Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/230

is disallowed by Peschel and others. The Jordan valley, with the Sea of Tiberias and the Dead Sea, lie on the line of an extensive fault, and it is claimed that this depression in the surface occurred with the production of the fault. Further evidence in support of the statement that the Dead Sea was never connected with the sea is of a negative character, and consists chiefly in the fact that marine forms have not been found in the waters of the Jordan or of Lake Tiberias, and that silver is absent from the waters of the Dead Sea.

A former connexion with the ocean is claimed for a number of the Swiss and Italian lakes by Dr Forel and Professor Pavesi, and the Norwegian lakes by Loven and Sars, on the ground of the occurrence of marine forms of the crustaceans and other classes. For a summarized account of these researches see Pavesi, Arch. de Genève, 1880, iii. 1.

Temperature of Lakes.—The earliest reliable temperature observations in lakes or seas are those of Saussure, and they are to be found in his charming Voyage dans les Alpes. He was the first to obtain thoroughly trustworthy observations in the deeper waters of the lakes. He used for this purpose an ordinary thermometer whose bulb was covered over with several thicknesses of cloth and wax, so as to render it very slowly conducting. He was in the habit of leaving it down fourteen hours, and then bringing it up as quickly as possible and immediately reading the temperature. He did not, however, trust to his thermometer not changing its reading while being brought up, but by an elaborate series of experiments he obtained corrections, to be applied when the thermometer had to be drawn through more or less water of higher temperature. His observations are collected in the following table along with those of Jardine in some of the Scottish lakes, at the beginning of the century:—

Name of Lake.		Date.	Temperature of		Depth.	Height above Sea.
			Surface.	Bottom.
			° F.	° F.	Feet.	Feet.
Geneva		February.	42·1	41·6	1,013	1,230
Neuchâtel		17th July.	73·7	41·4	346	1,304
Bourget		October 1784.	64·0	42·1	256	...
Annecy		14th May 1780.	57·9	42·1	174	1,426
Joux		...	55·6	51·3	85	350
Bienne		...	69·3	44·4	231	1,419
Constance		25th July 1784.	64·6	39·6	394	1,250
Lucerne		28th July.	68·4	40·8	640	1,380
Thun		7th July 1783.	66·2	41·0	373	1,896
Brienz		8th July 1783.	68·0	40·5	533	...
Maggiore		19th July 1783.	78·1	44·1	357	...
Lomond		8th Sept.	59·5	41·5	600	25
Katrine	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	7th Sept. 1812.	57·3	41·0	480	364
		3d Sept. 1812.	56·4	41·3	...	...

An exceedingly important and valuable series of observations was made by Fischer and Brunner^[1] in the Lake of Thun throughout the course of a whole year (March 1848 to February 1849). They used, after Saussure’s method, thermometers protected by non-conducting envelopes, which were pulled up as quickly as possible. The depth of the water where they observed was 540 feet, and they made a series of observations of the temperature at that depth, at the surface, and at eleven intermediate depths, and repeated the series of observations at eight different dates over the year. From these series, which afford the first information of the yearly march of temperature at different depths, we learn that the lake as a whole gains heat till the end of September, then loses it until the month of February, when it begins to warm again, though slowly. The maximum temperature occurs in October at depths from the surface to 70 feet, in November at depths from 70 to 120 feet, in December from 120 to 200 feet, and in February at 500 feet. As the whole yearly variation of the temperature at 200 feet is less than a degree, the epoch at which the greater depths attain their maximum and minimum temperatures cannot be certainly deduced from one year’s observations. The minimum temperature of depths from the surface to 80 feet is attained in the month of February, at greater depths in the month of March. During the course of the whole year the temperature at the bottom varied between 40°·7 and 40°·9 Fahr., and in the month of February the whole of the water from the surface to the bottom was between 40°·7 and 41° Fahr.

These and other observations showed that, from depths of 400 feet, the variation of temperature with increasing depth is quite insignificant, so that even though the lake might be 1000 feet deep the temperature at 400 feet is only one or two tenths of a degree different from that of the bottom; further, on many of the thermometers recently used, it is impossible to distinguish with certainty temperatures differing by less than half a degree, consequently it was not difficult to believe that in all deep lakes there is a considerable stratum of water which remains constantly at the same temperature, all the year and every year, and that in winter this stratum thickens so as often to fill the lake, and gets thinner again in summer. By the improvement of the instruments both of these suppositions have been shown to be erroneous. In summer and in temperate latitudes, however deep the lake may be, its temperature falls as the depth increases, first rapidly and then very slowly, and the bottom temperature observed in any summer depends on the nature of the winter which preceded it, and may vary from year to year by one to two degrees. It was also believed that the deep water of a lake preserved constantly the mean winter temperature or the mean temperature of the six coldest months of the year in the locality. This was deduced from some observations by Sir Robert Christison in Loch Lomond, who found the bottom temperature at Tarbet to be 41°·4 Fahr., agreeing with the mean of the six winter months as observed at Balloch Castle, which, however, is about 15 miles distant. Although the theorem may be accidentally true for Loch Lomond, it has been proved not to hold for other lakes. Thus Simony (Wien. Sitz. Ber., 1875, lxxi. p. 435) gives the following table, comparing the temperature of the bottom water in the Gmünder See with the winter (October to March) air temperature:^[2]—

	Winter Period. Mean Temperature.			Summer Period. Mean Temp.	Bottom Temp., Gmünder See.	Date of Observation of Bottom Temperature.
	Oct.–Mar.	Dec.–Feb.
	° F.	° F.		° F.	° F.
1867–68	37·5	32·9	1868	64·4	40·5	6th Oct. 1868.
1868–89	40·1	36·8	1869	63·1	40·5	1st Oct. 1869.
1869–70	35·0	29·3	1870	60·8	40·2	26th Sept. 1870.
1871–72	35·2	27·8	1872	62·2	40·0	3d Oct. 1872.
1872–73	41·0	35·0	1873	60·2	40·5	5th Oct. 1873.
1873–74	39·0	32·7	1874	61·9	40·4	25th Sept. 1874.
1874–75	33·8	28·2	...	...	39·1	10th April 1875.

It will be seen that, with the exception of the end of 1872, the mean winter temperature is below that of the bottom water, and generally very markedly so.

During 1877–81 observations have been made by the present writer on the distribution of temperature in lakes forming part of the Caledonian Canal. The monthly mean temperatures at Culloden and at Corran Ferry lighthouse, which cannot differ much in climate from Loch Ness and Loch Lochy respectively, have been supplied by Mr Buchan of the Scottish Meteorological Society. The bottom temperatures are those observed in the deepest part of the lakes, namely, 120 fathoms in Loch Ness, and 80 fathoms in Loch Lochy. The connexion between bottom temperature

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1. Mém. Soc. Phys. Genève, xii. p. 255.
2. These air temperatures are those of the observatory at Vienna, corrected for difference of level.