224 FKAGMENTS OF SCIENCE. 



The annexed sketch of a model (p. 225) will enable 

 the reader to grasp the essential features of the prob- 

 lem and its solution. Glen Gluoy and Glen Eoy are 

 lateral valleys which open into Glen Spean. Let us 

 suppose Glen Spean filled from v to w with ice of a uni- 

 form elevation of 1,500 feet above the sea, the ice not 

 filling the upper part of that glen. The ice would 

 thrust itself for some distance up the lateral valleys, 

 closing all their mouths. The streams from the moun- 

 tains right and left of Glen Gluoy would pour their 

 waters into that glen, forming a lake, the level of 

 which would be determined by the height of the col at 



A, 1,170 feet above the sea. Over this col the water 

 would flow into Glen Eoy. But in Glen Eoy it could 

 not rise higher than 1,150 feet, the height of the col at 



B, over which it would flow into Glen Spey. 



The water halting at these levels for a sufficient 

 time, would form the single road in Glen Gluoy and the 

 highest road in Glen Eoy. This state of things would 

 continue as long as the ice dam was sufficiently high 

 to dominate the cols at A and B; but when through 

 change of climate the gradually sinking dam reached, 

 in succession, the levels of these cols, the water would 

 then begin to flow over the dam instead of over the 

 cols. Let us suppose the wasting of the ice to con- 

 tinue until a connection was established between Glen 

 Eoy and Glen Glaster, a common lake would then fill 

 both these glens, the level of which would be deter- 

 mined by that of the col c, over which the water would 

 pour for an indefinite period into Glen Spean. Dur- 

 ing this period the second Glen Eoy road and the high- 

 est road of Glen Glaster would be formed. The ice 

 subsiding still further, a connection would eventually 

 be established between Glen Eoy, Glen Glaster, and 

 the upper part of Glen Spean. A common lake would 



