A UMNOLOGICAL STUDY OF THE FINGER EAKES. 
573 
No discussion of these facts would be possible if it was based on the single series of 
ODservations made on each of these lakes, but these can be interpreted in the light of 
the almost innumerable observations on Wisconsin lakes, and considered in this light 
they are extremely interesting. 
The influence of the small size of the lake is apparent in the shallow epilimnion — 
7 meters, or a little more than half its thickness in the larger lakes. The same general 
relation is shown by other similar facts. The temperature of the lower water is raised 
above 4° by mixture; and the depth at which this water reaches temperatures of 10° 
or 15° in August will give, like the position of the thermocline, the approximate value 
of the mixing power of the wind. In 1910 the temperature of 15° lay at about 10 meters 
in Canadice Lake, ii meters in Otisco, 18.5 meters in Cayuga, 15 meters in Seneca and 
Owasco. The temperature of 10° lay at 14 meters in Canadice, from 20 to 25 meters in 
the other lakes. In Otisco the bottom water at 20 meters had a temperature of 12°. 
The bottom of the epilimnion marks the lower limit of the direct distribution of heat in 
summer, and its position in the various lakes is the best measure of the relative influence 
of the wind on them. The depth of the successive isotherms also marks the approximate 
levels of wind influences. As would be expected, these levels are higher in the smaller 
lakes, and their smaller dimensions form the first reason for their smaller gain of heat. 
The second reason lies in the smaller mean depth of the lake and the smaller reduced 
thickness of each stratum. In a shallow lake the heating surface is greater in proportion 
to the depth than in a deeper lake, and it might therefore be expected that the former 
lake would be proportionately higher in temperature, and that the number of calories 
gained per square centimeter of surface would be the same in the two lakes so that the 
product Dm (Tm®-4) would be nearly constant for all lakes as it is for those of the first 
class. It might be expected, for instance, that if Canadice Lake (Dm =16.4 meters) 
gained 19,400 calories of wind-distributed heat, then in Otisco Lake, with a mean depth 
of 1 1. 2 meters, Tm®-4 would be high enough to make the product about the same, so 
that the two lakes would gain the same amount of heat from an equal heating surface. 
This might be expected the more readily as Otisco is *the larger lake and has a relatively 
larger surface; but so far from reaching this result, Otisco Lake has gained only about 
17,000 calories, or nearly 9 per cent less than the deeper lake. 
There are two reasons for this disadvantage of a shallow lake. First, the tem- 
perature of the epilimnion is determined not only by the relation of insolation and wind 
action, but even more by losses to the air. A shallow epilimnion ordinarily reaches a 
higher temperature than a thicker one, but the difference in the temperature is not so 
great as in the reduced thickness, so that the total amount of heat in the epilimnion is 
smaller. The losses to the air prevent the temperature from rising above a certain 
point. If, for instance, the epilimnion of Canadice Lake were to have as much heat as 
even that of Keuka, whose epilimnion is the thinnest of the six major lakes, Tm-4 
would have to be 22.3°, and Tm 26.3°.. This is an obviously impossible temperature 
as the mean of any considerable stratum, since in our latitudes it is reached only by 
a very thin surface layer in the hottest part of bright calm days. It rarely persists 
overnight. 
60289° — Bull. 32 — 14 37 
