ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 301 



5. That the observations did not afford data sufficiently decided 

 for the establishment of any law as to the progression of 

 the increase of temperature. 



The experiments instituted in the Saxony mines were under th 

 careful direction of Reich, and made with very good thermome- 

 ters, sunk forty inches into the solid rock, and with every possible 

 precaution. They yielded the following results.* 



1. That the temperature increases decidedly with the depth. 



2. That the temperature is invariable at every one point of 



observation. 



3. That the average distance, corresponding to an increase of 



one degree Reaumur in temperature, is 129 feet. 



4. That a general law with regard to the relative increase of 



temperature cannot be deduced from these experiments. 



5. That the rock in the underground workings and in the course 



of time becomes somewhat cooled by the air of the mine, 

 and that on the whole the cooling influences overbalance 

 the heating ones. 



Among other observations the following may be mentioned :— 

 The distance corresponding to an increase of temperature of one 

 degree was determined by : 



Oldham in Waterford, Ireland, as 165 Feet. 



Phillips in New Castle 100 " 



Hodgkinson in Manchester... '..... 115 " 



Einzeau in Belgium 102 " 



Gordier near Ganneaux Ill "f 



It will be observed that in the observations mentioned above, 

 the depth corresponding to an increase of 1° varies from 92.3 

 to 167 feet. 



Conclusive as are the experiments in mines with regard to 

 the increase of temperature, they after all refer only to compara- 

 tively slight depths. The depths at which observations have been 

 made in artesian wells exceed those of the mine experiments. As 

 is well known, by means of these artesian wells, a vent is opened 

 whereby the water of subterranean reservoirs or springs, confined 



* Reich : Beobachtungen iiber die Temperatur des Gesteines, 1834. 

 f Naumann ; Lehrbuch der Geognosie, I, 54. 



