agents with volcanic phenomena that are still, at first sight, so different and almost contrasted. The temperature of springs is generally nearly equal to the mean temperature of the ground from which they issue. But there are some exceptions to this usual condition, which are called thermal; a term which should be applied, not only to waters manifestly hot or warm, but also to those which by thermometrical indications differ by only two or three degrees from the normal temperature. Thermal springs are not always, therefore, distinctly separated from ordinary springs.
The extreme variations of temperature which we feel so vividly, according to the seasons, penetrate the ground very slowly and gradually subside, till they become insensible at a depth which is measured at Paris as of about twenty-five metres. Below this stratum of invariable temperature, the heat gradually increases as we descend; a fact which is not confined to temperate regions, but has been observed in countries near the equator and near the poles. It has been demonstrated by observations made in mines, in tunnels, and in artesian wells.
It is evident that this internal heat can not emanate from the sun nor from any cause exterior to our globe, for, if it did, it would not increase as we descend. It appears to be the resultant and continuation of the heat through which our planet has formerly passed. In radiating toward the celestial spaces, which are colder than anything of which we know, the outer masses are necessarily consumed first, while the heat continues intense in the central masses. In consequence of this general increase of heat, there are present in all parts of the interior of the globe, even far away from active volcanoes, rocks, contact with which heats water in a greater or less degree.
We have now to examine the various ways in which the structure of the rocks permits water, after it has descended to great depths, to return to the surface. The simplest way is by a turning back of the strata. The water of the artesian wells of Paris has been forced, having entered at the outcrops of the beds, to descend, between impenetrable strata, to a depth of which it has taken the temperature. The existence of a vast thermal bed under a part of the north of France would not have been revealed without the borings which have opened a way of return to its waters. But if the strata to which we refer, instead of being disposed in a vast concave basin, are subjected to a bending which would bring them up again to the surface, their thermal water would return with them, as if drawn through a siphon. This is the kind of a disposition which Nature has made real in countries where the strata have been bent under strong mechanical action. Such a structure may be recognized in the cases of the springs of Barbotan, Baden, Schinznach, Aix-la-Chapelle, Bercette, and in the Appalachian Mountains in Virginia.
Another mechanism of Nature, yet more closely resembling a siphon, is furnished by the large, nearly vertical fractures called