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On the Cause of the Temperature of Hot and Thermal Springs ; 

 and on the bearings of this subject, as connected with the ge- 

 neral question regarding the Internal Temperature of the 

 Earth. By Professor Gustav Bischof of Bonn. Commu- 

 nicated by the Author. (Concluded from p. 164.) 



Chap. XVIII. — In what proportion to the height above the lowest 

 points of the Earth's surface is the decrease of the Temperature 

 of the Air and of the Soil ? and is this Decrease the same at all 

 parts of the Earth ? 



There are various causes which influence the decrease of tem- 

 perature with the height. Such are, 



1st, The configuration of the earth's surface. 



2dly, Local circumstances and climate. 



3dly, The seasons. 



4thly, The time of day. 



Let us examine these causes more closely. 



De Saussure* already remarked the more rapid decrease of 

 the temperature on steep mountains, and Von Humboldt was 

 led to the same conclusion, by comparing the temperature of 

 the air on elevated plateaux, with that of the summits of moun- 

 tains. He found, namely, that such plateaux, or narrow val- 

 leys, were warmer than the steep declivities of mountains, or 

 than such strata of air which, instead of being in immediate con- 

 tact with projecting parts of the earth's surface, rested upoa 

 other strata of air. On the other hand, the decrease of tempe- 

 rature from the level of the sea will be slower on extensive and 

 gently rising chains of mountains, than on mountains having a 

 steep ascent. As the lofty American chains of the Peruvian 

 and Mexican Andes, to the height of St Gothard, form one im- 

 mense mass, and up to that height comprise a surface of 120 

 square geographical miles, the decrease of temperature in the 

 inhabited regions o* the Andes is slow, but becomes suddenly 

 more rapid where the less massive rocks rise 10,800 to 12,000 

 feet in isolated cones upon their broad basis. Thus Von Hum- 

 boldt found a decrease of temperature of 2°.25 in 996 feet, be- 

 tween the level of the sea and the Indian village of Calpi ; 



* Voyages dans les Alpes, iv. § 935- 



