296 ANNUAL OF SCIENTIFIC DISCOVERY. 



motion of a glacier was that of a viscous fluid, was never able to reconcile 

 himself to the notion of the viscosity of ice ; and accordingly, in a lecture 

 delivered before the Royal Institution, in January 1857, he proposed another 

 theory, by which the viscous motion of glaciers might be explained. This 

 theory is based upon a fact announced by Mr. Faraday in 1850, that two 

 pieces of ice at 32 will freeze together when brought into contact, either 

 with or without pressure, a phenomenon which Dr. Hooker has named 

 " Regelation." By experiments with small masses, he showed that ice can 

 be moulded by pressure into any given form; and he asserted that the plas- 

 ticity of ice, whether upon the large or small scale, was owing, not to its 

 viscosity, but to fracture and regelation. 



Without pretending to arbitrate on a question disputed by such eminent 

 authorities, we may observe, that we are not disposed to attach so much im- 

 portance to the difficulty based upon the brittleness of small masses of ice. 

 Tlrat ice is not perfectly rigid is shown by its frequent bending beneath the 

 weight of the skater. Other bodies, scarcely less brittle than ice, Avill How 

 down slopes with precisely the motion of treacle, or any other viscous fluid: 

 as is proved by an instance quoted in Professor Forbes's volume, in which 

 Stockholm pitch was observed to flow very slowly out of a barrel, when it 

 was sufficiently hard to break into fragments under the blow of a hammer. 

 It would seem, therefore, that the properties of hardness and brittleness are 

 not incompatible with that of viscosity, or quasi-fluidity. But, after all, the 

 difference between the two theories is only one of degree. If we discard the 

 term viscous altogether, and substitute for it the alias plastic, which, indeed, 

 Professor Forbes seems to prefer, the same designation may be applied to 

 both theories. Both agree in asserting that, by the subjection of glacier-ice 

 to a peculiarly violent strain, solution of continuity is produced, which is 

 afterwards repaired when the disjoined surfaces are brought into contact by 

 pressure; but, according to Forbes, the solution of continuity is only partial; 

 while, according to Tyndall, it is complete. Professor Tynclall may deny 

 the viscosity of ice, but he will hardly deny its plasticity, since he has him- 

 self succeeded in moulding it by pressure into a variety of forms; and, 

 whether solution of continuity which it undergoes in the process be partial 

 or complete, is a circumstance by which the ultimate fact of its plasticity is 

 not affected. 



Hitherto we have considered the different glacier theories with reference to 

 the one point of glacier motion; there are, however, several other phenom- 

 ena connected with glaciers, whose explanation must be included in any 

 complete theory on the subject, all of which, according to Professor Forbes, 

 arc fully accounted for by his hypothesis. We have not space to enter into 

 a detailed enumeration of these phenomena, the explanation of which may, 

 in most cases, be deduced with facility from the plastic theory ; but we must 

 dwell briefly upon two points, which appear to be closely connected with each 

 other, and of which, in one case at least, the theoretical explanation is cer- 

 tainly less clear. We allude to the mode in which glacier ice is formed, and 

 to the peculiar structure which it exhibits. We have already seen that the 

 glacier proper issues from, and is, in fact, fed by, the vast snow-fields which 

 occupy the higher plateaux of the mountains. It is to these snow-fields that 

 the term nere'or firn is applied. In the mass of neve a succession of sti'ata 

 of more and less crystalline snow is observed, which is generally admitted to 

 be owing to the successive falls of snow by which it is formed. The ques- 

 tion, therefore, arises, What is the process by which the granular snow of the 



