240 Professor Forbes's Sixth Letter on Glaciers. 



water ; the latter percolating freely through the crevices of 

 the former, to all depths of the glacier ; and as it is matter of 

 ocular demonstration that these crevices, though very minute^ 

 communicate freely with one another to great distances, the 

 water with which they are filled communicates force also to 

 great distances, and exercises a tremendous hydrostatic pres- 

 sure to move onwards in the direction in which gravity urges 

 it, the vast, porous^ crackling mass of seemingly rigid ice, in 

 which it is, as it were, bound up. 



But farther than this, the experiments first announced in 

 the earliest of these letters, shewed, that whatever be the con- 

 stitution of a glacier, and whatever be the cause of its motion, 

 THE FACT IS, that it does not move like a solid body sliding 

 down a bed or channel, but that the velocity of each part of 

 its breadth is different. It was demonstrated by the most 

 clear and plain geometrical measurements, that whilst the 

 Centre of a glacier moves 500 feet, the side of the glacier moves 

 only 300 ; consequently, the portions of ice which started to- 

 gether soon part company, and the central molecule has com- 

 pleted its course, or arrived in the lower valley, whilst the 

 other, which was its companion, has advanced only three-fifths 



ABC D E F G 



^ 

 ^ d 



d 



a g 



y 



c , e 

 d 



h f 



of the distance, or remains perhaps several miles behind. 

 Thus it has been shewn from multiplied measurements of the 

 most precise and accordant kind, that a series of stones or 

 marks being supposed to be laid across a glacier in the line 

 ABCDEFG ; they will be found, after a certain time, in the 



