334 Mr. W. Mathews on Glacier-motion. 



of shear of ice, or, taking the latter at 75 lbs., to a pressure of 

 900 lbs. The problem is, how is this pressure to be generated 

 out of the gravitation of an ice-plank the entire weight of which 

 is less than 32 lbs. ? I am curious to see the mathematical rea- 

 soning by which the Canon will perform this feat of mechanical 

 legerdemain. If he declines it, may I invite him to consider the 

 inverse problem, and to calculate the shearing-force of ice from 

 the behaviour of an ice-plank under strain ? 



My own mathematical resources are not adequate to the task; 

 but I venture to submit the following as an approximate solution. 

 If the whole weight of the plank were employed in shearing it, 

 and in shearing it only along the supporting edges, two surfaces, 

 each with an area of 12 square inches, would be sheared by a 

 force of 32 lbs., or 1^ lb. per square inch. But, of the whole 

 weight of the plank, one portion is employed in exerting a pres- 

 sure upon the bearers, a second in exerting a stress upon the 

 upper half of the plank and a strain upon the lower, leaving only 

 a third portion available for producing shear. And this third 

 portion is exerted, not merely at the supporting edges, but in 

 shearing successive slices of the plank continuously from end 

 to end. The shearing-force therefore must be less, probably 

 very considerably less, than 1^ lb. per square inch. 



In the Philosophical Magazinefor January 1870 Canon Moseley 

 has given us a shearing-force varying, in round numbers, from 

 119 to 98 lbs. per square inch. In a subsequent experiment, 

 described in his recent paper, he has sheared ice with a force of 

 63*6 lbs. per square inch. Surely, by judicious management, he 

 may make still further progress in the same direction. The 

 whole of his experiments on shearing, as hitherto conducted, are 

 irrelevant to the problem of glacier-motion. In an actual glacier 

 the dislocated masses of which it is composed are subject to lon- 

 gitudinal strains strikingly similar to those of a suspended ice- 

 plank. It is possible that under this condition the resistance to 

 shearing may be largely decreased ; and this condition was ex- 

 cluded in Canon Moseley's experiments. 



I have compared a glacier to a series of ice-planks, placed 

 upon their edges across the channel of the glacier, and occasion- 

 ally fracturing into transverse prisms. The Canon objects that 

 a real glacier is much more like a single ice-plank, placed upon 

 its face with its length parallel to the axis of the channel. I do 

 not concur in this opinion ; and if he would take the trouble to 

 walk up the Mer de Glace, from the Montanvert to the Col du 

 Geant, I think he would see cause to modify it. 



There are other points in Canou Moseley's paper which are 

 open to exception ; and these I propose to make the subject of a 

 future communication. 



