Lakes 303 



dorf and Wadensweil. At the end of December the 28th, the 

 ice covered it entirely, but only for a single day. On the 2Qth 

 it thawed, and the lake remained partially free of ice until the 

 middle of January. It froze over completely on the 22nd 

 January, and on the 25th the ice was four inches thick in the 

 centre of the lake." Of the larger Swiss lakes, Morat, Zurich, 

 Zug, Xeuchatel, Constance, and Annecy were frozen in 1880 ; 

 Thun is known to have been frozen four times, namely in 1363, 

 1435, 1685, and 1695; Brienz has only once been frozen, in 

 1363 ; Lucerne freezes partially in very severe winters, and 

 Geneva in its western and shallower part, whilst Wallenstadt 

 and Bourget are not known to have ever been frozen. 



For further information on the temperature of frozen lakes, 

 see Buchanan, Nature, March 6, 1879; Forel, Arch, de Go 

 1880, iv. i; Nichols, Proc. Boston Soc. of Nat. Hist. 1881, 

 xxi. p. 53. 



Changes of Level. As the water supply of lakes depends on 

 the rainfall, and as this varies much with the season, and from 

 to year, we should expect, and indeed we find, fluctuation 

 of level in all lakes. There are, however, other changes of 

 level which are independent of the water supply, and which 

 resemble tides in their rhythmic periods. They have long been 

 known and observed in Switzerland, and especially on the lake 

 of Geneva, where they are known. by the name of "seiches." 

 The level of the lake is observed to rise slowly during twenty 

 >i thirty minutes to a height which varies from a few centimetres 

 to as many decimetres; it then falls again slowly to a corre- 

 sponding depth, and rises again slowly, and so on. These 

 movements were observed and much studied at the end of 

 last century by Jallabert, Bertrand, and Saussmv. and at 

 the beginning of this century they formed the subject of an 

 instructive memoir i>\ Vau< II.T. who eliminated the following 

 law connecting tin- Miches with tin- movements of the baro- 

 " The amplitude of seiches is small when t IK atmosphere 

 is at rc^ lies are greater the more v.ni.ible is the 



atmosphere's pressure; they are the greatest when the baro- 

 in. -ter i> tallinu'." Vaucher recognised the existence of sei> 

 in the Lakes of Geneva, NeuchatH /un< \\, Constant , Am. 



