4-12 Prof. Forbes's Reply to Mr. Hopkins 



distortion be gently effected, it takes place at an infinite 

 number of points, and without visible discontinuity in the 

 substance. It is no good to cite Mr. Hodgkinson's observa- 

 tion, that in the case of cast iron crushing force produced a 

 single line of fracture, which, besides, was very far from being 

 always the case. This shows only that under the circumstances 

 of temperature and crushing force, iron was not a substance 

 calculated to illustrate the analogy of glaciers, which might 

 have been easily foreseen. It is evidently a begging of the 

 question to say, that because iron did not yield at many points 

 under a crushing force, therefore glacier-ice will not. The 

 difference too is palpable ; a cubic inch of ice may be crushed by 

 a small weight, that of iron only by many tons. Then again, 

 Mr. Hopkins, taking up an illustration of my own, says a sheet 

 of paper whose edges are pulled in opposite directions tears 

 but in one place*. This is true for the writing-paper which 

 Mr. Hopkins uses, but let him trace the same paper in the 

 successive stages of its manufacture until it becomes a pulp, 

 he will find the lines of disseverment multiply, and at length 

 become all but infinite, yet still dependent on the slorvness of 

 the action; the slower, the more numerous; yet in all Mr. 

 Hopkins's mathematical investigations time never enters, and 

 for obvious reasons could not be introduced. In one word, 

 a rending pressure may produce in different bodies, all having 

 the essential characters of solids in small masses, (1) an abrupt 

 disseverment or fracture, (2) an imperceptible moulding or 

 plasticity, or, (3) intermediate between these, a contusion or 

 bruise, arising from the partial solution of continuity at innu- 

 merable points. Nay, all this may be seen in one and the same 

 body at different temperatures, or in the same body at the 

 same temperature, depending on the velocity or violence of 

 the crushing force. The manner of the fracture therefore be- 

 comes " a question of degree," and cannot be specified without 

 a specification of the viscosity or softness. It is not a "re- 

 sultat de calcul;" it is a consequence of physical principles 

 not yet compassed by algebra. Until Mr. Hopkins shall 

 find the equation to a bruise, we must prefer our own conclu- 

 sions to his. 



I have shown that a glacier does move as a bruised mass, — 

 by the yielding of different parts of its transverse section at 

 a great, though finite number of points in its breadth f. Their 



* Phil. Mag., March, p. 242, note. 



t See the experiments on the plasticity ofglaciericein my Eighth Letter 

 on Glaciers. The angle which the lines of tearing make with the axis or 

 side of a viscid stream will be found, I believe, to depend on its stiffness, 

 or rather on the ratio of its viscosity to the rapidity of motion. 



