ON THE STRUCTURE AND MOTION OF GLACIERS vi 
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 
continuity 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 different 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 hundred 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 semi- 
circle under the hydraulic press ; the change in the curvature of the ice 
would closely approximate 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 experiment 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 cord 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 viscosity. It may, however, be remarked, that the 
slight and irregular variations to which Professor Forbes alludes, and 
which are such as would occur if the motion were such as we suppose 
it to be, are likely to throw much light upon the problem. It is also 
extremely probable that the motion, if effected in the manner referred 
to, will be sometimes accompanied by an audible crackling of the 
mass. To this we paid but little attention when on the ground ; for 
the significance of this, as well as of many other points, was first 
suggested by the experiments made after our return. It is, however, 
