on the Moiio?i of Glaciers. 417 



ential movements of points in the transverse section occur, and 

 secondarily, lines of disruption or transverse crevasses perpen- 

 dicular to the former. Now Mr. Hopkins denies that the 

 former "ribboned structure" is produced by differential 

 motion (p. 331, first paragraph). 1 have only to request him, 

 when he next repeats the experiment, to watch the motions of 

 the coloured particles on the surface, which is easily done, and 

 he will perceive (with greatest distinctness in the higher parts 

 of the model, where the tearing force is most violent) that the 

 parts do visibly and mechanically slide past one another in 

 such directions. No person who sees the model made, or has 

 even been told how it was made, and inspects the " ribboned 

 structure " on its surface, can, I think, unless influenced by 

 previous theoretical views, entertain any other opinion. 



The experiments, however, on which Mr. Hopkins lays 

 most stress in his last paper, were made to corroborate " the 

 conclusions at which he arrived by mathematical investiga- 

 tion," and which do not profess to imitate the physical condi- 

 tions of a glacier, because the primary cause of its motion, 

 gravity producing Jltiid pressure, is left out of account. The 

 central part of a semi-solid mass is mechanically drawn away 

 from the sides, without any provision being made for the con- 

 tinuity of the motion by a supply of the material from the 

 source or origin of motion, as in my models and as in the real 

 glacier. I cannot agree with Mr. Hopkins (p. 328), that "it 

 is immaterial whether the motion be produced by gravity or 

 any other cause." I hold it to be in the highest degree ma- 

 terial. The results of such experiments, irrespective of the 

 physical cause of the motion, may confirm Mr. Hopkins's 

 merely abstract mechanical investigations, as no doubt on 

 many points they do; but as to affording any illustration, 

 whether positive or negative, of the actual problem before us, 

 namely the physical cause of glacier motion, they are mani- 

 festly, and by his own admission, incompetent, since they dis- 

 play mere results of motion, independent of the cause. 



Mr. Hopkins's experiment, however, illustrates two things; 

 Jirst, that forces exerted over large surfaces of even tolerably 

 solid bodies, produce, not a si7igle fissure or discontinuity, but 

 a system of simultaneous parallel fissures (p. 329), whose 

 number and distances must depend on the state of aggrega- 

 tion of the mass ; secondly, that the phaenomena about which 

 Mr. Hopkins's reasonings and experiments are almost entirely 

 conversant, are the effects of strains at the instant of the com- 

 7nencemetit ofmotio7i in a rigid mass; but they do not account 

 for the continuity of the motion so as to satisfy the condition 

 of the immense central, compared to the lateral, velocity. He 



Phil. Mag. S. 3. Vol. 26. No. 1 74. May 1 845. 2 F 



