186 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
du Piget, a little way above the source of the Arveiron, and therefore still deeper in 
the valley, has a mean velocity nearly four times less, arising solely from the dimi- 
nished slope and volume of the glacier in that part*. Hence there must be a conden- 
sation of the ice here, a pressure a tergo, the quicker moving ice pressing against the 
slower, consolidating it, remoulding its plastic material and sealing the crevasses; 
and a slight examination of the state of the glacier at the points in question will 
show this to be the case. 
III. All that has now been said with respect to the two stations on the Glacier des 
Bois may be repeated with only numerical differences with respect to the two stations 
on the Glacier des Bossons ; the one set of observations confirming the other. 
IV. In both glaciers the summer motion exceeds the winter motion in a greater 
proportion, as the station is lower, that is, exposed to more violent alternations of heat 
and cold ; this we shall find to be general. 
Before continuing our deductions, we would call attention to the close relation 
which may be established between the mean temperature of any portion of the year 
and the velocity of the glacier corresponding to it. This is done in figure 1, Plate 
XI., exactly in the same way as I did when comparing my observations in the 
summer of 1842 with the corresponding changes of temperature -j~. That is to say, 
I have projected by periods (corresponding to the intervals of observation on the 
glaciers) the mean temperatures as observed at Geneva and at the Great St. Bernard, 
which are regularly published in the Bibliotheque Universelle, the average of which 
(separately deduced from the mean of daily maxima and minima, and projected in 
the upper part of the figure) may represent not inaptly the average temperature to 
which the glaciers in question, and especially the middle and lower regions of them, 
are exposed ; and further, this average possesses the advantage of being derived from 
data wholly unconnected with the place or parties where and by whom the observa- 
tions on the motion of the glaciers were made, and therefore are free from the re- 
motest suspicion of either in any degree influencing the other. 
* This explains a circumstance which has always hitherto been a difficulty to me ; the united testimony of 
the best-informed inhabitants, not only at Chamouni but elsewhere (as at Zermatt and at the Simplon), 
to the effect that during winter the lowest end of a glacier, which terminates in a valley, does not greatly pro- 
trude, nor force the snow before it. This arises in fact from the comparative smallness of the motion which 
the tongue of such a glacier appears to possess, especially in winter. 
t Travels, p. 141 . 
