174 



descend as it actually does, if the resistances opposed to its descent were 

 twelve times greater than the resistances opposed to the descent of 

 the lead If its descent were resisted by a friction, for instance, having 

 twelve times the coefficient of that of the lead on the board, or such as 

 would cause it to rest without slipping on an incline having twelve 

 times the tangent of the inclination of the board ; or if the variations 

 of temperature were less and the resistance greater in any proportion 

 which would retard the descent twelve times as much. So that 

 we may suppose in the case of the glacier a far greater resistance in 

 proportion than that sustained by the lead upon the board, and 

 variations of temperature far less, without passing the limits within 

 which a probability is created by the experiment that the descent of 

 the glacier is due to the same cause as that of the lead. 



In the act of descending on the board, the slab of ice of which we 

 have spoken could not but be thrown into a state of extension in 

 some parts and of compression in another. The conditions of the 

 descent being in other respects given, the amount of this extension or 

 compression might be at any point determined. If at any point the 

 extension exceeded the tenacity of the ice, the slab would there sepa- 

 rate across its length ; and if at any point the compression exceeded 

 the resistance to crushing, it would there crush. 



Supposing it to be thinner at the sides than in the middle, the sur- 

 face-motion of the middle would be faster than that of the sides, and 

 from this differential motion would result cracks oblique to the axis of 

 the slab, the explanation of which, as they exist in glaciers, is one of 

 the most successful attempts yet made at the solution of the me- 

 chanical problem of glacier-motion. These conditions of the descent 

 of the slab, when referred to a glacier, explain the formation of trans- 

 verse and lateral crevasses, and the fact of a glacier crushing itself 

 through a gorge. 



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 f. It moves fastest in the hottest months, and in those 

 months varies its motion the most, because in them the variations of 

 temperature are the greatest. It moves most slowly in the coldest 

 months, and in those varies its motion the least, because in those 

 months the variations of temperature are the least. These differences 



* Forbes, ' Occasional Papers,' p. 12. 



t Ibid. p. 129. Tyndall, Glaciers of the Alps/ p. 294. 



