Mr. J. Croll on the Cause of the Motion of Glaciers. 167 



the ice, and that this work in all probability is the expansion of 

 the ice. Or has he determined directly that his lens, after reach- 

 ing the temperature which is considered to be the melting-point 

 of ice, actually continued to expand as the rays passed into it ? 

 It is absolutely essential to Mr. Moseley's theory of the motion 

 of glaciers, during summer at least, that ice should continue to 

 expand after it reaches the melting-point ; and it is therefore in- 

 cumbent upon him to afford us some evidence that such is the 

 case ; or he need not wonder that we cannot accept his theory, 

 because it demands of us the adoption of a conclusion so con- 

 trary to all our previous conceptions. But, as a matter of fact, 

 it is not strictly true that when rays pass through a piece of ice 

 there is no melting of the ice in the interior. Experiments made 

 by Professor Tyndall show the contrary*. 



There is, however, one fortunate circumstance connected with 

 Canon Moseley's theory. It is this; its truth can be easily 

 tested by direct experiment. The ice, according to this theory, 

 descends not simply in virtue of heat, but in virtue of change of 

 temperature. Try, then, Hopkins's famous experiment, but keep 

 the ice at a constant temperature ; then, according to Moseley's 

 theory, the ice will not descend. Or try Mr. Mathews's experi- 

 ment, but keep the ice-plank at a constant temperature, and the 

 plank ought not to sink in the middle. But let it be observed 

 that although the ice under this condition should descend (as 

 there is little doubt but it would), it would show that Mr. Mose- 

 ley's theory of the descent of glaciers is incorrect, but it would 

 not in the least degree affect the conclusions which he has lately 

 arrived at in regard to the generally received theory of glacier- 

 motion. It would not prove that the ice sheared, in the way 

 generally supposed, by its weight only. It might be the heat, 

 after all, entering the ice, which accounted for its descent, although 

 gravitation (the weight of the ice) might be the impelling cause. 



The present state of the question. 



The condition which the perplexing question of the cause of 

 the descent of glaciers has now reached seems to be something 

 like the following. The ice of a glacier is not in a soft and 

 plastic state, but is solid, hard, brittle, and unyielding. It ne- 

 vertheless behaves in some respects in a manner very like what 

 a soft and plastic substance would do if placed in similar cir- 

 cumstances, inasmuch as it accommodates itself to all the inequa- 

 lities of the channel in which it moves. The ice of the glacier, 

 though hard and solid, moves with a differential motion ; the 

 particles of the ice are displaced over each other, or, in other 

 words, the ice shears as it descends. It had been concluded that 

 * See Philosophical Transactions, December 1857. 



