34 PRACTICAL FOEESTKY. 



mentioned above the station outside the forest was -within less than 

 a mile of it, and thus likely to be influenced by the cooler air currents 

 flowing from it; that is, the real effect of the presence or absence of 

 woods oyer large stretches of cotrntry is probably greater than these 

 observations show. 



A system introduced in Austria is expected to give a clearer idea of 

 the distance to which the forest influence reaches. It consists of 

 Knes of stations beginning in the center of a large forest and extending 

 step by step into the open country beyond. 



MOISTURE IN rOBEST AIB. 



The moisture of the air is greater in the forest than outside. The 

 absolute quantity of water vapor in a cubic foot of air is generally 

 the same in both places, but the forest air is cooler, and therefore its 

 relative humidity is greater. Relative humidity is the amount of 

 vapor actually in the air, expressed as so much per cent of all it could 

 hold at the same temperature. The amount of water that the air 

 can hold changes when the temperature changes, but in such a way 

 that air cooled until it is only half as warm as before can hold much 

 less than half as much vapor. If a hot and a cold stream of air, both 

 saturated with water vapor, meet and mix, the mixture can no 

 longer hold as much vapor as the two streams separately, and a part 

 is condensed, usually in the form of rain or snow. German and Swiss 

 observations, have shown that the average humidity is greater in the 

 forest by from 3 to 10 per cent. This difference increases with the 

 altitude above sea level and the density of the forest cover. The 

 increase of humidity explains why dew is more frequent in the neigh- 

 borhood of the forest than at a distance. 



EVAPORATION. 



The water which falls to the earth from the atmosphere had first 

 to be evaporated, so that year by year the quantity of water which 

 the air takes from the surface of the globe by evaporation is the 

 same as that which falls upoil it in the shape of rain, hail, snow, 

 and dew. The effect of the forest on this great movement of water 

 is to detain more of it on those portions of the earth which are shel- 

 tered by trees. It does this partly by tending to increase the rain- 

 fall, but its effect in lessening the loss of water through evapora- 

 tion is probably much more important. The colder and moister 

 air of the forest has less capacity for taking up water vapor than 

 that of the open country. It is also quieter, which means that the 

 winds are less active in replacing saturated air with air which can 

 still take up more water. The forest acts powerfully in checking 



358 



