132 Mr. H. H. Howorth on 



below, and retains no trace of the fissure, leaving the rock an 

 islet in the icy flood ; a substance which moves in such a 

 fashion cannot in any true sense of the word be termed a 

 rigid solid, and must be granted to be ductile, viscous, plastic, 

 or semi-fluid, or to possess qualities represented by any of 

 these terms which we may choose to adopt as least shock- 

 ing to our ordinary conception of the brittleness of ice," 

 and it was no doubt with some satisfaction that Forbes 

 quoted the words of Mousson in his Die Gletscher der 

 Jetztzeit,^. 162, speaking of the plastic theory " Er steht 

 noch heute unangefochten da," and that he numbered among 

 his warmest supporters Darwin and Whewell. 



While it was generally admitted that the phenomena 

 accompanying the motion of glaciers had been shown by 

 Forbes to exactly reproduce the conduct of a plastic body 

 which can mould itself to its boundaries when in motion, and 

 which has a continuous differential motion, the conduct of 

 hand specimens continued to be a stumbling block. As 

 Tyndall urged, a slight blow, if properly directed, will split 

 open a block of ice 10 or 15 cubic feet in volume, and as Mr.. 

 McGee says, "on a cold, still night the steel runner of a boy's- 

 skate initiates a fracture miles in length in the ice bridging 

 a river, which shews that if ice is plastic that it is also very 

 rigid under some conditions," but the fact which weighed most 

 with observers was the experimental test applied by Professor 

 Moseley, of Cambridge. These experiments were described in 

 a paper read before the Royal Society in 1869, in which 

 he published some very interesting results on the shearing 

 force of ice, in which, according to one experiment, the shear 

 per square inch, or unit of shear, was 72*433lb., and in another 

 case 766 1 91b., the mean being about 7 5 lb., while, according 

 to his calculations for a glacier to move by its own weight,, 

 as Tyndall had seen it move in the Mer de Glace, the unit 

 of shear should not have been more than 1-3 1931b., whence 

 he concluded that the weight of a glacier alone is insufii- 



