PRACTICAL FORESTRY. 33 



been deprived of as much heat as would be given off if the whole tree 

 were burned. 



The effect of the cooler air of the forest is felt to some distance in 

 the open coimtry. During the day, in calm summer weather, when 

 the air is warmer than the tree tops, it is gradually cooled by contact 

 with the cooler leaves and twigs. In cooling it becomes heavier and 

 falls toward the ground. A rising current of warmer air is formed to 

 supply its place, and so the colder air flows off along the surface into 

 the open country and causes local breezes. At night the air currents 

 are reversed. The air in the forest is then warmer than the air out- 

 side, because the cover checks the radiation of heat, and so the colder 

 air moves from the open country toward the woods. In these ways 

 the influence of the forest is felt at a distance. 



The amount of this coohng of the air has been measured in certain 

 places. It is naturally found to be greatest in summer; while in 

 winter and at night the air in the tree tops is a little warmer than in 

 the open. It is important to add that the cooling effect of the forest 

 is greater than the average in the mountains, and less in the plains. 



EXTREMES OF HEAT AND COLD. 



The extremes of heat and cold are moderated by the forest. Obser- 

 vations on this point have been made, for example, in Bavaria and 

 Wiirttemberg. They showed that the lowest temperature of every 

 day in the year was higher, on an average, by nearly 2° in the forest, 

 while the highest temperature was lower by nearly 4°. The greatest 

 heat of the day in the summer was 1\° less in the forest than outside. 

 Prussian observations showed that for fen years the greatest heat of 

 the day in July was, on an average, nearly 6° lower in the forest, and 

 the greatest cold of the night in January nearly 3° less than outside. 

 It should not be forgotten that the latitude, the elevation, and the' 

 exposure had a powerful influence on these differences, which are also 

 greatly affected by the land of trees and the density of the forest. 



It must be borne distinctly in mind that the figures given above 

 are rehable only for the places in central Europe where they were 

 observed. But the principles on whi(5h they depend are just as true 

 in America as they are in Europe. Natural laws are the same the 

 world over. It is safe to conclude, then, that in the United States 

 the forest modifies the temperature of the air in certain ways and 

 for certain reasons, both of which we have seen. Just how great this 

 influence is in different parts of this continent it is as yet impossible 

 to tell. But it is probably greater on the average than these observa- 

 tions indicate, for two reasons : First, the extremes of heat and cold, 

 moisture and dryness, are much greater here than in central Europe, 

 and changes are more sudden; second, in most of the double stations 



358 



