213 



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 well-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-bubbles 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 

 supposed to take place by pressure and differential motion, indepen- 

 dently, as far as Mr. Hopkins understood, either of infiltration or 

 the melting of any portions of the ice. 



Both Professor Forbes and Dr. Tyndall had endeavoured to eluci- 

 date the phaenomena of glacial motion by means of a semifluid sub- 

 stance descending down a trough inclined to the horizon. For the 

 purpose of ascertaining the direction of greatest extension and com- 

 pression of the substance when thus put in motion, the latter gen- 

 tleman described circles on its siirface while still at rest, and ob- 

 served the compressions and extensions of the radii when the mass 

 was in motion. He thus found that the lines of greatest extension 

 were inclined at angles of 45° to the axis of the trough ; each such 

 line pointing centrally and downwards, or laterally and upwards, 

 while the lines of greatest compression were perpendicular to them. 

 In the case of a glacier descending a canal-shaped valley, the former 

 of these directions would manifestly be that of greatest tension ; 



