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XXVI. On the Physical Cause of the Motion of Glaciers. 

 By James Croll, of the Geological Survey of Scotland*. 



I HAVE just seen an abstract of a most interesting paper 

 by the Reverend Canon Moseley " On the Mechanical 

 Possibility of the Descent of Glaciers by their weight only/' 

 which was read before the Royal Society on the 7th of January 

 last t« I n that memoir he arrives at the conclusion that, owing 

 to the great resistance offered by the solid ice to shearing, it is 

 impossible that glaciers can descend by their weight alone. 



" All the parts/' he remarks, " of a glacier do not descend 

 with a common motion ; it moves faster at its surface than 

 deeper down, and at the centre of its surface than at its edges. 

 It does not only come down bodily, but with different motions 

 of its different parts ; so that if a transverse section were made 

 through it, the ice would be found to be moving differently at 

 every point of that section There is a constant displace- 

 ment of the particles of the ice over one another and alongside 

 one another, to which is opposed that force of resistance which 

 is known in mechanics as shearing -force." 



He determines by calculation the amount of shearing-force 

 which must not be exceeded if the displacement of the particles 

 is to be effected by the weight of the ice alone. In the case of 

 the Mer de Glace at the Tacul, the shearing-force of the ice must 

 not exceed 1*3193 lb. per unit surface of one square inch, if that 

 glacier descends merely by its weight, at the rate observed by 

 Professor Tyndall. From experiments which he has made, 

 he finds that the actual shearing-force of ice per unit surface is 

 about 75 lbs. Consequently he concludes it is impossible that the 

 motion of the glacier can be due to its weight alone; there must 

 be some other force in addition to the weight impelling the ice 

 forward. And he calculates that the amount of work performed 

 by this unknown force is thirty -four times the amount performed 

 by the weight of the glacier. 



This is a most important conclusion. It is quite decisive 

 against the generally received opinions regarding the descent of 

 glaciers by their own weight. 



Rut although it is thus demonstrated that glaciers cannot de- 

 scend by means of their weight alone in the manner generally 

 supposed, still, I venture to think that, notwithstanding the 

 demonstration, gravitation after all may be the only force moving 

 the ice. 



* Communicated by the Author. 



t Proceedings of the Royal Society, vol. xvii. p. 202. [See p. 229 of 

 our present Number, Ed. Phil. Mag?\ 



