336 Professor Forbes’s Ninth Letter on Glaciers. 
will be a swelling one, as acen, which might even rise to-- 
f 
MMMM OL 
wards valley, but generally need only be less sloped than the 
bed. The effect of superficial thaw, and internal subsidence, 
diminishes this again, and gives it the form of the dotted 
curve an’. 
In these diagrams the varying velocity in different parts 
of the transverse section is, for simplicity, kept out of view. 
A retardation of the foremost portion of a viscid stream, 
and consequent heaping of its surface, is exactly imitated in 
the models formed of plaster of Paris, which I have else- 
were described, and which, though of uniform fluidity from 
end to end, and therefore not subject to the objection arising 
from the cooling of lava, where a precisely similar fact is 
observed, reproduce faithfully the motions of the glacier of 
the Aar. 
The fact established on the glacier of the Aar satisfactorily 
refutes the notion that a predominant state of compression 
in a glacier is incompatible with the existence of transverse 
crevasses. 
Il. Continuity of motion. Until recently it was a question 
entirely unresolved, whether the glaciers move by insensible 
and nearly uniform degrees, or whether they start forward 
by short jerks, as might be expected if the movement in 
their irregular channels were effected by piecemeal fractures, 
local subsidences, and the justling of independent fragments. 
Accordingly when I determined, for the first time, in June 
1842, the absolute continuity of the motion down even to the 
interval of an hour,* it seemed impossible to reconcile this 
* See first Letter on Glaciers in this Journal, October 1842, p. 340. 
