Lakes 301 



the middle, thus establishing a circulation consisting in broad 

 lines of a surface movement from the sides to the middle of the 

 lake, and a movement in the opposite direction below the 

 surface. Even if the current of air were not sufficient of itself 

 to produce a surface current in the water, it would do it in- 

 directly. For, as it first strikes the water at the edges, the 

 water would get cooled most rapidly, and under suitable circum- 

 stances would form a fringe of ice ; the water so cooled would be 

 lighter than the warmer water farther out, and would have a 

 tendency to flow off towards the middle, or with the current 

 of air. Now, although, when compared with other seasons, 

 there is in a hard frosty winter not much wind, still, even in 

 the calmest weather there is almost always sufficient motion 

 in the atmosphere to enable the meteorologist to state that 

 the wind is from a particular quarter; this will assist the 

 circulation which has just been described as taking place in a 

 calm lake, though it will somewhat distort its effects. It will 

 produce excessive cooling at the side nearest the wind, and, 

 when the lake freezes, it will have a tendency to begin at the 

 windward side. 



The extent to which this circulation affects the deeper 

 waters of a lake depends on local circumstances, and generally 

 we may say that the more confined a lake is the more easily 

 will it freeze, and the higher will be the mean temperature of 

 it- waters. In the very cold winter 1878-79 the writer was 

 able to make observations on the temperature of the water 

 under the ice in Linlithgow Loch and in Loch Lomond. In 

 the following winter, which, though mild in Scotland, was 

 excessively severe in Swit/rrland, Dr Forel made observations 

 in the Lakes of Morat ;md Xinich. continuing the writ 

 observations of the unexpectedly low temperature of the 

 water. The freezing of so deep a lake as that of /uric h was a 

 fortunate cirnmManre, because in it the bottom is actually at 

 the temperature of maximum density. The majority of the 

 whi< li freeze are so ^hallow as to admit of the whole of 

 their water being cooled considerably below the temperature 

 ximuni 



lion of temperature in !i/,n lak - will be 



