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On the Old Road Volume 2 (of 2), essay(s) by John Ruskin |
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Notes On Natural Science - The Temperature Of Spring And River Water. 1836 |
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_ OBSERVATIONS ON THE CAUSES WHICH OCCASION THE VARIATION OF TEMPERATURE BETWEEN SPRING AND RIVER WATER.--BY J. R.[31]
275. Many instances might be given of the strong effect of this interior heat. The glaciers of the Alps, for instance, frequently cover an extent of three or four square leagues, with a mass of ice 400, 500, or even 600 feet deep, thus entirely preventing the access of exterior heat to the soil; yet the radiation of heat from the ground itself is so powerful as to dissolve the ice very rapidly, and to occasion streams of no inconsiderable size beneath the ice, whose temperature, in summer, is, I believe, as far as can be ascertained, not many degrees below that of streams exposed to the air; and the radiation of heat from the water of these streams forms vaults under the ice, which are frequently 40 ft. or 50 ft. above the water; and which are formed, as a glance will show, not by the force of the stream, which would only tear itself a broken cave sufficient for its passage, but by the heat which radiates from it, and gives the arch its immense height, and beautifully regular form. These streams continue to flow in winter as well as in summer, although in less quantity; and it is this process which chiefly prevents the glacier from increasing in size; for the melting at the surface is, in comparison, very inconsiderable, even in summer, the wind being cold, the sun having little power, and slight frosts being frequent during the night. It is also this melting beneath the ice (subglacial, suppose we call it) which loosens the ice from the ground, and occasions, or rather permits, the perpetual downward movement, with which
277. In another gallery, 564 feet below the surface, the water and air had likewise the same temperature, 12-1/2 deg., either 54-4/5 or 6O-1/4 Fahr. The water at the bottom, 677 feet, was 14 deg., 57-1/2 or 63-1/4 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? HERNE HILL, _Sept. 2, 1836._
[Footnote 31: London's _Magazine of Natural History_, vol. ix., pp. 533-536.--ED.] [Footnote 32: The query was as follows:-- _An Inquiry for the Cause of the Difference in Temperature of River Water and Spring Water, both in Summer and Winter._--In the summer time the river water is much warmer than that from a spring; during the severe frosts of winter it is colder; and when the stream is covered over with ice, the spring, that is, well or pump water is unaffected by frost. Does this difference proceed from the exposure of the surface of the river water, in summer, to the sun's direct influence, and, in winter, to that of frost; while the well water, being covered, is protected from their power? Or is there in river water, from the earthy particles it contains, a greater susceptibility of heat and cold?--_Indigena_. _April 19, 1836._--ED.] _ |
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