Masterpieces of Science 



it encounters is as the sixth power of its speed. 

 It is not worth while for us to pause in our im- 

 aginary journey to consider whether the third 

 power or the sixth be the rate at which the effi- 

 ciency in the carrying power of the stream in- 

 creases with its speedier flow. It is enough for 

 us to know that the water, with very slight in- 

 crease in its velocity, is able to carry a very much 

 larger stone than it could before its speed was 

 increased. 



The sides of these mountain torrents are gen- 

 erally steep. It is rare indeed that the slopes 

 which lead to them are much less inclined than 

 the roofs of ordinary houses. Over all the sur- 

 face on either side of the torrent, frost and other 

 agents of decay are constantly at work breaking 

 out bits of stone or forming soil. This mass of 

 broken-up rock is constantly slipping down the 

 sides of the valley. Every time the winter frost 

 seizes it, it expands a little, and is thus shoved 

 downward; frequently, when soaked with water, 

 great sheets of it slip swiftly, as mud-avalanches, 

 into the stream. In this way the torrent is 

 always provided with fragments which it may 

 grind up into pebbles, sand, and mud, and bear 

 onward to the fields .below. In times of drought, 

 these stream-beds are occupied by rivulets of 

 clear water, and at such periods the observer 

 gains no idea of the vigour with which the mill 

 works; but in times of heavy rain he will find 

 the water turbid with sediment made by the 

 attrition of pebbles against the bordering walls of 

 144 



