56 



rat has been repeated by a great number of 

 travellers. When we descend from a high 

 chain of mountains, and advance toward the 

 poles, we find at first in plains of little height, 

 and finally in the regions near the coasts, the 

 same arborescent plants*, which in the low la- 

 titudes cover only the heights near the perennial 

 snows. 



In estimating the rapidity with which the 

 mean temperature of the atmosphere diminishes 

 in proportion as we proceed from the equator 

 to the poles, or from the surface of the earth to 

 the high regions of the aerial ocean, I have con- 

 sidered the decrement of heat as following an 

 arithmetrical progression. This supposition is 

 not perfectly accurate with respect to the air-f- ; 



* In the study of the geographical relations of plants, we 

 must distinguish between those vegetables, the organization 

 of which resists great changes of temperature and barometric 

 pressure, and those plants which appear to belong only to 

 certain zones at certain heights. This difference is still more 

 sensible in the temperate zone than under the tropics, where 

 the herbaceous plants are less frequent, and where the trees 

 are stripped of their leaves only by the effect of the dryness 

 of the air. We see some vegetables push their migrations 

 from the northern coasts of Africa over the Pyrenees as far 

 as the downs near Bordeaux, and the basin of the .Loire ; for 

 instance, the merendera, the late-flowering hyacinth, and the 

 hoop-petticoat narcissus, narcissus bulbocodium. Annales du 

 Mus., t. iv, p. 401. 



t The mean temperatures augment from the equator to the 

 poles, nearly as the square of the sine of the latitude, (Journ. 



