168 Prof. J. D. Forbes's Sixteenth Letter on Glaciers, 



communicated to the readers of your Journal. As my stay 

 was limited by imperative engagements to little more than 

 a week, I was prevented from undertaking a continuous se- 

 ries of observations on the movement of the ice. I was for- 

 tunate, however, in obtaining materials for the correction 

 and extension of certain parts of my Map of the Merde Glace, 

 which were deficient in my former observations, especially as 

 to the exact form of the basin of the great Glacier du Geant, 

 which I had only visited once before, on occasion of the pas- 

 sage of the Col of that name in 1842. This year I traversed 

 again all the difficult part of that glacier, and took angles 

 with the theodolite from the upper part of the basin, imme- 

 diately under the Aiguille du Geant. But as these observa- 

 tions can have little interest until reduced into the form of a 

 corrected edition of the Map, I shall say nothing of them here. 

 It will be recollected by some of your readers that a re- 

 markable stone called " La pierre platte,"* was one of the 

 earliest points whose position was ascertained by me in 1842. 

 Its daily motion was watched by me during that summer, f 

 and its annual motion was ascertained by renewed observa- 

 tions in 1843, 1844, 1846, and again this year. I measured 

 the distance along the ice from the original position of the 

 ** Pierre platte" on the 27th June 1842 (ascertained by re- 

 ference to fixed marks on the rocks) to its position on the 

 12th July 1850, and found it to be 2520 feet. But, of this 

 distance, 1212 feet had been travelled at my previous obser- 

 vation on the 21st July 1846, leaving 1308 feet during the 

 last four years against 1212 in the first four. When more 

 accurately stated and compared, the mean annual and daily 

 motions will stand as follows : — 



Daily motion, in Inches, 

 Annual motion, in Feet, 



We cannot infer, with absolute certainty, that the slight in- 



* Lying on the surface of the Glacier de Lechaud (in the upper part of the 



Mer de Olace), and carried along by the motion of the ice. It is marked C in 

 my Map of the Glaciers. 



t Travels in the Alps of Savoy, '2d edition, p. 139-40. , 



