1907.] 



on Seiches in the Lakes of Scotland. 



667 



Essai Theorique sur les Seiches, an approximate method for calculating 

 the periods of a seiche. He treats the seiche as the interference of 

 two solitary waves travelling backwards and forwards in the lake, the 

 velocity of propagation being at each section that due to the greatest 

 depth there. He thus arrives at the formula 



T. 





clx 



JigK) 



J^i- dx . 

 ,^— -- simply means the time that a man would take 

 « V C^ ^) 

 to travel from one end of the lake to the other along the line of 

 greatest depth, his speed at each point being that which a stone would 

 have after it has fallen from rest through a distance equal to half the 

 depth at that point. 



This formula is exact for a lake of uniform breadth and depth, 

 but errs in excess for a lake having a concave, and in defect for a 

 lake having a convex bottom. But the approximation gets better as 

 the nodality rises ; and, for that and other reasons, his rule is very 

 useful in limnographic calculations. 



Results of Seiche Observations, and Comparison with Theory. — I 

 propose next to give you a few results, selected here and there from 

 various seiche observers. In the first place, I show you a table 

 of the periods (in minutes) of some foreign lakes. As you see, they 

 vary greatly — from the 14 hours' period of the fair weather seiches of 

 Erie, which is 250 miles long, to the 14 seconds' period of the seiches 

 observed by Endros in a pond about 120 yards long. 



Some Foreign Lakes. 



The next table shows the periods found for such of the Scottish 

 lakes as have hitherto been examined. Except for Treig, Xess, Earn 

 and Tay, the determinations are very rough. 



