40 



COSMOS. 



at a depth of 1496 feet below the level of the sea), we shall 

 find this remarkable result, that at four places widely sepa- 

 rated from one another an increase of heat of 1 F. varies 

 only between 54 and 58-6 feet;* such a coincidence in the 

 results can not, however, be always expected to occur when 

 we consider the nature of the means which are employed for 

 determining the internal heat of the earth at definite depths. 

 Although we may assume that the water which is infiltrated 

 in elevated positions through hydrostatic pressure, as in con- 

 nected tubes, may influence the rising of springs at points of 

 great depth, and that the subterranean waters acquire the 

 temperature of the terrestrial strata with which they are 

 brought in contact, the water that is obtained through bor- 

 ings may, in certain cases, when communicating with vertic- 

 ally descending fissures, obtain some augmentation of heat 

 from an inaccessible depth. An influence of this kind, which 

 is very different from that of the varying conductive power 

 of different rocks, may occur at individual points widely dis- 

 tant from the original boring. It is probable that the waters 

 in the interior of our earth move in some cases within limit- 

 ed spaces, flowing either in streams through fissures (on which 

 account it is not unusual to find that a few only of a large 

 number of contiguous borings prove successful), or else follow 

 a horizontal direction, and thus form extensive basins a re- 

 lation which greatly favors the labor of boring, and in some 

 rare cases betrays, by the presence of eels, muscles, or vege- 

 table remains, a connection with the earth's surface. Al- 

 though, from the causes which we ha^e already indicated, 

 the ascending springs are sometimes warmer than the slight 

 depth of the boring would lead us to anticipate, the afflux 

 of colder water which flows laterally through transverse fis- 

 sures leads to an opposite result. 



It has already been observed that points situated on the 

 same vertical line, at an inconsiderable depth within the in- 



* In a table of fourteen borings, which were more than one hundred 

 yards in depth, and which were situated in various parts of France, 

 Bravais, in his very instructive encyclopedic memoir in the Patria, 

 1847, p. 145, indicates nine in which an increase of temperature of 

 1 F. is found to occur for every 50-70 feet of depth, which would 

 give a deviation of about 10 feet in either direction from the mean 

 value given in the text. See also Magnus, in Poggen., Ann., bd. xxii., 

 1831, s. 146. It would appear, on the whole, that the increase of 

 temperature is most rapid in Artesian wells of very considerable depth, 

 although the very deep wells of Monte Massi, in Tuscany, and Neuffen, 

 on the northwest part of the Swabian Alps, present a remarkable ex- 

 ception to this rule. 



