Some very interesting information by Prof. John Le Conte, is given in
the _Overland Monthly_, being the result of some physical observations
made by the author at Lake Tahoe, in 1873. Lake Tahoe, also called Lake
Bigler, is situated at an altitude of 6,247 feet in the Sierra Nevada
Mountains, partly in California, partly in Nevada. The lake has a length
of 22 and a width of 12 miles. As regards its origin, the author regards
it as a "plication hollow," or a trough produced by the formation of two
mountain ridges, afterward modified by glacial agency. The depth of the
lake is remarkable; the observations taken at ten stations along the
length of the lake gave the following depths in feet: 900, 1,385, 1,495,
1,500, 1,506, 1,540, 1,504, 1,600, 1,640, 1645. This depth exceeds that
of the Swiss lakes proper--Lake Geneva, for example, has a maximum depth
of 1,096 feet--but is considerably less than that of Lakes Maggiore and
Como, on the Italian side of the Alps. A series of observations of the
temperature of the water were taken between the 11th and 18th of August.
The average corrected results are as follows:
Depth in feet. Temp. (C.)
0(surface).................................19.4
50..........................................17.2
100..........................................12.8
150..........................................10.0
200.......................................... 8.9
250.......................................... 8.3
300.......................................... 7.8
330 (bottom)................................. 7.5
400.......................................... 7.2
480 (bottom)................................. 6.9
500.......................................... 6.7
600.......................................... 6.1
772 (bottom)................................. 5.0
1506 (bottom)..............,.................. 4.0
The temperature, therefore, diminishes with increasing depth to about
700 or 800 feet, and below this remains sensibly the same down to 1,506
feet; or in other words, a constant temperature of 4° C. prevails at all
depths below about. 820 feet. This is in accordance with the theory, the
temperature named being that of the maximum density of water, and it
confirms the recent observations of Prof. Forel in Switzerland; he
found, for example, that a constant temperature of 4° C. was reached in
Lake Zurich at a depth of nearly 400 feet, the lake being then covered
with 4 inches of ice. The explanation of the observed fact that Lake
Tahoe does not entirely freeze over even in severe winters is found in
the extreme depth; and the fact that the bodies of drowned persons do
not rise to the surface after the lapse of the usual time is explained
by the low temperature prevailing near the bottom, which does not allow
the necessary decomposition to go forward so as to produce the ordinary
result.
The water of Lake Tahoe is remarkable both for its transparency and
beauty of color. A series of observations made at the close of August or
beginning of September showed that a horizontally adjusted dinner plate
of about 9½ inches diameter was visible at noon at a depth of 108 feet.
The maximum depth of the limit of visibility as found by Prof. Forel, in
Lake Geneva, was 56 feet. He showed, moreover, that this limit is much
greater in. winter than in summer, as explained in part by the greater
absence of suspended matter and in part by the fact that increase of
temperature increases the absorbing power of water for light. The
maximum depth of visibility in the Atlantic Ocean, as found by Count
de Pourtales, was 162 feet, and Prof. Le Conte states his belief that
winter observations in Lake Tahoe would place the limit at even a
greater depth than this. The author gives a detailed and interesting
discussion in regard to the blue color of lake waters, reviewing in full
the results of previous writers on the subject, and concludes that while
pure water unquestionably absorbs a larger part of the red end of the
spectrum, and hence appears blue by transmitted light, the color seen by
diffuse reflection is mainly due to the selective reflection from the
fine particles suspended in it.
The last subject discussed by the author is that of the rhythmical
variations of level, or "seiches," of deep lakes; he applies the usual
formula to Lake Tahoe, and calculates from it the length of a complete
longitudinal and of a transverse "seiche;" these are found to be
eighteen or nineteen minutes in the first case and thirteen minutes in
the second.
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A catalogue, containing brief notices of many important scientific
papers heretofore published in the SUPPLEMENT, may be had gratis at this
office.
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