Mr. J. Croll on the Cause of the Motion of Glaciers. 163 



you must be wrong when you assert that the glacier descends by 

 its weight only ; for, as I have demonstrated, the mere weight of 

 the glacier alone is not sufficient to do this." Canon Moseley has 

 not, however, proved that the glacier cannot absolutely descend 

 by its weight alone ; he has only proved that z/ the glacier shears 

 in the way that it is generally supposed to do, it cannot de- 

 scend by its weight alone. Had it been established that the 

 ice of the glacier shears in the way that it is generally supposed 

 to do, Mr. Moseley's results would leave us no other alternative 

 than to conclude that there must actually be some other cause 

 in addition to the weight of the glacier impelling it forward ; and 

 we should be obliged to seek in heat or in something else for this 

 additional impelling power. 



I presume that Canon Moseley has not duly considered this 

 point, and that consequently he has been led to the conclusion 

 that, if his late remarkable results be received (which no doubt 

 they will ere long), we shall then be obliged to adopt his own 

 theory of glacier-motion, or some other similar theory which calls 

 in the aid of forces more powerful than that of gravitation to 

 impel the glacier downwards. That he supposes that we are 

 forced to this alternative is, I think, apparent from the way in 

 which he has lately introduced his theory. " The ice of a gla- 

 cier," he says, " behaves itself in its descent exactly as the lead 

 did in my experiment. The Mer de Glace moves faster by day 

 than by night. Its mean daily motion is twice as great during 

 the six summer as during the six winter months. The con- 

 nexion between its rate of motion and the external temperature 

 is most remarkable. It has been carefully observed, and the 

 results, as recorded by Professor Forbes,leave no doubt of the fact, 

 that no change of external mean temperature is unaccompanied 

 by a corresponding change of glacier-motion. From this it fol- 

 lows that the two are either dependent on some common cause, or 

 that the one set of changes stands in the relation of a cause to the 

 other. That both sets of phenomena (the changes of the sun's 

 heat and the changes of glacier-motion) should be due to some 

 common independent cause seems impossible. We are forced, 

 therefore, on the conclusion that one is caused by the other. And 

 as the changes in the glacier-motion cannot cause the changes 

 of solar heat, it must be the changes of solar heat which cause the 

 changes of glacier-motion"*. 



It is certainly true that the fact that the glacier moves more 

 rapidly during the day than during the night, and during sum- 

 mer than during winter, proves that there must be some physical 

 connexion between the heat of the sun and the motion of the 



* Proceedings of the Bristol Naturalists' Society, vol. iv. p. 38 (new 

 series). 



