140 Professor Forbes's Thirteenth Letter on Glaciers. 



A very fine block of granite, marked R in oil paint on 

 three sides, about the same distance as Q from the west mo- 

 raine (300 feet), had its position fixed on the 26th August 

 1844, and was found this year to have moved in 693 days, to 

 20th July 1846, 1108 feet, or at the rate of no less than 

 19-2 inches daily, or 533*6 feet annually, a remarkably rapid 

 motion, but which was carefully verified by two concordant 

 measures. The rate of motion of this block (engaged as it was 

 in the great crevasses near the Angle), from the 23d to the 

 30th July 1846, was 11 feet 4-5 inches, or 207 inches daily. 



B. At points not before observed. 



I had long wished to ascertain the motion of the ice, which 

 issues from the very remarkable basin-shaped glacier of 

 Talefre. That an oval circus or amphitheatre, whose length 

 may be roughly stated at 4000 yards, its breadth at 2000, 

 entirely filled with snow and ice (excepting the rocky island 

 of the Jardin), should disgorge itself by an icy stream through 

 a chasm less than 700 yards wide in its broadest part, ap- 

 pears to me, as I have elsewhere stated at large,* a plain 

 demonstration of the viscous theory of glaciers ; the outlet in 

 this case acting precisely as a stream does to a lake of inde- 

 finite extent, as a mere overflow, or trop-pleiUf a fact only 

 consistent with the quasi-fluid motion of the mass so dis- 

 charged. 



The great distance of the Talefre from habitable spots, 

 renders it inconvenient for prolonged experiments, but hav- 

 ing last summer twice spent the greater part of a day in its 

 neighbourhood, I have been enabled to determine with exact- 

 ness its rate of motion. 



The part which I selected for experiment, will be under- 



* London Philosophical Magazine, May 1845, page 415. Compare the map 

 of the Mer de Glace in Ti-avels in the Alps of Savoy. The outlet of the glacier 

 is seen in Plate 11., fig. 2, accompanying the present paper. 



