Mr. J. Ball on the Cause of the Descent of Glaciers. 3 



tion to this, that of the weight of the portion of the body driven 

 up the slope. In the opposite direction, the weight of that por- 

 tion which moves down the inclined plane acts along with the 

 expansive force to overcome the resistance of friction. The in- 

 verse process occurs when the body contracts under the influence 

 of cold ; the larger portion of the mass will descend, and the 

 smaller part only will ascend. Hence every alternation of tem- 

 perature must cause a proportional, however slight, movement of 

 the centre of gravity in a downward direction, and a correspond- 

 ing descent of the whole mass. 



In order to deduce from this observation an explanation of 

 the motion of glaciers, it was necessary to assume that a glacier 

 lying in an alpine valley is a continuous solid, comparable in 

 structure to the sheet of lead upon a roof, that under the influ- 

 ence of sunshine, shade, radiation, &c. its temperature is sub- 

 ject to frequent alternations, and, finally, that the consequence 

 of such changes of temperature is to cause the entire mass of the 

 glacier to expand and contract in the direction of its length. 



I have never met any one practically conversant with the phe- 

 nomena of glaciers who could be brought seriously to discuss 

 this theory, for the simple reason that every one of these assump- 

 tions is at variance with the facts of nature. So far from being 

 a continuous solid mass like a sheet of metal, the ordinary con- 

 dition of glacier-ice is that of a mass of more or less imperfectly 

 welded separate portions, traversed by fissures, and whose upper 

 surface is very frequently cut by deep rents extending to a depth 

 very much greater than that subject to the influence of external 

 changes of temperature. 



In the next place we have every possible ground, both from 

 a priori reasoning and direct observation, for believing that the 

 temperature of the interior of a glacier is very nearly constant, 

 varying only by a small fraction of a degree from the freezing- 

 point. Professor Forbes long ago made the obvious objection that 

 the temperature of ice cannot rise above 32° F., while, on the 

 other hand, no cause has been suggested that can tend sensibly to 

 lower the temperature of the interior below that limit. Every one 

 familiar with glaciers is aware that nothing is so rare as to find 

 the surface of a glacier composed of moderately compact or nearly 

 transparent ice. The first effect of the sun shining on the sur- 

 face is to convert the superficial crust into a mass of crumbling 

 ice filled with cavities, so nearly opaque that it protects the inte- 

 rior of the mass from any but the most trifling influence of lumi- 

 nous heat, and absolutely cuts off all obscure radiation. Only 

 when the superficial crust has been washed away by heavy rain, 

 and before the sun has again acted on the surface, do we find 

 blue ice, more or less compact, appearing on the surface. It has 



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