1846.] ANNUAL MOTION OF THE MER DE GLACE. 123 



7. ON THE ANNUAL MOTION OF GLACIERS, AND ON THE 

 INFLUENCE OF SEASONS. 



The first estimate of the least authority on the advance of 

 any point of a glacier from year to year, was made by Hugi on 

 the glacier of the Aar, from 1827 to 1836. The method em- 

 ployed was to measure the distance of a well-marked block of 

 stone, resting on the ice from a transverse line determined by 

 the fixed objects on the shore. This is the only way, generally 

 speaking, practicable upon glaciers at a distance from habita- 

 tions, and where marks cannot be conveniently renewed in the 

 ice, from time to time, during the whole year. The velocity of 

 the part of the glacier immediately below the promontory, called 

 the Abschwung, was found to be about 240 feet per annum, 

 which, though neither confirmed nor invalidated by the discor- 

 dant measurements subsequently made by other observers on 

 the same glacier, has at length been substantially corroborated 

 by a professional surveyor, M. Wild, who has recently under- 

 taken the verification at M. Agassiz's request. 



After having myself observed the motion of several points 

 of the Mer de Glace of Chamouni during the summer of 1842, 

 I fixed the positions of two conspicuous blocks, one near Mon- 

 tanvert, marked D 7 ; and another opposite the Tacul, marked 

 C, or the Pierre Platte (see my Map of the Mer de Glace), by 

 means of which I hoped to ascertain the mean annual motion 

 in succeeding years. With respect to the latter, or the Pierre 

 Platte, I was successful ; for in September 1843 I ascertained 

 geometrically its change of position, subject, however, to the 

 uncertainty of a few yards, owing to the sliding of the block 

 from the pedestal of ice upon which it was so picturesquely 

 poised,* a circumstance which happens once or twice in the 

 course of every summer. 



From the 17th of September 1842, to the 12th of 



September 1843, the advance was (in 360 days) 256'8 feet. 

 Or reduced to the exact year of 365 days, . 260-4 feet. 

 Mean daily motion, . - . . . . 8*56 inches, 



* See Frontispiece to Travels through the Alps of Savoy. 



