326 THE LIFE OF JAMES D. FORBES. [CHAP. 



relatively to the height of the observed marks above the 

 supposed bed of the glacier. The results of Forbes' s 

 observations confirmed this. The progress of each point, 

 as well in direction as in amount, was rigorously deter- 

 mined by a trigonometrical process, reference being had 

 to two fixed stations, seventy-five feet apart. This was a 

 work of considerable difficulty, and even danger, as the 

 continual fall of blocks, which bounded with great 

 velocity down the terminal face of the glacier, placed 

 both the observer and his instruments in some jeopardy 

 indeed, during the course of the observations, Balmat 

 received a severe blow on the head from one of them. 

 In order to plant the wooden pins which marked the 

 points, it was necessary to commence by laboriously 

 removing the blocks and rubbish from the glacier above, 

 the fall of which would, at any instant, have threaten cd 

 the lives of his assistants, Balmat and Bellin a work 

 which occupied them for some hours. 



The results showed that less than fifty feet of 

 thickness between the lowest mark and the next, corre- 

 sponded to an apparent acceleration of nearly half the 

 motion of the lower point ; the acceleration of the central 

 upon the highest point being less considerable. Their 

 heights were approximately, eight, fifty-four, and 143 

 feet above the bed or floor of the glacier. 1 



Before quitting Chamounix, Forbes attempted to carry 

 out his project of a more complete exploration of the 

 Glacier du Ge*ant, and the clearing up of some doubtful 

 points in the geography of that glacier. This time, 

 however, he was baffled by the excessive difficulty of the 

 ice-fall, and it was not until 1850 that he succeeded in 

 obtaining the observations he desired. 



Journal, August 14th. 



' I started at five, with David Couttet and Auguste, for 

 the Glacier du Ge'ant. The morning was charming, and 

 by half-past seven we were well advanced on the Glacier 



1 These observations are given in a more detailed form in Forbes's 

 Eleventh Letter on Glaciers, dated September 16, 1846. 



