EXPERIMENTS ON EVAPORATION. 



n 



"Tbe MarSohal Vaillant has frequently drawn public attention 

 to ^rest hydrology. lie instituted experiments to measure the 

 quantity of rain which 'is arrested by the foliage of trees; arid the 

 fo|16wmg are some of the results he obtained ; the observations were 



made in t\xe lorest of Foritainebleau, during the year 1866 :— *^ 



Montha. 



QUANTITY OP BAINPALL. 



in open air. of 30 years' growth. 

 millimitrM, mm. 



Proportion. 



January,.., 35 24.5 or -70 



February,...., 78-5. 



Maroh,. 80-8. 



Spril, 67 . 



May, 46-2. 



June, 66 ., 



July 105 8. 



August 117 ., 



Sejrtember, 123-5.:. 



.63 



.58-2. 



.50.9. 



.31-5. 



.32 .. 



.53'8. 



.60 ., 



.65 . 



October, 31 14-2. 



November, 47-5 29 . 



December, 61-2 43-5. 



•81 

 •72 

 •76 

 •68 

 •48 

 •50 

 •51 

 •51 

 •45 

 •62 

 ,•70 



859-5 



524-7 



0-60 



" From this it is seen that the leaves arrested in winter 30 %, in 

 summer S0%, and, on an average, throughout the year 40% of the 

 rain which fell. 



" Under the Epio4as [Picea excelsa, Zinh] of 35 years' growth, 

 probably in dense clumps, the pluviomfetre received only 21% of the 

 years' rain-fall : the tree arrested 79 %. In a forest adjacent to 

 Versailles, the pluviomfetre, placed under a leafy wood, received 80% 

 of the year's rain: the foliage arrested only 20%. Under a timber 

 forest of oaks, of from 70 to 100 years of age, the instrument 

 received 85 % of the year's rain. 



" Perhaps there may be ground for some little discussion in regard 

 to these experiments : the position in which the pluviomfetre might 

 Jiave a marked influence on the results. Two instruments placed 

 under an umbrella, one towards the centre, the other at the circum- 

 ference, under the droppings of the ribs, would give a bad measure- 

 ment of the mean fall of water on the soil. 



* Eevue des Earn et Forets. 1867. P. 161. 



