CLOUDS AND RIVERS, ICE AND GLACIERS. 159 



reflecting that the upper wide part had become narrow, 

 and the narrow middle part again wide, Rendu observes, 

 i 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/ 



400. To fully test his conclusions, Rendu required 

 the accurate measurement of glacier motion. Had ha 

 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 excur- 

 sions 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 sub- 

 sequently measured more accurately, gave him a velocity 

 of 400 feet a year. Note his explanation of this dis- 

 crepancy: — 'The enormous difference of these two obser- 

 vations 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 friction.' So clear and definite were 

 Rendu'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 



