MECHANICS OF GLACIERS. 63 



Prof. J. Thomson has deduced from the mechanical theory of heat, 

 and Sir W. Thomson has verified by experiment, the law that the 

 freezing-point of water is lowered by pressure ; and Helmholtz * has 

 shown how it follows, as a corollary to this important law, that the 

 temperature of ice is lowered when it is subjected to pressure within 

 a confined space, the ratio of the liquid water to ice being at the 

 same time increased. The thermal energy which is generated by 

 pressure becomes latent in the newly liquefied ice, and so is not 

 available to affect the temperature of the mass. This liquefaction 

 must take place most in the lower layers of the glacier ; and owing 

 to the great latent heat of water (=nearly 80 thermal units at a 

 pressure of one atmosphere) the pressure, though great, melts only 

 a small proportion of the ice-mass, which is very large. Helmholtz f 

 has pointed out the bearing of this principle upon glacier-work. The 

 ice being pressed, and a small portion of it melted, the water is free 

 to escape. The temperature of the pressed ice is lowered, but not 

 that of the water, which, being free to escape, does not suffer any 

 lowering of temperature. " So we have, under these circumstances, 

 ice colder than 0°C. in contact with water at 0°C. The consequence 

 of this will be, that water is continually frozen around the pressed ice 

 and forms new ice, while a portion of the pressed ice is melted." Owing 

 partly to imperfect homogeneity of the ice of a glacier, partly to the 

 inequalities of its bed, pressure acts more upon some points and in 

 some directions than in others. Several results may follow. (1) If the 

 pressure is applied continuously and rapidly enough, and the tempera- 

 ture of the ice is below that required for liquefaction under the given 

 pressure, the ice cracks ; work is done in overcoming cohesion. (2) 

 Some ice is melted, mechanical force is transformed into heat, which 

 becomes latent in the melted ice, the water is squeezed out and 

 regelates in contact with the colder ice, its latent heat being given 

 up to the colder ice in contact with it, raising the temperature of 

 this ice, until it and the regelated film have acquired again a uniform 

 temperature. (3) Friction follows, if, as in the glacier, the force 

 continues to be applied, by the faces of the cracks sliding over one 

 another. Heat is generated; portions of the ice surfaces are lique-' 

 fled, the water trickling out as before and becoming regelated 

 in contact with colder ice, the thermal energy given up by it in 

 the act of regelation being diffused by slow conduction, as before. 

 (4) As the heat given up by the water in the act of regelation to 

 the contiguous ice is transmitted by slow conduction through the 

 neighbouring ice, it causes expansion, or a tendency to expand, 

 which can only be prevented by increase of resistance. (5) If ex- 

 pansion occurs, as in (4), or water trickles out, as in (2) and (3), its 

 tendency will be, under the influence of gravitation, rather down- 

 wards than in any other direction ; and so the centre of gravity of 

 the whole mass is lowered, though the mass may not be moving as 

 a rigid body. This I conceive to be the outline of the history of the 

 expenditure of that portion of the potential energy of the weight of 



* ' Eis und Gletscher.' t Loc. cit. 



