168 Mr. J. Croll on the Cause of the Motion of Glaciers. 



the mere weight of the glacier was sufficient to shear the ice. 

 Canon Moseley has investigated this point, and shown that it 

 is not. He has found that for a glacier to shear in the way that 

 it is supposed to do, it would require a force some thirty or forty 

 times as great as the weight of the glacier. Consequently, for 

 the glacier to descend, a force in addition to that of gravitation 

 is required. What, then, is this force ? It is found that the rate 

 at which the glacier descends depends upon the amount of heat 

 which it is receiving. This shows that the motion of the glacier 

 is in some way or other dependent upon heat. Is heat, then, 

 the force we are in search of? The answer to this, of course, is, 

 since heat is a force necessarily required, we have no right to 

 assume any other till we see whether or not heat will suffice. In 

 what way, then, does heat aid gravitation in the descent of the 

 glacier ? In what way does heat assist gravitation in the shear- 

 ing of the ice ? There are two ways whereby we may conceive the 

 thing to be done : the heat may assist gravitation to shear, by 

 pressing the ice forward, or it may assist gravitation by diminish- 

 ing the cohesion of the particles, and thus allowing gravitation to 

 produce motion which it otherwise could not produce. Every at- 

 tempt which has yet been made to explain how heat can act as a 

 force in pushing the ice forward, has failed. The fact that heat can- 

 not expand the ice of the glacier may be regarded as a sufficient 

 proof that it does not act as a force impelling the glacier forward ; 

 and we are thus obliged to turn our attention to the other 

 conception, viz. that heat assists gravitation to shear the ice, not 

 by direct pressure, but by diminishing the cohesive force of the 

 particles, so as to enable gravitation to push the one past the 

 other. But how is this done ? Does heat diminish the cohesion 

 by acting as an expansive force in separating the particles ? Heat 

 cannot do this, because it cannot expand the ice of a glacier; 

 and besides, were it to do this, it would destroy the solid and 

 firm character of the ice, and the ice of the glacier would not then, 

 as a mass, possess the great amount of shearing-force which ob- 

 servation and experiment show that it does. In short it is because 

 the particles of the ice are so firmly fixed together at the time 

 that the glacier is descending, that we are obliged to call in 

 the aid of some other force in addition to the weight of the gla- 

 cier to shear the ice. Heat does not cause displacement of the 

 particles by making the ice soft and plastic ; for we know that 

 the ice of the glacier is not soft and plastic, but hard and brittle. 

 The shearing-force of the ice of the moving glacier is found to 

 be by at least from thirty to forty times too great to permit of the 

 ice being sheared by the mere force of gravitation ; how, then, is 

 it that gravitation, without the direct assistance of any other 

 force, can manage to shear the ice ? Or to put the question 



