Mr. J. Ball on the Cause of the Descent of Glaciers. 7 



amount ? The utmost amount of differential motion yet observed* 

 shows a relative displacement of one inch per day in a distance 

 of sixteen feet. In other words, if we take two points in the 

 glacier one inch apart, it has been shown that the one may slip 

 past the other in twenty-four hours to an extent measured by 

 the sixteenth part of a line, or about the thickness of a sheet of 

 note-paper. There is no doubt that, to accomplish this relative 

 displacement, the force that urged the glacier forwards must 

 overcome the resistance opposed by the mutual cohesion of the 

 particles of the ice. To determine the amount of this resistance, 

 or the shearing-force of ice, Canon Moseley devised the experi- 

 ment to which he has repeatedly referred in his writings. A 

 cylinder of solid ice is closely fitted into a hole passing through 

 two blocks of hard wood whose faces are brought into close con- 

 tact, one of which is fixed and the other moveable ; and weights 

 are applied to the moveable block until displacement is effected. 

 In short, to ascertain the resistance opposed to very slow changes 

 in the relative positions of the particles, so slight as to be insen- 

 sible at short distances, Mr. Moseley measures the resistance op- 

 posed to rapid disruption between contiguous portions of the 

 same substance. Without entering into a detailed examination 

 of the question, I should think that so familiar a fact as the 

 behaviour of sealing-wax at temperatures between 70° and 80° F. 

 sufficiently shows the fallacy of this experiment. A cylindrical 

 stick of sealing-wax half an inch in diameter requires a sharp 

 blow to break it, and will bear for some time a considerable 

 weight, placed close to the point at which it is horizontally sup- 

 poi-ted, without apparent yielding; but if placed in the same 

 position for twenty-four hours, with no other pressure act- 

 ing on it than its own weight, you will find it unable to sustain 

 that slight pressure. It will be bent or twisted, and relative 

 displacements of the particles far greater than occur in glacier- 

 ice will have ensued. 



I venture to hold that the doubts expressed by Mr. Mathews 

 in his excellent paper are more than justified. I am persuaded 

 that, in attempting to estimate experimentally the resistances 

 opposed by solid bodies to change of form, time is an essential 

 element, and that this is especially true when such bodies are 

 brought near to their melting-point. In the present case, with- 

 out adverting particularly to minor sources of error, I think that 

 it would be a sufficient answer to Canon Moseley to deny alto- 

 gether the relevancy of his observations on the shearing-force of 

 ice to the argument in hand. 



* In Professor TyndalPs observations at the Tacul, where the middle 

 stake, 31 feet above the lower one, advanced at the daily rate of 4*50 inches, 

 while the motion of the lower one was 2' 56 inches. 



