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

218 is disallowed by Peschel and others. The, with the  and the , lie on the line of an extensive , and it is claimed that this depression in the surface occurred with the production of the. Further evidence in support of the statement that the was never connected with the  is of a negative character, and consists chiefly in the fact that  have not been found in the  of the  or of, and that  is absent from the  of the. A former connexion with the is claimed for a number of the and  lakes by Dr Forel and Professor Pavesi, and the  lakes by Loven and Sars, on the ground of the occurrence of  of the  and other classes. For a summarized account of these researches see Pavesi, Arch. de Genève, 1880, iii. 1.

of Lakes.—The earliest reliable observations in lakes or s 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 of the lakes. He used for this purpose an ordinary whose bulb was covered over with several thicknesses of  and, so as to render it very slowly conducting. He was in the habit of leaving it down fourteen, and then bringing it up as quickly as possible and immediately reading the. He did not, however, trust to his not changing its reading while being brought up, but by an elaborate series of experiments he obtained corrections, to be applied when the  had to be drawn through more or less  of higher. His observations are collected in the following table along with those of Jardine in some of the lakes, at the  :—

An exceedingly important and valuable series of observations was made by Fischer and Brunner in the throughout the course of a whole (  to  ). They used, after Saussure’s method, s protected by non-conducting envelopes, which were pulled up as quickly as possible. The depth of the where they observed was 540, and they made a series of observations of the  at that depth, at the surface, and at eleven intermediate depths, and repeated the series of observations at eight different dates over the. From these series, which afford the first information of the ly march of at different depths, we learn that the lake as a whole gains  till the end of, then loses it until the  of , when it begins to warm again, though slowly. The maximum occurs in  at depths from the surface to 70, in  at depths from 70 to 120 , in  from 120 to 200 , and in  at 500. As the whole ly variation of the at 200  is less than a, the epoch at which the greater depths attain their maximum and minimum s cannot be certainly deduced from one ’s observations. The minimum of depths from the surface to 80  is attained in the  of, at greater depths in the  of. During the course of the whole the  at the bottom varied between 40°·7 and 40°·9, and in the  of  the whole of the  from the surface to the bottom was between 40°·7 and 41° These and other observations showed that, from depths of 400, the variation of with increasing depth is quite insignificant, so that even though the lake might be 1000 deep the  at 400  is only one or two tenths of a  different from that of the bottom; further, on many of the s recently used, it is impossible to distinguish with certainty s differing by less than half a, consequently it was not difficult to believe that in all deep lakes there is a considerable stratum of  which remains constantly at the same , all the  and every , and that in  this stratum thickens so as often to fill the lake, and gets thinner again in. By the improvement of the instruments both of these suppositions have been shown to be erroneous. In and in , however deep the lake may be, its  falls as the depth increases, first rapidly and then very slowly, and the bottom  observed in any  depends on the nature of the  which preceded it, and may vary from  to  by one to two s. It was also believed that the deep  of a lake preserved constantly the mean   or the mean  of the six coldest s of the  in the locality. This was deduced from some observations by Sir Robert Christison in, who found the bottom at  to be 41°·4 , agreeing with the mean of the six  s as observed at , which, however, is about 15 s distant. Although the theorem may be accidentally true for, it has been proved not to hold for other lakes. Thus Simony (Wien. Sitz. Ber.,, lxxi. p. 435) gives the following table, comparing the of the bottom  in the  with the  ( to )  : —

It will be seen that, with the exception of the end of, the mean is below that of the bottom, and generally very markedly so. During – observations have been made by the present writer on the distribution of in lakes forming part of the. The ly mean s at and at , which cannot differ much in  from  and  respectively, have been supplied by Mr Buchan of the   Society. The bottom s are those observed in the deepest part of the lakes, namely, 120 s in, and 80 s in. The connexion between bottom 