PEOrESSOE TYNDALL’S OBSEEVATIONS ON THE MEE DE GLACE. 277 
down which, the ice is shot in crags, pinnacles, wedges, and castellated masses, all tossed 
together in the utmost confusion. Eegarding this portion of the glacier, Professor 
Foebes writes as follows : “ Escaping from the rocky defile between the promontory 
of the Montanvert and the base of the Aiguille de Dru, it pours in a cascade of icy 
fragments, assuming the most fantastic forms, into the valley beneath.” Above the fall 
the ice is compact: Professor Forbes compares it to the dark unruffled swell of swift 
water rushing to precipitate itself in a mass of foam over a precipice. 
In fig. 6 I have protracted the inclination of the fall and of the glacier above it, one 
of them, BC, making an angle of 5° 10', and the other, BA, an angle of 22° 20' with the 
horizon. Supposing the ic’e to pursue the direction which it had previous to reaching 
the fall, it would, at the end of a certain time, reach the point d*-, but the ice is not 
rigid enough to do this, and the mass descends to e. Now if it be the viscosity of the 
substance which has carried it in a certain time from B to d, that same property ought, 
one would think, to enable it to drop down the vertical de without breaking. But so far 
from its being able to do this, the glacier descends the slope BA as “ a cascade of icy 
fragments.” The fact, therefore, adds its evidence to that already adduced against the 
viscosity of the substance. 
But the case will appear much stronger when we revert to other slopes upon the Mer 
de Glace. For example; the inclination of the glacier above I’Angle is 4°: it subse- 
quently descends a slope of 9° 25', and in doing so is so much fissured as to be abso- 
lutely impassable. The chasms cut the glacier from side to side, and present clear 
vertical faces of great depth ■|’. Subtracting the smaller of the above angles from the 
larger, the difference, 5° 25', gives the change of slope which produces the chasms. In 
fig. 7 the two adjacent slopes are protracted to a proper scale. Now the velocity of the 
Eiff. 7. ^ 
glacier here, in the dfrection of its length, is to the vertical velocity with which it would 
have to sink to reach its bed, as B^Z: de, or as the cosine of 5° 25' is to its sine, or as 
996 : 94, or, in round numbers, as 10 : 1. Hence if it be viscosity which enables the 
mass to move from B to <Z in a certain time, the same property ought, one would think, 
to permit it to sink through the space de, which is only one-tenth of B^Z, in the same 
I here assume that the general inclination of the surface of the glacier changes in accordance with that 
of its hed, which will hardly be questioned. 
t I once found myself alone upon this portion of the glacier towards the close of a day’s work, and expe- 
rienced great difficulty in escaping from the entanglement of chasms in which I had involved myself. 
MDCCCLIX. 9 p 
