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PEOFESSOES TTXDALL AXD HUXLEY OX 
fessor Forbes, “ has the candour not to treat his ingenious speculations as leading to 
any certain result, not being founded on expeiiments worthy of confidence My 
theory of glacial motion, then, is this : — A glacier is ax imperfect fluid or viscous 
BODY, WHICH IS URGED DOWX SLOPES OP A CERTAIX IXCLIXATIOX 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 surface of a sluggish river. In short, the analogies are put 
\ forth so clearly, so ably, and so persistently, that it is not surprising that this theory 
I stands at present without a competitor. The phenomena, indeed, are reaUy such as to 
render it difficult to abstain from forming some such opinion as to their cause. The 
resemblance of many glaciers to “ a pail of thickish mortar poured out;” the gradual 
changing of a straight line transverse to the glacier into a curve, in consequence of the 
^ swifter motion of the centre ; the bent grooves upon the surface ; the disposition of the 
/ \ dirt ; the contortions of the ice, a specimen of which, as sketched near the Heisseplatte 
^ !upon the Lower Grindelwald glacier, is 
igiven in fig. 1, and of which other stri- 
^iu g examples have been adduced by M. 
;|lscHER, in proof of the plasticity of the 
substance, — are all calculated to esta- 
blish the conviction, that the mass must 
be either viscous, or endowed with some 
other ^ro'perty mechanically equivalent to 
viscosity. The question then occurs, is 
the viscosity real or apparent Does 
any property equivalent to viscosity exist, in virtue of which ice can move and mould 
itself in the manner indicated, and which is still in harmony -with our experience of the 
non-viscous character of the substance % If such a property can be shoum to exist, the 
choice will rest between a quality which ice is proved to possess, and one wliich, in 
opposition to general experience, it is assumed to possess, in accounting for a series of 
phenomena which either the real or the hypothetical property might be sufficient to pro- 
duce. In the next section, the existence of a true cause vdll be pointed out, which recon- 
ciles the properties of ice, exhibited even by hand specimens, with the apparent endences 
Eig. 1. 
