RECENT THEORIES. 65 



moving but two feet a day, can not make this slight bend without rupt- 

 ure ; for at this point there are always large transverse fissures which 

 heal up below by pressure and regelation. In another place the gla- 

 cier is similarly broken by passing an angle produced by a change of 

 slope of only 2°. It seems almost impossible that a body having the 

 slightest viscosity should, be fractured under these circumstances. Tyn- 

 dall concludes, therefore, that the motion of glaciers is viscoid, but 

 the body is not viscous — the viscoid motion being the result, not of the 

 property of viscosity, but of fracture, change of position, and regela- 

 tion. 



Comparison of the Two Theories. — Forbes's theory supposes motion 

 among the ultimate particles, without rupture. TyndalPs supposes 

 motion among discrete particles by rupture, change of position, and 

 regelation. The undoubted viscoid motion is equally explained by 

 both : by the one, by a property of viscosity ; by the other, by a prop- 

 erty of regelation. There can be little doubt that both views are true, 

 and that both properties are concerned in glacial motion. 



Recent Theories. 



Croll's Theory. — Croll has recently, in his work on Climate and 

 Time, brought forward a theory which has attracted much attention. 

 Moseley had previously attempted to prove the untenableness of all 

 theories attributing the motion of glaciers to gravity, by showing ex- 

 perimentally that the shearing force of ice (the force necessary to 

 slide one layer on another, as in differential motion) is many times 

 greater than that portion of gravity which acts in the direction of the 

 slope of a glacial bed. Croll, accepting Moseley's view in regard to 

 the shearing force of ice, but accepting also gravity as the moving 

 force of glaciers, thinks to reconcile these by supposing that there is in 

 ice, when subjected to heat, a momentary loss of cohesion by melting, 

 which is transferred from molecule to molecule, giving rise thereby to 

 a kind of intestine molecular motion similar in its effects to viscosity. 

 The process is as follows : Heat falling on glacier-ice melts its sur- 

 face. The water thus formed runs down to a lower level, and is 

 again refrozen. Now, what takes place conspicuously on the surface 

 takes place molecularly in the interior of the ice. In every part the 

 ice-molecules are melting and refreezing. A molecule takes up heat 

 by melting, runs down to an infinitesimally lower point, refreezes, and 

 in so doing gives up its heat and melts another molecule, which in its 

 turn seeks a lower position, and, by refreezing, transfers its heat and 

 fusion to still another molecule, and so on. Thus the whole glacier is 

 in a state of molecular movement downward. 



The theory is ingenious, but somewhat obscure. We will, there- 

 fore, dismiss it with two remarks : 1. Moseley's objection to gravity as 



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