198 THE POPULAR SCIENCE MONTHLY. 



minimum of the forest being less than that of the field ; in other words, 

 the diurnal temperature is more uniform. 



But this is a matter involving sueh complicated and varying con- 

 ditions, that absolute propositions are. open to question, however true 

 they may be with proper qualifications. Rivoli has very recently 

 done much to make our knowledge of this subject definite. His ob- 

 servations were carefully made, under circumstances which eliminated, 

 as much as possible, all disturbing conditions ; there being no body 

 of water near, the country level, and the wind having a fair sweep in 

 all directions. We will state the results of his investigations as briefly 

 as possible. 



Influence of Forests on Winter Temperature. 1xl the winter-time, 

 the simplest relations of the forest to temperature prevail. During 

 this season of the year, when the wind passes into the north and be- 

 comes colder, the forest warms it ; when it passes into the south and 

 becomes warmer, the forest cools it. During winter the forest plays 

 the role of a bad conductor, and acts as an equalizer of temperature. 



Influence of Forests on Summer Temperature. In summer the 

 case is not so simple, owing to evaporation from the surface, tran- 

 spiration through the leaves, and radiation from them. At this season 

 of the year the atmosphere in the forest is usually warmer during the 

 nights and colder during the day than in the open field. The night is 

 warmer in consequence of the obstruction which the mass of foliage 

 presents to radiation from the surface beneath it ; the day is cooler in 

 consequence of the transpiration of vapor from the leaves, and the 

 obstruction interposed between the surface and the direct rays of the 

 sun. In the summer, as well as in the winter, the forest usually acts 

 as an equalizer of the temperature of the atmosphere. 



While, however, this is usually the case, there are exceptions. 

 During nights when it is calm, radiation from the leaves of the trees 

 may cool particles of air, which, descending toward the surface, form 

 just before daybreak a stratum of the atmosphere below, which is 

 colder than if the region had been destitute of trees. It is within the 

 experience of most cultivators of the soil that frosts sometimes strike 

 hardest near a wood. 



To the fact that under certain circumstances a forest may cause 

 frost in the fields near by, we may add the qualification that this can 

 only occur in the case of white frosts, and that whenever there is mo- 

 tion of the atmosphere, and the wind a cold one, the influence of the 

 forest is always protective. An orchard sheltered by a wood may 

 escape unhurt, while another in the same neighborhood not so protect- 

 ed may suffer the loss of its entire crop. This is believed to be not a 

 very uncommon occurrence in the case of peaches. 



If the fruit-growing interest of the country were to state its ac- 

 count with the forest, we should suppose it to be something like the 

 following : 



