312 Cambridge Philosophical Society :— 



This process had been designated bj^ the term " regelation ;" and 

 manifestly some corresponding term \vas required to designate the 

 property which, in ice, rendered that process so complete. Such 

 terms as viscous and plastic failed to express adequately the property 

 in question. At the same time it should be remarked that, so far as 

 glacial motion depended on the facility with which the glacial mass 

 might change its form, the manner in which that change was effected 

 was of secondary importance, and did not diminish whatever value 

 attached to Professor Forl)es's first recognition of this change as an 

 important cause of glacial motion. In one respect, however, the 

 mechanism of the motion would be in some measure aifected. Dr. 

 Tyndall contends, and in a paper recently presented to the Royal 

 Society has collected a considerable amount of evidence to show, 

 that glacial ice would bear no more linear extension, independently 

 of lateral compression, than ordinary specimens of ice would lead us 

 to suppose, and consequently, when acted on by extending forces, it 

 cracked, forming fissures and crevasses to a much greater extent 

 than would seem consistent with any property to which the term 

 viscous could be applied with strict propriety. 



Professor Forbes was also the first to make known to us, by 

 systematic and M'ell- directed observations, the facts and laws of the 

 veined structure in glacial ice. He also entered into elaborate specu- 

 lations on the causes which produced this structure, both in his 

 'Travels in the Alps,' and in letters written subsequently. Dr. 

 Tyndall has also put forward a theory suggested by an analogy be- 

 tween the veined structure of ice and the lamination of rocks. As 

 there appeared to have been some confusion as to the differences 

 between these two theories, Mr. Hopkins would endeavour, as far 

 as he was able, to explain them. 



Both these theories depended primarily on the internal tensions 

 and pressures to which the glacial mass might be subjected. The 

 different parts of a glacier, as was well known, move with different 

 velocities, the most general law being that the central move faster 

 than the lateral portions ; but whatever may lead to this unequable 

 motion, its manifest result" must be a tendency to drag the slower 

 moving portions of the ice after those which move more quickly. 

 Moreover, it was easy to see that, in certain directions, this drag- 

 ging might be greater on one portion of the ice than on a contiguous 

 portion, and might thus tend to give different motions to contiguous 

 vertical slices of the mass. This difference of motion, or differential 

 motion, was supposed by Professor Forbes to actually exist, and that 

 ruptures or breaches of continuity were absolutely produced between 

 these vertical thin slices of ice by the strain upon them, and that 

 these ruptures gave rise to the veined structure. His first idea ap- 

 peared to have been that water infiltrated into the small fissures thus 

 formed, where it afterwards froze and formed the veins of blue trans- 

 parent ice, while the intermediate vertical laminae contained a suffi- 

 cient quantity of air-bubt)les to render them white and opake. This 

 notion of infiltration appeared to have been subsequently given up by 

 the Professor, the conversion of the opake into transparent ice being 



