1845.] 



MOTION OF THE AAR GLACIER ACCOUNTED FOR. 



71 



internal melting, for that would produce merely a lowering of 

 the surface, and a subsidence of the level of the ice ; such as I 

 have shown* actually takes places in other glaciers whose 

 sections move with increasing velocity on the whole. There is 

 only a vis a tergo which can approximate the sections together, 

 and, as we read in the Comptes Rendus, squeeze the moraine 

 longitudinally, giving it a greater breadth,t and condense the 

 entire body of the ice so as to make it more compact in texture. \ 

 If we take a vertical section instead of a plan (see next page), 

 the slice abed must be condensed into the higher and shorter 

 solid cdef, and so of the rest, and the surface will be a swelling 

 one, as acen, which might even rise towards valley, but 



Fig. 18. 



generally need only be less sloped than the bed. The effect of 

 superficial thaw and internal subsidence diminishes this again, 

 and gives it the form of the dotted curve an'. 



In these diagrams the varying velocity in different parts of 

 the transverse section is, for simplicity, kept out of view. 



A retardation of the foremost portion of a viscid stream, 

 and consequent heaping of its surface, is exactly imitated in the 

 models formed of plaster of Paris, which I have elsewhere 

 described, and which, though of uniform fluidity from end to 

 end, and therefore not subject to the objection arising from the 

 cooling of lava, where a precisely similar fact is observed, repro- 

 duce faithfully the motions of the glacier of the Aar. 



* Travels, p. 153. 



f Les " Moraines medianes s'elargissent dans la meme proportion que le 

 mouvement se ralentit." Comptes Kendus, 9th Dec. 1844, p. 1301. 

 \ Ibid, p. 1306, line 29. 



