ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 299 



E, * the point of constant temperature is near the surface, because 

 the annual variations are comparatively small. The average 

 temperature of the warmest month in Singapore is 22.40 ° R., of 

 the coldest month 20.6 ° R. f The yearly variation therefore, 

 does not exceed 1.8 ° R., and consequently the point where the 

 extremes equalize themselves must be very near the surface. In 

 higher latitudes however, where the variations are greater, (London 

 11,80 P R., Paris 13.50° R., New York 21.70 ° R.,) the point 

 of invariable temperature lies deeper. In the temperate zone, the 

 daily variations disappear at a depth of from three to five feet, and 

 the annual variations at a depth of from 60 to 80 feet beneath 

 the surface. The celebrated thermometer placed 86 feet beneath 

 the surface in the vault of the national observatory at Paris in 

 1783, shews constantly a temperature of 9.60° R.J Since the 

 average temperature of Paris is 8.60 ° R., it would therefore appear 

 that even at this depth of 86 feet the influence of the central 

 heat begins to make itself felt. 



As early as the year 1678, the Jesuit Athanasius Kircher was 

 informed by Hungarian miners that a higher temperature existed 

 in the depths of mines, than on the surface of the earth, and 

 Von Trebra, in 1785, mentions the same fact.§ Not only was 

 practical experience of the existence of a subterranean source of 

 heat first obtained by miners, but the first experiments made 

 with the view of ascertaining the temperature of the earth's 

 crust at greater depth, were instituted in mines. The results of 

 these experiments constituted for a long time the only proofs of 

 the increase of the temperature with the depth. It cannot be 

 denied however that the observations made in the shafts and 

 underground working of mines are subject to various disturbing 

 influences, so that it would appear that at least the earliest 

 of these observations are less to be relied upon than those from 

 other sources. But since they shew a general coincidence they 

 furnish, when taken in connection with other observations, a com- 

 plete confirmation of the fact of the increase of temperature with 

 the depth. The results of the experiments instituted in mines, 

 differ in value according as they have reference to the tempera- 



* Pouillet ; Muller, Lehrbueh der Physik and Meteorologie, Vol II, 

 p-. -724. 



t Ibid II, 716. 



t Quenstedt, Epochen der Natur, p. 13. 



§ Ibid p. 12. 



