30 THE AMERICAN NATURALIST. (VOL: XXXII. 
temperate, and tropical, but bases the distinction upon bottom 
temperatures instead of surface temperatures, as follows: 
1. Tropical Type: Temperature of deep layers varies from 
and above maximum density. 
2. Temperate Type: Temperature of deep layers varies above 
and below maximum density. 
3. Polar Type: Temperature of deep layers varies from and 
below maximum density. 
He subdivides each type into two classes, deep and shallow, 
defining deep lakes as those which have a constant bottom 
temperature, and shallow lakes as those which have a variable 
bottom temperature. This subdivision is not a happy one, as 
observation shows that there are many lakes which would 
unquestionably be called “deep” which have a variable bottom 
temperature. , 
The temperature changes which take place in the nine classes 
of lakes according to our system of classification are exhibited 
in another manner in Fig. 3. These diagrams show by curves 
the surface and bottom temperatures for each season of the 
year, the times being plotted as abscissze and the temperatures 
as ordinates. The shaded areas show the difference between 
the surface and bottom temperatures, the wider the shaded 
area, the greater being the difference. 
A study of these diagrams brings out some interesting facts | 
concerning the phenomena of circulation and stagnation. In 
Fig. 2 it will be seen that the circulation periods occur when 
the curve showing the temperatures at various depths becomes 
a vertical line; that is, when the water all has the same tem- 
perature. The stagnation periods are shown by the line being 
curved, the top to the right when the warmer layers are above 
the colder, and to the left when the colder layers are above the 
warmer. In Fig. 3 the circulation periods are indicated by 
the surface and bottom temperature curves coinciding, and the 
stagnation periods by these lines being apart. The distance 
between the lines indicates, to a certain extent, the difference 
in density between the top and bottom layers, and we see that 
the farther apart the lines become the less likelihood there is 
that the water will be stirred up by the wind. 
G 
aig 
