Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/231

Rh (as observed in the second of ) and   can be judged of from the following table, where the mean s of  to, and also of  to , are given:—

From this table it is apparent that the bottom, even of lakes as deep as, is subject to considerable variation from to , that it depends on the of the previous , and that it is usually higher than that. The difference between the bottom and the mean   is greater the lower the   is. It is further interesting to notice that the mean  of – was about one  higher than that of –, yet the bottom s were 0°·25 lower in  than in, and this is no doubt due to the fact that the cold of – was more continuous than that of –, when the actual s observed were much lower. The of the bottom  depends not only on the  of the previous, and on the depth of the lake; it also depends on the nature of the  where it lies, and especially on its exposure to s. s drive the surface  before them, and if there were no return  it would be heaped up at the further end. The effect is to accumulate surface at one end, and to draw on deeper  to make up the deficiency at the other end. Hence the prevailing direction of the impresses itself on the distribution of  in the ; and this is well shown in the distribution of  as determined from observations at five stations on the same day in  in a  after a warm, and in one after a cold. In, warm is associated with southerly and westerly s, and cold  with northerly and easterly s. In the warm s we have accumulation of surface  at the north-eastern end, and of bottom  at the south-western end, producing in  a higher mean  of  at the north-east, and a lower mean  of  at the south-west end. In cold s the reverse is observed. Thus in, after a cold , the mean of the first 300  of  at the south-west end of  was 48°·8, and at the north-east end 44°·96, a difference of nearly four. In, after a comparatively mild , it was 48°·13 at the south-west end, and 47°·95 at the north-east end, or nearly identical s. Even at stations a few hundred from each other, great differences are often observed in the s observed at the same depth, and it is evident that the difference of  so produced must cause a certain amount of. There can be but little doubt that, under the influence of the varying of the, and of the , the  of a lake is thoroughly mixed once a. In lakes which do not consist of a single long trough like, but of several basins as , the bottom is different in the different basins, even when the depth is the same. consists of three principal basins of very unequal depth:—the large expanse of studded with  at the lower end, the Balloch basin; the middle or Luss basin; and the upper and deepest or Tarbet basin. In the last we have 600 of, in the Luss basin 200 , and in the Balloch basin a maximum of 72  of. On 23d  the bottom  in the Tarbet basin was 41°·4, and in the Luss basin 46°·4. , a much smaller lake, consists of three basins, each of them being from 100 to 120 deep, and in them we have bottom s of 46°·3, 46°·9, and 45°·2, the lowest  being nearest the outlet. It might have been expected that the bottom in lakes similar as regards size and depth would be lower at greater elevations and higher nearer the -level. This does not, however, hold universally; thus and  are very similar in size and depth; they are only 12 s from each other, but  is 450  and  1330  above the ; yet at 102  in  the  on the 18th   was 53°·9, and in  at the same depth on the 16th  1876 it was 45°·4. The difference of is nearly 900, and, instead of the higher lake holding the colder, its  is 8°·5 warmer than that of the lower one. Similarly in, 1153 above the , the bottom  at 324  was 44°·7, and in , 668  above , at the same depth it was 44°·0. These examples will suffice to show that many circumstances concur in determining the s of the s of lakes. There is one factor which is often neglected, namely, the amount of change of. This depends on the area of its , and necessarily varies greatly. In comparing the bottom in lakes with the mean s of the coldest half of the, we find that the two approach each other more nearly the higher these s are. When the of the  falls for a lengthened period below the  of maximum  of  (39°·2 ), then the mechanical effect produced is much the same as if the  had been raised. For, in virtue of the cooling above, the will have no tendency to sink; it will rather tend to float as a cold layer on the surface of the warmer and denser  below. Were a lake comparable with a glass of, that is, were its depth equal to or greater than its length or breadth, it would be possible to realize this ideal condition of things, which, until recently, was supposed to represent what really takes place when a lake is covered with , namely, that after the has all been cooled to a uniform  of 39°·2  further cooling affects only a small surface layer, which consequently rapidly freezes. If this were the case, we should expect to find the of the  below the  of a frozen lake increasing rapidly from 32° where it is in contact with the  to 39°·2 at a short distance from it, and we should expect to find the remainder of the  down to the bottom at the same. In fact, however, the depth of even the deepest lakes bears an insignificant proportion to their superficial dimensions, and observations in  show that the effective, that is, the  in so far as it is effective for the purpose under consideration, varies much over the surface of even very small lakes. The variations in distribution of produce variations in  which of themselves are sufficient to produce. Then, as a factor of, there are the s, which are the main mixing agents, and also the movement in the s caused by the inflow of at different points and the removal of the excess at one point. The effect of these mechanical agents, s and, is to propagate the  at the surface to a greater depth than would otherwise be the case. At the same time it must be remembered that in seasons of great cold there is rarely much. If we reflect, however, on what must take place when there is a large expanse of 