No. 3. 



Decrease of Temperature in the Atmosphere. 



77 



Decrea'-e of 'reini)crature in the superior 

 Strata of the Atmosphere. 



The temperature rises rapidly as we as- 

 cend in the atmosphere; places atuong the 

 mountains always possess a climate more 

 severe as tliey are higher above the level of 

 the sea. Even under the equator, height of 

 position modifies the seasons so much, that 

 the hamlet of Antisana, which is within one 

 degree of south latitude, but which is up- 

 wards of l;3,()00 feet above the sea level, 

 has a mean temperature which does not 

 .■differ much from that of St. Petersburgh. 

 Near it, but at a still greater height, the 

 summit of Cyambe, covered by an immense 

 mass of everlasting snow, is cut by the equi- 

 noctial 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 evapo- 

 ration 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 dif- 

 ferent climates. The temperature which 

 would be proper to a place perfectly iso- 

 lated, is necessarily modified by a consider- 

 able number of circumstances. Thus the 

 radiation of heated plains of considerable 

 extent, the nature of the colour of the rocks, 

 the thickness of the forests, the moistness 

 or dryness of the soil, the vicinity of gla- 

 ciers, the prevalence of particular winds, 

 hotter or colder, moister or drier, the accu- 

 mulation of clouds, &c., are so many causes 

 which tend to modify the meteorological 

 conditions of a country, whatever its mere 

 geographical position. The neighbourhood 

 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 Bogota, Santa 

 Rosa, 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 

 temperature, cent., 1.8° F.,* corresponds to 

 195 metres, or 649.4 feet of ascent among 

 the equatorial Andes. In Europe, it has 

 been ascertained that the decrease of tem- 

 perature 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. Bernard, to have the Fahrenheit ther- 



* 1° of Centigrade thermometer is equal to 1.8° F. 



mometer foil one degree, it is necessary to 

 ascend : 



In spring . 32G.1 feet. 



Ill summer 'XHiSt 



In autumn :)H2.2 



In winter 4'i;J.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 ob- 

 served in the 70° of N. lat., in calm wea- 

 ther; 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, occasioned by a rise above 

 the level of the sea, more remarkable than 

 among equatorial mountain ranges; and it 

 is not without astonishment that the Eu- 

 ropean, leaving the burning districts which 

 produce the banana and cocoa-tree, fre- 

 quently reaches, in the course of a few 

 hours, the barren regions which are covered 

 with everlasting snow. " Upon each par- 

 ticular rock of the rapid slope of the Cor- 

 dillera," 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 geographical distribu- 

 tion of vegetable forms." 



In the hottest countries of the earth, the 

 summits of very lofty mountains are con- 

 stantly covered with snow; in the elevated 

 and cold strata of the atmosphere, the watery 

 vapor is condensed, and falls in the state of 

 hail and snow. In the plain, hail melts 

 almost immediately; the fusion is slower 

 upon the mountains ; and for each latitude 

 there is a certain elevation where hail and 

 snow no longer melt perceptibly. This ele- 

 vation is the inferior limit of perpetual 

 snow. 



The accidental causes which tend to mo- 

 dify the temperature of a climate,, also act 

 in raising or lowering the snow-line. On 

 the southern slope of the Himalaya, for ex- 

 ample, the snow-line does not descend so 

 low as it does upon the northern slope ; and 

 in Peru, from 14° to 16° of S. latitude, Mr. 

 Pentland found the perpetual snow-line, at 

 an elevation of 1312 feet higher than it is 

 under the equator. 



Elevation above the level of the sea, con- 

 sequently, has the same effect upon climate 

 as increase in latitude. Upon mountain 

 ranges, vegetation undergoes modification 

 in its forms, becomes decrepit, and disap- 

 pears towards the line of perpetual snow, 

 precisely as it does within the polar circle, 

 and for no other than the same reason, viz., 

 depression of temperature. 



The constancy and the small extent of 



