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. * * * * * A catalogue, containing brief notices of many important scientific papers heretofore published in the SUPPLEMENT, may be had gratis at this office. * * * * * THE SCIENTIFIC AMERICAN SUPPLEMENT. PUBLISHED WEEKLY. TERMS OF SUBSCRIPTION, $5 A YEAR.