Prof. Forel on the " Seiches " of the Swiss Lakes. 447 

 tlie axis of x, and if s be the length of the arc measured from 



rrr 



the middle point (where 0= ± -r), the intrinsic equation to the 



4: 



free surface is tan $_ -\- e -» t 



the scale of linear magnitude being such that the initial and 

 final width of the streams is tt. 



Postscript, Nov. 14. — Although the accurate determination by 

 experiment of the pressure in the interior of a stream is a matter 

 of considerable difficulty, the theoretical value of the pressure in 

 the middle of a jet issuing through a slit in a thin plate has been 

 recently verified approximately by Mr. A. Mallock in my labora- 

 tory. The fraction of the original head still remaining in the plane 

 of the plate was found to be about -59, which agrees very closely 

 with the value obtained from theory, viz. *58. For a circular hole 

 the corresponding fraction is higher, about -67; but for this there 

 is at present no theory. The observations were made by introdu- 

 cing along the axis of the jet a fine glass tube in the side of which 

 was a small hole, the interior of the tube beiug in connexion with a 

 manometer. 



LY. Notes on the " Seiches " of the Swiss Lakes. 

 By Professor Forel. 



To the Editors of the Philosophical Magazine and Journal. 



GrENTLEMEN, 



I HAVE received the following notes from Professor Forel 

 concerning the " Seiches" of the Swiss Lakes. Professor 

 Forel was so good last summer as to communicate to the Phy- 

 sical Society of London the results of his observations on these 

 phenonema, which he regards as mononodal oscillations of the 

 water affected by the depths of the lakes. The present notes 

 of Dr. Forel will be of interest to those of your readers who are 

 acquainted with that gentleman's previous observations ; and 

 they have a direct bearing upon the study of stationary liquid 

 waves, some experiments on which were published by^ me in 

 vour Magazine. 



Your obedient Servant, 



Frederick Guthrie. 



-o- 



(Professor Forel to Frederick Guthrie). 



My very honoured Colleague, Morg-es, 29th Sept. 1876. 



I have the satisfaction of being able to inform you that 



I have at last found a formula which is perfectly adapted to 



the " Seiches " in our lakes, inasmuch as it takes into account 



their length and depth. 



