Prof. J. D. Forbes's Sixteenth Letter on Glaciers. 169 



crease of velocity here noticed since 1844 is due to a change 

 in the conditions of the glacier (although I believe that the 

 recurrence of several snowy seasons and the very marked in- 

 crease of the volume and extent of the glacier during these 

 years would produce such an effect), because it has moved 

 nearly half-a-mile from its position when first observed, and 

 the part of the glacier on which it now lies may be subject to 

 different accelerating and retarding causes. 



It is mentioned in my Thirteenth Letter, page 4, that I 

 marked a fine solitary block towards the centre of the Mer 

 de Glace opposite " Les Fonts" with the letter V in 1846, 

 and that I took angles for fixing its place with reference 

 to the adjacent rocks. It was then about 760 feet distant 

 from the west bank I had little difficulty in recognizing 

 the block in 1850, although it had travelled a great distance 

 and was considerably lower than the Montauvert. It had pre- 

 served its parallelism to the shore, for I found it at almost 

 the same distance from the west bank as at first ; and by 

 measuring carefully along the side of the glacier, I estimated 

 its progress in four years, from 30th July 1846 to 13th July 

 1850, at 3255 feet. This gives, for the mean motion in 365 

 days, 822*8 feet, or the mean daily motion 27*05 inches, which 

 is remarkably large. Its position is very near the point of 

 one of the " 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 

 observations 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 mauner until the 19th 

 February 1846, when they were interrupted by Balmafs ill- 

 ness, which was accompanied by inflammation of the eyes. 

 But in October of the same year they were resumed, 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 necessary toobserve 



