208 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
« The glacier struggles between a condition of fluidity and rigidity*.” “ A glacier 
is not a mass of solid ice, but a compound of ice and water more or less yielding, ac- 
cording to its state of wetness or infiltrationf .” “ The pressure communicated from 
one portion to the other, will not be the whole pressure of a vertical column of the 
material equal in height to the difference of level of the parts of the fluid considered ; 
the consistency or mutual support of the parts opposes a certain resistance to the 
pressure, and prevents its indefinite transmission. * * A glacier is not coherent 
ice, but a granular compound of ice and water*” “ When the semifluid ice inclines 
to solidity during a frost, the motion is checked; if its fluidity is increased by a 
thaw, the motion is instantly accelerated. * * It is greater in hot weather than in 
cold, because the sun’s heat affords water to saturate the crevasses Such were 
the terms in which within a few months after suggesting the viscous theory I ex- 
pressed my opinion of the influence of the compound structure of the glaciei, a mass 
composed* not of ice alone, but of ice including water in its countless capillaries never 
frozen || even in winter. The quality of plasticity or viscosity resulting from the union 
of a nearly perfect fluid with an imperfect solid is seen in very numerous and fami- 
liar instances, as for instance in sand, which is itself devoid of any tenacity until its 
interstices have been saturated with just so much water as to cause it to flow, or in the 
still more familiar instance of water-ice prepared for the table, in which the varying 
proportion of the solid and fluid ingredient gives to it every shade of consistency, 
from a brittle solid to a liquor including suspended solid grains. The prodigious effect 
of capillary infiltration in determining the motion of even the most solid and ponderous 
bodies, breaking up their parts, and giving to the motion of the whole a more or less 
river-like character, is seen in the frequent case of land-slips, as for instance that of 
Goldau. And scarcely less instructive are the numerous examples, cited in the first 
section of this paper, of huge masses ol almost cold and brittle lavas being pressed 
on with a uniform and graduated motion, by the almost unimpeded hydrostatical 
communication of pressure from the yet active fluid which circulates unseen in their 
pores. With this analogy before me, I replied in 1844 in the following terms to the 
question, “ How far a glacier is to be regarded as a plastic mass ? “Were a glacier 
composed of a solid crystalline cake of ice, fitted or moulded to the mountain bed 
which it occupies like a lake tranquilly frozen, it would seem impossible to admit such 
a flexibility or yielding of parts as should permit any comparison to a fluid or semi- 
fluid body transmitting pressure horizontally, and whose parts might change their 
mutual position so that one part should be pushed out whilst another remained behind. 
But we know in point of fact, that a glacier is a body very differently constituted. 
It is clearly proved by the experiments of Agassiz and others, that the glacier is not 
a mass of ice, but of ice and water ; the latter percolating freely through the crevices 
* Third Letter on Glaciers, August 1842. Appendix to Travels, p. 407. 
f Travels, 1st edit., 1843, p. 175. 
X Travels, p. 367., edit. 1843. 
§ Ibid. p. 372. 
|| Ibid. p. 361, 372. 
