178 INFLI7EI?"CE OF FORESTS ON HTJMIDITY. 



miditj of the air and tlie earth, and this climatic action it exerts 

 partly as dead, partly as living matter. By its interposition as a 

 curtain between the sky and the ground, it both checks evapora- 

 tion from the earth, and mechanically intercepts a certain propor- 

 tion of the dew and the lighter showers, which would otherwise 

 moisten the surface of the soil, and restores it to the atmosphere 

 by exhalation ; * while in heavier rains, the large drops which fall 

 upon the leaves and branches are broken into smaller ones, and 

 consequently strike the ground with less mechanical force, or are 

 perhaps even dispersed into vapor without reaching it.f 



* Mengotti had observed, and described in his usual pictm-esque way, the 

 retention of rain-water by the foliage and bark of trees, but I do not know 

 that any attempts were made to measure the quantity thus intercepted before 

 the experiments of Becquerel, communicated to the Academy of Sciences in 

 1866. These experiments embraced three series of observations continued re- 

 spectively for periods of a year, a month, and two days. According to Bec- 

 querel's measurements, the quantity falling on bare and on wooded soil re 

 spectively was as 1 to 0.67 ; 1 to 0.5 ; and 1 to 0.6, or, in other words, he found 

 that only from five-tenths to sixty-seven hundredths of the precipitation reached 

 the ground. — Gomptes Bendus de V Academic des Sciences, 1866. 



It seems, indeed, improbable that in rain-storms which last not hours, but 

 whole days in succession, so large a proportion of the downfall should continue 

 to be intercepted by forest vegetation after the leaves, the bark, and the whole 

 framework of the trees were thoroughly wet, but the conclusions of this em- 

 inent physicist appear to have been generally accepted until the very careful 

 experiments of Mathieu at the Forest-School of Nancy were made known. The 

 observations of Mathieu were made in a plantation of dedicuous trees forty- 

 two years old, and were continued through the entire years 1866, 1867 and 

 1868. The result was that the precipitation in the wood was to that in an open 

 glade of several acres near the forest station as 943 to 1,000, and the proportion 

 in each of the three years was nearly identical. According to Mathieu, then, 

 only 57 thousandths or 5.7 per cent, of the precipitation is intercepted by trees. 

 — SuRELL, Etude sur les Torrents, 2d ed., ii., p. 98. 



By order of the Direction of the Forests of the Canton of Berne, a series of 

 experiments on this subject was commenced at the beginning of the year 1869. 

 During the first seven months of the year (the reports for which alone I have 

 seen), including, of course, the season when the foliage is most abundant, as 

 well as that when it is thinnest, the pluviometers in the woods received only 

 fifteen per cent, less than those in the open grounds in the vicinity.— Risler, 

 in Revue des Eaux et Forets, of 10th January, 1870. 



+ We are not, indeed, to suppose that the condensation of vapor and the 

 evaporation of water are going on in the same stratum of air at the same time, 

 or, in other words, that vapor is condensed into rain-drops, and rain-drops 

 evaporated, under the same conditions ; but rain formed in one stratum may 



