378 Professor Forbes's Eighth Letter on Glaciers, 



in which it admits of being treated, namely, by the assiduous 

 observation of the motion and change of form of a small com- 

 pact space of ice on a glacier. The Mer de Glace of Cha- 

 mouni offers fewer fit points for such an experiment than 

 many other glaciers, since in all its middle and lower portions 

 the ice is excessively crevassed near the sides. There is one 

 spot, however, between the " Angle " and Trelaporte, below 

 the little glacier of Charmoz, where the ice is extremely flat 

 and compact for a space of about seventy yards in width, and 

 several hundred yards in length, which is wholly devoid of 

 open crevasses, and where I expected to find the variation of 

 velocity from the side towards the centre very sensible, be- 

 cause the veined structure is there more perfectly developed 

 than in any other part of the glacier. In this anticipation I 

 was not disappointed. The ice in question is separated from 

 the western moraine of the glacier by a space deeply crevassed 

 50 or 60 yards wide. The entire breadth of the glacier is 

 here at least 800 yards. The central part has great transver- 

 sal crevasses due to the rapid descent of the glacier where it 

 sweeps round the promontory of Trelaporte immediately 

 above. There is no trace of longitudinal fissures of any kind, 

 except the true blue veined or ribboned structure, which, as 

 already mentioned, is here exceedingly developed ; giving to 

 the even part of the glacier already specified the appearance 

 of exquisite veined chalcedony of an aqua-marine colour ; and 

 the vertical plates of ice thus subdivided are so distinct as to 

 produce a true cleavage when the ice is broken by a hammer 

 or cut with an axe. When the glacier is wet, the blade of a 

 knife may be introduced to a depth of some inches between 

 the laminae, which are commonly not more than a quarter of 

 an inch apart. 



I fixed in a line transverse to the axis of the glacier six 

 stations. Over the first of these the theodolite was regularly 

 centered, in order to observe the relative motions of the others 

 which were respectively 30, 60, 90, 120, and 180 feet distant. 

 Finding that, even in the course of a single day, the accelera- 

 tion of the more central parts was evident, and the six points 

 in question formed a portion of a continuous curve, I subdi- 

 vided the first 90 feet from the theodolite into 45 spaces of 2 



