170 Prof. J. D. Forbes's Sixteenth Letter on Glaciers. 



that the station on the glacier of Bossons was altogether" 

 changed after the above mentioned interruption, being trans- 

 ferred from the west to the east side (in the same region of the 

 glacier), and it was 340 feet from the bank. The station on the 

 Glacier des Bois was almost unchanged, and was about 280 

 feet from the north bank, between the Cote du Piget and the 

 acclivity of the Chapeau. I have added a column giving the 

 mean of the temperatures of the several periods of observa- 

 tion, carefully calculated from the published observations at 

 Geneva and the great St Bernard, on the same principle as 

 I have fully explained in my Fourteenth Letter above referred 

 to. The comparisons of the temperature and the rate of 

 motion lead to conclusions similar to those which I have 

 drawn in that paper from the earlier observations, the general 

 observation always holding that the acceleration in spring 

 is in a greater proportion to the temperature than at any 

 other season of the year, on account of the great influence of 

 the melting snows in imparting fluidity to the glacier masses. 

 I do not mean that the comparison leads always to consist- 

 ent results. I do not think that the causes of the compara- 

 tive acceleration of one glacier and retardation of another 

 have yet been clearly brought out, though I conceive that ac- 

 curate local observations, combined with such measurements, 

 would gradually but surely unveil them. Nor do I mean to 

 aftirm that measurements made with so much labour and 

 trouble, and under circumstances even of personal danger 

 at certain seasons of the year, are irreproachable in point of 

 accuracy. I think it even probable that oversights have oc- 

 curred ; but I have very strong reason for confiding in the 

 absolute fidelity with which the observations have been made 

 and transmitted to me. Circumstances have transpired since 

 my last publication which increase this confidence ; and I 

 should be ungrateful if I did not once more publicly acknow- 

 ledge, whilst giving to the world the sequel of observations 

 made under such circumstances that their resumption is 

 scarcely probable, the lasting obligations which I owe to the 

 zeal, fidelity, and disinterestedness of my worthy though 

 humble friend and guide. 



