7 ............... 1504 458.41
8 ............... 1600 487.67
9 ............... 1640 499.86
10 ............... 1645 501.39
These figures show that this lake exceeds in depth the deepest of the Swiss lakes (the Lake of Geneva), which has a maximum depth of 334 meters. On the Italian side of the Alps, however, Lakes Maggiore and Como are said to have depths respectively of 796.43 and 586.73 meters. These two lakes are so little elevated above the sea that their bottoms are depressed 587 and 374 meters below the level of the Mediterranean.
(2.) _Relation of Temperature to Depth_. By means of a self-registering thermometer (Six"s) secured to the sounding-line, a great number of observations were made on the temperature of the water of the Lake at various depths and in different portions of the same. These experiments were executed between the 11th and 18th of August, 1873. The same general results were obtained in all parts of the Lake. The following table contains the abstract of the average results, after correcting the thermometric indications by comparison with a standard thermometer:
Obs. in Feet in Meters F. deg. in C._
1 ...... 0-Surface 0-Surface 67 19.44
2 ...... 50 15.24 63 17.22
3 ...... 100 30.48 55 12.78
4 ...... 150 45.72 50 10.00
5 ...... 200 60.96 48 8.89
6 ...... 250 76.20 47 8.33
7 ...... 300 91.44 46 7.78
8 ...... 330 (Bottom) 100.58 45.5 7.50
9 ...... 400 121.92 45 7.22
10 ...... 480 (Bottom) 146.30 44.5 6.94
11 ...... 500 152.40 44 6.67
12 ...... 600 182.88 43 6.11
13 ...... 772 (Bottom) 235.30 41 5.00
14 ...... 1506 (Bottom) 459.02 39.2 4.00
It will be seen from the foregoing numbers that the temperature of the water decreases with increasing depth to about 700 or 800 feet (213 or 244 meters), and below this depth it remains sensibly the same down to 1506 feet (459 meters). This constant temperature which prevails at all depths below say 250 meters is about 4 degrees Cent. (39.2 Fah.). This is precisely what might have been expected; for it is a well established physical property of fresh water, that it attains its maximum density at the above-indicated temperature. In other words, a ma.s.s of fresh water at the temperature of 4 deg. Cent. has a greater weight under a 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 ma.s.s 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 sh.o.r.e 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 ma.s.s 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 ma.s.s of the liquid through an average temperature of 8 deg. Cent, requires a long time, and the cold weather is over before it is accomplished. In the shallower portions, the surface of the water may reach the temperature of congelation, but the agitations due to the action of strong winds soon breaks up the thin pellicle of ice, which is quickly melted by the heat generated by the mechanical action of the waves. Nevertheless, in shallow and detached portions of the Lake, which are sheltered from the action of winds and waves--as in Emerald Bay--ice several inches in thickness is sometimes formed.
[Ill.u.s.tration: Lily Lake]
[Ill.u.s.tration: Cave Rock, Lake Tahoe]
[Ill.u.s.tration: Pyramid Peak and Lake of the Woods]
[Ill.u.s.tration: Clouds Over the Mountain, Lake Tahoe]
(4.) _Why Bodies of the Drowned do not Rise_. A number of persons have been drowned in Lake Tahoe--some fourteen between 1860 and 1874--and it is the uniform testimony of the residents, that in no case, where the accident occurred in deep water, were the bodies ever recovered. This striking fact has caused wonder-seekers to propound the most extraordinary theories to account for it. Thus one of them says, "The water of the Lake is purity itself, but on account of the highly rarified state of the air it is not very buoyant, and swimmers find some little fatigue; or, in other words, they are compelled to keep swimming all the time they are in the water; and objects which float easily in other water sink here like lead." Again he says, "Not a thing ever floats on the surface of this Lake, save and except the boats which ply upon it."
It is scarcely necessary to remark that it is impossible that the diminution of atmospheric pressure, due to an elevation of 6250 feet (1905 meters) above the sea-level, could sensibly affect the density of the water. In fact, the coefficient of compressibility of this liquid is so small that the withdrawal of the above indicated amount of pressure (about one-fifth of an atmosphere) would not lower its density more than one hundred-thousandth part! The truth is, that the specific gravity is not lower than that of any other fresh water of equal purity and corresponding temperature. It is not less buoyant nor more difficult to swim in than any other fresh water; and consequently the fact that the bodies of the drowned do not rise to the surface cannot be accounted for by ascribing marvelous properties to its waters.
