338 Professor Forbes’s Ninth Letter on Glaciers. 
It is worthy of remark, that the entire annual motion of 
the part in question of the glacier of the Aar, was ascer- 
tained to be 60 metres, or 164 millimetres per day.* Now 
the mean motion during 35 days of August and September, 
was 203 millimetres, or but one-fourth above the annual 
mean (and during part of the time, we have seen, it fell to 
155 millimetres, or 6e/on the annual mean); proving suffici- 
ently that the annual motion is not entirely effected during 
the warm season, and that even in winter it must bear a 
very sensible proportion to its summer motion, as has been 
directly proved in the case of the Mer de Glace of Cha- 
mouni.t 
IV. The extreme inequality of motion of the central and 
lateral parts of glaciers is the best direct proof of the very 
considerable plasticity of their mass ; and in the paper before 
us this is shewn, in a still more striking manner, than in the 
experiments which I have published. A glacier, like the 
Mer de Glace of Chamouni, has sd considerable a velocity 
(on an average at least three times that of the glacier of the 
Aar), that the ice is impetuously borne along, and torn from 
the sides at the expense of innumerable lacerations and 
crevasses. So that in the whole extent of the middle and 
lower regions of that glacier, in no place do the ice and 
ground meet without the former being more or less fissured 
by rents. But the contrary is the case on the great glaciers 
which move on small slopes, and with smaller velocities ; 
and the discovery of this fact rewarded me for the labour of 
a short visit which I made the Great Aletsch glacier, in July 
1844, when I ascertained, not merely the small daily pro- 
gress of the mass of the glacier, but the astonishing retar- 
dation produced by the sides, whilst the surface remained 
compact and wholly undivided by longitudinal crevasses. 
In that case, I found that, “whilst the velocity of the iee at 
1300 feet, or about a quarter of a mile from the side, is 
14 inches in 24 hours, at 300 feet distant from the side it 
* Comptes Rendus, p. 1301, line 29. 
t Travels in the Alps, p. 151, 420; and Fifth Letter on Glaciers 
Edin. Phil. Journal, April 1844. 
