154 LIGHT SCIENCE FOR LEISURE HOURS. 



tents of the lake were safe] y carried off. It was hoped 

 that the process would continue, and the country be 

 saved from the danger which had been so long im- 

 pending over it. But as the heat of the weather in- 

 creased, the central part of the barrier slowly melted 

 away, until it became too weak to bear the enormous 

 weight of water which was pressing against it. At 

 length it gave way, so suddenly and completely that 

 all the water which remained in the lake rushed out in 

 half an hour. The downward passage of the water 

 illustrated, in a very remarkable way, the fact that the 

 chief mischief of floods is occasioned where water is 

 checked in its outflow. For it is related that, " in the 

 course of their descent the waters encountered several 

 narrow gorges, and at each of these they rose to a 

 great height, and then burst with new violence into 

 the next basin, sweeping along forests, houses, bridges, 

 and cultivated land." Along the greater part of its 

 course the flood resembled rather a moving mass of 

 rock and mud than a stream of water. Enormous 

 masses of granite were torn out of the sides of the 

 valleys and whirled for hundreds of yards along the 

 course of the flood. M. Escher relates that one of the 

 fragments thus swept along was no less than sixty 

 yards in circumference. At first the water rushed 

 onward at a rate of more than a mile in three minutes, 

 and the whole distance (forty-five miles) which sepa- 

 rates the valley of Bagnes from the Lake of Geneva 



