236 Professor Forbes's Sixth Letter on Glaciers. 



enough, to bear a comparison with the far less ductile body 

 of a glacier. In the heart of the mass where the same intes- 

 tinal motions take place (as I have shewn conclusively by 

 using coloured layers of plastic matter in the models formerly 

 exhibited to the Royal Society), the displaced particles re- 

 unite and consolidate into a homogeneous mass without any 

 trace of dislocation.* 



V. The convexity or concavity of a semifluid stream like a 

 current of lava or of a glacier, depends entirely upon the re- 

 lations or conditions in which it is placed. Upon the same 

 slope, a fluid of one degree of consistence will run off in a con- 

 cave stream, whilst a more viscid one, which must accumulate 

 in thickness, in order to overcome the resistance in front (just 

 as water which meets a sudden obstacle), rises into a convex 

 curve. This is perfectly seen in the case of a substance like 

 plaster of Paris, mingled with water, whose consistence may be 

 varied at pleasure, and a stream of which may be made either 

 concave or convex, or concave at its origin and convex at its 

 termination, as is the case with a glacier. The evidence 

 on this subject, afforded by the models formerly laid before 

 the Royal Society, is so complete and conclusive, that, how- 

 ever interesting it might be to put into a mathematical form 

 the relations of the constants of the effect of gravity, the vis- 

 cosity of the body, and the retardation of the sides, as affect- 

 ing the form of the surface, it is sufficient for my present pur- 

 pose to appeal to facts so familiar, and experiments so easy, 

 that their evidence may well be preferred to the more casual 

 and embarrassed case of lava streams, which, as I have already 

 observed, are seldom or never to be regarded, on a great scale, 

 as simple moving masses. I may, however, add, that when the 

 inclination is small the surface is convex, at a certain distance 

 from the origin. 



* The following passage from M. Dufrenoy's Account of Vesuvius, is 

 interesting, if it were only as recording his remark, that the variation of 

 velocity in different parts of a stream must produce longitudinal striae. 

 " La plupart des coulees presentent des bandes longitudinales assez paral- 

 leles entre elles ; ces larges stries saillantes sur la surface sont les traces 

 du mouvement de la lave qui ne s'avance pas d'une seule piece, mais 

 par bandes paralle'les." Sur les Environs de Naples, p. 324. 



