58 



diminished the sum-total of snow . and rain, we may well admit that it has lessened the 

 quantity which annually falls within particular limits. Various theoretical considerations 

 make this probable, the most obvious argument, perhaps, being that drawn from the gen- 

 erally admitted fact, that the summer and even 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 saturated current sweeps over it, it must produce precipi- 

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



"We must here take into the account a very important consideration. It is not. 

 universally or even generally true that the atmosphere 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, will 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 acquaititance with the laws of general atmospheric motion, but 

 of the origin and limits, the beginning and end of that motion, as it manifests itself at 

 any particular time and place, we know nothing. We cannot say where or when the 

 vapour, exhaled to-day from the lake on which we float, will be condensed and fall ; ; 

 whether it will waste itself on a barren desert, refresh upland pastures, descend in snow 

 on Alpine heights, or contribute to swell a distant torrent, which shall lay waste square 

 miles of fertile corn-land ; nor do we know whether the rain which feeds our brooklets is 

 due to the transpiration from a neighbouring forest or to the evaporation from a far-off' 

 sea. If, therefore, it were proved that the annual quantity of rain and dew is now as 

 great on the plains of Castile, for example, as it was when they were covered with the- 

 native forest, it would by no means follow that those woods did not augment the amount 

 of precipitation elsewhere. 



" The whole problem of the pluviometrical influence of the forest, general or local, 

 is so exceedingly complex and difficult that it cannot with our present means of know- 

 ledge be decided upon ci priori grounds. It must now be regarded as a question of fact 

 which would probably admit of scientific explanation if it were once established what the 

 actual fact is. Unfortunately the evidence is conflicting in tendency, .and sometimes 

 equivocal in interpretation, but I believe that a majority of the foresters and physicists, 

 who have studied the question are of opinion that in many, if not in all cases, the destruc- 

 tion of the woods has been followed by a diminution in the annual quantity of rain and 

 dew. Indeed, it has long been a popularly settled belief that vegetation and the conden- 

 sation and fall of atmospheric moisture are reciprocally necessary to each other, and even 

 the poets sing of 



* * ■* Afric's barren sand, 

 Where nought can grow, because it raineth not, 

 And where no rain can fall to bless the land, 

 Because nought grows there." 



Dr. Schacht, Professor at the University of Bonn, says in his well-known work,. 

 "Les Arbres" : — 



"The snow and ice which accumulate during the winter on the mountains, melt 

 rapidly under the spring sunshine — thus swelling the torrents whose mass of water makes 

 its way into the valleys with resistless force. But when the mountain sides- are covered 

 by forests, or where the arable plains are bordered by woods, the scene changes its aspect. 

 The greater part of the. snow is deposited on the trees or falls between them, and the- 

 water which results is absorbed by the soil formed by the accumulation of vegetable mat- 

 ter j but wherever the forests have disappeared, the spring inundations of the rivers have 



