298 Lakes 



that of 1880-81, when the actual temperatures observed were 

 much lower. The temperature of the bottom water depends 

 not only on the temperature of the previous winter, and on the 

 depth of the lake ; it also depends on the nature of the country 

 where it lies, and especially on its exposure to winds. Winds 

 drive the surface water before them, and if there were no return 

 current it would be heaped up at the further end. The effect 

 is to accumulate surface water at one end, and to draw on deeper 

 water to make up the deficiency at the other end. Hence the 

 prevailing direction of the wind impresses itself on the distri- 

 bution of temperature in the water ; and this is well shown in 

 the distribution of temperature as determined from observa- 

 tions at five stations on the same day in Loch Ness in a summer 

 after a warm winter, and in one after a cold winter. In 

 Scotland, warm weather is associated with southerly and 

 westerly winds, and cold weather with northerly and easterly 

 winds. In the warm years we have accumulation of surface 

 water at the north-eastern end, and of bottom water at the 

 south-western end, producing in summer a higher mean tem- 

 perature of water at the north-east, and a lower mean tem- 

 perature of water at the south-west end. In cold years the 

 reverse is observed. Thus in 1879, after a cold winter, the 

 mean temperature of the first 300 feet of water at the south- 

 west end of Loch Ness was 48-8, and at the north-east end 

 44-96, a difference of nearly four degrees. In 1880, after a 

 comparatively mild winter, it was 48-13 at the south-west 

 end, and 47-95 at the north-east end, or nearly identical 

 temperatures. Even at stations a few hundred yards from 

 each other, great differences are often observed in the tem- 

 peratures observed at the same depth, and it is evident that 

 the difference of density so produced must cause a certain 

 amount of circulation. There can be but little doubt that, 

 under the influence of the varying temperature of the seasons, 

 and of the winds, the water of a lake is thoroughly mixed once 

 a year. In lakes which do not consist of a single long trough 

 like Loch Ness, but of several basins as Loch Lomond, the 

 bottom temperature is different in the different basins, even 

 when the depth is the same. Loch Lomond consists of three 



