PEOFESSOE TYNDALL’S OBSEEVATIONS ON THE MEE DE GLACE. 
265 
Third Line (CC'). — Mean Daily Motion. 
East. West. 
No. of stakes. 12 34 56 789 10 11 12 
Motion.... 19i 22| 28f 301 33| 28i 24i 25 25 18 x 8^ 
Stake No. 1 was fixed in the ice, close to the eastern side of the glacier, and the 
retarding influence of this side is quite manifest from the measurements. A glance, 
however, reveals a fact confirmative of the former measm'ements ; the daily motion of 
the extreme eastern stake is 14^ inches behind the maximum, while the motion of the 
extreme western stake is 25-| inches behind it. The stake No. 5, which moved at the 
maximum rate, was also much nearer to the eastern than to the western side of the ice- 
stream ; the observation therefore corroborates those already made as regards the position 
of the point of maximum motion. 
How then is the fact to be accounted for, that the point of maximum motion of the 
Mer de Glace is thus thrown towards its eastern boundary 1 Reflection suggested to me 
that the efiect might be due to the curvature of the valley through which the Mer de 
Glace moves. At the place where the foregoing observations were made the glacier 
bends, turning its concave side to the Montanvert, and its convexity towards the Cha- 
peau. M. Rexdu insists on the complete analogy of the phenomena of a river and those 
of a glacier ; and the idea has been to a great extent corroborated by the measurements 
of Professor Foebes and M. Agassiz ; but let us make a bolder application of the ana- 
logy than any of them contemplated, confining our view to the influence of curvature 
merely. The point of maximum motion of a river moving through a channel similar 
to that occupied by the Mer de Glace, would lie on that side of the centre of the channel 
towards which the river turns its convex curvature. Can this be the case with the ice 1 
If so, the place of maximum motion ought to be different where the glacier bends in the 
opposite direction. Fortunately the Mer de Glace itself enables us to bring this idea to 
a test. 
Higher up the valley, and opposite to the passages called “ Les Fonts,” such a band 
occurs. Here the convexity is turned towards the Montanvert or western side of the 
vaUey. A line was set out across this portion of the glacier on the 25th of July, and 
its measurement upon the 26th gave the following results: — 
Fourth Line (DD'). — Mean Daily Motion. 
East. West. 
No. of Stakes. 1 2 3 4 5 6 9 10 11 12 13 14 15 16 17 
Motion.. 61 8 121 15i 15i 18f 19i 21 20i 23i 23i 21 22i 17i 15 
After the setting out of this line, its length was measured by Mr. Hikst ; and found 
to be 39 chains 25 links, which, as each chain is equal to 22 yards, gives 863 yards 
as the width of the Mer de Glace opposite the first “Powi^.” A mark on the rock crossed 
by this pont constituted indeed one of the fixed termini of the line. 
For the sake of stricter discussion, a copy of the notes of this measurement faces the 
next page. 
