GLACIERS AND ICE. 
47 
to fig. 1, where abode indicate medial moraines formed by 
the union of one more than a corresponding number of glaciers. 
Like a river, where the glacier rubs along its bed, its motion 
is impeded ; hence it moves with different velocities at different 
points on its surface. A row of stones lying straight across 
the glacier to-day, will not be in the same position to-morrow; 
the central ones will have crept forward some 20 to 30 inches, 
the marginal ones but 5 to 10 inches, and the others in 
proportion. The first reference to the speedier motion of the 
central parts of the glacier was made in the writings of 
a Bishop of Savoy, M. Rendu ; a year later, in 1842, Principal 
Porbes definitely established the fact, whilst immediately after- 
wards M. Agassiz demonstrated it by observations which then 
embraced a year's motion of the different parts of the glacier 
of the Aar. These results, confirmed by subsequent observers, 
are in exact accordance with the flow of rivers which are 
found to move fastest at their central superficial parts. Indeed, 
as already noticed, the strictness of the analogy between a 
glacier and a river is remarkable. When a current rounds a 
curve, its point of swiftest motion shoots for a while beyond 
the centre, regaining its central position if the river continues 
straight, but passing again to the opposite extreme if it turns 
in a contrary direction — its course resembling the path taken 
by a dog pursuing a hare, which swerves from side to side, 
a figure usually shown in natural philosophy diagrams to 
illustrate the influence of inertia. Just in the same manner 
does the glacier behave when its course is deflected. Professor 
Tyndall first drew attention to this interesting fact, proving 
it by a series of measurements on the curvatures of the Mer- 
de- Glace between Trelaporte and Montanvert, and arrived at 
the conclusion that “ the line of maximum motion in a glacier 
is a curve more deeply sinuous than the valley itself, crossing 
the axis of the valley at each point of contrary flexure." * 
The central parts of a glacier being those in most rapid 
motion, the sides must necessarily be in a state of strain from 
the constant drag taking place towards the centre. This is 
the case, and as ice is incapable of stretching, the glacier 
breaks at right angles to the line of tension. These cracks, 
gradually opening by the continued pull, form the so-called 
marginal crevasses , indicated by the double border lines in 
fig. 3. The crevasses must thus point down the valley, at first- 
sight appearing as if the glacier moved more rapidly at its 
edges than at its centre. It was this anomalous appearance 
which so greatly puzzled the early observers. Recently, how- 
ever, it has been shown by a celebrated English geometer. 
* Phil. Trans., 1859, p. 268. 
