Professor Forbes’s Ninth Letter on Glaciers. 337 
to the only modification of De Saussure’s theory applicable 
to the case, and the fact seemed to point, as a necessary 
consequence, to an insensible yielding of parts throughout 
the whole mass, which therefore moves as a whole, and not 
by jerks occasioned by strains upon a nearly rigid mass when 
they attain the limit consistent with the small play of flexi- 
bility of the particles, as some authors would have us be- 
lieve.* 
It is satisfactory to have an entire confirmation of these 
particulars, from the observations made on the glacier of the 
Aar in 1844. The observations were made “ to the accuracy 
of a millimetre on the movement of the glacier from hour to 
hour,’+ and “ at the lower extremity of the glacier, as in the 
upper part of its course, the glacier does not advance ab- 
ruptly, by jerks (saccades) as formerlyt supposed, but its 
march is gradual and continuous.” 
Il. The influence of warm and damp weather in accelerating 
the continuous march of the glacier, and of cold weather in 
checking it, I deduced in 1842 from a careful comparison of its 
motion for three months with the state of the thermometer, 
and exhibited the result in numbers and diagrams. Here, 
again, the observers on the Aar glacier have confirmed this 
fact, so important in a theoretical point of view. “The ad- 
vancement of the glacier,” they say, “ was far from uniform ; 
it varied considerably, according to the condition of the 
atmosphere.” During nine days of cold snowy weather in 
August 1844, the mean daily advance was 155 millimetres, 
but during the sixteen fine days which followed, it moved 
through 230 millimetres per day. 
* Mr Hopkins’s experiment of a box full of ice descending an even 
plane, does not apply to this case, because, though it moves as a whole, 
it does so without change of figure, and without the resistance arising 
from the irregularity of the channel ofa glacier; and hence the seeming 
analogy to a glacier entirely fails. 
t Comptes Rendus, p. 1302, line 12. 
{ Jadis. It is to be inferred that the writer meant previous to 
1842. 
§ Comptes Rendus, p. 1303, line 9. 
|| Travels in the Alps, p. 148. 
“1 Comptes Rendus, p. 1301, at the bottom. 
VOL. XXXVII. NO. LXXVI.—APRIL 1845. Y 
