"The glacier's cold and restless mass
Moves onward day by day."
276. But more forcible and striking evidence is afforded by experiments made in mines of great depth. Between 60 ft. and 80 ft. down, the temperature of the earth is, I believe, the same at all times and in all places; and below this depth it gradually increases. Near Bex, in the Valais, there is a perpendicular shaft 677 ft. deep, or about 732 ft. English, with water at the bottom, the temperature of which was ascertained by Saussure. He does not tell us whether he used Réaumur's or the centesimal thermometer; but the result of his experiment was this:—In a lateral gallery, connected with the main shaft, but deserted, and, therefore, unaffected by breath or the heat of lamps, at 321 ft. 10 in. below the surface, the temperature of the water and the air was exactly the same, 11½°; or, if the centesimal thermometer was used, 52⅘ Fahr.; if Réaumur's, 57⅞ Fahr.
277. In another gallery, 564 feet below the surface, the water and air had likewise the same temperature, 12½°, either 54⅘ or 6O¼ Fahr. The water at the bottom, 677 feet, was 14°, 57½ or 63¼ Fahr. The ratio in which the heat increases, therefore, increased as we descend, since a difference of 113 feet between the depth of the bottom of the shaft and the lowest gallery makes a greater difference in temperature than the difference of 243 feet between the lowest and upper gallery. This heat is the more striking when it is considered that the water is impregnated with salt; indeed, Saussure appears inclined to consider it accidental, perhaps occasioned by the combustion of pyrites, or other causes in the interior of the mountain ("Voyages dans les Alpes," tom. iv., c. 50). All experiments of this kind, indeed, are liable to error, from the frequent occurrence of warm springs, and other accidental causes of increase in temperature. The water at the bottom of deep lakes is always found several degrees colder than the atmosphere, even when the water at the surface is warmer: but that may be accounted for by the difference in the specific gravity of water at different temperatures; and, as the heat of the sun and atmosphere in summer is greater than the mean heat of the earth at moderate depths, the water at the bottom, even if it becomes of the same heat with the earth, must be colder than that at the surface, which, from its exposure to the sun, becomes frequently warmer than the air. The same causes affect the temperature of the sea; and the greater saturation of the water below with salt renders it yet more susceptible of cold. Under-currents from the poles, and the sinking of the water of low temperature, which results from the melting of the icebergs which float into warmer latitudes, contribute still farther to lower the temperature of the deep sea. If, then, the temperature of the sea at great depths is found not many degrees lower than that at the surface, it would be a striking proof of the effect produced by the heat of the earth; but I am not aware of the results of the experiments which have been made on this subject.
278. We must, then, rest satisfied with the well-ascertained fact, that the temperature of the earth, even at depths of a few feet, never descends, in temperate latitudes, to the freezing point; and that at the depth of 60 feet it is always the same, in winter much higher, in summer considerably lower, than that of the atmosphere. Spring water, then, which has its source at a considerable depth, will, when it first rises, be of this mean temperature; while, after it has flowed for some distance, it becomes of the temperature of the atmosphere, or, in summer, even warmer, owing to the action of the sun, both directly and reflected or radiated from its bottom. Besides this equable temperature in the water itself, spring or well water is usually covered; and, even if exposed, if the well is very deep, the water will not freeze, or at least very slightly; for frost does not act with its full power, except where there is a free circulation of air. In open ponds, wherever bushes hang over the water, the ice is weak. Indigena's supposition, that there are earthy particles in river water, which render it more susceptible of cold than spring water, cannot be true; for then the relative temperatures would be the same in winter and in summer, which is not the case; and, besides, there are frequently more earthy particles in mineral springs, or even common land springs, than in clear river water, provided it has not been fouled by extraneous matter; for it has a tendency to deposit the earthy particles which it holds in suspension.
279. It is evident, also, that the supposition of Mr. Carr (Vol. v., p. 395) relative to anchor frosts, that the stones at the bottom acquire a greater degree of cold, or, to speak more correctly, lose more heat, than the water, is erroneous. J. G. has given the reasons at p. 770; and the glaciers of Switzerland afford us an example. When a stone is deposited on a glacier of any considerable size, but not larger than 1 foot or 18 inches in diameter, it becomes penetrated with the heat of the sun, melts the ice below it, and sinks into the glacier. But this effect does not cease, as might be supposed, when the stone sinks beneath the water which it has formed; on the contrary, it continues to absorb heat from the rays of the sun, to keep the water above it liquid by its radiation, and to sink deeper into the body of the glacier, until it gets down beyond the reach of the sun's rays, when the water of the well which it has formed is no longer kept liquid, and the stone is buried in the ice. In summer, however, the water is kept liquid; and circular wells, formed in this manner, are of frequent occurrence on the glaciers, sometimes, in the morning, covered by a thin crust of ice.
Thus, the stones at the bottom of streams must tend to raise, rather than lower, this temperature. Is it possible that, in the agitation of a stream at its bottom, if violent, momentary and minute vacua may be formed, tending to increase the intensity of the cold?
FOOTNOTES:
[31] London's Magazine of Natural History, vol. ix., pp. 533-536.—Ed.
[32] The query was as follows:—