Professor Forbes on the Leading Phenomena of Glaciers, 249 



and, (2.) to prove from direct experiment that the matter of 

 a glacier is plastic on a great scale, a fact which seems so re- 

 pugnant to first impressions as lately to have been urged in a 

 most respectable quarter,* as rendering the doctrine of semi- 

 fluid motion untenable. No one had a right to maintain the 

 theory of fluid motion as more than a conjecture, until at 

 least these preliminary obstacles were removed by direct ob- 

 servations. 



These observations have been made, and the result is the 

 viscous or plastic theory of glaciers, as depending essentially 

 on the three following classes of facts, all of which were ascer- 

 tained for the first time by observations in 1842, of which the 

 proofs are contained in this work. 



1. That the different portions of any transverse section of a 

 glacier move with varying velocities, and fastest in the centre. 



2. That those circumstances which increase the fluidity of 

 a glacier, — namely, heat and wet, — invariably accelerate its 

 motion. 



3. That the structural surfaces occasioned by fissures which 

 have traversed the interior of the ice, are also the surfaces of 

 maximum tension in a semisolid or plastic mass, lying in an 

 inclined channel. 



There is only one other point to which I would invite atten- 

 tion, and it is this. We have noticed the enormous depression 

 which the surface of the ice undergoes during the waraier 

 months of the year. We may be sure that, in some manner 

 or other, this is made up for during winter and spring. I al- 

 ready suggested, in my fourth letter to Professor Jameson (Edin. 

 New Phil. Journal, vol. xxxiv. p. 1), that this may be partly 

 owing to the dilatation of the ice during winter by the conge- 

 lation of the water in its fissures, producing, at the same time, 

 '• the veined structure." The glacier is very far indeed from 

 being frozen to the bottom in winter, for we have seen that 

 physical principles are opposed to this, as well as the fact that, 

 the motion continues during all that period, shewing that a 



* BihliGtheque UniverseUef January 1 84.3. 



