146 Professor Forbes' s Thirteenth Letter on Glaciers* 



and position will be understood from a reference to the gene- 

 ral Map of the Mer de Glace. It is shaped somewhat like 

 a tongue, and is steep, without being much crevassed or very 

 uneven, at least in its middle region, where it is about 1000 

 feet broad, where cattle are every year compelled to traverse 

 it; yet even there it has an inclination of 18° or 20° near its 

 centre. This glacier terminates at a considerable height 

 above the Mer de Glace, and gives rise to the Nant Blanc, a 

 torrent which gives its name to the glacier, and whose vo- 

 lume and appearance is subject to remarkable changes, de- 

 pending upon the state of the weather. It is supported on a 

 vast pile of blocks which it has brought down, and which 

 forms its moraine ; and whilst on the left bank the ice over- 

 hangs the moraine, on the right, the latter being elevated in 

 a mound, forms a barrier to the former. This glacier has a 

 very small and crevassed neve, taking its origin amongst the 

 rugged rocks between the Aiguilles of Dru and Bochard. 

 The veined structure of the ice of the glacier is perfectly nor- 

 mal, exhibiting the parabolic loops with the frontal dip in- 

 wards. Its general form resembles that of the Glacier des 

 Bossons. 



I stationed my theodolite on the northern moraine, and 

 fixed a station towards the centre of the glacier, or rather 

 on the axis of the structural bands, which usually intimates 

 the point of swiftest motion. The inclination of the glacier 

 was here, as already mentioned, 18° or 20°. Notwithstanding 

 this great declivity, the mean motion from August 4 to Au- 

 gust 15, was only 9*9 inches daily. 



Glacier du Miage, — In order to complete here the observa- 

 tions on the velocity of motion of different glaciers, I shall 

 include those made also on the south side of Mont Blanc. 

 I had intended to have made a considerable number of experi- 

 ments on the remarkable Glacier du Miage, fully described in 

 my Travels, Chap. X., but I found it less suitable than I ex- 

 pected. I renewed, however, my survey of all the middle 

 region of the glacier ; and I ascertained the motion of its 

 central part, between the two medial moraines at its issue 

 into the valley from between the colossal summits of the 

 Aiguilles Rouges an(J Mont Broglia ; in fact, at a point near 



