406 FOKESTS AND THEIR CLIMATIC INFLUENCE. 



A tree (triiuk, brauclies, and leaves) must, as has been said above, 

 become warmer or cooler, like all bodies immersed in air, according as 

 the sun is above or below the horizon. In the first case, it grows warm 

 from the eflect of the solar radiation ; in the second, it grows cool from 

 thilt of the nocturnal radiation, and this process goes on until the tree 

 acquires an equilibrium of temperature with the surrounding medium. 

 A\'!!en nocturnal radiation commences, if the sky be without clouds, in 

 proiwrtion as the upper branches and leaves become cool, those which 

 are underneath yield up their heat in succession to those above through 

 the process of radiation. From this it will readily be conceived that 

 the strata of air w^hich envelop the tree retain during a great part of 

 the night a temperature higher than that of the strata of air which are 

 remote from it. 



A tree which has been warmed by the effect of solar radiation so far 

 acts as a body imi)arting warmth to the air, that, when a rain occurs 

 suddenly', the temperature of the air is more lowered at some distance 

 from the tree than immediately around its periphery. Of this we Avill 

 cite an exemplification. On the 9th of May, at one o'clock, after a strong 

 insolation or free exposure to the sun's rays, the following temperatures 

 were observed : 



Temperature above the chestnut tree ' 19°.4 (67° F.) 



Temperature at a certain distance 18°.3 (65° F.) 



Difference lo.l (2° F.) 



Half an hour afterward, a rain fell, and the temperatures changed : 



Temperature above the chestnut tree 17°.5 (64° F.) 



Temperature beyond it 15o.2 (60° F.) 



Difference 2o.3 (4° F.) 



Thus, in the interval of half an hour, the air which surrounded the 

 tree had been cooled by only I'^.O, (3° F.,) while that which was a little 

 distant from it was cooled to the extent of 3°.l, (GOF.;) it follows 

 that the tree had radiated heat so as to impart warmth to the ambient 

 atmosphere. The sun having reappeared after some moments, the 

 temperatiu'e at both stations rose, but somewhat less above the chest- 

 nut tree than at a certain distance from it. These temperatures, at 3 

 o'clock, were as follows : 



Above the tree 20°.8 (69° F.) 



At a certain distance 19°.2 (67° F.) 



Difference lo.6 (2° F.) 



To give an idea of the warmth imparted to the air through the pres- 

 ence of leaves, we will take, for an example, the temperature of the air in 

 July, 18C3, at 9 o'clock in the morning, and at 3 and 9 o'clock in the 

 evening; for the monthly mean the following results were obtained: 



At 9 o'clock in tlie morning 21°.56 (71° F.) 



At 3 o'clock in the evening 26°.76 (80° F/ 



At 9 o'clock in the evening 19° .20 (67° F.) 



Here we see that the temperature of the air was at its maximum at 3 

 o'clock, and that it had diminished by nearly a fourth at 9 in the even- 

 ing. The radiation of the internal heat of the trunk and branches of 

 the tree continued, on the other hand, to repair the losses sustained by 



