360 Wisconsin Academy of Sciences , Arts, and Letters. 
different years; but it is not possible to compare work done 
on different lakes. 
Work for single strata may be stated in similar fashion 
with the same advantages and limitations. 
Other reference planes may be used than the surface of the 
lake and this is necessary in certain cases, as illustrated on a 
preceding page. 
Work may be stated in ergs instead of g. cm. The result in 
g. cm. multiplied by 981 gives work in ergs. 
14. EFFECT OF THE PENETRATION OF THE SUN’S RAYS INTO 
THE WATER. 
In this discussion I have not observed the limitations indi¬ 
cated in the quotation with which I opened the paper. I 
have not confined the discussion of the work of the wind to a 
consideration of the work found in that region of the lake 
which “lies below the direct influence of the sun.” On the 
contrary all computations and discussions have proceeded 
on the assumption that all distribution of heat from the sur¬ 
face down is due to the wind. This method has been followed 
for two reasons. First, we have as yet little knowledge of the 
quantitative relations of the forces which distribute heat and 
clearness of thought on the subject may be furthered by 
selecting one agent at a time and analysing its possible pow¬ 
ers, rather than by attempting a simultaneous analysis of 
numerous interrelated forces. The second reason is that the 
possible help of the sun in distributing heat is so much more 
complex than the wind and is so little known that we find it 
difficult to make even a possible quantitative statement 
about it. 
We know indeed that a large share of the heat found in the 
upper meters of the water must be delivered there by the 
sun, and therefore, that a considerable part of the work as¬ 
signed to wind in these strata really belongs to sun. The 
larger part of the area MAI in pi. VII, for instance, belongs to 
the sun; a smaller share of the corresponding area for the 
1 m.-2 m. stratum belongs to the sun; and so on in a ratio 
that rapidly declines and becomes zero before the thermo- 
cline is reached. We know also that the wind-currents 
which convey heat to the deeper water do not take all of it 
from the surface but pick up some of it at a greater or smaller 
