148 Canon Moseley on the Mechanical Impossibility 



rod remained perfect throughout. Their values are recorded in 

 the fourth column of the Table. 



That this perfect elasticity was not, in point of fact, pre- 

 served is plain from the variation of the modulus. If it were, 

 and the experiments could in all respects be relied upon, the 

 elasticity of this rod of ice would be about one third that of cast 

 iron, three times that of deal, and a little greater than that of 

 tin*. 



I am far, however, from putting these experiments forward 

 as sufficient in accuracy for determining a physical constant of 

 so much importance as the modulus of elasticity of ice. The 

 elasticity of this ice-rod would have been different at a different 

 temperature — or possibly if it had been taken out of another pond, 

 or even if taken out of a different part of the same pond. The 

 elasticity of such a rod of ice remains perfect only within certain 

 exceedingly small limits of deflection, which it is not easy to 

 observe with precision in the open air in a temperature consider- 

 ably below freezing. To that end an apparatus better adapted 

 to the purpose than mine would be necessary. 



Mr. Matthews has referred in just terms of respect to the 

 contribution of the late Mr. W. Hopkins to the science of 

 glacier- motion, and particularly to that well-known experiment 

 in which placing a block of ice on an inclined plane of sandstone, 

 the ice descended (melting at its surface of contact with the 

 sandstone) with a velocity which, when the inclination of the 

 plane did not exceed a certain limit, was uniform. 



I have repeated Mr. Hopkins's experiments in various ways. 

 I have used rough paving-stones for the inclined plane, and 

 stones pierced with holes to allow of the escape of the water 

 melted from the ice at the surfaces of contact, which seemed 

 to lubricate them. With the same view I have used soft sand- 

 stone to imbibe the water, and I have caused ridges to be 

 cut in its surface in the direction of the descent and across 

 it. In all these variations of his experiments I have found a 

 perfect confirmation of Mr. Hopkins's results. His experiments 

 shows it wo/ to be mechanically impossible that a block of ice 

 should descend on an inclined plane by its weight alone — and 

 therefore not to be mechanically impossible that a glacier should 

 descend by its weight alone, if it descended as the block of ice 

 did in his experiment. But it does not. There is an essential 



* The modulus of elasticity of hot-blast cast iron (Buffery) is, accDrding 

 to Ilodgkinson, 15381200; that of deal (Memel), according to Barlow, 

 1535200; and that of cast tin, according to Tredgold, 4608000. I 

 think the best means of determining the modulus of elasticity of ice would 

 be found ra the acoustic properties which at low temperatures it probably 

 possesses. 



