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energy, industry, and ability by the late Principal James Forbes, whose 
various works on glaciers have exhausted the whole field of observation 
and supply most of the facts on which the true solution of the problem, 
whenever it is arrived at, must be founded. When however at another stage 
of the enquiry, it came to be discovered by Faraday and Tyndall that 
ice, when broken up, was capable of being united again by sufficient 
pressure, so as to become as perfectly solid and homogeneous as it was 
before, it became evident that supposing a sufficient pressure to be exerted 
on the glacier, in the direction of its descent, and its substance to be 
thus crushed through the contractions and gorges of its channel, and 
over the irregularities in its bed, it would re-form itself and solidify, 
and become a compact mass again as it was before, when it had passed 
the obstructions. The fact of the more rapid descent of the ice at the 
surface than at the bottom, and at its centre than at its sides, has more- 
over since been shewn to be not incompatible with a solid state of the ice 
by the remarkable experiments of M. Tresca, on what he calls the flow of 
solids. Forcing lead and other metals (and also ice) under enormous pressure, 
through a strong hollow cylinder of a smaller diameter from one of a larger, 
he found that the continuity of the mass was preserved, as it would be if it were 
a liquid, and that the particles of the solid nearest the axis of the lesser 
cylinder were made to move faster through it, than those further from the 
axis, as those of a liquid would. Supposing a sufficient pressure to thrust it 
forward, to act in the direction of the length of a glacier, we are no longer 
obliged, therefore, to have recourse to the supposition that it is viscous to 
account for its descent. The whole question resolves itself into that of the 
amount of the downward pressure and its sufficiency to thrust the glacier 
forwards, not with a common motion, but with that differential motion 
which has been described. Is the weight of the glacier sufficient for 
this or is it not? The mass of a glacier is so enormous and it reaches 
so high up into the mountains, that, thinking of the question in a loose 
way, the pressure of its weight seems to be enough for anything. Closer 
consideration shows us, however, a fallacy in this estimate of it. I 
will try to give a popular illustration of this fallacy. Imagine for an 
instant the ice of the Mer de Glace to be all removed, and the bed of 
the glacier to be laid bare up to the "Col du Geant." Let a line of 
men be supposed to stand across the bed of the channel opposite Montan- 
vert, and another line close behind them, and another behind them, and so 
on up to the Col du Geant. Every man of that multitude would stand as 
firmly in his place on the floor of rock as though he were the only one no 
pressure would be produced on the first line by putting the second behind 
