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surements, and experiments designed to clear up this subject, final 

 conclusions covering the whole field have not yet been established. 

 In this country almost nothing has ever been done to secure ac- 

 curate data for the investigation of this problem as a whole. 

 Some light, however, has been thrown on the subject by means of 

 a series of observations which have been going on for several 

 years in the San Bernardino mountains in southern California. It is 

 the purpose of the present article to make clear what are the vari- 

 ous factors entering into the problem, and to state some of the 

 more important facts that these observations in southern California 

 reveal. 



In the San Bernardino mountains records of precipitation for 

 several years, at a large number of stations, show that differences 

 in forest cover are closely correlated with differences in rainfall. 

 This correlation is so close that it is possible to judge the mean 

 annual precipitation with a fair degree of accuracy from the 

 appearance of the forest alone. In these mountains forests cover 

 the slopes wherever the mean annual rainfall exceeds 20 to 24 

 inches ; however, on southern and western slopes forests of equal 

 density represent a larger rainfall than on nothern and eastern 

 slopes. 



Other things being equal, regions having the greatest rainfall 

 bear forests of the greatest density and luxuriancy of growth ; but 

 where the mean annual rainfall falls below 18 to 20 inches, types 

 of vegetation in which trees predominate are replaced by those 

 in which shrubs and herbage predominate. 



WHAT CAUSES RAINFALL. 



Because rainfall is most abundant where forests grow, many 

 believe that forests exert an important influence on the amount of 

 precipitation. A more reasonable inference, however, is that 

 rainfall is the great factor in controlling the distribution and density of 

 forests. 



Precipitation occurs whenever the air is suddenly cooled below 

 the dew-point. The most effective cause of this is the expansion 

 of air in ascending. This upward movement is caused very largely 

 by cyclonic storms. 



Whether forests have any appreciable effect in cooling the air 

 to below the dew-point is uncertain. From the known effect of 

 forests on the temperature and relative humidity of the air, it is 

 reasonable to infer that they may have some such effect, at least 

 to a small degree, and consequently that they have some influence 

 in increasing precipitation. The present evidence, however, 

 derived from many series of observations conducted in Europe 

 and elsewhere, is so conflicting that a definite answer to this 

 question, having the stamp of scientific accuracy, is not possible. 



WHAT BECOMES OF THE RAINFALL. 

 That the excessive destruction of forests is followed by the 



