16 



* dirt-bands,' but a little nearer the western bank. It lies, however, 

 on the band. 



" I shall now give the sequel of my guide Auguste Balmat's obser- 

 vations on the motion of the Glacier des Bois (the outlet of the Mer 

 de Glace), and of the Glacier des Bossons, since the period to which 

 the table in my Fourteenth Letter extends, which will be found to 

 embrace continuous observations, by periods of a few weeks from the 

 2d October 1844 to the 21st November 1845. They were continued 

 in like manner until the 19th February 1846, when they were in- 

 terrupted by Balmat's illness, which was accompanied by inflamma- 

 tion of the eyes. But in October of the same year they were re- 

 sumed, and were continued without intermission until the end of 

 June 1848, embracing altogether a period of nearly four years, with 

 only eight months' intermission. It is necessai-y to observe that the 

 station on the glacier of Bossons was altogether changed after the 

 above mentioned interruption, being transferred 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 un- 

 changed, 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 

 observation, 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 con- 

 clusions 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 consistent results. 

 I do not think that the causes of the comparative acceleration of one 

 glacier and retardation of another have yet been clearly brought out, 

 though I conceive that accurate local observations, combined with 

 such measurements, would gradually but surely unveil them. Nor 

 do I mean to affirm that measurements made with so much labour 

 and trouble, and under circumstances even of personal danger at cer- 

 tain seasons of the year, are irreproachable in point of accuracy. I 

 think it even probable that oversights have occurred ; but I have 



