150 ICE AND GLACIERS. 



a portion of the more powerfully compressed ice will melt ; and 

 the latent heat necessary for tliis will be supplied by the ice 

 which is less strongly compressed, and by the water in contact 

 with it. Thus ice would melt at the compressed places, and water 

 would freeze in those which are not pressed : ice would thus be 

 gradually transformed and yield to pressure. It is also clear 

 thitt, owing to the very small conductivity for heat which ice 

 possesses, a process of this kind must be extremely slow, if the 

 compressed and colder layers of ice, as in glaciers, are at con- 

 siderable distances from the less compressed ones, and from the 

 water which furnishes the heat for melting. 



To test this hypothesis, I placed in a cylindrical vessel, between 

 two discs of ice of three inches in diameter, a smaller cylindrical 

 piece of an inch in diameter. On the uppermost disc I placed a 

 wooden disc, and this I loaded with a weight of twenty pounds. 

 The section of the narrow piece was thus exposed to a pressure 

 of more than an atmosphere. The whole vessel was packed 

 between pieces of ice, and left for five days in a room, the tem- 

 perature of which was a few degrees above the freezing-point. 

 Under these circumstances the ice in the vessel, which was ex- 

 posed to the pressure of the weight, should melt, and it might be 

 expected that the narrow cylinder on which the pressure was 

 most powerful should have been most melted. Some water was 

 indeed formed in the vessel, but mostly at the expense of the 

 larger discs at the top and bottom, which being nearest the 

 outside mixture of ice and water could acquire heat through the 

 sides of the vessel. A small welt, too, of ice, was formed round 

 the surface of contact of the narrower with the lower broad 

 piece, which showed that the water, which had been formed in 

 consequence of the pressure, had again frozen in places in which 

 the pressure ceased. Yet under these circumstances there was 

 no appreciable alteration in the shape of the middle piece which 

 was most compressed. 



This experiment shows, that although changes in the shape of 

 the piecf'S of ice must take place in the course of time in accord- 

 ance with J. Thomson's explanation, by which the more strongly 

 compressed parts melt, and new ice is formed at the places which 

 are freed from pressure, these changes must be extremely slow 

 when the thickness of the pieces of ice through w^hich the heat 



