176 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



and the atmosphere influences the temperature of the ground 

 only to a limited depth below the surface. It was determined 

 during the eighteenth century that external influences are 

 perceptible only within depths of abou.t 30 feet, or as far 

 down as 80 feet, according to the geographical position of the 

 locality. At the so-called "neutral" zone, or critical horizon 

 of depth, there is a constant temperature which practically 

 corresponds with the average annual temperature of the par- 

 ticular place. Below this zone of constant temperature, the 

 temperature increases in mines, and the increase can only be 

 attributed to the earth's own heat. This increase of tempera- 

 ture had already been noted by Kircher and Boyle in the 

 seventeenth century, but it was not until 1740 that definite 

 observations were made by Gensanne in the lead-mines of 

 Giromagny in the Vosges. Gensanne's result demonstrated 

 an increase of i -C. for 114 feet of depth. Measurements 

 were made in 1790 and 1791 in the Freiberg mines by Freies- 

 leben and Alexander von Humboldt ; Lean took observations 

 in the Cornwall mines, Fantonetti in Italian mines, and 

 Alexander von Humboldt in South American and Mexican 

 mines. All these observations were based upon the tempera- 

 ture of the air in the mines. But, as it was pointed out by 

 Cordier and Reich, this temperature is influenced by air 

 currents, by the mining work, and by the breath of the miners 

 and of animals. Cordier and Reich then placed the thermo- 

 meter in the rock itself, and taking necessary precautions for 

 correction of experiments, arrived at results of a more reliable 

 character. Cordier reports from French mines an average 

 increase of temperature of i C. for 25 metres (circa 77 feet), 

 while Reich reports grades of 41.84 metres (circa 129 feet). 



Since 1828, temperature observations have been continuously 

 taken in the mines of Saxony and Prussia, and these yield an 

 average of i C. for 167 feet, but as the variations range from 

 48 to 355 feet, it is impossible to draw any definite law. In 

 England, the British Association for the Advancement of 

 Science about twenty years ago appointed a special commis- 

 sion for investigations of the ground temperatures, and the 

 relative capacities of heat conduction shown by different 

 rocks. A great number of observations have also been con- 

 tributed by other lands, but as yet no definite results have 

 been obtained. The ground-borings made in various countries 

 have afforded a means of taking observations on the increase 



