198 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
in the breadth of a glacier is proportional to the absolute velocity at the time of the 
ice under experiment*.” (4) The progression of velocity from the side to the centre 
is marked by insensible gradations-^. (5) When we compare the motion of a given 
point of a glacier any day of one year and the same day of another, the probability 
is that the velocity will be exactly the same, if the season be equally hot or cold; 
hence, surely, a most unexpected result, which I first announced in 1842, that a few 
days observation of a glacier will enable any one to compare its mean rate of motion 
over its various parts and with different glaciers. Thus, the motion of a point marked 
D 2 on the Mer de Glace, was in 1842, from August 1 to August 9, 16^ inches daily ; 
from August 9 to September 16, 18 inches ; now next year, 1843, one observation at 
the same point in August gave 16 inches; and in 1844, owe observation in September 
gave \7\ inches. But still further, (6) the very law of flexure of the ice is the same 
from year to year : a series of stations across the ice at the Montanvert gave, in 1842, 
the following (simultaneous) relative velocities^; : — 
1-000 1-332 1-356 1'36/. 
The same points being recovered in 1844, the relative motions were (by a single ob- 
servation of the space moved over in five days) — 
1-000 1-339 1-362 1*374, 
ratios almost the same but slightly increasing, which corresponds with the fact men- 
tioned above (3), that when the absolute velocities are greater, the relative velocities 
are so too, which was here the case, for the velocity denoted by 1-000 was a little 
greater in the second case than in the first. 
Tensions and Thrusts. — The occurrence of open crevasses plainly indicates the 
existence of strains in the ice of glaciers producing disruption, at least partially. 
Hence some writers have precipitately inferred that the whole glacier must be in a 
state of tension ; an uncertain inference surely in a problem of singular complexity, 
and one which is not warranted by a more accurate analysis. Yet for a time rival 
theories seemed poised on the inappropriate question, “ Are glaciers in a state of in- 
ternal tension or compression?” Even if the glacier moved as a mass of fragments, 
therefore without tension, the cohesion must first have been broken before it could 
be reduced into fragments. I have been inconsiderately censured for quoting, with 
approbation^, the observation of M. Elie de Beaumont, that a glacier appears to 
be rather in a state of distension than compression, whilst I adopted a hydrostatic- 
pressure, acting from the origin as the source of motion. A careful examination of 
the passages in question will show that my assent to the view of M. Elie de Beau- 
mont was limited to portions of the glacier, and especially to those portions most cre- 
vassed, the parts, namely, which connect the sides and centre, and which serve to 
drag the more sluggish, because retarded, lateral portions after the freer central part 
* Travels, p. 148. f See § 5 of this paper. 
I Travels, 1st edit., p. 146. § Travels, 1st edit., pp. 178, 370; 2nd edit., p. 370 and note. 
