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



motion ; the latter is the function attributed to heat in the theory 

 of glacier-motion which I ventured to advance some time ago*. 

 It results, therefore, from Canon Moseley's own theory, that the 

 longer the time that is allowed for the pressure to shear the ice, 

 the less will be the pressure required; for, according to his 

 theory, a very large proportion of the displacement is produced 

 by the motive power of heat entering the ice ; and, as it follows 

 of course, other things being equal, the longer the time during 

 which theheat is allowed to act, the greater will be theproportionate 

 amount of displacement produced by the heat ; consequently the 

 less will require to be done by the weight applied. In the case of 

 the glacier, Mr. Moseley concludes that at least thirty orforty times 

 as much work is done by the motive power of heat in the way of 

 shearing the ice as is done by mere pressure or weight. Then, 

 if sufficient time be allowed, why may not far more be done by 

 heat in shearing the ice in his experiment than by the weight 

 applied ? In this case how is he to know how much of the shear- 

 ing is effected by the heat and how much by the weight. If the 

 greater part of the shearing of the ice in the case of a glacier is 

 produced, not by pressure, but by the heat which necessarily 

 enters the ice, it would be inconceivable that in his experiments 

 the heat entering the ice should not produce, at least to some 

 extent, a similar effect. And if a portion of the displacement 

 of the particles is produced by heat, then the weight which is 

 applied cannot be regarded as the measure of the force employed 

 in the displacement, any more than it could be inferred that the 

 weight of the glacier is the measure of the force employed in the 

 shearing of it. If the weight is not the entire force employed 

 in shearing, but only a part of the force, then the weight cannot, 

 as in Mr. Moseley's experiment, be taken as the measure of the 

 force. 



How, then, are we to determine what is the amount of force 

 required to shear ice ? in other words, how is the unit of shear 

 to be determined ? If we are to measure the unit of shear by 

 the weight required to produce displacement of the particles of 

 the ice, we must make sure that the displacement is wholly 

 effected by the weight. We must be certain that heat does not 

 enter as an element in the process. But if time be allowed to 

 elapse during the experiment, we can never be certain that heat 

 has not been at work. It is impossible to prevent heat entering 

 the ice. We may keep the ice at a constant temperature, but 

 this would not prevent heat from entering the ice and producing 

 molecular work. True that, according to Moseley's theory of 

 glacier-motion, if the temperature of the ice be not permitted to 

 vary, then no displacement of the particles can take place from 

 * Philosophical Magazine for March 1869. 



