GLACIER OF THE SECOND ORDER. 
179 
referred to a mark cut on the rock where it stood. Two marks were then fixed on 
the glacier ; one was a pole stuck in at A, several feet into the snow of the avalanche 
already described as traversing the length of the glacier. The slope of the snow at 
the point A was about 10°; and the distance of A from the station St., by an ap- 
proximate measurement, 340 feet. 350 feet further in the same direction, a hole 
was made with a blasting iron into the solid ice at B, where the inclination was 20°. 
The precise position of these marks being determined relatively to the visual line, 
the observation was finished at 4 o’clock p.m. 
On the 23rd of July we returned. The mark A in the snow (which was so firmly 
driven in that it could not be withdrawn without breaking the pole) had advanced 
in the direction of the slope exactly four inches at 1 p.m., or in sixty-nine hours ; 
whilst the mark B in the ice had advanced 5^ inches in the same time ; whence we 
have 
Velocity of A in twenty-four hours . . . . T4 inch. 
Velocity of B in twenty-four hours . ... 1*8 inch. 
The result was what I had anticipated, although it must be confessed it might be 
expected to be nearly the same upon any theory of glacier motion yet proposed. 
The slope of a glacier, per se, is not an index of what should be the velocity of 
motion on the viscous theory. No doubt, other things being equal, the velocity will 
be proportional to some function of the declivity, and such we have seen to be fully 
borne out by experiments on the Mer de Glace of Chamouni ; and in the present 
case, the velocity under a slope of 20° was about one-third greater than that under a 
slope of 10°. But the analogy of a river, as well as theoretical considerations, show 
that the slope is but one of numerous considerations ; such as (1) the mass of the 
viscous body; the smaller the mass the smaller the velocity on a given slope*; (2) 
the state of infiltration or wetness of the glacier altering its resistance to change of 
form-f-. Without mentioning other causes, these are quite sufficient to account for 
the small velocity observed, when we recollect the very insignificant mass of this 
glacier and its dry state arising from its great elevation, its northern exposure, and 
even the very inclination of its bed which keeps it in a state of perfect drainage and 
leaves it always in a state tending to the snowy , rather than that of imbibition. 
§ 7. On the Annual Motion of Glaciers, and on the Influence of Seasons. 
The first estimate of the least authority on the advance of any point of a glacier 
from year to year, was made by Hugi on the glacier of the Aar, from 1827 to 1836. 
The method employed was to measure the distance of a well-marked block of stone, 
resting on the ice from a transverse line determined by the fixed objects on the shore. 
This is the only way, generally speaking, practicable upon glaciers at a distance 
from habitations, and where marks cannot be conveniently renewed in the ice from 
time to time during the whole year. The velocity of the part of the glacier imme- 
* Travels in the Alps of Savoy, 2nd edit., p. 387. . t Ibid. pp. 148. 371. 
