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.