8i8 THE POPULAR SCIENCE MONTHLY. 



agents with volcanic phenomena that are still, at first sight, so dif- 

 ferent and almost contrasted. The temperature of springs is gen- 

 erally 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 thermo- 

 metrical indications differ by only two or three degrees from the nor- 

 mal temperature. Thermal springs are not alwa^^s, 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 grad- 

 ually subside, till they become insensible at a depth which is measured 

 at Paris as of about twenty-five metres. Below this stratum of inva- 

 riable temj)erature, 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 demon- 

 strated 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 fi'om any cause exterior to our globe, for, if it did, it would not 

 increase as we descend. It appears to be the resultant and continua- 

 tion 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 conse- 

 quence of tliis 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 im- 

 penetrable 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 tip again to the surface, their 

 thermal water Avould return with them, as if drawn tlirough a siphon. 

 This is the kind of a disposition which Nature has made real in coun- 

 tries where the strata have been bent under strong mechanical action. 

 Such a structure may be recognized in the cases of the springs of ]>ar- 

 botan, Baden, Schinznach, Aix-la-Chapelle, Bercette, and in the Ap- 

 palachian Mountains in Virginia, 



Another mechanism of Nature, yet more closely "resembling a 

 siphon, is furnished by the large, nearly vertical fractures called 



