Vol. 51.] MODE OF FLOW OF A VISCOUS FLUID. 361 



27. An Experiment to Illustrate the Mode of Flow of a Viscous 

 Fluid. By W. J. Sollas, M.A., D.Sc, LL.D., F.B.S., F.G.S., 

 Professor of Geology in the University of Dublin. (Bead 

 April 24th, 1895.) 



Now that the theory of glacier-movement propounded by James 

 Forbes stands alone, undisturbed by conflicting hypotheses, we 

 more readily perceive its simplicity and beauty ; and we cannot re- 

 fuse a tribute of admiration to the clarity of thought, no less to the 

 skill in observation, which led that distinguished investigator to 

 success in the treatment of a problem which his famous contem- 

 poraries had found so baffling. 



Whatever may be the ultimate explanation, there can be no 

 question of the fact, that the ice of glaciers behaves precisely like a 

 plastic solid or highly viscous liquid ; and it is consequently by 

 enquiries into the laws of viscous flow that we may justly seek to 

 extend our knowledge of the movements of flowing ice. 



No substance is probably better suited for experiments in this 

 direction than solid pitch, or that substance commonly known as 

 ' cobbler's wax/ in which it occurs as the chief constituent. Under 

 a sudden pull or pressure this material behaves like the most brittle 

 solid, but when stress is slowly applied it yields continuously to 

 deformation. A small fragment kneaded between the finger and 

 thumb may be slowly drawn out into a thread of indefinite length, 

 but if suddenly pulled it snaps across with a clean fracture ; it may 

 be readily impressed with the finger-nail, but a smart tap with the 

 point of a needle shivers it into angular fragments. 



While it thus essentially resembles ice in the manner in which it 

 reacts against deforming forces, it possesses the advantage of being 

 less viscous, and this, as well as a higher fusion-point, render it 

 singularly well adapted for investigations in the laboratory. 



The first to call attention to the similarity in the movement of 

 flowing pitch and glacier-ice was Prof. Lewis Gordon (Phil. Mag. 

 vol. xxvi. 1845, p. 206), and his observation, now fifty years old, 

 is repeatedly cited by Forbes, always with approval ; in one place 

 indeed he speaks of it as 'a beautiful experiment,' though experi- 

 ment it was not. 



Since that time models of glaciers in pitch have frequently been 

 made, and I myself, some fifteen years ago, had a model of the 

 basin of the Mer de Glace carved in wood, and this, when supplied 

 with pitch neve and glaciers, illustrated most of the phenomena of 

 glacier-movement. But it is not merely for the purposes of class- 

 room demonstrations that this substance may be employed ; by its 

 means we are enabled to study, at ease and with precision, the 

 nature of those hidden movements which take place in the inac- 

 cessible interior of a glacier, or, more important still, of an ice- 

 sheet. 



