36 



LAKES OF NORTH AMERICA. 



way accurate measurements of temperature at various depths have been 

 made in a number of lakes, both in America and in Europe, with remark- 

 ably consistent results. Of the observations thus far made in this country, 

 the most instructive are by Professor John Le Conte,i in Lake Tahoe, 

 California. From the report of these observations I quote the following : 



"These experiments were executed between the 11th and 18th of August, 1S73. 

 The same general results were obtained in all parts of the lake. The following table 

 contains an abstract of the average results, after correcting the thermometric indications 

 by comparison with a standard thermometer : 



"It will be seen from the foregoing numbers that the temperature of the water 

 decreases with increasing depth to about 700 or 800 feet (213 or 244 meters), and 

 below this depth it remains sensibly the same down to 1,506 feet (459 meters). This 

 constant temperature which prevails at all depths below say 250 meters is about 

 4° C. (39.2° Fahr.). This is precisely what might have been exjiected ; for it is a 

 well-established jihysical jiroperty of fresh water, that it attains its maximum density at 

 the above-indicated temperature. In other words, a mass of fresh water at the tempera- 

 ture of 4° C. has a greater weight under a given volume (that is, a cubic incli unit 

 of it is heavier at this temperature) than it is at any temperature either higher or 

 lower. Hence, when the ice-cold water of the snow-fed streams of spring and summer 

 reaches the lake, it naturally tends to sink as soon as its temperature rises to 4° C. ; 

 and, conversely, when winter sets in, as soon as the summer-heated surface-water is 



1" Physical Studies of Lake Tahoe," Overland Monthly, 2d Series, vol. 2, 1883, pp. 

 500-510, 595-612 ; vol. 3, 1894, pp. 41-46. 



