480 METEOROLOGY. — TEMPERATURE. 



In the hottest countries of the earth, the summits of very lofty 

 mountains are constantly 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 elevation is the inferior limit of perpetual 

 snow. 



The accidental causes which tend to modify the temperature of a 

 climate, also act in raising or lowering the snow-line. On the south- 

 em slope of the Himalaya, for example, the snow-line does not de- 

 scend 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, consequently, has the same 

 eSect upon climate as increase in latitude. Upon mountain ranges, 

 vegetation undergoes modification in its forms, becomes decrepit, 

 and disappears 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 variation which occurs in 

 the temperature of the atmosphere under the equator, enables us to 

 indicate with some precision the point of mean temperature below 

 which there is no longer any vegetation. In ascending Chimbora- 

 zo I met with this point at the height of 15,774.5 feet, where the 

 mean temperature approached 35° F., and where consequently the 

 saxifrages, which root among the rocks, must still receive a temper- 

 ature of from 41° to 43^ F. during the day, inasmuch as far beyond 

 the inferior snow-line, at an elevation of 19,685 feet above the sea- 

 line, I saw a thermometer suspended in the air, and in the shade 

 mark 44.6° F. 



In considering the extension of vegetation towards the polar re- 

 gions, we discover plants growing in very high latitudes in places 

 which have a mean temperature much below that which I believe to 

 be the limit of vegetable life on the mountains of the equatorial region. 

 In these rigorous climates vegetation is suspended by the severity 

 of the cold during the greater portion of the year ; it is only during 

 the brief and passing heat of summer that the vegetable world 

 wakes from its long winter sleep. Nova Zembla, lat. 73° N., the 

 mean temperature of whose summer is between 34° and 35^^ F., is, 

 perhaps, like the perpetual snow-line of the equator, the term of 

 vegetable existence. It is also to the very remarkable heat of the 

 summer in countries situated at the nothern extremity of the con- 

 tinent of Asia, remarkable if it be contrasted with the intensity of 

 the winter cold, that man succeeds in rearing a few culinary vegeta- 

 bles in those dreadful climates. At Jakoustk, in 62° of N. lat., and 

 where mercury is frozen during two months of the year, the mean 

 temperature of summer is very nearly 64^ F. We have here as M. 

 de Humboldt observes, " a well-characterized continental climate," 

 examples of which indeed are frequent in the north of America. At 



