PRECIPITATION OVER WOODED AND TREELESS DISTRICTS. 



Ill 



precipitatio:n^ ofer wooded and treeeess dis- 

 tricts. 



This branch of the forest meteorological problem is the most impor- 

 tant of all, has been most discussed, and the discussion has resulted in 

 all shades of opinion, from those absurdly favorable to trees to those 

 utterly adverse. Direct comparison of observations is hero not possible, 

 or at least comparable observations have not been carried on, and, as 

 in the case of temperatures over wooded and treeless districts, there is 

 always occasion for doubt whether the differences found at the selected 

 different stations are not due to other things than the presence or 

 absence of forests. The difficulty is, in fact, greater here than in the 

 case of temperatures, because the rainfall is especially sensitive to all 

 sorts of conditions and surroundings, and is also apparently very capri- 

 cious (changing from month to month or year to year by large amounts 

 and without any discoverable cause). In addition to all this, when a 

 difference of rainfall corresponding to a difference of forest conditions 

 has been found, there is still occasion for doubt as to which is cause 

 and which effect. There is every reason to believe that with increased 

 rainfall, other things being favorable, there will be an increased growth 

 of trees. 



The facts at hand do not prove, with entire conviction, that forests 

 increase the rainfall. The historical method is lacking generally in 

 the character of the data for the beginning of the comj)arison. Be- 

 sides, where a change of rainfall is actually shown to be coincident 

 with a change in the forest growth it is not entirely certain that 

 the former is due to the latter; it may be due to what are called 

 secular changes of the rainfall, the reasons for which lie beyond our 

 knowledge. The geographical method is not entirely satisfactory, for 

 the reasons already mentioned. The entirely convincing method de- 

 l)ends on observations above forests and with systems of radial sta- 

 tions as proposed by Dr. Lorenz-Liburnau, and from these there is not 

 yet a sufficient amount of x^ublished results. 



It will be of interest, however, to see what is the character of the 

 geog-raphical method so far as it relies on purely meteorological facts. 



In lS0(i-'«)8 M. Becquerel took a series of observations of rainfall at 

 five stations, at Moutargis and Cliutillon-sur-Loing, and their vicinity. 

 They are in the department of Loiret, 50 to 75 miles (80-121 kilom.) 

 south of Paris, and contained within a range of 25 or 30 miles (40-48 

 kilom.) of a country with fields and woods interspersed. His observa- 

 tions gave results as follows: 



