3G8 ANNUAL OF SCIENTIFIC DISCOVERY. 



presence of mountains. Although mountainous districts do not invariably 

 receive a Drearer annual amount of rain than other places equally near to 

 the equator and the ocean, it may be considered a general rule, that they are 

 visited by more numerous showers, and are more exempted from the occur- 

 rence of long-continued droughts. Very extensive plains, on the contrary, 

 generally receive their supply of rain in a few excessive showers, and very 

 frequently suffer much from the long continuance of dry weather. These 

 different' peculiarities seem to show that mountains serve not only to cool 

 the air, but also to neutralize the insulating power of its lower stratum, and 

 to prevent the accumulation of electricity in the region of the clouds. As we 

 have reason to believe that the violence of storms is, in part, due to electri- 

 cal action, which serves to maintain ascending currents in the air, and to 

 cause the condensation of vapor to take place more abundantly, mountains 

 must evidently have the effect of preventing excessive showers, and of dis- 

 pensing the aqueous resources of the atmosphere with more economy for the 

 purposes of vegetation. 



It is to the growth of perennial plants that long-continued droughts are 

 most detrimental. Though the grass may be parched on a dry summer, the 

 injury is scarcely felt during the following year, if rains be abundant; but 

 in the vegetation of trees the result is different. The imperfect layer of wood 

 formed under such unfavorable circumstances, will have a great tendency to 

 decay prematurely; and, as the decay must quickly extend to the whole 

 vegetable structure, the droughts of a single summer may destroy the work 

 of a century. From several facts it appears that the lignifying process in 

 trees is dependent on feeble currents of electricity, which are made to circu- 

 late along their tissues by the evaporation from the leaves, combined with 

 the chemical changes transpiring in the soil; and accordingly, when this 

 evaporation is checked on account of the want of rain, an imperfect forma- 

 tion of wood will be the inevitable consequence. The health of trees will 

 therefore depend on the frequency of rain, on the extent of foliage which 

 they possess, and also on the fertility of the soil ; for when the vegetable 

 nutriment is more abundantly supplied, a greater amount of elaborating 

 energy is required to convert it into wood. 



Whatever opinion may be formed respecting these theoretical views, the 

 characters of arborescent vegetation in different regions exhibit the effects of 

 the meteoric conditions which I have noticed. The most durable timber 

 and the oldest trees are to be found in islands, in mountainous districts, in 

 lands contiguous to the sea, and in other places where they are favored by 

 the frequency of rain. It is well known that the oaks of the British Isles are 

 much superior in strength and durability to those produced in the interior of 

 Europe; and many trees in England are remarkable for their great age, 

 which in some cases is known to exceed one thousand years. More extraor- 

 dinary instances of vegetable longevity are to be found in the island of Ten- 

 erifFe, in Sicily, and on the coast of Africa. The mountains on the Syrian 

 coast have been celebrated for the great age of their cedars. But perhaps no 

 forests furnish a greater abundance of trees, or more durable timber, than 

 those of Guiana; and scarcely any region on the globe is visited by more 

 frequent rains. Doubtless the age and durability of trees depend on the 

 species to which they belong; but when the comparison is confined to trees 

 of the same species, it will be seen that they are affected, in a very serious 

 degree, by the meteoric influences to which they are exposed. 



That the absence of mountains is unfavorable to the health of arbores- 



