138 LIGHT SCIENCE FOR LEISURE HOURS. 



heavy rains fall upon the less elevated portions of the 

 Alpine snow, they not only melt that snow much more 

 rapidly than the summer sun would do, but they wash 

 down large masses, which add largely to the destruc- 

 tive powef of the descending waters. 



The most destructive floods which have occurred in 

 Switzerland have usually been those which take place 

 in early summer. The floods which inundated the 

 plains of Martigny in 1818 were a remarkable instance 

 of the effects which result from the natural damming 

 up of large volumes of water in the upper parts of the 

 Alpine hill-country. The whole of the valley of 

 Bagnes, one of the largest of the lateral branches of the 

 main valley of the Rhone above Geneva, was converted 

 into a lake, in the spring of 1818, by the damming up 

 of a narrow pass into which avalanches of snow and 

 ice had been precipitated from a lofty glacier over- 

 hanging the bed of the river Dranse. The icy barrier 

 enclosed a lake no less than half a league in length 

 and an eighth of a mile wide, and in places two hundred 

 feet deep. The inhabitants of the neighbouring vil- 

 lages were terrified by the danger which was to be 

 apprehended from the bursting of the barrier. They 

 cut a gallery seven hundred feet long through the ice, 

 while the waters had as yet risen to but a moderate 

 height ; and when the waters began to flow through 

 this channel, its course was deepened by the melting of 

 the ice, and at length nearly half the contents of the 

 lake were safely carried off. It was hoped that the 

 process would continue, and the country be saved from 



