A LIMNOLOGICAL RECONNAISSANCE 19 



fore heavier water below. The wind causes both waves and 

 currents ; the former efficient in mixing the surface strata ; 

 the currents piling up the warmer water on the leeward side 

 of the lake, mixing it with the colder water below it and in 

 spring even setting the whole mass of water in the lake into 

 a sort of rotation. Thus the warmer and lighter water is 

 forced downward and mixed with the lower and colder 

 strata and heat is thereby conveyed from the surface into 

 the lake for a greater or less distance. Such an operation 

 clearly involves work against gravitation. 



We shall see that direct insolation is confined to the upper 

 water and that it is quite inappreciable below the depth of 

 ten meters. The bottom water therefore, which at thirty- 

 five to forty meters may have the temperature of 10.8°, 

 owes all heat above 4° to the action of the wind. How much 

 work is involved in such warming? The process has not 

 been done at once, but in numerous stages, and the net 

 amount of work may be represented by the amount of en- 

 ergy necessary to push down through water at its maximum 

 density a stratum of water of the smaller density possessed 

 by water at 10.8°. If the density of water at 4° is taken as 

 1.000,000, that of water at 10.8° is 0.99652; one liter has 

 lost by warming 348 mg. in weight. For each liter there- 

 fore of warmed water carried from the surface to the 35 

 m.-40 m. stratum a weight of 348 mg. must be moved a 

 mean distance of 37.5 meters. If the volume of the stratum 

 is known, the amount of work can easily be computed. 



There is, however, a more convenient method of computa- 

 tion. Since the heat is expressed in calories per square 

 centimeter of the surface the work needed to distribute it 

 is best expressed in the same way. Then the volume of a 

 stratum is represented by its reduced thickness stated in 

 centimeters, which is equivalent to the weight in grams of 

 a column of water whose base is a square centimeter and 

 whose height is the reduced thickness of the stratum. The 

 formula for computing the work is 



RT -Dm -(1.000,000-0) 

 in which RT is the reduced thickness of the stratum stated 

 in centimeters and therefore equal to the v.^eight in grams 



