2 FOKESTS, WOODS, AND TKEES 



and stream-flow, and on the erosion of the soil on slopes of 

 hills and mountains. The positive results of these observa- 

 tions, which are detailed below under separate headings, 

 may be considered to hold good with regard to the British 

 Isles and temperate regions generally. It is only fair to 

 state that the beneficial action of the forest in increasing 

 the rainfall, in diminishing the run-off water, and in pre- 

 venting the erosion of the soil is not universally admitted. 

 The main arguments against the ordinary view were 

 published by H. M. Chittenden (1) in 1908. The attention 

 of foresters and engineers may also be directed to the im- 

 portant monograph, entitled Boschi e Acque, which was 

 published at Rome in 1916 by M. Giandotti (2), Director 

 of the Hydrographic Office of the river Po. This is a 

 complete study of the whole question of the relation of 

 forests to rainfall and stream-flow. 



The investigations carried out in India on the influence 

 of forests on atmospheric and soil moisture, which have 

 been the subject of a recent ofiicial report (Indian Forest 

 Bulletin, No. 33, 1916), throw no new light on the problem 

 as regards tropical regions. The general conclusions arrived 

 at in India were : " The influence of forests on rainfall is 

 probably small, but the denudation of the soil, owing to 

 the destruction of forests, may be regarded as an established 

 fact in India." Dr. Gilbert Walker, in an appendix to 

 this bulletin, points out the difficulty of such investigations, 

 one cause of error being the tendency of the annual rainfall 

 to run in spells of excessive and deficient years, so that the 

 exact influence of forest growth or of forest destruction is 

 not readily arrived at. Dr. Hugh R, Mill, in Nature, 

 2nd August 1917, p, 446, advocates a study of the relation 

 of the isohyetal (3) lines to the configuration of the land 

 on wooded and treeless districts of similar character ; and 

 instances from the report of the rainfall in the Geological 

 Survey's Water Supply Memoirs on Hampshire that the 

 district of the New Forest shows a considerably higher general 

 rainfall than its elevation above sea-level appears to suggest. 



It will be convenient to state now under four distinct 



