288 Prof. Bischof o;i the Temperature of 



have influenced the observations. In this manner he found, 

 for an increase of temperature of 2°.25 F. (1° R.), a depth of 

 171 — 6.9 feet. In order to judge of the probability of a con- 

 stant error, he examined whether the warming or the cooling 

 influences predominated. He found that the error was on the 

 side of the latter, and that the increase of temperature was con- 

 sequently found too small, which seems also to accord with the 

 observations in other places. Although in several mines ob- 

 servations have been made at more than two points, one below 

 the other, it cannot be determined whether the increase of tem- 

 perature is proportional to the depth. Sometimes the tempe- 

 rature increases, sometimes decreases with the depth ; some- 

 times it decreases at first, and afterwards increases again, and 

 vice versa. 



The thermometries observations hitherto made in mines 

 have, then, given such very various values for the increase of 

 temperature towards the interior, that it is impossible from them 

 to discover the law. This is by no means surprising when we 

 consider how numerous are the causes which tend to discom- 

 pose that law, and which have been treated of in Chapters IX, 

 XI, XII, to XVII. We have already seen, from the reflec- 

 tions in Chap. XVII. what a considerable influence is due to 

 the configuration of the earth. Some of the observations may 

 have been made in mines excavated on the declivity of steep 

 mountains, in which case, as is shewn in Fig. V, PI. II, a 

 very protracted increase of temperature would take place. A 

 shaft sunk from C to e, for example, would have given an in- 

 crease of 2°.25, 2°.65— 4°.4 F. (1° R.) in 249 feet, whilst shafts 

 at c and d would have given 122 — 6.7 feet. If the configura- 

 tion of the surface of the earth, at each point of observation, 

 were known, we should be enabled, by means of the formula 

 developed in Chap. XVII., to correct the modifications of the 

 law thereby effected. But how is it possible to bring into cal- 

 culation the numerous other influences .-• 



However, almost all observations indicate that there exists 

 an inward heat, which is peculiar to the earth itself, which does 

 not proceed from the sun's rays, and increases rapidly with the 

 depth ; for the few observations which have given a contrary 

 result cannot be brought forward as proofs against the gene- 



