374 Wisconsin Academy of Sciences , Arts , antf Letters. 
Plate II. Temperature curve and density curve of lake Mendota. 
This is the mean of the daily observations Aug. 9-15, 1910. Ordi¬ 
nates represent depth and abscissas temperature or the value of 
1 — D. The scale for 1 — D is 100 times as great as for temperature 
and its zero starts at 4°. For purposes of comparison 4° should also 
be the starting point of temperature and loss of density should be 
compared with increase of heat as measured from that point. It 
is more convenient to treat 1 — D as a whole number, except in 
multiplying and to think of it as representing loss of weight— 
milligrams per liter, for instance; In this case a gain of 20° above 
4° involves a loss of 2677 points in density, while a similar gain of 
10° or half as much heat, reduces the density less than one-third 
as much, 729 points. 
It is this fact which makes it worth while to make a study of the 
work involved in distributing heat. If the density varied 
directly as the heat, the results of a change of density would not 
differ from those of a change of heat. 
