366 Professors Tyndall and Huxley on the Structure 



form of questions which he confessed his inabihty to answer. 

 " M. Rendu," says Professor Forbes, " has the candour not to 

 treat his ingenious speculations as leading to any certain result, 

 not being founded on experiments worthy of confidence. . . . My 

 theory of glacial motion, then, is this : — A glacier is an imperfect 

 fluid or viscous body, which is urged down slopes of a certain incli- 

 nation by the mutual pressure of its parts." 



" The sort of consistency to which we refer," proceeds Profes- 

 sor Forbes, " may be illustrated by that of moderately thick 

 mortar, or the contents of a tar-barrel poured into a sloping 

 channel." Treacle and honey are also referred to as illustrative 

 of the consistency of a glacier. The author of the theory endea- 

 vours, with much ability, to show that the notion of semifluidity 

 as applied to ice, is not an absurdity, but on the contrary, that 

 the motion of a glacier exactly resembles that of a viscous body. 

 Like the latter, he urges, it accommodates itself to the twistings 

 of valleys, and moves through narrow gorges. Like a viscous 

 mass, it moves quickest at its centre, the body there being 

 most free from the retai-ding influence of the lateral walls. He 

 refers to the "Dirt-Bands" upon the surface of the glacier, 

 and shows that they resemble what would be formed on the 

 surface of a sluggish river. Li short, the analogies are put 

 forth so clearly, so ably, and so persistently, that it is not sur- 

 prising that this theory stands at pi-esent without a competitor. 

 The phsenomena, indeed, are really such as to render it diflicult 

 to abstain from forming some such opinion as to their cause. 

 The resemblance of many glaciers to " a pail of thickish mortar 

 poured out ;" the gradual changing of a straight line transverse 

 to the glacier into a curve, in consequence of the swifter motion 

 of the centre ; the bent grooves upon the surface ; the disposi- 

 tion of the dirt, the contortions of the ice, a specimen of which. 



Fig. 1. 



as sketched near the Heisscplattc upon the Lower Griudelwald 

 glacier, is given in iig. 1, and of which other striking examples 



