60 EFFECTS OF FORESTS ON HUMIDITY. 



wind from the north-east — the former being saturated with moisture, 

 while the latter has been deprived of it."* 



And having stated what the eflFect of this is on the flow and 

 delivery of rivers, he goes on to say, — " If such be the important 

 part played by evaporation, it is evident that forests exercise an 

 appreciable influence on water-courses, according as they promote or 

 retard, to whatever degree it may be, evaporation. Bat in what do 

 they exercise this action 1 On this point the whole world seems to 

 be at sixes and sevens. 



" Throughout all time the poets have sung of the cool shade — the 

 moist shade of extensive woods. A nave of verdure thrown over a 

 road keeps it moist, and as a consequence protects it or injures it, 

 according to the climate or the seasons. Tn every page of works on 

 forestry may be met allusions to the humidity, which is favourable 

 to the seedlings, and which the shade maintains. 



" If any one entertain a doubt, let him follow in the suite of a 

 forest-guard the day after a storm of rain ; when, although the open 

 country and a fortiori the roads are already dry, he may procure for 

 himself a cold bath in penetrating into the clumps of copse-wood. 

 At his feet the tall herbs are little better than a stream, and on his 

 head every shaken branch will pour down a douche, which will make 

 itself be felt." And in explanation he adds in a foot-note, — " Any 

 one may observe that on the leaves of certain vegetables, but not of 

 all, the rain-drops take a spheroidal form ; in this state they resist 

 evaporation energetically, as one may see globular drops run about 

 on red-hot iron without ebullition. The foliage of other plants has 

 been dried for a long time, while these pearls sparkle still on the 

 herbs which bear them. Certain species preserve thus the rainfall 

 until the agitation of the leaves cause these spheroidal drops to fall." 

 And he goes on to say, " It seems then natural to believe that forests 

 oppose themselves to evaporation, and, consequently, acquire for the 

 ground the product of the rain. But objections are not awanting. 



" Attention is called to the circumstance that the foliage of trees 

 arrests a notable portion of the water falling as rain. When a light 

 rain-storm follows some days of dry weather, it may be seen that each 

 tree casts a dry shadow, which on the wet ground of the road is 

 marked out by a dust-covered patch. The water arrested by the 

 leaves is lost to the ground underneath ; the tree or the air will 

 absorb it, and the importance of this is such as is not to be 

 overlooked. 



* Schmit : Qrundriss der Meteorologie, 



