x.J ALPINE TRAWLS, 1846. 325 



The remainder of tins letter is taken up by the 

 description of a remarkable analogy between the condi- 

 tion of a portion of the glacier which was embayed 

 behind a promontory of rock, and that of a plastic body 

 acted upon by hydrostatic pressure. 



Forbes had measured the diurnal advance of the 



Glacier de Miage, and found it to be 9 '9 inches the 



YI TV same quantity as he had obtained in the case of 



the small hanging Glacier du Nant Blanc. The identity 



hese results was curious, as proving that two glaciers 



Mcalculably different bulk might have the same rate 



of motion, their conditions being different. In this case, 



tlif motion of the smaller glacier was accelerated by its 



"epness, while that of the larger was retarded by 



the slight declivity of its bed, and the enormous frontal 



-e to its exit into the Allde Blanche, already 



imbered with its ponderous masses of debris. He 



found on that occasion among the westerly moraines 



the glacier, specimens of dark blue lias slate, which 



: have come from the very axis of the granite chain, 



whTe its existence had been previously unsuspected by 



geologists. 



- then returned to Chamounix, and began a series 



of difficult experiments for the purpose of determining 



ce of movement between the surface of a 



acr, and the lower portions of its ice. On the sloping 



tongue, or extremity of the Glacier des Bois, where the 



i ina 1 face of compact ice was inclined at an angle of 



''<> to the hori/on, three marks were made, one 



In-hind (and eon-e|uently above) the. other, in a direction 



which was judged to !. nearly that of the progiv>H\ v 



on of tin- jr.- : any variation, then-fore, in the motion 



ints, could only lie imputed to the friction of 



the ; by the motion of the glacier, <-ach would 



pass in ...n over the same spot; and it might be 



supp>-d fr.'in the analogy of the lateral friction of 



gla< i the ph. Qomena of riven, that the 



mark upon the one above it 

 would 1 1. . but that this n-tardati- n would den- 



