Birge—Seat Budgets of American and European Lakes . 187 
8. No clear relation can be discovered between latitude and 
size of heat budget. 
9. More important than any of these specific results is the 
conclusion that, by the method employed, it is possible to ex¬ 
press in similar units and to compare the heat budgets of lakes 
which differ widely in area, depth, etc.; that, even from the im¬ 
perfect temperature records now available, important generali¬ 
zations can be made as to the heat cycle of these lakes; and that 
it is plainthatmoreabundant data will enable us to correct and 
to extend these generalizations. 
Figure 2 and figure 3 are diagrams showing tne quantity of 
heat in the several 10 m. strata of Cayuga lake and Wiirm-See. 
Each of the larger divisions of the network represents 2° hori¬ 
zontally and 5 m. vertically. The horizontal lines marked 10, 
20, etc. are so placed as to show the ‘‘reduced thickness’’ of the 
several 10-m. strata; that is, the thickness of each 10-m. stratum 
if its area were extended to that of the surface of the lake and 
if its sides were vertical. This thickness is computed by divid¬ 
ing the volume of the stratum by the area of the surface of the 
lake. 
The area in the diagram, bounded by the lines representing 
winter and summer temperature and by any two of the 10-m. 
boundary lines is proportional to the amount of heat delivered 
to that stratum from the surface of the lake. Each of the 
larger squares included in such a quadrilateral represents 1000 
gr. cal. per sq. cm. of the surface of the lake delivered 
to the stratum and each small square represents 40 such 
calories. The area bounded by any two of the 10-m. lines, the 
temperature line of 4°, and the temperature curve gives the sum¬ 
mer heat-income of the stratum in question. 
On the dates selected for summer temperatures the tempera¬ 
ture of the two lakes at the surface was almost identical. In 
Cayuga lake the temperature of the epilimnion falls very slowly 
and has declined less than a degree at 15 m. In Wiirm-See the 
temperature declines rapidly below 5 m. and at 15 m. is more 
than 10° below that of Cayuga. This lake was also warmer at 
depths immediately below 15 m., the difference falling to 0.4° 
at 40 m. At 50 m., 60 m., 70 m., and 80 m. Wiirm-See was 
