14 THE UTILITY OF FORESTS. 



be greater than that of air in the open, more especially in summer. 

 Observations show, near the 50th degree of northern latitude, that 

 the increase in per cent, may amount to 5-70 in spring, 9-28 in 

 summer, 5-22 in autumn and 5-24 in winter, with an annual 

 average of 6-36. There is, however, a decided difference according 

 to altitude ; the effect is smaller in low lands and greater in 

 mountains. Further differences have been recorded according to 

 species. Beech acts most powerfully in summer and firs in 

 winter, while the action of pines is considerably smaller. This 

 action of forests explains why dry air currents passing through 

 woodlands become relatively moist, so that precipitations may 

 take place, if not of actual rain, of dew. 



(b) Precipitations, or Rainfall. — The question whether, and in 

 how far, forests affect the rainfall has been actively discussed for 

 years past. That forests can affect precipitations follows from 

 the facts that their temperature is somewhat lower and their air 

 relatively moister, while the trees mechanically obstruct the 

 movement of air currents ; but, on the other hand, the rainfall 

 depends chiefly on other much more powerful agencies, in com- 

 parison with which the effect of forests is small. Numerous 

 observations have been made, but those which seem to indicate 

 a decided effect of forests on the rainfall are in many cases of 

 doubtful value. The great difficulty is to separate the effect of 

 forests from other influences. More particularly, elevation above 

 sea level affects the rainfall most powerfully, as air cools on rising, 

 causing increased precipitations, whether there are forests or not. 

 For instance, the observations made at the Prussian stations show 

 an increase of the rainfall in forests as compared with that on open 

 land between the sea and 328 feet elevation of 1-25 per cent., 

 between 328 and 656 feet 14-2 per cent., between 1,969 and 

 2,297 feet 19 per cent., and between 2,297 and 2,625 feet 46 per 

 cent. It is impossible to accept these effects as being produced 

 by the presence of forests alone. All they seem to show is that ?t 

 low elevations forests have very httle effect upon the rainfall, if 

 any at all, but that their influence becomes considerable with 

 elevation in mountainous districts. 



The results of 33 years' observations made at the Nancy stations 

 show an increase of rainfall in the forest equal to 23 per cent, over 

 that in the open ground. Objection may, however, be taken to 



