134 Mr. H. H. Howorth on 



proof that ice at o° C. will not stand permanently on a 

 vertical cliff of any considerable height. It gives way 

 gradually, but still it gives way " {Proc. Roy. Soc, 

 XXXVIII. 103). 



These were, however, subsidiary issues. The main one 

 converged upon Mr. Moseley's famous experiments, and no 

 wonder that they led to a wide revolt against the plastic 

 theory, and that a great impetus was given to various 

 transcendental views, such as those of Thomson and 

 Croll, already described, and when these were answered 

 led to another suggestion, namely that of Tyndall. 



Tyndall's most importantpfonouncement in criticism of 

 Forbes's views is contained in the following sentence, pub- 

 lished as an obiter dictum. "The very essence of viscosity," he 

 says, "is the ability to yield to a power of tension^ the texture 

 of the substance, after yielding, being in a state of equilibrium, 

 so that it has no strain to recover from." . . . He then goes on 

 to urge that ice will not stretch like well-known plastic 

 bodies, and while it yields to pressure it does not do so to 

 tension {Proc, Roy. Inst., 1858, 551 — 553). "Viscosity," 

 he says elsewhere, " consists in the power of being drawn out 

 when subjected to a force of tension, the substance, after 

 stretching, being in a state of molecular equilibrium, or in 

 other words, devoid of that elasticity which would restore it 

 to its original form" {Glaciers of the Alps, 312). He then 

 goes on to urge that on dislocated slopes, where tar or 

 treacle would flow without breaking across, ice breaks, and 

 he cites certain cases in the Alps. He similarly refers to 

 crevasses as breaks inconsistent with a viscous character, 

 since the maximum strain upon the ice is comparatively so 

 small ; and he further adds, that no single experiment on 

 great masses or small shows that ice possesses in any sensible 

 degree that power of being drawn out which seems the very 

 essence of viscosity. Professor Tyndall carefully guards him- 

 self, however, against being supposed to believe that a glacier 



