PEOFEFSOE TYNDALL’S OBSEEVATIONS ON THE MEE DE GLACE. 
273 
qiience of this difference of action upon the two branches, is that the side of the glacier 
which is subjected to the augmented tension does not yield to the strain as a viscous body 
would do, but breaks. In the words of Professor Foebes, the glacier at this place 
becomes “ excessively crevassed.” This fact, therefore, as far as it goes, is opposed to the 
idea of viscosity as above defined. 
2. The fact that the centre of a glacier moves more quickly than the sides, is that on 
which the viscous theory is chiefly based : let us examine the circumstances connected 
with this motion, availing ourselves while doing so both of the figure and the reasoning 
of Mr. Hopkins. Let ABCD, fig. 4, be a sloping canal, into which is poured a 
Fig. 4. 
quantity of treacle, honey, tar, or melted caoutchouc, all of which have been referred 
to as illustrative of the character of ice ; and let the mass move down the slope in the 
direction of the arrow. Let ST be a narrow segment of the viscous substance; this 
segment, as it moves downwards, will take the form S'T'. Supposing Ti to be a square 
element of the mass, it will be distorted lower down into the lozenge T'i', and the line 
Ti will become T'i'. Now the analogy between such a substance and ice fails in this 
respect ; in the viscous mass the short diagonal of the square stretches to the long one 
of the lozenge, but, in the glacier, the ice breaks at right angles to the tension, and 
marginal crevasses are formed. It was by means of the simple diagram here sketched 
that Mr. Hopkins showed why the marginal crevasses of a glacier are inclined towards 
its sourcTe*. This fact, therefore, so far as it goes, is also opposed to the idea of 
viscosity. 
But it is known that in the case of a substance confessedly viscous, a sudden shock or 
strain may produce fracture. Professor Foebes justly urges, “ that seahng wax at mode- 
rate atmospheric temperatures, wiU mould itself (with time) to the most delicate inequal- 
ities of the surface on which it rests but may, at the same time, be shivered to 
atoms by a blow with a hammer f.” Hence, in order to estimate the weight of the 
objection, that the glacier breaks when subjected to strain, we must know the conditions 
under which the force is applied. 
The fifteenth station on the line (EE') at Trelaporte stands on the lateral moraine of the 
^ glacier; between it and the fourteenth, a distance of 300 links, or 190 feet, intervenes, 
and within this distance the glacier suffers its maximum strain. Let AB (fig. 5) be the 
* Philosophical Magazine, vol. xxvi. page 160. 
t Philosophical Magazine, Fourth Series, toI. x. p. 201. Proceedings of the Eoyal Society, June 14, 1855. 
