EXHALATIOT^ OF VAPOE BY TREES. 185 



mean precipitation during the period of forest growth — that is, 

 from the sweUiug of the buds in the spring to the ripening of 

 the fruit, the hardening of the young shoots, and the full perfec- 

 tion of the other annual products of the tree — exceeds on the 

 average twenty-four inches. Taking this estimate, the evapora- 

 tion from the forest would be equal to a precipitation of two hun- 

 dred and forty inches, or very neai-ly one hundred and fifty stand- 

 ard gallons to the square foot of surface. 



The first questions which suggest themselves upon this state- 

 ment are : what becomes of this immense quantity of water, and 

 from what source does the tree derive it ? We are told in reply 

 that it is absorbed from the air by the humus and mineral soil of 

 the wood, and supplied again to the tree through its roots, by a 

 circulation analogous to that observed in Ward's air-tight cases. 



When we recall the effect produced on the soil even of a thick 

 wood by a rainfall of one inch, we find it hard to believe that 

 two hundred and forty times that quantity, received by the 

 ground between early spring and autumn, would not keep it in 

 a state of perpetual satm-ation, and speedily convert the forest 

 into a bog. 



No such power of absorption of moisture by the earth from the 

 atmosphere, or anything approaching it, has ever been shown by 

 experiment, and all scientific observation contradicts the supposi- 

 tion. Schiibler found that in seventy-two hours thoroughly dried 

 humus, which is capable of taking up twice its own weight of wa- 

 ter in the hquid state, absorbed from the atmosphere only twelve 

 per cent, of its weight of humidity ; garden-earth five and one- 

 fifth per cent, and ordinary cultivated soil two and one-third per 

 cent. After seventy-two hours, and, in most of his experiments 

 with thirteen different earths, after forty-eight hours no further 

 absorption took place. Wilhelm, experimenting with air-dried 

 field earth, exposed to air in contact with water and protected by 

 a bell-glass, found that the absorption amounted in seventy-two 

 hours to two per cent, and a very small fraction, nearly the whole 

 of which was taken up in the first forty-eight hours. In other ex- 



and again after an exposure to the air for three minutes, and computing the 

 superficial measure of all the leaves of the tree, concludes that an oak tree 

 evaporates, during the season of growth, eight and a half times the mean 

 amoxmt of rain fall on an area equal to that shaded by the tree. 



