1857.] Observations on Glaciers. 327 



the patches of dirt are squeezed longitudinally and drawn out 

 laterally, being thus converted into stripes of discoloration, which, 

 owing to the speedier motion of tlie centre, are convex towards the 

 lower extremity of the glacier. On consulting Professor Forbes's 

 map of the Mer de Glace, it will be seen that the " bands " com- 

 mence at the base of the icefall of the Taltjfre, while none exist 

 above the fall. Those shown on the Glacier du Geant, we are led 

 to infer, commence at the base of the cascade of La Noire, which, 

 however is not sketched on the map. The theory of Professor 

 Forbes is, that a glacier throughout its entire length is composed of 

 alternate segments of hard and porous ice, that the dirt is washed 

 from the former, but finds a " lodgment " in the latter. The expe- 

 riments on which this important conclusion is founded were un- 

 known to the speaker, who finds observation and experiment in 

 harmony with the explanation given above. 



It may be urged that, after all, the foregoing experiments on the 

 yielding of ice do not prove the viscous theory to be wrong. The 

 mere fact of bending a prism of ice by fracture and regelation 

 does not prove that it is non-viscous. This is perfectly true, nor 

 was it conceived that the onus rested on the speaker to prove the 

 negative here. All that was claimed for the experiments is the 

 referring of certain observed phenomena to true causes, instead 

 of to imaginary ones. An illustration may help to place this 

 question in its true light. By Newton's calculation the velocity 

 of sound was found to be one-sixth less than observation made it, 

 and to account for this discrepancy he supposed that the sound 

 passed instantaneously through the particles of air themselves, time 

 being required only to accomplish the passage from particle to 

 particle. He supposed the diameter of each air particle to be 

 -i^^ths of the distance between two particles ; and nobody ever 

 proved him wrong. Still, when Laplace assigned a vera cattsa for 

 the discrepancy, the hypothesis of Newton, and other ingenious 

 suppositions, were at once relinquished. The proof, indeed, in 

 such cases consists in the substitution of facts for conjectures, and 

 whether this has been done in the case now under consideration the 

 intelligent reader must himself consider. 



In the foregoing remarks, Mr. Tyndall has expressed his dissent 

 from some of the theoretic views of Professor Forbes ; but he feels 

 great pleasure in recording the high value which he attaches to 

 the experimental labours and observations of this philosopher. 

 Indeed it is the fact of Professor Forbes having done so much 

 that rendered such frequent reference to him necessary. To the 

 same frank and friendly criticism the views propounded this evening 

 were submitted ; and a hope was entertained that the discussion of 

 the question would result in a more exact acquaintance with the 

 structure and motion of glaciers. 



[J. T.] 



