990 Wisconsin Academy of Sciences, Arts, and Letters . 
foundation of all considerations on the distribution of heat in 
lakes. It is an equally well known fact that the decrease in 
density corresponding to one degree increase of temperature 
is not constant but increases as the temperature departs from 
4°, either toward 0° or toward a higher temperature. Gen¬ 
eral references to this fact have been made by writers on lake 
temperatures, but no one called especial attention to it, so far 
as I am aware, until Groll (’05) showed its application in the 
production of convection currents. I desire to apply the same 
fact in the reverse direction and to show its relation to the 
thermal resistance offered by the warmer upper strata of the 
lake to the distribution through its mass of the heat received 
by its surface. Convection currents are far less important 
agents for distributing heat than are mechanical currents 
caused by wind. Indeed, it would be difficult to show that 
convection currents have any such efficiency in carrying heat 
as to make them worth serious consideration. Currents caused 
by wind do more work in equalizing temperatures and in carry¬ 
ing heat to the deeper strata than do all other agencies com¬ 
bined. Any factor which seriously modifies or limits their 
action has corresponding importance in the temperature changes 
of a lake. 
The following table corresponds in part to that given by 
Groll (’05, p. 48). Column II shows the density of water 
from 0° to 30°, as given by Landolt and Bomstein.* The 
numbers show also the weight of a liter of water at the tem¬ 
perature stated. Thus a liter weighs 1.000000 kg. at 4°; at 
10° it weighs 0.999727 kg. or 999,727 mg. The numbers in 
columns III-VI do not stand opposite the numbers of, column 
II but are opposite the spaces between these numbers. Each 
represents the result of a change of temperature in a unit 
volume of water, corresponding to the degrees in column I im¬ 
mediately above and below the number in question; or it re¬ 
lates to a column of water whose surfaces have the temperatures 
immediately above or below. 
* Physicalische chemische Tabelien, 3rd ed., 1905, p. 37. 
