PEOFESSOE TTM)ALL’S OBSEEVATIONS ON THE MEE HE GLACE. 
276 
open with extreme slowness. While standing one evening, in company with Mr. Hiest, 
on the Glacier du Geant, both of us were startled by a sound like a heavy explosion in 
the body of the glacier, underneath the place where we stood. This was instantly followed 
by a succession of loud cracks, accompanied by a low singing noise. The ice continued 
cracking for an hour ; but notwithstanding the manifest breaking of the glacier, which 
was to some extent awe-inspiring, we could not, for a long time, detect any trace of 
rupture. The escape of air-bubbles from the surface first informed us of the position 
and direction of the incipient crevasse, for such it was. It was so narrow that the 
thinnest blade of my penknife would not enter it. 
On another occasion, our guide, while engaged in setting out one of our lines, observed 
the ice to break beneath his feet, and a rent to propagate itself suddenly, with loud 
cracking, to a distance of 50 or 60 yards across the glacier. These fissures are produced 
by tension, and the velocity with which they widen is a measure of the amount of relief 
demanded by the glacier. The crevasse last alluded to required several days to attain 
a width of 3 inches, and the opening of the one on the Glacier du Geant was far 
slower than this. This is their general character. They form suddenly and open slowly^ 
and both facts are demonstrative of the non-viscosity of the ice. For were the substance 
capable of stretching^ even at the small rate at which they widen, there would be no neces- 
sity for their formation*. 
There is another point of view from which the question of viscosity may be examined ; 
but as the observations which bear upon it possess a general value, I will devote a 
special section to them ; choosing afterwards those which more particularly apply to the 
case now under consideration. 
§ 4. On the Inclinations of the Mer de Glace. 
By calculation from heights and distances. Professor Foebes obtained approximately 
the inclinations of some portions of the Mer de Glace f, but no direct observations on 
the subject have been hitherto made. On the 4th of August we transported our theo- 
dolite to the Jardin, for the purpose of ascertaining its inclination, and that of the Gla- 
cier du Talefre. From the green space on which visitors to the place usually repose, 
the angle of elevation to the top of the Jardin is 24° 7', and from the same place down- 
wards to the bottom of the Jardin the inclination is 30°. From the bottom of the Jardin, 
for some distance along its medial moraine, the ice is nearly level, its inclination being 
only 21'. A succession of slopes then follows, enclosing with the horizon the following 
angles of depression : — 3° 6' ; 4° 25' ; 6° 50' ; 8° 5' and 9° 40', which last brings us to the 
brow of the ice cascade. The inclination of the fall is 25°, — producing a line drawn 
along the centre of the cascade until it cuts the moraine between the Talefre and Lechaud : 
the inclination along this line, from the base of the cascade downwards, is 7° 30'. 
* For an interesting account of the formation of a number of new crevasses, see Agassiz, ‘ Systeme Gla- 
ciaire,’ p. 310. 
t Travels, p. 117. 
