440 APPENDIX. 



by means of air currents passing through and above the for- 

 ests being modified in temperature and moisture conditions ; 

 the mechanical effect upon such air currents by which they 

 are retarded in their progress may also be effective in chang- 

 ing their climatic value. 



(2) Local air currents are set up by the difference in tem- 

 perature of the air within and without the forest, analogously 

 to those of a lake or pond, cooler currents coming from the 

 forest during the day in the lower strata and warmer currents 

 during the night in the upper strata. The latter currents, 

 being warmer and moister, can be of influence on the tem- 

 perature and moisture conditions of a neighboring field by 

 moderating temperature extremes and increasing the humidity 

 of the air. 



This local circulation is the one most important difference 

 between forest and other vegetation. How far away from the 

 forest this circulation becomes sensible is not ascertained. In 

 winter time, when the temperature differences become small, 

 no such circulation is noticeable. 



(3) The general air currents in their lower portions are cut 

 off entirely by the forest, which acts as a wind-break. This in- 

 fluence can of course be experienced only on the leeward side. 

 How far this protection reaches it is difficult to estimate, but 

 it certainly reaches farther than that of a mere wind-break, 

 since by the friction of the air moving over the crowns a 

 retardation must be experienced that would be noticeable for 

 a considerable distance beyond the mere wind-break effect. 

 Deforestation on a large scale would permit uninterrupted 

 sweep of the winds, a change more detrimental where the 

 configuration of the ground does not fulfil a similar function 

 — in large plains more than in hilly and mountainous regions, 

 and at the seashore more than in the interior. 



In an experiment made by F. W. King in Wisconsin the 

 evaporation increased with the distance from the woods up to 

 300 feet ; the difference in the amount at a station only 20 

 feet from the protecting forest being over 66 per cent. Even 



