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

open water in the middle of a country covered with snow, and exposed to the rigours of a winter night, we see that the air in contact with the surface of the water must get warmed and form an ascending current, its place being taken by fresh air drafted from the cold land surface, which not only cools the water but forces it out towards the middle, thus establishing a circulation 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 current of air were not sufficient of itself to produce a surface current in the water, it would do it indirectly. For, as it first strikes the water at the edges, the water there would get cooled most rapidly, and under suitable circumstances would form a fringe of ice; the water so cooled would be lighter than the warmer water farther out, and would have a tendency to flow off towards the middle, or with the current of air. Now, although, when compared with other seasons, there is in a hard frosty winter not much wind, still, even in the calmest weather there is almost always sufficient motion in the atmosphere to enable the meteorologist to state that the wind is from a particular quarter; this will assist the circulation 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 wind, and, when the lake freezes, it will have a tendency to begin at the windward side.

The extent to which this circulation affects the deeper waters 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 temperature of its waters. In the very cold winter 1878–79 the writer was able to make observations on the temperature of the water under the ice in Linlithgow Loch and in Loch Lomond. In the following winter, which, though mild in Scotland, was excessively severe in Switzerland, Dr Forel made observations in the Lakes of Morat and Zürich, confirming the writer’s observations of the unexpectedly low temperature of the water. The freezing of so deep a lake as that of Zürich was a fortunate circumstance, because in it the bottom is actually at the temperature of maximum density. The majority of the lakes which freeze are so shallow as to admit of the whole of their water being cooled considerably below the temperature of maximum density.

The distribution of temperature in frozen lakes will be apparent from the table given below. Of the Lakes of Zürich and Morat and Loch Lomond the mean temperatures are in the order of their depth. Linlithgow is altogether peculiar. Its high temperature, which increased steadily all the time it was covered with ice, was due to chemical action amongst the filth which has been allowed to accumulate at its bottom. When the ice broke up the dead fish were taken away in carts.

Dr Forel gives the following particulars about the frozen Swiss lakes. “The Lake of Morat has a surface of 27·4 square kilometres and a maximum depth of 45 metres (147 feet); it is 1425 feet above the sea; and its mean latitude is 49° 56′ N. The ice overspread its whole surface suddenly in the night of the 17th to the 18th December, and it remained frozen till the 8th March. The Lake of Zürich has a superficies of 87·8 square kilometres, a maximum depth of 468 feet and altitude of 1338 feet, and a mean latitude of 47° 16′ N. Its congelation was gradual, and not sudden like that of the Lake of Morat. First the upper part of the lake was covered with ice between Männedorf and Wädensweil. At the end of December, the 28th, the ice covered it entirely, but only for a single day. On the 29th it thawed, and the lake remained partially free of ice until the middle of January. It froze over completely on the 22d January, and on the 25th the ice was 4 inches thick in the centre of the lake.” Of the larger Swiss lakes, Morat, Zürich, Zug, Neuchâtel, 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.

Table of Temperatures in Frozen Lakes.

Depth (in feet).		Temperature in Degrees Fahr.
		Zürich, 25th Jan. 1880.	Morat, 23d Dec. 1879.	Lomond, 29th Jan. 1879.	Linlithgow.
					11th Jan. 1879.	25th Jan. 1879.
	3	...	...	33·00	35·90	36·00
	6	...	...	33·50	36·30	36·80
	18	...	35·06	33·95	36·90	37·80
(Bottom)	48	36·95	36·14	35·20	39·85	42·05
(Bottom)	65	37·25	36·30	36·30	...	...
	100	37·76	36·68	...	...	...
(Bottom)	150	38·39	37·04	...	...	...
	200	38·66	...	...	...	...
	300	38·84	...	...	...	...
(Bottom)	435	39·20	...	...	...	...
Mean		38·40	36·00	34·46	37·22	38·28

For further information on the temperature of frozen lakes, see Buchanan, Nature, March 6, 1879; Forel, Arch. de Genève, 1880, iv. 1; 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 year 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 or thirty minutes to a height which varies from a few centimetres to as many decimetres; 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 end of last century by Jallabert, Bertrand, and Saussure, and at the beginning of this century they formed the subject of an instructive memoir by Vaucher, who enunciated the following law connecting the seiches with the movements of the barometer. “The amplitude of seiches is small when the atmosphere is at rest; the seiches are greater the more variable is the atmosphere’s pressure; they are the greatest when the barometer is falling.” Vaucher recognized the existence of seiches in the Lakes of Geneva, Neuchâtel, Zürich, Constance, Annecy, and Lugano, and Dr Forel of Morges, from whose papers, published principally in the Bibliothèque Universelle et Revue Suisse 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. They 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 Sécheron, 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 enormously. For instance, at Geneva, 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 August 1763 Saussure measured seiches of 1·48 metres, and on the 2d and 3d October 1811 the seiches observed by Vénié were as much as 2·15 metres.