Mr. J. Ball on the Cause of the Descent of Glaciers. 9 



Canon Moseley's objections to the received theory be admitted 

 as valid, they will be found to apply with equal force to his own 

 theory. Observation abundantly proves that the resistance 

 offered by the bed of great glaciers to the sliding of the lower 

 surface is very much less than even the smallest amount of shear- 

 ing-force derived from his several discordant observations. If 

 the mass of the glacier were dragged onwards, as he contends, 

 by the alternate expansion and contraction of the superficial 

 strata, it is certain that the lower surface would slide on its bed 

 at the same rate as the upper part. It is equally easy to show 

 that the differential motion found in going from the central part 

 to either bank would be an impossibility. The whole would 

 move forward as one mass, like the sheet of lead upon the roof, 

 and only where some small lateral portion of the ice encountered 

 a fixed obstacle would the resistance lead to fracture — just as in 

 Canon Moseley's cylinder experiments. The larger mass would 

 move on, the smaller fragment would be left behind. 



If I might presume to estimate the net results of this renewed 

 discussion of the causes of glacier-motion, I should say that they 

 are not considerable, but yet are far from worthless. Canon 

 Moseley's experiments have added something to our knowledge, 

 and especially those on the tenacity of ice, w r hich have some bear- 

 ing on the origin of crevasses. Of far greater importance are 

 the observations on ice-planks made by Mr. William Mathews. 

 The first of these, published in the ' Alpine Journal/ gave pro- 

 minence to a fact which had long been familiar to myself, and 

 probably to many others. I have often found that long icicles 

 placed in an inclined position, and supported only at the upper 

 end, will gradually resume the vertical direction, and I had, per- 

 haps too lightly, assumed that this was a particular instance of 

 the process by which ice changes its form through fracture and 

 regelation. In Mr. Mathews's first experiment, conducted during 

 a thaw, a thick plank of ice supported at each end was deflected 

 at the middle through a space of 7 inches in as many hours. Al- 

 though none but very minute fissures were observed, the facts did 

 not seem to me altogether inconsistent with that explanation. In 

 the second series of observations, made during the severe frost 

 of February last, Mr. Mathews found that at temperatures 

 notably below the freezing-point a plank of ice, supported as 

 before, subsides slowly between the points of support under the 

 sole influence of its own weight. The deflection under these 

 circumstances was about 1^ inch in twenty-four hours. Taking 

 this observation in connexion with a multitude of facts recently 

 brought to light, and especially the researches of M. Tresca, we 

 are led to admit that ice, in common with very many apparently 

 rigid bodies, does possess a certain degree of plasticity which is 



