PEOFESSOE TYNDALL ON THE VEINED STEUCTIJEE OE GLACIEES. 
297 
From a station lower down, chosen in a couloir along which the stones are discharged 
from the end of a secondary glacier which hangs upon the slope of Mont Tacul, I set 
out a second line (No, 2) transverse to the axis of the glacier. 
A thii’d line (No. 3) was set out across the glacier about a quarter of a mile still lower*. 
The mean daily motion of the centres of these three lines is given in the annexed 
Table, and also their distances apart. 
Mean daily motion of three points 
upon 
inches. 
No. 1 ... . 
20-55 
No. 2 ... . 
15-43 
No. 3 ... . 
12-75 
the axis of the Glacier du Geant. 
Distances apart. 
} . 2477 links. 
I . 2215 links. 
The advance of the hinder lines upon these in front is most strikingly shown by these 
measurements ; and the proof that the Glacier du Geant is in a state of longitudinal 
compression is thus complete. 
Here then we have a vast ice press, and here we have the pure snow filling the trans- 
verse channels of the streams. We are thus furnished with an experimental test on a 
grand scale of the pressure theory of the veined structure. In 1857 I examined a great 
number of these seams of white ice, and found in many of them a finely developed lenti- 
cular structure. In 1858 I also examined the seams, and found some of them “rib- 
boned ” in the most exquisite manner by the blue veins ; indeed I had never seen the 
veins more sharply and beautifully developed, 
Tim structure was observed in portions of the seams at and near the centre of the gla- 
cier., where the differential motion observed at the sides does not exist. This fact, I 
think, throws grave difficulties in the way of any theory which makes the veined struc- 
ture dependent on differential motion., and more especially a theory which requires “ a 
very considerable amount of this differential motion to produce any sensible degree of 
stratification in the vesicles.” 
^9. On the flattening of Air-bubbles in Glacier Ice., and its relation to the 
Veined Structure. 
Those who have given their attention to the subject, know that the bubbles contained 
in glacier ice are, in general, not spherical, but flattened ; and that from their shape 
conclusions of the greatest import have been drawn regarding the internal pressures of 
glaciers. 
M. Agassiz draws attention to this subject in the following words ; — “ The air-bubbles 
undergo no less curious modifications; in the neighbourhood of the neve, where they 
are most numerous, those which one sees on the surface are all spherical or ovoid, but 
by degrees they begin to be flattened, and near the end of the glacier there are some 
that are so flat that they might be talren for fissures when seen in profile. The drawing, 
* These three lines are drawn upon the sketch map at page 268 of Part I. of these researches (FF', GG', HH') . 
