64 EFFECTS OF FORESTS ON HUMIDITY. 



At the station of Belle-Fontaine, M. Mathieu measured tbe evapora- 

 tion by means of vessels exactly comparable placed outside of a w66d, 

 and under' a'leafy wood. During the months of January and' February 

 no observations could be made on account of the frost. But in the 

 subsequent months the following results were obtained : — 



WATEB EVAPOBATED. 

 Months. Outside the wood. Under the wood. Proportion, 



millimUrts. "mm. mm. 



March, 33 9 3.66 



April, 50 19. 2.63 



May, 105 23 4.56 



June, 107 19 5.63 



July, 95 10 9.50 



August, 75 8 9.37 



September, 55 11 5. 



October, 12 3 4. 



November, 2 9. 



December, 8 4 2. 



Totals, 542 106 5-11 



From this it is seen that during ten months in 1868 the evapora- 

 tion was more than five times greater in the open space than it was 

 in the forest, which consists of high perches, dense and close, of horn- 

 beams, beeches, oaks, and ashes of sixty-two years' growth. 



There are cited by Marsh some valuable observations by Eisler on 

 the evaporation from cultivated soils and the exhalation and exuda- 

 tion of humidity by field plants and forest trees, given in the ArcKives 

 ■des Sciences (Bihliotheque universclle de Geneve) for Sept. 15, 1869 ; 

 March 25, 1870; and Nov. 15, 1871, which seem to lead to this 

 general conclusion, that forests evaporate less than an equal extent of 

 pasturage, and that if we suppose a mean precipitation of two aud a 

 half millimetres per day, of which two millimetres penetrate into the 

 soil, the forest takes up less than one half of this supply, the i:e- 

 mainder descending into the sub-soil and percolating through earth 

 and rock until it issues in the form of springs. He found an evapora- 

 tion of one and one tenth millimetre per day to be the maximum from 

 a forest of firs under exceptionally favourable conditions of a fertile 

 and humid soil aud abundance of sunlight. 



Eisler, in the experiments referred to found that in 1867, not far 

 from Geneva, no water escaped from a parcel of ground thoroughly 



