26 ALTITUDE. 



the forest. This phenomenon is often noticed upon 

 different sides of the same hill. In the latter case, 

 it cannot certainly be the soil which causes such 

 varied results; but we must refer them to atmos- 

 pheric agencies. 



Among the most important facts to be determined 

 is the effect of different altitudes upon vegetation. 

 It does not require more than a child's discrimina- 

 tion to discover that the temperature upon the sum- 

 mit of a mountain is much lower than at its base, 

 and therefore that a fruit which flourishes at its 

 foot would not do so if planted in the most pro- 

 pitious place upon its top. Naturalists tell us that 

 if we start where the first glimpses of vegetation 

 appear at the frigid zone, and approach the tropics, 

 we shall perceive all the different stages of the vege- 

 table creation, in the same order in which they were 

 placed upon the earth. As we ascend a mountain 

 the effect is reversely the same. Hence it becomes 

 evident that to determine properly the limit for the 

 cultivation of a plant, it would be useless to draw 

 parallel lines, like those which we use for latitude, 

 because the same line would pass over valley and 

 mountain in which the temperature would vary 

 several degrees. 



In assigning a position to the isothermal lines, 

 now quite common in geographical maps, there 

 are several o-tlier facts to be considered. 



Aspect^ or exposure, exerts a powerful influence 



