996 Wisconsin Academy of Sciences, Arts, and Letters. 
When the temperature rises to 15° the resistance is nearly 
doubled as compared with 10°, and the efficiency of the dis¬ 
tributing agents is correspondingly reduced. It is not surpris¬ 
ing, therefore, that the distributing agents can work very ef¬ 
fectively during the spring and carry the heat received by the 
surface to considerable depths. It is not surprising also that 
during the spring they can distribute this heat so rapidly as 
to prevent the surface temperature from rising so fast as to 
offer considerable resistance to their action; while they lose a 
great part of their power as the season advances. 
There are, of course, other factors which work in the same 
direction, aiding to increase the efficiency of the forces which 
distribute heat in the early part of the season and checking 
this distribution as the summer advances. Students of lake 
temperature, however, have felt that these forces were not ade- 
quate to explain the observed facts. I believe that if the in¬ 
creased thermal resistance is also taken into account the 
phenomena will find a full explanation. 
A second point where this principle finds important appli¬ 
cation is at the thermocline. Ho one fact in lake temperatures 
so arouses surprise in the mind of the student as does the ease 
with which the thermocline can be disturbed and the difficulty 
with which it can be permanently displaced. Violent winds 
in summer may raise or depress its surface by several meters 
in the larger inland lakes, yet it returns to its old position 
with barely a trace of change. In Lake Mendota the ther¬ 
mocline may be reduced to a temperature amplitude of 2°, or 
less in late September or October, and may lie within a meter 
or two of the bottom. This position it may retain for days, 
if not for weeks, unless an unusually vigorous wind upsets 
it. Ho such slight difference of temperature would, or does, 
persist in the spring. The reason is that the temperatures 
in the spring are in the region of 6° or 8°, while in the fall 
they lie at 12° or 14°, and therefore offer much more resist¬ 
ance to mixture. 
At the junction of thermocline and epilimnion the fall of 
temperature is rapid. A decline of 4° or 5° in a meter is 
not uncommon and this is from a high temperature, 20°, 
