THE STEUCTUEE AND MOTION OF GLACIEES. 
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conclusion of the experiments the prism came out^ lent to a transparent semi-ring of 
solid ice. 
In this way, by the proper application of force, all the bendings and contortions 
observed in glacier ice, and adduced in proof of its viscosity, can be accurately imitated. 
Any observer, seeing a straight bar of ice converted into a continuous semi-ring without 
being aware of the quality referred to, and having his attention fixed on the changes of 
external form alone, would be naturally led to the conclusion that the substance is 
wscous. But it is plainly not viscosity, properly so called, which enables it to change its 
shape in this way, but a property which has hitherto been entirely overlooked by writers 
upon glaciers. 
It has been established by observation, that a vertical layer of ice originally plane, and 
perpendicular to the axis of a glacier, becomes bent, because the motion of its ends is 
retarded in comparison with that of its centre. This is the fact upon which the viscous 
theory principally rests. 
In the experiments with the straight prism of ice, four successive moulds, gradually 
augmenting in curvature, were made use of. In passing suddenly from the shape of one 
to that of the other, the ice was fractured, but the pressure brought the separated 
surfaces again into contact and caused them to freeze together, thus restoring the con- 
tinuity of the mass. The fracture was in every case both audible and tangible ; it could 
be heard and it could be felt. A series of cracks occurred in succession as the difierent 
parts of the ice-prism gave way, and towards the conclusion of the experiment, the 
crackling in some instances melted into an almost musical tone. But if instead of 
causing the change to take place by such wide steps as those indicated ; if instead of four 
moulds, forty, or four hundi’ed were made use of ; or better still, suppose a single mould 
to have the power of gradually changing its curvature from a straight line to a semicircle 
under the hydraulic press ; the change in the curvatm’e of the ice would closely approxi- 
mate to that of a truly plastic or viscous body. This represents the state of things in 
a glacier. A transverse plate of ice, situated between the mass in front of it and the 
mass behind, is virtually squeezed in a press of the description which has been just 
imagined. The curvature of the ice-mould does change in the manner indicated, and so 
slowly, that the bending closely resembles what would take place if the substance were 
viscous. The gradual nature of the change of curvature may be inferred from an expe- 
riment made by Professor Forbes on an apparently compact portion of the Mer de Glace. 
He divided a distance of 90 feet transverse to the axis of the glacier into spaces of two 
feet each, and observed with a theodolite the gradual passage of this straight line into a 
bent one. The row of pins bent gradually so as to form a curve convex towards the 
lower extremity of the glacier; their deviations from a perfect curve were slight and 
irregular, nor was any great dislocation to be observed throughout their whole extent. 
After six days the summit of the curve formed by the forty-five pins was one inch in 
advance of the straight chord which united its two ends. It is not surprising if, with 
this extremely gradual change, the motion should have appeared to be the result of 
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