Temperatures — The Thermocline. 
297 
and the isotherm of 15° penetrated nearly 10 meters of 
the lake in a week; it went down 3 meters further in another 
week, but thereafter moved downward at a rate little exceeding 
one meter per month. In 1896 the 15° isotherm was in¬ 
cluded in the May depression of temperature, but in late May 
it moved downward nearly 15 meters in one week, 1.5 in the 
week following, and only one meter in the next two and a half 
months. As the temperature of the surface rises above 15° 
the warmth penetrates to a distance increasingly small and 
the isotherms accordingly bend toward the horizontal at a 
level nearer the surface. The gain of heat, however, becomes 
rapidly distributed through the upper water to a depth of 8 to 10 
meters, so that the thermocline becomes permanent at about 
these depths. When the thermocline has once been formed it 
moves downward very slowly. Beginning at about 8 meters in 
late June, it descends somewhat rapidly to about 10 meters, 
but after that moves downward slowly and irregularly, its 
descent depending rather upon the wind than upon the tem¬ 
perature of the air. In both years the thermocline reached 
the bottom of the lake in the last of September, which would 
make its downward movement about 4 meters per month, but 
the last 5 or 6 meters were passed very rapidly in consequence 
of the gales of late September. 
In 1895 the 18° isotherm was near the center of the ther¬ 
mocline; it oscillated about the 9 meter level in late June, 
sank nearly 3 meters in July, about 2.5 meters in August, 
and 4.5 in September, the last 3 in the latter half of the month. 
In 1896 the 20° isotherm was near the center of the ther¬ 
mocline at the outset and crossed the 6 meter level about 
July 1st. It lay at 7.5 meters during the first week of July, 
reached 9 meters about the 20th of the month, oscillated be¬ 
tween 9 and 10 meters for more than three weeks following that 
date — weeks of unusually hot weather — until the middle of 
August. At that time the weather changed and continued cool 
with much northerly wind, under whose influence the thermo¬ 
cline rapidly sank more than 2 meters during the last half of 
the month and continued this downward movement through Sep¬ 
tember until it disappeared in the latter part of the month. 
