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. 



