Mr. J. Ball on the Cause of the Descent of Glaciers* 5 



would amount to 140 feet. What is the value of a theory which, 

 in order to escape the legitimate consequences of its own assump- 

 tions, is forced to invoke the intervention of modifying causes so 

 considerable as to reduce the supposed effects to a small fraction 

 (one-twentieth or perhaps one-fiftieth) of what should be their 

 amount. 



I pass over many obvious objections that must occur to any 

 one familiar with the phenomena — one of them being that, 

 according to the " crawling theory," the rate of advance of a 

 glacier should, under similar conditions, be proportioned to its 

 length — an inference utterly unsupported by observation — to 

 note one simple fact which alone is sufficient to upset that 

 theory. Universal experience taught mankind, long before ther- 

 mometers were invented, that snow is one of the most perfect 

 non-conductors of heat. Observations in the arctic regions have 

 proved that a moderate thickness of snow is a protection against 

 the utmost rigour of the polar winter ; and we know it to be ab- 

 solutely opaque to radiant heat (luminous or non -luminous). 

 During the long alpine winter the glaciers are wrapped in a thick 

 mantle of snow — absolutely protected against the alternations of 

 external temperature. In winter the glaciers continue to advance 

 at a rate about one-half their pace in summer. If there be any 

 force in physical reasoning, we must admit that the winter mo- 

 tion of glaciers does not depend upon oscillations of temperature, 

 and that the " crawling theory " is powerless to explain it. 



As for the observations on the dilatation of ice, to which Canon 

 Moseley so frequently refers, I simply deny their relevancy. The 

 results were obtained by observing the effect of changes of tem- 

 perature (always below the freezing-point) on blocks of solid ice, 

 carefully prepared from water that had had the air expelled by 

 boiling. It is conceivable, though I believe that no evidence is 

 available to support the supposition, that ice may continue to 

 expand to a limited extent at temperatures above freezing. Ob- 

 servations carefully made by exposing blocks of ice to non-lumi- 

 nous heat may possibly give interesting results ; but they will 

 have no application to glacier-ice and its phenomena. 



When Canon Moseley triumphantly asks what becomes of the 

 force which reaches the surface of a glacier under the form of 

 radiant heat, I reply that if he had ever passed a fine morning 

 upon a glacier, after rain had cleared away the crust of rotten 

 ice and had exposed a moderately compact surface, he would have 

 no difficulty in answering his own question. The beautiful ob- 

 servations of Professor Tyndall upon the condition of ice exposed 

 to the sun*s rays suggest the true solution. The first effect of 

 the radiant heat, to whatever depth it may penetrate, is to un- 

 build the crystalline structure of the ice, wherever this is weak- 



