273 



Before taking up these two chief beneficial effects of a pro- 

 tection forest, let it be said that a large number of other bene- 

 ficial influences have been ascribed to the forest. Some of these, 

 while rather intangible, are certain, but others are somewhat ex- 

 travagant and without much sure basis. 



Beneficial Influences. 



The time is too brief for a detailed discussion of these asser- 

 tions but they may thus be briefly summed up : 



It has not yet been definitely proved by mathematical means 

 that forests produce rain, because both instruments and methods 

 of meteorological inquiry are as yet unsatisfactory. Wlien, for in- 

 stance, rain gages will, according to their construction, the manner 

 of their position and the character of the \\nm\ and rain, during the 

 same storm, register amounts varying from 7 to 40 per cent, we 

 are wnthout any means of applying a constant factor of correc- 

 tion, and it would appear that no reliance can be placed on such 

 measurements for the purpose of determining the dift'erence of 

 rainfall within and without a forest. Several observations must 

 therefore be relied upon. 



The forest, however, does exclude the sun and wind from the 

 soil and the air temperatures and air humidity are modified. 

 The annual evaporation within the forest is about one-half of 

 that in the open field, while the quantity of moisture thrown into 

 the air by transpiration from the leaves in the forest is some- 

 times three .times that from horizontal water surface of the 

 same extent. 



From these statements we would expect as a consequence of 

 deforestation an effect on the climate of a deforested area in 

 three directions, namely : 



1. Extremes of temperature of air as well as soil are aggra- 

 vated. 



2. The average humidity of the air is lessened and, possibly. 



3. The distribution of precipitation throughout the year, if 

 not its quantity, is changed. The tendency of a forest growth, 

 therefore, would be, on account of its cooling effect, to keep the 

 air within and to some extent the air above it nearer saturation, 

 and as a consequence it might occur that moisture-bearing cur- 

 rents passing over it would precipitate their moisture more readily 

 above or near the forest growth. This influence is only of a local 

 character, for, to make an appreciable difference in the amount 

 of rainfall, it would appear that the forest area must be of con- 

 siderable extent. It cannot be put in comparison wath that of 

 the large oceans, the great air currents and the extensive moun- 

 tain ranges which determine the general climate. The size and 

 character of the forest, its density, height, situation, and compo- 

 sition, are of great importance in determining its influence. 



