12 



matter, 17,290 pounds of water must be absorbed by roots from the soil and 

 evaporated from the leaves — for all mineral matter in plants comes from 

 •water containing it in solution — in other words, to produce one pound of 

 vegetable matter, 368 pounds of water must be evaporated. Now, if we 

 assume, about 6000 pounds as the annual product of an acre of land in wood, 

 leaves, &c., a very simple calculation will show, that a rain fall of nine inches 

 is required to supply the water which must necessarily be consumed to pro- 

 duce the mineral matter annually formed or deposited in the tissues of plants ; 

 a quantity exceeding one-fourth of the whole annual quantity of rain falling 

 in this state. 



The process of evaporation of water is very much accelerated by wind ; for 

 when the air is still it soon becomes so nearly saturated with moisture that 

 no more can be absorbed from the soil, or from leaves : but when this moist 

 air is removed by the wind the absorption of moisture continues. 



The ground under the forests being shaded and kept cool, the evaporation 

 from the surface is very much diminished, and the water of rains remains 

 longer in the ground to, supply the absorplion of roots, within the forests, 

 than upon open fields. 



Another source of increased moisture under trees is the coolness of the 

 leaves at night causing a deposition of dew, sometimes in such quantities as 

 to cause drops of water to fall to the ground like rain. 



HOW TREES AFFECT THE SPRINGS, STREAMS AND RAIN OF A 



COUNTRY. 



From what has already been said it is evident that dearing away the forests 

 diminislm tlie Jlcm of water from springs In the woods the water is retained 

 in the soil, evaporation being prevented by the shade, wlyle in the fields the 

 water is rapidly evaporated, or, not being impeded by vegetation, runs off to 

 the river and water-courses more rapidly, thus giving less time to penetrate 

 the soil and supply subterranean passages to springs. There are well attested 

 instances where springs have been dried up in consequence of the clearing 

 of adjacent forests. 



Forest growths are regarded by many of our ablest physicists as exerting 

 a marked influence over the amount of rainfall in such a region. The fact 

 that all countries abundantly clothed with forests are also well supplied with 

 rain and the rain is equally distributed through the season of vegetable 

 growth, would naturally lead to the conclusion that forest growths have some 

 agency in determining this rainfall. Others have claimed that the equable 

 rainfall was the effect of other causes, and that the abundant forest growth 

 was the effect of the abundance and distribution of the rain, and not the de- 

 termining cause of the rain. But whichever may be placed as antecedent or 

 consequent, abundant and well distributed rain is found associated with large 

 forest growths; and if these forest growths are extensively removed, the 

 rains diminish or become capricious, droughts and flood alternating, while in 