The distribution of temperature with depth affords a natural and satisfactory explanation of the phenomenon, and renders entirely superfluous any a.s.sumption of extraordinary lightness in the water. The true reason why the bodies of the drowned do not rise to the surface is evidently owing to the fact that when they sink into water which is only 4 deg. Cent. (7.2 deg. Fah.) above the freezing temperature, the gases usually generated by decomposition are not produced in the intestines; in other words, at this low temperature the bodies do not become inflated, and therefore do not rise to the surface. The same phenomenon would doubtless occur in any other body of fresh water under similar physical conditions.[2]
[Footnote 2: It should be noted that since 1874 there have been remarkably few deaths from drowning in Lake Tahoe, and that the major cases of those referred to by Dr. LeConte were of workmen and others who were generally under the influence of intoxicants.]
(5.) _Transparency of the Water_. All visitors to this beautiful Lake are struck with the extraordinary transparency of the water. At a depth of 15 to 20 meters (49.21 to 65.62 feet), every object on the bottom--on a calm sunny day--is seen with the greatest distinctness. On the 6th of September, 1873, the writer executed a series of experiments with the view of testing the transparency of the water. A number of other experiments were made August 28 and 29, under less favorable conditions. By securing a white object of considerable size--a horizontally adjusted dinner-plate about 9.5 inches in diameter--to the sounding-line, it was ascertained that (at noon) it was plainly visible at a vertical depth of 33 meters, or 108.27 English feet. It must be recollected that the light reaching the eye from such submerged objects must have traversed a thickness of water equal to at least twice the measured depth; in the above case, it must have been at least 66 meters, or 216.54 feet.
Furthermore, when it is considered that the amount of light regularly reflected from such a surface as that of a dinner-plate, under large angles of incidence in relation to the surface, is known to be a very small fraction of the incident beam (probably not exceeding three or four per cent.), it is evident that solar light must penetrate to vastly greater depths in these pellucid waters.
Moreover, it is quite certain that if the experiments in relation to the depths corresponding to the limit of visibility of the submerged white disk had been executed in winter instead of summer, much larger numbers would have been obtained. For it is now well ascertained, by means of the researches of Dr. F.A. Forel of Lausanne, that the waters of Alpine lakes are decidedly more transparent in winter than in summer. Indeed, it is reasonable that when the affluents of such lakes are locked in the icy fetters of winter, much less suspended matter is carried into them than in summer, when all the sub-glacial streams are in active operation.
Professor Le Conte goes into this subject (as he later does into the subject of the color of Lake Tahoe) somewhat exhaustively in a purely scientific manner and in too great length for the purposes of this chapter, hence the scientific or curious reader is referred to the original articles for further information and discussion.
_Color of the Waters of Lake Tahoe_. One of the most striking features of this charming mountain Lake is the beautiful hues presented by its pellucid waters. On a calm, clear, sunny day, wherever the depth is not less than from fifty to sixty meters, to an observer floating above its surface, the water a.s.sumes various shades of blue; from a brilliant Cyan blue (greenish-blue) to the most magnificent ultramarine blue or deep indigo blue. The shades of blue increasing in darkness in the order of the colors of the solar spectrum, are as follows: Cyan-blue (greenish blue), Prussian-blue, Cobalt-blue, genuine ultramarine-blue, and artificial ultramarine-blue (violet blue). While traversing one portion of the Lake in a steamer, a lady endowed with a remarkable natural appreciation and discrimination of shades of color declared that the exact tint of the water at this point was "Marie-Louise blue."
The waters of this Lake exhibit the most brilliant blueness in the deep portions, which are remote from the fouling influences of the sediment-bearing affluents, and the washings of the sh.o.r.es. On a bright and calm day, when viewed in the distance, it had the ultramarine hue; but when looked fair down upon, it was of almost inky blackness--a solid dark blue qualified by a trace of purple or violet. Under these favorable conditions, the appearance presented was not unlike that of the liquid in a vast natural dyeing-vat.
A clouded state of the sky, as was to be expected, produced the well-known effects due to the diminished intensity of light; the shades of blue became darker, and, in extreme cases, almost black-blue. According to our observations, the obscurations of the sky by the interposition of clouds produced no other modifications of tints than those due to a diminution of luminosity.
