126 



THE FORMS OF WATER 



has since been established by measurement. 

 Looking at the glacier of Mont Dolent, which 

 resembles a sheaf in form, wide at both ends 

 and narrow in the middle, and reflecting tbat 

 *he upper wide part had become narrow, and 

 the narrow middle part again wide, Rcndu 

 observes, " There. is a multitude of facts 

 which seem to necessitate the belief that gla- 

 eiar ice enjoys a kind of ductility which en- 

 ables if, to 'mould itself to its locality, to thin 

 out, to swell, and to contract as if it were a 

 soft paste." 



400. To fully test his conclusions, Rendu 

 required the accurate measurement of glacier 

 motion. Had he added to his other endow- 

 ments the practical skill of a land-surveyor, 

 he would now be regarded as the prince of 

 glacialists. As it was he was obliged to be 

 content with imperfect measurements. In 

 one of his excursions he examined the guides 

 regarding- the successive positions of a vast 

 rock which he found upon the ice close to 

 the side of the glacier. The mean of five 

 years gave him a motion for this block of 40 

 feet a year. 



401.' Another block, the transport of which 

 he subsequently measured more accurately, 

 gave him a velocity of 400 feet a year. Note 

 his explanation of this discrepancy : '* The 

 enormous difference of these two ol)eerva- 

 tions arises from the fact that one block 

 stood near the centre of the glacier, which 

 moves most rapidly, while the other stood 

 noar the side, where the ice is held back by 

 friction. " So clear and definite were Rcndu's 

 ideas of the plastic motion of glaciers, that 

 had the question of curvature occurred to 

 him, I entertain no doubt that he would 

 have enunciated beforehand the shifting of 

 the point of maximum motion from side to 

 side across the axis of the glacier ( 25). 



402. It is right that you should know that 

 scientific men do not always agree in their 

 estimates of the comparative value of facts 

 ?md ideas ; and it is especially right that you 

 should know that your present tutor attaches 

 a very high value 'to ideas when they spring 

 from the profound and persistent pondering 

 of superior minds, and are not, as is too 

 often the case, thrown out without the war- 

 rant of either deep thought or natural capac- 

 ity. It is because I believe Rendu's labors 

 fulfil this condition that I ascribe to them so 

 high a value. But when you become older 

 and better informed, you may differ from 

 i-iie ; and I write these words lest you should 

 too readily accept my opinion of Rendu. 

 Judge me,' if you care to do so, when your 

 knowledge is matured. I certainly shall not 

 ! ear your verdict. 



403. But, much as I prize the prompting 

 idea, and thoroughly as I believe that often 

 in it the force of genius mainly lies, it would, 

 in my opinion, be an error of omissioa of the 

 gravest kind, and which, if habitual, would 

 insure the ultimate decay of natural knowl- 

 lge, to negL-ct verifying our ideas, and giv- 

 ng them outward reality and substance when 

 '.lie means of doing so are at hand. In sci- 

 j nce, thought, as far as possible, ought to foe 



wedded to fact, This was attempted by 

 Rendu, and in great part accomplished by 

 Agassiz and Forbes. 



GO. Viscous THEORY. 



404. Here indeed the merits of the distin- 

 guished Racialist last named rise conspicu- 

 ously to view. From the able and earnest 

 advocacy of Professor Forbes, the public 

 knowledge of this doctrine of glacial plastic- 

 ity is aUnost wholly derived. He gave the 

 doctrine a more distinctive form ; he first 

 applied the term viscous to glacier ice, and 

 sought to found upon precise measurements 

 a "viscous Theory" of glacier motion. 



405. I am here obliged to state facts in 

 their historic sequence. Professor Forbes 

 when he began his investigations was ac- 

 quainted with the labors of Rendu. In his 

 earliest work upon the Alps he refers to 

 those labors in terms of flattering recogni- 

 tion. But though as a ma te:- of f act lien- 

 du's ideas were there to prompt him, it would 

 be too much to say that he needed their in- 

 spiration. Had Rcndu not preceded him, 

 he might none the less have grasped the idea 

 of viscosity, executing his measurements and 

 applying his knowledge to maintain it. Be 

 that as it may, the appearance of Professor 

 Forbes on the Unteraar glacier in 1841, and 

 on the Mer cle Glace in 1842, and his labors 

 then and subsequently, have given him a 

 name not to be forgotten in the scientific his- 

 tory of glaciers. 



406. The theory advocated by Professor 

 Forbes was enunciated by himself in these 

 words : "A glacier is an imperfect fluid, or 

 viscous body, which is urged down slopes of 

 certain inclination by the natural pressure of 

 its parts." In 1773 Bordier wrote thus: 

 "As the glaciers always advance upon the 

 plain, and never disappear, it is absolutely 

 essential that new ice shall perpetually take 

 the place of that which is melted : it must 

 therefore be pressed forward from above. 

 One can hardly refuse then to accept the as- 

 tonishing truth, that this vast extent of haul 

 and solid ice moves as a single piece down- 

 ward." In the passage already quoted he 

 speaks of the ice being pressed as a fluid 

 from abore. Tirap constitute, I believe, 

 Bordier's contributions to this subject. The 

 quotations show his sagacity at an early 

 date ; but, in point of completeness, his 

 views are not to be compared with those of 

 Rendu nnd Forbes. 



407. I must not omit to state here that 

 though the idea of viscosity has not been es- 

 poused by M. Agassiz, his measurements, 

 and maps of measurements, on the Unteraar 

 glacier have been recently cited as the most 

 clear and conclusive illustrations of a quality 

 which, at all events, closely resembles vis- 

 cosity. 



408. But why, with proofs before him 

 mem; copious and characteristic than those of 

 any other observer, does M. Agassiz hesitate 

 to accept the idea of viscosity as applied to 

 ice ? Doubtless because he believes the no- 

 tion to be contradicted by our cvery-day ex- 



