A LIMNOLOGICAL STUDY OF THE FINGER LAKES. 
551 
would not be exceeded in ordinary summer weather. The surface temperature of 
steep-sided lakes is necessarily lower than that of lakes with shallow margins, and 
that of the smaller lakes is, for similar reasons, usually higher than that of the larger. 
If we consider the six main lakes only, the mean thickness of the epilimnion in 1910 
was II. 5 meters (9 to 15 meters); in 1911 it was 13.7 meters (ii to 16 meters). In 
twelve series of temperatures taken in six lakes in 1910 and 1911, the mean thickness 
was 12.6 meters. No constant difference due to area or depth appears in the list. 
Keuka Lake had the thinnest epilimnion in both years, a condition undoubtedly due 
to the fact that it is narrower than any of the other lakes, and that its shores are on the 
whole higher and steeper than the others. The wind has therefore less opportunity 
to distribute the warm surface water. The effect of the same cause is found also in the 
fact that the annual gains of heat in Keuka Lake are the smallest of the six, as will be 
shown later, and also in its relatively low bottom temperature. Cayuga Lake indicates 
rather doubtfully a tendency toward an epilimnion a little thicker than the others. 
I believe this will be found to be the fact when a sufficient number of observations 
have been made. There is nothing conclusive, however, in the observations given, 
as they are quite within the range of accidental variation. 
I have added to the table of New York lakes the facts for Green Lake, Wisconsin, 
which lies in a climate not essentially different from that of central New York. These 
show that the epilimnion in these years had a thickness almost exactly the same as 
those of the New York lakes. Green Lake has a length of about 11.4 kilometers while 
those in New York vary from about 18 kilometers to more than 60 kilometers. Their 
mean breadth is of the same order of magnitude as that of Green Lake, and is therefore 
relatively less. This, however, makes little difference in the effect of wind in favor 
of Green Lake, since the long axis of Green Lake lies across the prevailing direction of 
the wind, or at least obliquely to it, while the New York lakes are much more nearly 
parallel to it. This fact, coupled with the greater length of the lakes, should make the 
influence of the wind as great for any of the lakes and much greater for some of them. 
In Seneca Lake, for instance, the currents induced by wind are often so strong that 
even when no wind is blowing a deep-sea thermometer will not sink perpendicularly 
unless extra weight is attached to the line. This condition never occurs in the Wisconsin 
lakes. 
It appears, therefore, that 12 to 15 meters is about the maximum thickness which 
can be expected in the epilimnion of an inland lake before the temperature of the water 
begins to decline. Such a statement applies only to lakes which lie under the topographic 
and climatic conditions of the lakes discussed. Variations are found and the thickness 
must be measured by meters and not by centimeters; but in any ordinary season the 
observer may confidently expect to find the thermocline about where he found it in 
previous years. It may be a little higher or lower, but the thickness of the epilimnion 
in the same lake will always be of the same order of magnitude. 
Thermocline . — ^We understand by the thermocline that thermal region of the lake 
lying immediately below the epilimnion, in which the temperature falls rapidly. This 
