74 Review of the Controversy Regarding ike Motion of the Glacier. 



ing, and at first appears in the liighest degree improbable, yet, when its 

 one great postulate has been granted, it explains many of the 

 phenomena more satisfactorily than any previously promulgated. 



Ice, on this, which is called the viscous theory, is assumed to be a 

 body possessing the same proi:)erty of plasticity as pitch or treacle, 

 and capable, like them, of flowing in a channel and of spreading itself 

 out over a flat surface. The glacier, in this view, resembles a river, 

 and flows according to the same laws and in the same manner, subject 

 only to differences which result from the lowuess of the degree of 

 viscosity assigned to ice even by the strongest advocates of the theory 

 in question. 



Possibly it was the absence of any plausible competitor, as mucli as 

 its own logical claims, which enabled this theory to command so large 

 a measure of acceptance during so many years. It was ably expounded 

 by its originator, and as ably defended by others equally or more 

 eminent in j)hysical science in its original form.* 



But it can scarcely be said to exist in its original form at the present 

 day. It has fallen ; and the blow beneath which it fell, came from a mathe- 

 matician. In assuming that ice flows as water flows in essence, and that 

 the flovvof the two difler only in degree, Forbes had made an assumption 

 that could be proved or disproved. Liquidity is a consequence of vrant 

 of cohesion between the molecules of a substance, or else of the subjec- 

 tion of their mutual cohesion to the influence of heat. In any case, a 

 certain force is needed to make the most mobile liquidjflow, and this force 

 increases with the cohesion, until the substance ceases to be liquid or even 

 viscous. The amount of force required to make molecule slide over 

 molecule, as occurs in the flow of liquid and semi-liquid bodies, is called 

 the shearing force, and must be determined by experiment in every case. 

 In the solid bodies this shearing force is very high, amotfnting in wrought 

 iron to 50,000 lbs. per square inch, and great pressure is needed to 

 make their particles slide one over the other. The solidity of ice ren- 

 dered it doubtful, how far the motion of a glacier could be attributed 

 to the yielding of this substance' under the mere force of gravitation, 

 and the late Henry Moseley, Canon of Bristol, England, investigated 

 the subject, and published his researches in a paper printed in the 

 Proceedings of tlie Roijxl /S'oc/ef?/ 0/ io/;(Zo?^, for March, 18(J9; in two 

 papers, contributed to the London, Edinburgh and Dublin Philosophi- 

 cal Magazine for JMay and August of the same year ; and in a paper 

 read before the British Naturalists' Society, and published in their 

 proceedings, for December, 18G9. He determined the shearing force 



*We say, " in its original form," because the question at issue is not, " Does ice behave 

 like a viscous body?" which is not doubted, but, '• Is ice viscous?" 



