9i EFFECTS OF FORESTS ON MOISTURE 



Sect. I. — Gases in which the Extensive Destruction of Forests does not 



appear to have Perceptibly Affected the Quantity of the 



Rainfall over a Wide Expanse of Country. 



The subject of this chapter is the effect of forests ou the moisture 

 over a wide expanse of country, that of this sectiou is only the 

 apparently negative effect, in certain cases of the extensive destruc- 

 tion of forests, on the rainfall. Attention is called to the difference, 

 that it may be noted. The rainfall is treated neither as the source 

 nor as the measure of the moisture, but solely as one of the indications 

 of the humidity of the atmosphere, it being the indication of this to 

 which most importance is generally attached, and that which is most 

 generally referred to in popular discussions on the meteorological 

 effects of forests. 



Both Europe and America supply illustrations of apparently little 

 effect having been produced upon the deposit of moisture in the form 

 of rain over a wide expanse of country by the extensive destruction 

 of forests. 



There are many indications of Europe having been formerly 

 covered much more extensively and densely with forests than now. 

 In France, the destruction of these forests within the last two 

 hundred years as well as before has been considerable, yet there does 

 not appear to have been within that time any very great diminution of 

 the rainfall over some extensive districts in which observations on the 

 rainfall have been made. 



This point has been discussed at considerable length by M. Cezanne, 

 in h\.^Se(j[uel to Etude sur les Torrents des Ilautes Alpes, par Alexandre 

 Surell. It is mentioned by him that some writers on the subject, to 

 whom he afterwards refers more particularly, in endeavouring to demon- 

 strate the action of forests on the rainfall, have adduced as proof that 

 the destruction of forests, and the extensive plantation of these, have 

 both of them affected the regime of the rainfall in France; some, that 

 the rainfall has increased at Viviers contemporaneously with the de- 

 hoisement or destruction of forests on the mountains of the Cevennes j 

 others, that it has increased at Bourdeaux contemporaneously 

 with the rehoisement, or replanting of forests, on the Laudes : facts, 

 if facts they be, which, though not incompatible with each other, 

 point to different conclusions. In reference to these observations, 

 M. Cezanne writes : — " If facts of this kind were clearly established, 

 they would be unanswerable ; but before admitting them wc must 



