NOTES. 441 



behind a hedgerow, 6 to 8 feet high, a difference of 30 per cent 

 was noted in the same distances. Extensive observations 

 made by the Canadian Agricultural Experiment stations show 

 very considerable differences in crop production due to the 

 effect of wind-breaks. 



The upper air strata can be modified only by the conditions 

 existing near and above the crowns. At the same time they 

 must carry away the cooler and moister air there and create 

 an upward movement of the forest air, and thereby in part 

 the conditions of this become also active in modifying air cur- 

 rents. The greater humidity immediately above the crowns 

 is imparted to the air currents, if warm and dry, and becomes 

 visible at night in the form of mists resting above and near 

 forest areas. These strata protect the open at least against 

 insolation and loss of water by evaporation, and have also a 

 greater tendency to condensation as dew or light rain if con- 

 ditions for such condensation exist. This influence can be 

 felt only to the leeward in summer time, and with dry, warm 

 winds, while the cooling winter effect upon comparatively 

 warmer moist winds is not noticeable. Theoretical considera- 

 tions lead to the conclusion that in mountain regions only the 

 forest on the leeward slope can possibly add moisture to a 

 wind coming over the mountain, but this does not necessarily 

 increase the precipitation on the field beyond. Altogether, 

 the theoretical considerations are as yet neither proved nor 

 disproved by actual observations, and as to rainfall, the ques- 

 tion of influence on the neighborhood is still less settled than 

 that of precipitation upon forest areas themselves. Wherever 

 moisture-laden winds pass over extensive forest areas the 

 cooler and moister condition of the atmosphere may at least 

 not reduce the possibility of condensation, which a heated 

 plain would do ; but observations so far give no conclusive 

 evidence that neighboring fields receive more rain than they 

 otherwise would. 



(4) With regard to comparative temperatures in forest 

 stations and open stations that are situated not far apart fi-om 



