Section IV, ISSY. [ 99 ] Trans. Roy. Soc. Canada. 



XI.— On the Theory of Glacial Motion. 

 By Prof. J. W. Spencer, F.a.S. 



(Communicated by Dr. R. Bell, May 25, 1887.) 



As the foregoing paper contains some observations bearing upon the character of 

 glacial motion, the correlation of these observations and a short consideration of the latter 

 subject forms an appropriate supplement to it. 



As glaciers disport themselves like rivers, in that they are constantly flowing, with 

 greater velocity at their centre than their margins, above than below ; as they form pools 

 and rapids, and conform themselves to channels. Prof Forbes was led to propose the theory 

 that : " A glacier is an imperfect fluid or viscous body which is urged down slopes of a 

 certain inclination by mutual pressure of its parts."' ' He explained the veined structure 

 of glaciers as being diie to the differential movement of its parts. 



Against this view, it was urged that ice is a brittle solid, which in the laboratory 

 cannot be moulded as a semi-fluid, or even in nature, when in passing over a change of 

 declivity of even 4J° it becomes ruptured. Consequently, Prof Tyndall applied Faraday's 

 " Law of Regelation," ^ that ice when broken and moistened, reunited and could be 

 moulded into any form by repeated crushing and pressure, and proposed the " Fracture 

 and Eegelation theory." He explained the veined structure of glaciers as being analogous 

 to the slaty cleavage of certain rocks — the result of transverse pressure. 



Canon Moseley ^ calculated that the resistance of ice to descent is thirty-four times 

 gravitation, and, therefore, fractiire and gravitation could not be maintained. He likened 

 the motion to the creeping of a leaden roof, owing to the expansion and contraction from 

 change of temperature, which expansion Dr. Croll ' modified in assuming the transmission 

 of heat from molecule to molecule with successive liquifaction and solidification of the 

 glacial waters. 



Malleability, plasticity and viscosity are different degrees of the same property. Prof 

 Heim "' distinguishes between these last two semi-fluid forms. In plastic bodies, the 

 internal cohesion is less than internal resistance, and, therefore, under pressure these will 

 flow, but under tension they are not drawn out, but are brittle. In viscous bodies, the 

 internal cohesion is greater than internal resistance, and, therefore, they will not only 

 flow under pressure, but in tension thej' are drawn out before rupture. He concludes 

 that glaciers are plastic bodies, and explains the veined structure as being due to partial 

 liquefaction under compression in passing through narrow channels, as it had been dis- 



' Travels in the Alps, 1843. '' Forms of Water. 



' Proc. Royal Society, 1869. * Climate and Time. 



* Handbucli der Gletscherkunde von Dr. Albert Heim, Stuttgart, 1885. 



