1869.] 



and Viscous Solids by " Shearing. 



313 



to the Royal Society. On a smaller scale I have made experiments on round 

 bars of brittle sealing-wax, hardened steel, similar steel tempered to various 

 degrees of softness, brass, copper, lead. 



Sealing-wax and hard steel bars exhibited the spiral fracture. All the 

 other bars, without exception, broke as Mr. Kirkaldy's soft steel bars, 

 right across, in a plane perpendicular to the axis of the bar. These expe- 

 riments were conducted by Mr. Walter Deed and Mr. Adam Logan in the 

 Physical Laboratory of the University of Glasgow ; and specimens of the 

 bars exhibiting the two kinds of fracture are sent to the Royal Society 

 along with this statement. I also send photographs exhibiting the spiral 

 fracture of a hard steel cylinder, and the "shearing" fracture of a lead 

 cylinder by torsion. 



These experiments demonstrate that continued " shearing " parallel to 

 one set of planes, of a viscous solid, developes in it a tendency to break 

 more easily parallel to these planes than in other directions, or that a viscous 

 solid, at first isotropic, acquires "cleavage-planes" parallel to the planes 

 of shearing. Thus, if C D and AB (fig. 2) represent in section two sides 

 of a cube of a viscous solid, and if, by " shearing " parallel to these planes, 

 C D be brought to the position C D', relatively to A B supposed to remain 

 at rest, and if this process be continued until the material breaks, it breaks 

 parallel to A Band CD'. 



The appearances presented by the specimens in Mr. Kirkaldy's museum 

 attracted my attention by their bearing on an old controversy regarding 

 Forbes's theory of glaciers. Forbes had maintained that the continued 

 shearing motion which his observations had proved in glaciers, must tend 

 to tear them by fissures parallel to the surfaces of " shearing." The 

 correctness of this view for a viscous solid mass, such as snow becoming 

 kneaded into a glacier, or the substance of a formed glacier as it works 

 its way down a valley, or a mass of debris of glacier-ice, reforming as a 

 glacier after disintegration by an obstacle, seems strongly confirmed by 

 the experiments on the softer metals described above. Hopkins had argued 

 against this view, that, according to the theory of elastic solids, as stated 

 above, and represented by the first diagram, the fracture ought to be at 

 an angle of 45° to the surfaces of "shearing." There can be no doubt of 

 the truth of Hopkins's principle for an isotropic elastic solid, so brittle 

 as to break by shearing before it has become distorted through more than 

 a very small angle ; and it is illustrated in the experiments on brittle 

 sealing-wax and hardened steel which I have described. The various 

 specimens of fractured elastic solids now exhibited to the Society may be 

 looked upon with some interest, if only as illustrating the correctness of 

 each of the two seemingly discrepant propositions of those two distin- 

 guished men. 



VOL. XVIT. 



