156 Mr. J. Croll on the Cause of the Motion of Glaciers, 



seen advanced to the contrary can, so far as I perceive, overturn 

 Mr. Moseley's conclusion, that the glacier cannot descend by its 

 weight only. The interesting experiment described by Mr. Ma- 

 thews*, of a plank of ice supported horizontally at each end being 

 deflected in the middle without any weight being applied to the ice, 

 does not appear to me to prove any thing either in favour of the 

 generally received theory or against Canon Moseley's conclusion, 

 — for this very simple reason, that whatever theory we may adopt 

 as to the cause of the motion of glaciers, the deflection of the 

 plank in the way described by Mr. Mathews follows as a neces- 

 sary consequence. Although no weight was placed upon the 

 plank, it does not necessarily follow that the deflection was 

 caused by the weight of the ice alone ; for, according to Canon 

 Moseley's own theory of the motion of glaciers by heat, the 

 plank ought to be deflected in the middle, just as it was in Mr. 

 Mathews's experiment. A solid body, when exposed to variations 

 of temperature, will expand and contract transversely as well as lon- 

 gitudinally. Ice, according to Canon Moseley's theory, expands 

 and contracts by heat. Then if the plank expands transversely, 

 the upper half of the plank must rise and the lower half descend. 

 But the side which rises has to perform work against gravity, 

 whereas the side which descends has work performed upon it by 

 gravity ; consequently more of the plank will descend than rise, 

 and this will, of course, tend to lower or deflect the plank in the 

 middle. Again, when the plank contracts, the lower half will 

 rise and the upper half will descend ; but as gravitation, in this 

 case also, favours the descending part and opposes the rising 

 part, more of the plank will descend than rise, and consequently 

 the plank will be lowered in the middle by contraction as well 

 as by expansion. Thus, as the plank changes its temperature, 

 it must, according to Mr. Moseley's theory, descend or be de- 

 flected in the middle, step by step — and this not by gravitation 

 alone, but chiefly by the motive power of heat. I do not, of 

 course, mean to assert that the descent of the plank was thus 

 actually caused by heat ; but I assert that Mr. Mathews's expe- 

 riment does not necessarily prove (and this is all that is required 

 in the mean time) that gravitation alone was the cause of the 

 deflection of the plank. Neither does this experiment prove 

 that the ice was deflected without shearing; for although the 

 weight of the plank was not sufficient to shear the ice, as Mr. 

 Mathews, I presume, admits, yet Mr. Moseley would reply that 

 the weight of the ice, assisted by the motive power of heat, was 

 perfectly sufficient. 



Had Mr. Mathews laid his plank horizontally across an inclined 

 plane and fixed the two ends of the plank so as to prevent them 



* Alpine Journal for February 18/0; ' Nature' for March 24, 1870. 



