METEOROLOGY. — ^TEMPERATURE. 479 



covered by an immense mass of everlasting snow, is cut by the 

 equinoctial line itself. 



The cold which prevails among lofty mountains, is ascribed to the 

 dilatation which the air of lower regions experiences in its upward 

 ascent, to a more rapid evaporation under diminished pressure, and 

 to the intensity of nocturnal radiation. 



Places which are situated upon the same mountain-chain, nearly 

 in the same latitude, and at the same height, have often very differ- 

 ent climates. The temperature which would be proper to a place 

 perfectly isolated, is necessarily modified by a considerable number 

 of circumstances. Thus the radiation of heated plains of considera- 

 ble extent, the nature of the color of the rocks, the thickness of the 

 forests, the moistness or dryness of the soil, the vicinity of glaciers, 

 the prevalence of particular winds, hotter or colder, moister or drier, 

 the accumulation of clouds, &c., are so many causes which tend to 

 modify the meteorological conditions of a country, whatever its 

 mere geographical position. The neighborhood of volcanoes in a 

 state of activity does not appear to affect the temperature sensibly : 

 thus Purace, Pasto, Cumbal, which have flaming volcanoes towering 

 over them, have not warmer climates than Bogata, Santa Eosa, De 

 Osos, Le Param de Herve, &c., situated on sand-stone or syenite. 



From the whole series of observations which I had an opportunity 

 of making on the Cordilleras, it appears that one degree of tempera- 

 ture, cent., 1.8° F., corresponds to 195 metres, or 649.4 feet of 

 ascent among the equatorial Andes. In Europe, it has been ascer- 

 tained that the decrease of temperature in ascending mountains, is 

 more rapid during the day than during the night — during summer 

 than during the winter ; for example, between Geneva and Mount 

 St. Lernard, to have the Fahrenheit thermometer fall one degree, it 

 is necessary to ascend : 



In spring 826.1 feet. 



In summer 886.6 



In autumn • 882.2 



In winter. 422.2 



It sometimes happens, however, that in winter, in a zone of no 

 great elevation, the temperature increases with the elevation — a fact 

 which Messrs. Bravais and Lottin observed in the 70° of N. lat., in 

 calm weather ; at an elevation between 1312 and 1640 feet, the rise 

 amounted to as many as 6° centigrade, 10.8° Fahrenheit. 



In no part of the globe is the diminution of temperature, occasion- 

 ed by a rise above the level of the sea, more remarkable than among 

 equatorial mountain ranges ; and it is not without astonishment that 

 the European, leaving the burning districts which produce the banana 

 and cocoa-tree, frequently reaches, in the course of a few hours, the 

 barren regions which are covered with everlasting snow. " Upon 

 each particular rock of the rapid slope of the Cordillera," says M. 

 de Humboldt, " in the series of climates superimposed in stages, we 

 find inscribed the laws of the decrease of caloric, and of the geo- 

 graphical distribution of vegetable forms."* 



 Humboldt's Central Asia, vol. lit, p. 286. 



