Nichols.] 
64 
[October 6, 
greatest density of the water is not wholly true, because in many 
deep lakes the temperature of the water at the bottom lies appre- 
ciably above that of the greatest density. This has been shown 
conclusively by various observers, Simony, Fischer-Ooster and 
C. Brunner, Forel, and others. For instance, Fischer-Ooster and 
C. Brunner, 1 in their observations on Lake Thun, found that at a 
depth of four hundred and fifty (Swiss) feet, the temperature 
varies during the year only between 4°.82 C. and 4°.95 C., not 
reaching, however, the temperature of the greatest density. 
Forel 2 has shown that in the Lake of Geneva the temperature, 
even at a depth of three hundred meters, and during the severe 
winter of 1879-80, did not fall below 5° C., and that it is extremely 
doubtful that the water of the open lake has ever reached 4° C. 
The temperature, however, below one hundred and sixty meters 
varies only a few tenths of a degree from summer to winter. 
If a lake has a sufficient depth so that a temperature may be 
reached which varies only to the extent of a few tenths of a 
degree, the depth at which such a temperature may be reached, 
as well as the temperature at the bottom, must depend upon the 
locality and upon other conditions. Le Conte found 8 a tempera- 
ture of 4° C. in August, 1873, at the bottom of Lake Tahoe (Lat. 
39° N., Long. 120° W.) in a depth of fifteen hundred and six feet 
(four hundred and fifty-nine meters), but at a depth of seven hun- 
dred and seventy-two feet (two hundred and thirty-five meters) 
the temperature was 5° C., and from this point arose until it 
reached 19°.44 at the surface. 
In the case of our great lakes, the water of which is cool, even 
in summer, and which are in some winters covered to a consider- 
erable extent with ice, it is probable that in summer the bottom 
water is generally that of the greatest density. To what extent 
the water of the open lake is regularly or occasionally cooled, I 
cannot say, as I have not been able to obtain any observations 
made in winter. Indeed the only reliable observations with the 
details of which I am acquainted are those made by the U. S. 
!Mem. Soc. Phys. de Gen&ve, xn (1849), pp. 255-276. 
2 Bibl. Univ., Arcli. Science, in (1880), p. 505. 
3 Overland Monthly, xi (1873), p. 475. 
