190 n^FLTJElSrCE ON PRECIPITATION. 



the opinion of perhaps the greatest number of observers. la 

 deed, it is almost impossible to suppose that, under certain condi- 

 tions of time and place, the quantity and the periods of rain 

 should not depend, more or less, upon the presence or absence 

 of forests ; and without insisting that the removal of the forest 

 has diminished the sum total of snow and rain, we may well ad- 

 mit that it has lessened the quantity which annually falls within 

 particular limits. Yarious theoretical considerations make this 

 probable, the most obvious argument, perhaps, being that drawn 

 from the generally admitted fact, that the summer and even the 

 mean temperature of the forest is below that of the open country 

 in the same latitude. If the air in a wood is cooler than that 

 around it, it must reduce the temperature of the atmospheric 

 stratum immediately above it, and, of course, whenever a satu- 

 rated current sweeps over it, it must produce precipitation which 

 would fall upon it or at a greater or less distance from it. 



"We must here take into the account a very important consid- 

 eration. It is not universally or even generally true, that the at- 

 mosphere returns its condensed humidity to the local source from 

 which it receives it. The air is constantly in motion, 



howling tempests scour amain 



From sea to land, from land to sea ; * 



and, therefore, it is always probable that the evaporation drawn 

 up by the atmosphere from a given river, or sea, or forest, or 

 meadow, wiU be discharged by precipitation, not at or near the 

 point where it rose, but at a distance of miles, leagues, or even 

 degrees. The currents of the upper air are invisible, and they 

 leave behind them no landmark to record their track. We know 

 not whence they come, or whither they go. "We have a certain 

 rapidly increasing acquaintance with the laws of general atmos- 

 pheric motion, but of the origin and Kmits, the beginning and 

 end, of that motion as it manifests itself at any particular time 

 and place, we know nothing. "We can not say where or when the 

 vapor, exhaled to-day from the lake on which we float, will be 



* Und Stilrme brausen um die Wette 

 Yom Meer aufs Land, vom Land aufs Meer. 



Goethe, Faust, Song of the Archa/ngda, 



