CLOUDS AND KIVERS, ICE AND GLACIERS. 87 



rotten and unsafe. Test such ice with your penknife : 

 you can dig the blade readily into it, or cut the ice with 

 ease. Try good sound ice in the same way : you find 

 it much more resistant. The one, indeed, resembles 

 soft chalk; the other hard stone. 



222. Now the Mer de Glace in summer is in this 

 thawing condition. Its ice is rendered soft and yielding 

 by the sun; its motion is thereby facilitated. We 

 have seen that not only does the glacier slide over its 

 bed, but that the upper layers slide over the under 

 ones, and that the centre slides past the sides. The 

 softer and more yielding the ice is, the more free will 

 be this motion, and the more readily also will it be 

 forced through a defile like Trelaporte. 



223. But in winter the thaw ceases; the quantity of 

 water reaching the bed of the glacier is diminished or 

 entirely cut off. Tbe ice also, to a certain depth at 

 least, is frozen hard. These considerations would justify 

 the opinion that in winter the glacier, if it moves at 

 all, must move more slowly than in summer. At all 

 events, the summer measurements give no clue to the 

 winter motion. 



224. This point merits examination. I will not, how- 

 ever, ask you to visit the Alps in mid- winter; but, if 

 you allow me, I will be your deputy to the mountains, 

 and report to you faithfully the aspect of the region 

 and the behaviour of the iee. 



