136 LIGHT SCIENCE FOR LEISURE HOURS. 



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 Khone 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 ice 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 villages 

 were terrified by the danger which was to be appre- 

 hended 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 

 the danger which had been so long impending over it. 

 But as the heat of the weather increased, 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 



