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Lakes 



and Lugano, and Dr Forel of Morges, from whose papers, 

 published principally in the Bibliotheque Universelle et Rente 

 > during the last five years, the facts regarding the seiches 

 have been taken, has observed them in every lake where he 

 had looked for them. It is in every way likely that they are 

 to be found in all lakes of notable extent and depth. Tlu \ 

 have been studied principally on the Lake of Geneva, where 

 Dr Forel, at Morges, about the middle of the lake on the north 

 shore, and M. Plantamour, at Secheron, about a mile from 

 Geneva on the north shore, have had self-registering tide 

 gauges in operation for a number of years. In the writings of 

 the Swiss observers the seiche is the complete movement of 

 rise above and fall below the mean level, the amplitude is the 

 extreme difference of level so produced, and the duration of the 

 seiche is the time in seconds measured from the moment when 

 the water is at the mean level until it is again at the mean 

 level, after having risen to the crest and sunk to the trough of 

 the wave. The amplitude of the seiches is very variable. At 

 the same station and on the same day successive seiches are 

 similar. When the seiches are small they are all small, when 

 they are large they are all large. At the same station and on 

 different days the amplitudes of the seiches may vary enor- 

 mously. For instance, at Geneva, where the higher seiches 

 have been observed, they are usually of such a size as to 

 be imperceptible without special instruments; yet on the 

 3rd August 1763 Saussure measured seiches of 1-48 metres, and 

 on the 2nd and 3rd October 1841 the seiches observed by 

 Venie were as much as 2-15 metres. They are greater at the 

 extremities than at the middle of lakes, at the head of long gulfs 

 whose sides converge gently than at stations in the middle of 

 a long straight coast, and in shallow as compared with deep 

 lakes or parts of a lake. They also appear to increase with 

 the size of the lake. The duration of the seiches is found to 

 vary considerably, but the mean deduced from a sufficient 

 number of observations is fairly constant at the same locality. 

 Thus, for Morges, Dr Forel has found it to be for the half 

 seiche 315 9 seconds. At different stations, however, on 

 the same lake and on different lakes it varies considerably. 



