Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/232

220 open in the middle of a  covered with, and exposed to the rigours of a  night, we see that the  in contact with the surface of the  must get warmed and form an ascending , its place being taken by fresh  drafted from the cold  surface, which not only cools the  but forces it out towards the middle, thus establishing a  consisting in broad lines of a surface movement from the sides to the middle of the lake, and a movement in the opposite direction below the surface. Even if the of air were not sufficient of itself to produce a surface  in the, it would do it indirectly. For, as it first strikes the at the edges, the  there would get cooled most rapidly, and under suitable circumstances would form a fringe of ; the  so cooled would be lighter than the warmer  farther out, and would have a tendency to flow off towards the middle, or with the  of air. Now, although, when compared with other, there is in a hard frosty not much , still, even in the calmest  there is almost always sufficient motion in the  to enable the  to state that the  is from a particular quarter; this will assist the  which has just been described as taking place in a calm lake, though it will somewhat distort its effects. It will produce excessive cooling at the side nearest the, and, when the lake freezes, it will have a tendency to begin at the ward side. The extent to which this affects the deeper s of a lake depends on local circumstances, and generally we may say that the more confined a lake is the more easily will it freeze, and the higher will be the mean of its s. In the very cold  – the writer was able to make observations on the  of the  under the  in  and in. In the following, which, though mild in, was excessively severe in , Dr Forel made observations in the and , confirming the writer’s observations of the unexpectedly low  of the. The freezing of so deep a lake as that of was a fortunate circumstance, because in it the bottom is actually at the  of maximum. The majority of the lakes which freeze are so shallow as to admit of the whole of their being cooled considerably below the  of maximum.

The distribution of in frozen lakes will be apparent from the table given below. Of the and and  the mean s are in the order of their depth. is altogether peculiar. Its high, which increased steadily all the time it was covered with, was due to action amongst the filth which has been allowed to accumulate at its bottom. When the broke up the dead  were taken away in carts. Dr Forel gives the following particulars about the frozen lakes. “The has a surface of 27·4 and a maximum depth of 45  (147 ); it is 1425  above the ; and its mean  is 49° 56′ N. The  overspread its whole surface suddenly in the night of the 17th to the 18th, and it remained frozen till the 8th. The has a superficies of 87·8, a maximum depth of 468  and altitude of 1338, and a mean  of 47° 16′ N. Its congelation was gradual, and not sudden like that of the. First the upper part of the lake was covered with between  and. At the end of, the 28th, the covered it entirely, but only for a single. On the 29th it thawed, and the lake remained partially free of until the middle of. It froze over completely on the 22d, and on the 25th the was 4  thick in the centre of the lake.” Of the larger  lakes, , , , , , and  were frozen in ;  is known to have been frozen four times, namely, in , , , and ;  has only once been frozen, in ;  freezes partially in very severe s, and  in its western and shallower part, whilst  and  are not known to have ever been frozen.

Table of in Frozen Lakes. For further information on the of frozen lakes, see Buchanan, Nature, 6, ; Forel, Arch. de Genève,, iv. 1; Nichols, Proc. Boston Soc. of Nat. Hist.,, xxi. p. 53.

Changes of Level.—As the supply of lakes depends on the, and as this varies much with the , and from  to , 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 supply, and which resemble s in their rhythmic periods. They have long been known and observed in, and especially on the , where they are known by the name of “seiches.” The level of the lake is observed to rise slowly during twenty or thirty to a height which varies from a few  to as many ; it then falls again slowly to a corresponding depth, and rises again slowly, and so on. These movements were observed and much studied at the by Jallabert, Bertrand, and Saussure, and at the  they formed the subject of an instructive memoir by Vaucher, who enunciated the following law connecting the seiches with the movements of the. “The amplitude of seiches is small when the is at rest; the seiches are greater the more variable is the ’s ; they are the greatest when the  is falling.” Vaucher recognized the existence of seiches in the, , , , , and , and Dr Forel of , from whose papers,  principally in the Bibliothèque Universelle et Revue Suisse during the , 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. They have been studied principally on the, where Dr Forel, at , about the middle of the lake on the north shore, and M. Plantamour, at , about a from  on the north shore, have had self-registering  s in operation for a number of s. In the writings of the  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  in  measured from the moment when the  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. The amplitude of the seiches is very variable. At the same station and on the same 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 s the amplitudes of the seiches may vary enormously. For instance, at, where the highest seiches have been observed, they are usually of such a size as to be imperceptible without special instruments; yet on the 3d  Saussure measured seiches of 1·48 , and on the 2d and 3d   the seiches observed by Vénié were as much as 2·15. 