160 HEAT OF HIGHER LAYERS OE AIR, 



till they have reached the mouth of the glacier, 

 where, piled up for thousands of years, and added 

 to every year, they form the Moraines of the 

 glaciers. Where there are not crevices in the mass, 

 numberless little rivulets, fed by the melting ice, 

 course over its surface, till either, received by hol- 

 lows closed at bottom, they fill them to overflow- 

 ing, or, pouring into deep clefts, they fall to the 

 bottom of the valley, and then, flowing onwards 

 under the glacier, and, joining with many others 

 into a large stream, gush out again into the open 

 air, at the lower end of the glacier. In spots 

 where large blocks or heaps of stone lie upon the 

 ice, it is screened from the action of the sun's 

 rays. While, then, the thawing goes on around, 

 there remain standing under the shelter of these 

 coverings pillars of ice, on which the pieces of 

 rock rest from year to year, while they grow, as it 

 were, to considerable heights above the surface, 

 till at length the treacherous props, fretted away 

 by the weather, fall under their burden. Leaves 

 and other thin organic bodies, which fall by 

 chance upon the ice, work just the contrary effect. 

 They have, as has long been shown experimentally, 

 in a very far higher degree than ice, the power of 

 turning the sun's rays into sensible heat. But 

 since these must, if they are of small thickness, 

 immediately send on to the ice beneath them the 

 heat which they take up, this ice must melt more 



