THE GLACIERS AND THEIR INVESTIGATORS. 751 



priest and canon when he wrote, and afterward Bishop of Annecy. In 1841 

 Rendu laid before the Academy of Sciences of Savoy his ' Theory of the Gla- 

 ciers of Savoy,' a contribution forever memorable in relation to this subject. 



"Eendu seized the idea of glacier plasticity with great power and clearness, 

 and followed it resolutely to its consequences. It is not known tbat he bad 

 ever seen the work of Bordier ; probably not, as he never mentions it. Let me 

 quote for you some of Rendu's expressions, which, however, fail to give an ade- 

 quate idea of his insight and precision of thought : ' Between the Mer de Glace 

 and a river there is a resemblance so complete that it is impossible to find in 

 the glacier a circumstance which does not exist in the river. In currents of 

 water the motion is not uniform, either throughout their width or throughout 

 their depth. The friction of the bottom and of the sides, with the action of 

 local hindrances, causes the motion to vary, and only toward the middle of the 

 surface do we obtain the full motion.' 



" This reads like a prediction of what has since been established by meas- 

 urement. Looking at the glacier of Mont Dolent, which resembles a sheaf in 

 form, wide at both ends and narrow in the middle, and reflecting that tbe upper 

 wide part had become narrow, and the narrow middle part again wide, Rendu 

 observes : ' There is a multitude of facts which seem to necessitate the belief 

 that glacier-ice enjoys a kind of ductility, which enables it to mould itself to its 

 locality, to thin out, to swell, and to contract, as if it were a soft paste.' 



" To fully test his conclusions, Rendu required the accurate measurement 

 of glacier motion. Had he added to his other endowments 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 forty feet a year. 



"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 observations arises from 

 the fact that one block stood near the centre of the glacier, which moves most 

 rapidly, while the other stood near the side, where the ice is held back by fric- 

 tion.' So clear and definite were Rendu's ideas of the plastic motion of gla- 

 ciers, that, had the question of curvature occurred to him, I entertain no doubt 

 that be would have enunciated beforehand the shifting of the point of maximum 

 motion from side to side across the axis of the glacier ( 25). 



" It is right that you should know that scientific men do not always agree 

 in their estimates of the comparative value of facts and ideas ; and it is espe- 

 cially 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 

 warrant of either deep thought or natural capacity. 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 me ; 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 fear your verdict. 



"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 omission of the gravest kind, and which, if habitual, would insure tbe ulti- 

 mate decay of natural knowledge, to neglect verifying our ideas, and giving them 

 outward reality and substance when the means of doing so are at hand. In 

 science, thought, as far as possible, ought to be wedded to fact. This was at- 

 tempted by Rendu, and in great part accomplished by Agassiz and Forbes. 



" Here, indeed,, the merits of the distinguished glacialist last named rise con- 

 spicuously to view. From the able and earnest advocacy of Prof. Forbes, the 

 public knowledge of this doctrine of glacial plasticity is almost 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. 



"I am here obliged to state facts in their historic sequence. Prof. Forbes, 



