CLOUDS AND RIVERS, ICE AND GLACIERS. 113 



285. Now clcth is what is called a bad conductor. 

 It does not permit heat to travel freely through it. 

 Bat where it has merely to pass through the thickness 

 of a single bit of cloth, a good quantity of the heat 

 gets through. But if you double or treble or quintuple 

 the thickness of the cloth ; or, what is easier, if you put 

 several pieces one upon the other, you come at length 

 to a point where no sensible amount of heat could get 

 through from the upper to the under surface. 



286. What must occur if such a thick piece, or such 

 a series of pieces of cloth, were placed upon snow on 

 which a strong sun is falling? The snow round the 

 cloth is melted, but that underneath the cloth is pro- 

 tected. If the action continue long enough the in- 

 evitable result will be, that the level of the snow all 

 round the cloth will sink, and the cloth will be left 

 behind perched upon an eminence of snow. 



287. If you understand this, you have already mastered 

 the cause of the moraine-ridges. They are not produced 

 by any swelling of the ice upwards. But the ice under- 

 neath the rocks and rubbish being protected from 

 the sun, the glacier right and left melts away and leaves 

 a ridge behind. 



288. Various other appearances upon the glacier are 

 accounted for in the same way. Here upon the Mer de 

 C J lace we have flat slabs of rock sometimes lifted up on 

 pillars of ice. These are the so-called Glacier Tahles. 

 They are produ^d, not by the growth of a stalk of ice 



