no remaining doubt as to 

 the plasticity of the glacier on the great scale. 



Lest, however, the convexity should have been too small, 

 in so short a time, to admit of measurement, I had provided 

 another test, in order to show that the progressive advance- 

 ment of the line of marks was due to the actual deformation 

 of the ice, and not to the mass of the glacier in this part 

 revolving round some fixed or moveable centre. For this 

 purpose, I fixed a mark in the glacier, 20 feet from the 

 theodolite, and in a direction perpendicular to the before- 

 mentioned line of marks. It was, therefore, seen from the 

 theodolite in the direction of the length of the glacier, and, 

 consequently, was not liable to displacement by its motion. I 

 measured, from time to time, the angle between this mark 

 and the several marks transverse to the glacier, and I found 



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