Prof. Forel on the " Seiches of the Swiss Lakes. 449 



The formula gives A = 130 metres, a value which I therefore 

 assign as mean depth to the whole of the lake, including the 

 little lake near Geneva. Including this additional lake, 

 £=2069". 



Longitudinal seiches of Lake Brienz. 



1= 13700 2* = 574" t=2$7" 



Greatest depth 261 metres. 



Mean depth calculated 233 metres, and so on. 



In my observations, however, I have to notice two excep- 

 tions, which I hope may be accounted for by errors of observa- 

 tion. They relate to the Lakes of Wallenstadt and Zug. As 

 soon as I have time, I shall endeavour to reexamine my ex- 

 periments on these two lakes. This, dear Mr. Guthrie, forms 

 my communication. The interest which you have been good 

 enough to take in my researches, which are so closely related 

 to yours, will be my excuse for troubling you with them. If 

 you think that these formulae have an interest for the Physical 

 Society, I beg you to communicate them in my name. 



&e. &c. 



Pkofessor Forel. 



My dear Mr. Guthrie, Merges, 4th Oct. 1876. 



I kept back for some days the above letter (which I 

 wrote on the 29th of September), because I had decided to 

 verify some of the observations which did not agree with my 

 formula. I have returned to day from Wesen, where I have 

 been examining the Lake of Wallenstadt, and I am happy to 

 inform you that the question which perplexed me is now fully 

 cleared up. 



According to my former observations, the duration of the 

 seiches on the Lake of Wallenstadt was 371", the length being 

 15,000 metres. Calculations from the formula gave a mean 

 depth of 128 metres. Now the only depth given, and which 

 was considered the maximum, was 114 metres. Accordingly 

 either the formula must be defective or the real depth must be 

 greater. The latter is the case. Soundings which I took 

 yesterday in this lake showed me that there is a great basin 

 of comparatively even bottom, having at different points the 

 depths of 97, 104, 118, 128, 133, 136, 138 metres. These, as 

 you see, have a mean value which approaches much more 

 closely to the value 120, which the formula gives. 



I am happy in being able to give you this new confirmation 

 of my hypothesis, and &c. &c. H< FoRE L. 



Phil. Mag. S. 5. Vol. 2. No. 13. Dec. 1876. 2 G 



