caloric from the air and the water; or because a great mass of water
does not follow with an equal rapidity the changes in the temperature
of the atmosphere, and the lake receives streams which rise from
several cold springs in the neighbouring mountains. I have to regret
that, notwithstanding its small depth, I could not determine the
temperature of the water at thirty or forty fathoms. I was not
provided with the thermometrical sounding apparatus which I had used
in the Alpine lakes of Salzburg, and in the Caribbean Sea. The
experiments of Saussure prove that, on both sides of the Alps, the
lakes which are from one hundred and ninety to two hundred and
seventy-four toises of absolute elevation* (* This is the difference
between the absolute elevations of the lakes of Geneva and Thun.)
have, in the middle of winter, at nine hundred, at six hundred, and
sometimes even at one hundred and fifty feet of depth, a uniform
temperature from 4.3 to 6 degrees: but these experiments have not yet
been repeated in lakes situated under the torrid zone. The strata of
cold water in Switzerland are of an enormous thickness. They have been
found so near the surface in the lakes of Geneva and Bienne, that the
decrement of heat in the water was one centesimal degree for ten or
fifteen feet; that is to say, eight times more rapid than in the
ocean, and forty-eight times more rapid than in the atmosphere. In the
temperate zone, where the heat of the atmosphere sinks to the freezing
point, and far lower, the bottom of a lake, even were it not
surrounded by glaciers and mountains covered with eternal snow, must
contain particles of water which, having during winter acquired at the
surface the maximum of their density, between 3.4 and 4.4 degrees,
have consequently fallen to the greatest depth. Other particles, the
temperature of which is +0.5 degrees, far from placing themselves
below the stratum at 4 degrees, can only find their hydrostatic
equilibrium above that stratum. They will descend lower only when