142 Prof, Bischoff on the Temperature of 



of temperature of 2°.25 occurred in 123.3 feet, and from observa- 

 tions in two metalliferous mines, the depth corresponding to an 

 increase of 2°.25 was 325.5 feet. 



Supposing that the temperature in the upper station of the 

 metalliferous mines, where observations on the increase of 

 temperature were made, should be found 2''.92 higher than in 

 the coal mines, whilst the temperature in the inferior station 

 of both mines would be the same, the depth, corresponding to 

 an increase of 2°.25, would be in the metalliferous mines also 

 123.3 feet. 



It would be difficult to make a correction for this not unim- 

 portant influence on the increase of temperature, proceeding 

 from the unequal power of conducting heat of different rocks. 



Chap. XVII. — Can the increase of Temperature towards the in- 

 ferior of the Earth Jollow the same law in all parts of the Earth, 

 departing from points all situated in the same level? 



According to Chap. VIII., the influence of the external tem- 

 perature under the equator scarcely extends to the depth of 

 twelve inches below the surface of the earth ; whilst in the 

 temperate zones it extends to a depth which has not yet been 

 ascertained, but which in our latitudes, perhaps, amounts to 

 sixty feet. Curves may, therefore, be supposed to be drawn 

 from the equator to the poles ; at the former reaching to with- 

 in one foot of the surface ; in our latitude lying at a depth of 

 about sixty feet, and extending at the poles to a still greater 

 depth, and thus describing the limit of the influence of the 

 external temperature. A constant regular increase of tempe- 

 rature, dependent only on the interior temperature of the earth, 

 can, therefore, only be supposed to commence below this limit ; 

 the crust of the earth above the line which marks the limit 

 being exposed at the same time to the influence of the external 

 temperature of the air, and of the interior heat of the earth. 

 But under the equator the former influence disappears, and the 

 increase of temperature depends solely upon the interior heat 

 of the earth. In higher latitudes, on the other hand, botli in- 

 fluences are found to operate. This we find by observations 

 on the temperature of those springs which have their origin 

 within this limit ; namely, those springs observed by me, and 



