64 EFFECTS OP FORESTS ON HUMIDITY. 



At the station of Belle-Fontaine, M. Mathieu measui'ed the evapora- 

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

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

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

 subsequent months the following results were obtained : — 



WATER EVAPORATED. 

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



miUimUres. 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 Risler on 

 the evaporation from cultivated soils and the exhalation and exuda- 

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

 des Sciences ( JbiUiotheque universdLe 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 and 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 re- 

 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 aud one tenth millimetre per day to be the maximum from 

 a forest of firs under exceptionally favourable conditions of a fertile 

 and humid soil and abundance of sunlight. 



Risler, in the experiments referred to found that in 1867, not ftir 

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



