226 |Wisconsin Academy of Sciences, Arts, and Letters. 
part of the day and the greater part of this difference was 
found in the upper 10 in. The middle or insulating stratum 
had not yet become definitely defined thermally, but the results 
obtained from dissolved gases showed that the lower strata no 
longer took part in the general circulation of the water. The 
decrease in the alkalinity between 75m. and 100 in. indicates 
that this stratum was at the lower limit of the circulation. As 
the season advanced and the upper water became warmer, this 
transition zone undoubtedly came much nearer the surface, very 
probably lying at a depth not exceeding 20 m. to 30 m. 
The effectiveness of the slight differences in temperature in 
preventing the mixture of the water at all depths doubtless finds 
its explanation in the fact that the waters had such a high 
temperature. The thermal resistance to mixture due to a rise 
in temperature of water from 19° to 20° is 25 times as great as 
that due to a rise from 4° to 5 0 . 1 That is, it would require 
twenty-five times as much work to mix two given volumes of 
water at temperatures of 19° and 20° as it would take to mix 
the same volumes of water if their temperatures were 4° and 5° 
respectively. Thus, since the lowest temperature in these trop¬ 
ical lakes was about 19°, a slight rise in the temperature of the 
upper water would produce a marked increase in its thermal 
resistance to mixture with the somewhat cooler water below. 
The maximum rise in the temperature of the upper water dur¬ 
ing the summer most probably does not exceed a few degrees 
because day and night are about equal in length in this latitude, 
thus giving as much time for the radiation as for the accumula¬ 
tion of heat. But this relatively small increase in the tempera¬ 
ture of the upper water undoubtedly causes the three strata to 
become sharply defined. In fact, it seems probable that they 
become so well marked that the resistance to mixture offered 
by the transition zone, or middle stratum, is comparable to that 
offered by the thermocline or mesolimnion of the deeper lakes 
of the temperate zone in summer. 
The waters of lake Ilopango were warmest and those of lake 
Atitlan were coolest. (See table II, p. 244.) The differences 
in temperature were due chiefly to differences in the altitude of 
the various lakes, the former lying at the lowest altitude, while 
the latter lies at the highest. The maximum difference between 
surface and bottom temperature was found in lake Coatepeque. 
Birge, Trans. Wis. Acad. Sci., Arts, & Letts., Vol. XVI, Part 2, 1910, p. 992. 
