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mass, by dispersion from the regions of greatest to those of least 

 fluid pressure. Thus the fluid pressure is relieved in those portions 

 in which the compression and liquefaction of the ice had set in, 

 accompanied by the lowering of temperature. On the removal of 

 this cause of liquidity the fluid pressure, namely, the cold which 

 had been evolved in the compressed parts 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, in its turn, acts in like 

 manner ; and, on the whole, a continual succession goes on of pres- 

 sures being applied to particular parts liquefaction in those parts 

 dispersion of the water so produced, in such directions as will relieve 

 its pressure, and recongelation, by the cold previously evolved, of 

 the water on its being relieved from this pressure. Thus the parts 

 recongealed 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 enter again on a similar cycle of 

 operations. 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 permeated by minute quantities of water. 



POSTSCRIPT received 22nd April, 1857. 



It will be observed that in the course of the foregoing communica- 

 tion, I have supposed the ice under consideration to be porous, and 

 to contain small quantities of liquid water diffused through its sub- 

 stance. 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 plas- 

 ticity 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 centigrade, then, as soon as 

 pressure is applied to it, pores occupied by liquid water must in- 



