2 ON THE STRUCTURE AND MOTION OF GLACIERS 
the main divisions of the problem of glacier structure and motion. 
An account of the experiments and observations, and our joint 
reflections on them, are embodied in the memoir now submitted to 
the Royal Society, 
§ 2. On the Viscous Theory of Glaciers. 
A glacier is a mass of ice which, connected at its upper extremity 
with the snow which fills vast mountain basins, thrusts its lower 
extremity into the warm air which lies below the snow-line. The 
glacier moves. It yields in conformity with the sinuosities of its walls, 
and otherwise accommodates itself to the inequalities of the valley 
which it fills. It is not therefore surprising that the glacier should 
have been regarded as an ice-river by those who dwelt in its vicinity, 
or that this notion should have found a place in the speculations 
of writers upon the subject. The statements of M. Rendu in con- 
nexion with this point are particularly distinct. ‘There are,” he 
writes, “a multitude of facts which seem to necessitate the belief that 
the substance of glaciers enjoys a kind of ductility which permits it to 
model itself on the locality which it occupies, to become thin and 
narrow, and to elongate itself like a soft paste.”! But this observer 
put forward his speculations with great caution, and often in the form 
of questions which he confessed his inability to answer. “M. Rendu,” 
says Professor Forbes, “has the candour not to treat his ingenious 
speculations as leading to any certain result, not being founded 
on experiments worthy of confidence... ... My theory of glacial 
motion, then, is this:—A GLACIER IS AN IMPERFECT FLUID OR 
VISCOUS BODY WHICH IS URGED DOWN SLOPES OF A CERTAIN 
INCLINATION BY THE MUTUAL PRESSURE OF ITS PARTS.” 
“The sort of consistency to which we refer,” proceeds Professor 
Forbes, “may be illustrated by that of moderately thick mortar, or 
the contents of a tar-barrel poured into a sloping channel.” Treacle 
and honey are also referred to as illustrative of the consistency of a 
glacier. The author of the theory endeavours, with much ability, 
to show that the notion of semifluidity, as applied to ice, is not 
an absurdity, but on the contrary, that the motion of a glacier exactly 
resembles that of a viscous body. Like the latter, he urges, it 
accommodates itself to the twistings of valleys, and moves through 
narrow gorges. Like a viscous mass, it moves quickest at its centre, 
the body there being most free from the retarding influence of the 
lateral walls. He refers to the “ Dirt-Bands ” upon the surface of the 
glacier, and shows that they resemble what would be formed on the 
1 Théorie des Glaciers de la Savoie, p. 84. 
