420 Mr. W. Mathews on Glacier-motion. 



line, are plotted to scale. The diagram exhibits the very small 

 motion of points close to the side, whence the curves extend 

 with their concavity downwards as far as the point of maximum 

 differential velocity, where they become convex, and gradually 

 increase in curvature up to about one fourth of the width of the 

 glacier, whence they sweep across to the corresponding point on 

 the opposite side in a curve so flattened as to be scarcely distin- 

 guishable from a straight line. 



The above considerations lead to the following conclusions 

 ■upon the five fundamental propositions of Canon Moseley. 



1. It is probable that every molecule of a glacier moves with 

 a very slow differential motion, which, whenever the ice is con- 

 tinuous, is continuous from molecule to molecule, and from 

 moment to moment of time. 



2. The hypothesis that the differential motion is uniform 

 from centre to side is wholly contrary to fact. The semi- 

 surface of every glacier may be roughly divided into two equal 

 longitudinal strips, through the lateral of which from 80 to 90 per 

 cent, of the differential motion is distributed, while the central 

 strip moves downwards almost like a rigid body, with a large 

 reserve of gravitating force capable of affecting the sides. 



Canon Moseley is of opinion that this divergence between 

 theory and fact greatly strengthens his position ; but he has not 

 made good this part of his case. 



3. 4. The Canon has failed, as it seems to me, to establish any 

 analogy between the disruption of adjacent surfaces of a solid 

 body in a shearing-machine and the slow relative displacements 

 of the molecules of a glacier. He has yet to prove that, because 

 he was obliged to employ a force of 75 lbs. per square inch to 

 shear asunder adjacent surfaces of a solid cylinder of ice through 

 a space of 1 inch in half an hour, it would require as great a 

 force to produce the relative displacements which occur in an 

 actual glacier, — the latter having been shown, from the observa- 

 tions above described, to range for molecules the tenth of an 

 inch apart, and an interval of twenty-four hours, from the 2130 



to the 70,000 of an inch *' 



5. On the other hand, the slow continuous displacements of 



the molecules of a glacier are undistinguishable in kind from the 

 displacements of the molecules of an ice-plank, supported at its 

 extremities and allowed to subside under the influence of its 

 weight, displacements which require for their production, as I 

 have shown in the Philosophical Magazine for November 1871, 

 a force considerably less than 1£ lb. per square inch— less, there- 

 fore, than the very force which the Canon considers sufficient to 

 shear the Mer de Glace if it descends by its own gravitation. 



* This objection has been forcibly urged by Mr. Ball in the Philoso- 

 phical Magazine for July 1870. 



