Rhythmical Variations of Level in Lakes:  or “Seiches."—­As might be expected, the waters of Lake Tahoe are subject to fluctuations of level, depending upon the variable supplies furnished by its numerous affluents.  In mid-winter, when these streams are bound in icy fetters, the level falls; while in the months of May and June, when the snows of the amphitheater of mountain-slopes are melting most rapidly, the level of the Lake rises, and a maximum amount of water escapes through its outlet.  According to the observations of Capt.  John McKinney, made at his residence on the western shore of this Lake, the average seasonal fluctuation of level is about 0.61 of a meter; but in extreme seasons it sometimes amounts to 1.37 meters.  The Lake of Geneva, in like manner, is liable to fluctuations of level amounting to from 1.95 to 2.60 meters, from the melting of the Alpine snows.

But besides these variations of level due to the variable quantities of water discharged into them by their affluents, many lakes of moderate dimensions are liable to rhythmical oscillations of level of short duration, which are, obviously, but produced by fluctuations in the supply of water.  It is to this kind of species of variation of level that our attention will be directed in the sequel.

This interesting phenomenon was first recognized in the Lake of Geneva; but was subsequently found to be common to all the Swiss lakes, as well as to those of Scotland.  It is, therefore, a general phenomenon, which may be observed in all lakes of moderate dimensions.  The inhabitants of the shores of the Lake of Geneva have long designated this rhythmical oscillation of the level of the water by the term of Seiche; and this designation has been adopted by scientific writers.

These Seiches were first signalized in the Lake of Geneva in 1730, by Fatio de Duillier, who ascribed them to the checking of the flow of the waters of the Rhone on the shoal near Geneva by the force of the wind at mid-day.  Addison and Jallabert, in 1742, supposed them to be caused by sudden increments in the discharge of the affluents, due to the augmentation in the amount of snow melted after mid-day; or to the sudden increase in the flow of the Arve, checking the outflow of water by the Rhone.  Bertrand supposed that electrified clouds might locally attract and elevate the waters of the lake, and thus produce oscillations of level.  H.B. de Saussure, in 1799, attributed the phenomenon to rapid local variations of atmospheric pressure on different parts of the lake.  J.P.E.  Vaucher, in 1802 and 1804, adopted de Saussure’s explanation, and confirmed it by many excellent observations.  He, moreover, established that Seiches, more or less considerable, occur in all the Swiss lakes; and that they take place at all seasons of the year, and at