Variation of Motion at different Seasons. 335 



temperature of the air of the valley to which in this part of its course 

 it is exposed ; but this last cause is alone insufficient ; for 



II. We find that the lowest part of the same glacier immediately 

 behind the Cote du Piget, a little way above the source of the Ar- 

 veiron, and therefore still deeper in the valley, has a mean velocity 

 nearly four times less^ arising solely from the diminished slope and 

 volume of the glacier in that part.* Hence there must be a conden- 

 sation of the ice here, a pressure d 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 shew 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 differ- 

 ences 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 mo- 

 tion 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 tempera- 

 ture of any portion of the year and the velocity of the glacier cor- 

 responding to it. This is done in Plate V., exactly in the same 

 way as I did when comparing my observations in the summer 

 of 1842 with the corresponding changes of temperature.f 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 pub- 

 lished in the Bibliotheque Universelle, the average of which (sepa- 

 rately 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 espe- 

 cially the middle and lower regions of them, are exposed ; and fur- 

 ther, this average possesses the advantage of being derived from data 



* This explains a circumstance which has always hitherto been a diflSculty 

 to me ; the united testimony of the best informed inhabitants, not only at Cha- 

 mouni but elsewhere (as at Zerraatt and at the Simplon), to the effect that du- 

 ring winter the lowest end of a glacier, which terminates in a valley, does not 

 greatly protrude, 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. 



