EXPERIMENTS ON EVAPORATION. 6^; 



"The Mardchal Vaillaut has frequently drawn publio attention 

 to forest hydrology. He instituted experiments to measure the 

 quantity of rain which is arrested by the foliage of trees ; and the 

 following are some of the results he obtained ; the observations were 

 made in the forest of Fontainebleau, during the year 1866 : — *_ 



QUANTITY OP RAINFALL. 

 Months. In open air. Sfo'lar&th. P-PO'^on, 



millimetres. mm. mm. 



January, 35 24.5 or -70 



February...... 78-5 63 -81 



March, 808 582 -72 



April, 67 50.9 -76 



May, 46-2 31-5 -68 



June, 66 32 -48 



July, 105 8 53-8 -50 



August, 117 60 -51 



September, 1235 65 -51 



October, 31 142 -45 



November, 47-5 29 ..,-62 



December, 61-2 435 -70 



859-5 524-7 0-60 



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

 summer 50%, and, on an average, throughout the year 40% of the 

 rain which fell. 



" Under the Epicdas [Picea excelsa, Linh'\ of 35 years' growth, 

 probably in dense clumps, the pluviom^tre received only 21% of the 

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

 Versailles, the pluviom^tre, 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 pluviometre mi'>ht 

 have a marked influence on the results. Two instruments placed 

 under an umbiella, 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. 



* jRewte des Eaux et Fonts. 1867. P. 161. 



