he maximum density of
fresh water.

(3.) _Why the Water does not freeze in Winter_. Residents
on the shore of Lake Tahoe testify that, with the exception
of shallow and detached portions, the water of the Lake never
freezes in the coldest winters. During the winter months, the
temperature of atmosphere about this Lake must fall as low,
probably, as 0 degrees Fah. (-17.78 deg. Cent.). According to
the observations of Dr. George M. Bourne, the
minimum temperature recorded during the winter of 1873-74 was
6 deg. Fah. (-14.44 deg. Cent.). As it is evident that during
the winter season the temperature of the air must frequently
remain for days, and perhaps weeks, far below the freezing
point of water, the fact that the water of the Lake does not
congeal has been regarded as an anomalous phenomenon. Some
persons imagine that this may be due to the existence of
subaqueous hot springs in the bed of the Lake--an opinion
which may seem to be fortified by the fact that hot springs do
occur at the northern extremity of the Lake. But there is
no evidence that the temperature of any considerable body of
water in the Lake is sensibly increased by such springs. Even
in the immediate vicinity of the hot springs (which have in
summer a maximum temperature of 55 deg. C. or 131 F.), the
supply of warm water is so limited that it exercises no
appreciable influence on the temperature of that portion of
the Lake. This is further corroborated by the fact that no
local fogs hang over this or any other portion of the Lake
during the winter which would most certainly be the case if
any considerable body of hot water found its way into the
Lake.

The true explanation of the phenomenon may, doubtless, be
found in the high specific heat of water, the great depth of
the Lake, and in the agitation of its waters by the strong
winds of winter. In relation to the influence of depth, it
is sufficient to remark that, before the conditions preceding
congelation can obtain, the whole mass of water--embracing a
stratum of 250 meters in thickness--must be cooled down to
4 deg. Cent.; for this must occur before the vertical
circulation is arrested and the colder water floats on the
surface. In consequence of the great specific heat of
water, to cool such a mass of the liquid through an average