40 REPORT—1857. 
of the ice and water, freezes the water again in new positions, and thus a change of 
form, or plastic yielding of the mass of ice to the applied pressures, has occurred. 
The newly-formed ice is at first free from the stress of the applied forces, but the 
yielding of one part always leaves some other part exposed to the pressure, and that 
part, in its turn, melts and falls in temperature ; and, on the whole, a continual suc- 
cession goes on, of pressures being applied to particular parts—liquefaction occurring 
in those parts accompanied by evolution of cold,—dispersion of the water so pro- 
duced in such directions as will relieve its pressure,—and re-congelation, by the cold 
previously evolved, of the water on its being relieved from this pressure. The cycle 
of operations then begins again, for the parts re-congealed, after having been melted, 
must, in their turn, through the yielding of other parts, receive pressures from the 
applied forces, thereby to be again liquefied, and to proceed through successive ope- 
rations as before. ‘The succession of these processes must continue as long as the 
external forces tending to change of form remain applied to the mass of porous ice per- 
meated by minute quantities of liquid water. The ice is thus shown to be incapable 
of opposing a permanent resistance to the pressures, and to be subject to gradual 
changes of form while they act on it; or, in other words, it has been shown to be 
possessed of the quality of plasticity. In the foregoing, I have supposed the ice 
under consideration to be porous, and to contain small quantities of liquid water dif- 
fused through its substance. Porosity and permeation by liquid-water are generally 
understood, from the results of observations, and from numerous other reasons, to 
be normal conditions of glacier ice. It is not, however, necessary for the purposes 
of my explanation of the plasticity of ice at the freezing-point, that the ice should be, 
at the outset, in this condition; for even if we commence with the consideration of 
a mass of ice perfectly free from porosity, and free from particles of liquid water 
diffused through its substance, and if we suppose it to be kept in an atmosphere at 
or above 0° Centigrade, then, as soon as pressure is applied to it, pores occupied by 
liquid water must instantly be formed in the compressed parts, in accordance with 
the fundamental principle of the explanation which I have proposed—the lowering, 
namely, of the freezing- or melting-point, by pressure, and the cognate fact, that ice 
cannot exist at 0° Centigrade under a pressure exceeding that of the atmosphere. I 
would further wish to make it distinctly understood, that no part of the ice, even if 
supposed at the outset to be solid, or free from porosity, can resist being permeated 
by the water squeezed against it from such parts as may be directly subjected to the 
pressure; because, the very fact of that water being forced against any portions of 
the ice supposed to be solid, will instantly subject them to pressure, and so will 
cause melting to set.in throughout their substance, thereby reducing them imme- 
diately to the porous condition. Thus it is a matter of indifference, as to whether 
we commence with the supposition of a mass of porous or of solid ice. 
Mr. Thomson then referred to an experiment made by Prof. Christie, late Secre- 
tary to the Royal Society, showing the plasticity of ice in small hand specimens, 
and also to more recent experiments by Prof. Tyndall to the same effect, and very 
interesting on account of the striking way in which they exhibit the phenomena. 
He also stated that another very important quality of ice was brought forward by 
Faraday in 1850 (see ‘Atheneum,’ No 1181). It was that two pieces of moist ice 
will consolidate into one on being laid in contact with one another, even in hot wea- 
ther. The theory he had just propounded, he said, afforded a clear explanation of 
this fact as follows :—The two picces of ice, on being pressed together at their point 
of contact, will, at that place, in virtue of the pressure, be in part liquefied and reduced 
in temperature, and the cold evolved in their liquefaction will cause some of the liquid 
film intervening between the two masses to freeze. It is thus evident, he added, that 
by continued pressure fragmentary masses of ice may be moulded into a continuous 
mass; and a sufficient reason is afforded for the reunion, found to occur in glaciers, 
of the fragments resulting from an ice cascade, and for the mending of the crevasses or 
deep fissures which result occasionally from their motion along their uneven beds. 
Secular Variations in Lunar and Terrestrial Motion from the influence of 
Tidal Action. By D. Vaucuan, of Cincinnati, Ohio. a 
Laplace eoncludes from his elaborate investigations, that the rotation of the earth 
