Professor Forbes's Eighth Letter on Glaciers. 370 



feet, each of which was marked by a perforation in the ice 

 into which short pins could be accurately fitted, and the de- 

 formation of this straight line of 90 feet in length was care- 

 fully observed at short intervals. The errors of the original 

 places of the marks were determined by a simple but nice 

 process, and 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 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 conti- 

 nuity 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") J they had a convexity whose versed sine was 

 about an inch. All this, viewed in prospective with the theo- 

 dolite, left no remaining doubt as to the plasticity of the gla- 

 cier 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 shew 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 re- 

 volving round some fixed or moveable centre. For this pur- 

 pose, I fixed a mark in the glacier, 20 feet from the theo- 

 dolite, and in a direction perpendicular to the before men- 

 tioned line of marks. It was, therefore, seen from the theo- 

 dolite in the direction of the length of the glacier, and, con- 

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



