Vll 



recently by M. Rendu, then Canon, afterwards Bishop of Annecy ; but to 

 Forbes belongs certainly the merit of proving its general truth by careful 

 and prolonged experimental measurements of the rate of progression of 

 glaciers at different parts. But whilst it is plain that the ice of a moving 

 glacier behaves in the gross like a plastic substance, there has been a ques- 

 tion by what intestine changes or motions of its particles its change of figure 

 is brought about or accompanied ; and later inquirers maintain that the ice 

 yields by breaking up into minute fragments, which speedily reunite by 

 partial melting and regelation, thus permitting of change of form in the 

 mass. The ribboned or veined' structure of glacier ice, which had been but 

 little attended to by previous writers, was carefully studied by Forbes. 

 Having seen that the velocity of movement increases from the sides to the 

 middle of the glacier, he ascribed the production of the ribboned structure 

 to " differential motion" between adjacent laminar sections of its substance, 

 a process which has since been termed " shearing/' Others have com- 

 pared the phenomenon to the lamination of slaty rocks now very generally 

 regarded as caused by pressure in a direction perpendicular to the planes 

 of lamination. 



But while there may be difference of opinion as to the physical explana- 

 tion of the observed phenomena, there can be no question of Forbes' s 

 signal merit in connexion with the scientific history of glaciers ; and it is 

 pleasing to know that it was generously acknowledged in his lifetime by a 

 distinguished rival in the same field, who thus speaks of him*: — "The 

 more his labours are compared with those of other observers, the more 

 prominently does his comparative intellectual magnitude come forward. 

 The speaker would not content himself with saying that the book of Prof. 

 Forbes was the best book which had been written on the subject. The 

 qualities of mind, and the physical culture invested in that excellent work, 

 were such as to make it, in the estimation of the physical investigator at 

 least, outweigh all other books upon the subject taken together." 



While Switrerland was the main region of Forbes' s explorations, he did 

 not neglect his own land. In the summer of 1845 he traversed the rugged 

 hills of Skye, and proved that the bare scarps of the Cuchullius had been 

 ground down by the same kind of glaciers as those which are now wearing 

 down the gorges of the Alps. 



The last important scientific labour he was permitted to undertake was 

 on the subject of thermal conductivity. He was the first to point out — 

 and this at a very early period of his career — the fact that the conducting- 

 powers of the metals for electricity are approximately proportional to their 

 conducting-powers for heat. Now, heat diminishes materially the electric 

 conducting-power — does it also affect the thermal conductivity ? Forbes 

 showed that (at least in the case of iron, the only metal his failing health 

 left him strength to examine) the conductivity for heat diminishes as the 

 temperature increases. Another result of the same investigation, and one 



* Eeport of a Lecture delivered by Professor Tyndall at the Eoyal Institution , 

 June 4, 1858. 



