given volume (that is, a cubic unit of it is heavier at this temperature) than it is at any temperature either higher or lower.  Hence, when the ice-cold water of the snow-fed streams of spring and summer reaches the Lake, it naturally tends to sink as soon as its temperature rises to 4 deg.  Cent.; and, conversely, when winter sets in, as soon as the summer-heated surface water is cooled to 4 deg., it tends to sink.  Any further rise of temperature of the surface water during the warm season, or fall of temperature during the cold season, alike produces expansion, and thus causes it to float on the heavier water below; so that water at 4 deg.  Cent., perpetually remains at the bottom, while the varying temperature of the seasons and the penetration of the solar heat only influence a surface stratum of about 250 meters in thickness.  It is evident that the continual outflow of water from its shallow outlet cannot disturb the mass of liquid occupying the deeper portions of the Lake.  It thus results that the temperature of the surface stratum of such bodies of fresh water for a certain depth fluctuates with the climate and with the seasons; but at the bottom of deep lakes it undergoes little or no change throughout the year, and approaches to that which corresponds to the 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