In places where the depth is comparatively small and the bottom is visibly white, the water a.s.sumes various shades of green; from a delicate apple-green to the most exquisite emerald-green. Near the southern and western sh.o.r.es of the Lake, the white, sandy bottom brings out the green tints very strikingly. In the charming _cul-de-sac_ called "Emerald Bay," it is remarkably conspicuous and exquisitely beautiful.
In places where the stratum of water covering white portions of the bottom is only a few meters in thickness, the green hue is not perceptible, unless viewed from such a distance that the rays of light emitted obliquely from the white surface have traversed a considerable thickness of the liquid before reaching the eye of the observer.
The experiments with the submerged white dinner-plate, in testing the transparency of the water, incidentally manifested, to some extent, the influence of depth on the color of the water. The white disk presented a bluish-green tint at the depth of from nine to twelve meters; at about fifteen meters it a.s.sumed a greenish-blue hue, and the blue element increased in distinctness with augmenting depth, until the disk became invisible or undistinguishable in the surrounding ma.s.s of blue waters. The water intervening between the white disk and the observer did not present the brilliant and vivid green tint which characterized that which is seen in the shallow portions of the Lake, where the bottom is white.
But this is not surprising, when we consider the small amount of diffused light which can reach the eye from so limited a surface of diffusion.
In studying the chromatic tints of these waters, a hollow pasteboard cylinder, five or six centimeters in diameter, and sixty or seventy centimeters in length, was sometimes employed for the purpose of excluding the surface reflection and the disturbances due to the small ripples on the water. When quietly floating in a small row-boat, one end of this exploring tube was plunged under the water, and the eye of the observer at the other extremity received the rays of light emanating from the deeper portions of the liquid. The light thus reaching the eye presented essentially the same variety of tints in the various portions of the Lake as those which have been previously indicated.
Hence it appears that under various condition--such as depth, purity, state of sky and color of bottom--the waters of this Lake manifest nearly all the chromatic tints presented in the solar spectrum between greenish-yellow and the darkest ultramarine-blue, bordering upon black-blue.
It is well known that the waters of oceans and seas exhibit similar gradations of chromatic hues in certain regions.
Navigators have been struck with the variety and richness of tints presented, in certain portions, by the waters of the Mediterranean Sea, the Atlantic and Pacific Oceans, and especially those of the Caribbean Sea. In some regions of the oceans and seas, the green hues, and particularly those tinged with yellow, are observed in comparatively deep waters, or, at least, where the depths are sufficiently great to prevent the bottom from being visible. But this phenomenon seems to require the presence of a considerable amount of suspended matter in the water. In no portion of Lake Tahoe did I observe any of the green tints, except where the light-colored bottom was visible. This was, probably, owing to the circ.u.mstance that no considerable quant.i.ty of suspended matter existed in any of the waters observed.
_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 sh.o.r.e 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 quant.i.ties 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 sh.o.r.es 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 all times of the day; but, in general, more frequently in spring and autumn. As regards the cause of the phenomenon, Vaucher shows how rapid local alterations of atmospheric pressure would produce oscillations in the level of the lake, and compares them to the vibrations of a liquid in a recurved tube or siphon. Finally, Arago maintained that _Seiches_ may arise from various causes, and traced the a.n.a.logy between them and certain remarkable oscillations of the sea, including those arising from earthquakes.
But physical science is indebted to Professor F.A. Forel, of Lausanne, for the most complete and exhaustive investigation in relation to the phenomena of _Seiches_. This accomplished physicist began his researches in 1869, and has continued them up to the present time. He has been able to demonstrate that these rhythmical oscillations occur in nearly all the Swiss Lakes (he studied the phenomena in nine of them), and that they follow in all cases the same general laws. Those of the Lake of Geneva have received the most elaborate and prolonged investigation. In March, 1876, Forel established a self-registering tide-gauge (_limni-metre enregistreur_) on the northern sh.o.r.e of this lake, at Morges; and, with the cooperation of P. Plantamour, another one was installed in June, 1877, at Secheron, near the city of Geneva, at the southern extremity. Since these dates, these two instruments have, respectively, been registering oscillations of the level of the water of the Lake of Geneva; and they are so sensitive as to indicate the waves generated by a steamer navigating the lake at a distance of ten or fifteen kilometers.