300 Lakes 



raised. For, in virtue of the cooling above, the water will 

 have no tendency to sink; it will rather tend to float as a cold 

 laver on the surface of the warmer and denser water below. 

 \\ re a lake comparable with a glass of water, 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 ice, namely, that after the 

 \v iter has all been cooled to a uniform temperature of 39-2 Fahr. 

 further cooling affects only a small surface layer, which con- 

 sequently rapidly freezes. If this were the case, we should 

 expect to find the temperature of the water below the ice of a 

 frozen lake increasing rapidly from 32 where it is in contact 

 with the ice to 39-2 at a short distance from it, and we should 

 expect to find the remainder of the water down to the bottom 

 at the same temperature. In fact, however, the depth of even 

 the deepest lakes bears an insignificant proportion to their 

 superficial dimensions, and temperature observations in summer 

 show that the effective climate, that is, the climate 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 temperature produce variations in density 

 which of themselves are sufficient to produce convection 

 currents. Then, as a factor of climate, there are the winds, 

 which are the main mixing agents, and also the movement in 

 the waters caused by the inflow of water at different points and 

 the removal of the excess at one point. The effect of these 

 mechanical agents, winds and currents, is to propagate the air 

 temperature 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 wind. If we 

 reflect, however, on what must take place when there is a 

 large expanse of 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 



