134 ICE AND GLACIERS. 



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 temperature of which was a few 

 degrees above the freezing-point. Under these circumstances the ice 

 in the vessel, which was exposed 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 ex- 

 pense 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 conse- 

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

 pressure ceased. Yet under these circumstances there was no ap- 

 preciable alteration in the shape of the middle piece which was most 

 compressed. 



This experiment shows that although changes in the shape of the 

 pieces of ice must take place in the course of time in accordance 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 which the heat is conducted is at all con- 

 siderable. Any marked change in shape by melting in a medium the 

 temperature of which is everywhere 0, could not occur without 

 access of external heat, or from the uncompressed ice and water ; 

 and with the small differences in temperature which here come into 

 play, and from the badly conducting power of ice, it must be ex- 

 tremely slow. 



That on the other hand, especially in granular ice, the formation 

 of cracks, and the displacement of the surfaces of those cracks, render 

 such a change of form possible, is shown by the above-described ex- 

 periments on pressure; and that in glacier ice changes of form thus 

 occur, follows from the banded structure, and the granular aggrega- 

 tion which is manifest on melting, and also from the manner in which 

 the layers change their position when moved, and so forth. Hence, I 

 doubt not that Tyndall has discovered the essential and principal 



