FIFTH NATIONAI, CONSERVATION CONGRESS 189 



transpiration of water by the leaves appreciably increases the moisture content of 

 the air within or near the forest and, second, because the temperature of the air in 

 the forest is lower and therefore nearer its saturation point. 



PRECIPITATION. 



Forests increase both the abundance and frequency of local precipitation over 

 the areas they occupy, the excess of precipitation as compared with that of ad- 

 joining unforested areas amounting in some cases to more than 25 per cent. 



Observations upon the influence of forests on local precipitations began as 

 early as the middle of the last century, but systematic observations did not start 

 until the second half of the 60's (Bavaria, France, and Switzerland) and in 

 many places are still being carried on. This excess of precipitation over forested 

 areas varies from a fraction of one per cent to 35 per cent. Such wide variation 

 is due partly to differences of geographic situation, altitude, character of the 

 forest, etc. At the forest experiment station at Nancy 33 years' observations show 

 an average excess of precipitation on forested areas of 23 per cent, while Eber- 

 mayer, in Germany ; Bouvard, at Moumal, and Blandford, in India, compute it as 

 being 12 per cent. Some meteorologists are inclined to ascribe the difference in 

 the amount of precipitation over forests and open fields to the imperfection of the 

 rain gauges. Hellman's experiments, for instance, showed that the ordinary rain 

 gauge in a wind of medium velocity registers 19 per cent less of precipitation than 

 actually falls. It is possible that rain gauges in the forest, being protected from 

 wind, will catch more rain and therefore show a greater amount of precipitation 

 than when placed in the open. That the greater amount of precipitation over 

 forest areas as compared with open fields can not, however, be ascribed entirely 

 to this has been clearly brought out by Miittrich, who, during four years of care- 

 ful observation with Hellman's improved rain gauges, found that the difference 

 of precipitation in the forest and outside the forest still amounted to 6 per cent. 



French observers are practically unanimous in recording a larger amount 

 of precipitation over forests than over fields. This conclusion is the result of ex- 

 periments carried on at the forest school at Nancy, in the forests of Haye, by 

 Fautrat in the forest of Halatte (Oise), and by de Pons in the forest of Troncais 

 (Allier). Most of those carried on in Germany, Austria, Russia, and India have 

 forced similar conclusions. 



Regular observations taken at Nancy for 33 years since 1866 at stations in- 

 side, on the edge of, and outside the forest show that, without exception, more 

 rain has fallen inside than outside the forest and that during 8 or 10 years more 

 rain fell on the edge of the forest than outside. If the amount of the rainfall ai 

 the center of the forest be designated at 100, then the amount of rainfall at the 

 edge of the forest would be represented by.93.9 and the rainfall outside the forest 

 by 76.7. 



The influence of mountains upon precipitation is increased by the presence of 

 forests. The influence of forests upon local precipitation is more marked in th'^ 

 mountains than in the plains. 



The difficulty of bringing out clearly the influence which forests have upon 



