26 Proceedings of the Boy at Society 
requiring to remain twelve hours at the bottom, to arrive at the 
temperature of the surrounding water. But the zeal and patience 
of the philosopher were a match for this trial, and his construction 
had probably the advantage of securing his bulb and tube against 
the disturbing influence of pressure, which must have been great in 
some of his experiments, but which, as he never refers to it, must 
not have occurred to him as a condition to be provided against. 
He then, between 1779 and 1784, made a number of observations 
on the lakes of G-eneva, De Joux, Annecy, Thun, Bourget, Brienz, 
Lucerne, Constance, and Lago Maggiore; always reaching the 
bottom at depths varying from 80 English feet to 163, 240, 335, 
350, 370, 500, 600, 620, and 950 feet. His observations were 
generally made at midsummer, a few in February, and a few also 
in October. Excluding the experiment in Lac de Joux, whose 
depth of 80 feet excludes it from the category of deep lakes, and 
that of Maggiore in a warmer latitude and locality than the Swiss 
lakes, we find that he never got a higher temperature than 42°T, 
and once he got it so low as 39°-6. The deepest lakes on the 
whole gave the lowest temperatures, but by no means always in 
exact proportion. In the lake of G-eneva the bottom temperature 
at 950 feet was 41 0, 7 ; and in that of Lake Constance it was 39°*6, 
at 370 feet only. He thought the time of the year made little 
difference ; but he did not try the same lake in the same place in 
different months. He tries to show that locality did not much 
affect the question of temperature. But this is surely a mistake; 
for the vicinity of snow-clad mountains, and the hard winter they 
occasion, are the probable causes why the cold at the bottom of 
the deep lakes there is greater than is observed in so much higher 
a latitude as Scotland. 
In fact, the deep temperature of a very deep -lake must be ruled 
far more by the cold of winter than by the heat of summer. The 
cold water must continue to descend as long as the cold months 
last. The colder these months are, the longer that cold lasts, the 
greater must be the cold at the bottom, and the thicker the 
stratum of cold water. The warmth of the air in summer and 
autumn acting only on the water by conduction, cannot move the 
deep cold substratum upwards. The only other heating influence 
from above, a far more penetrating influence, is the sun’s rays. 
