M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 323 



It should therefore be found, as already remarked, that a tree, with 

 its trunk, branches, and leaves, becomes warm or cool, like any other 

 bodies immersed in the air, according as the sun is above or below the 

 horizon. In the first they are warmed by the solar rays, and iu the 

 second they are cooled by the radiations of the night, until the tree is 

 brought to an equilibrium of temperature with the air around it. The 

 cooling of the night then begins, and if the sky is clear of clouds, it 

 goes on in degree proportioned to the amount of cooling of the upper 

 branches and the leaves, which gradually lose their heat by radiations 

 into space. We may see from this, how the strata of air that envelop 

 the tree maintain, during a great part of the night, a temperature 

 higher than that of the air which is some distance off". 



A tree which has been warmed by the effect of the sun's rays acts in 

 turn by warming the air around it, and when it comes to rain suddenly, 

 the temperature of the air becomes reduced, notwithstanding, at some 

 distance around. 



We will cite an instance: On the 9th of May, at one o'clock, after a 

 strong electrical insolation, we had the following conditions: 



Temperature above a horse-chestnut 19°. 4 (C.) 



Temperature to a little distance T. 18 .3 



Difference 1 .1 



Half an hour after a rain fell, and the temperatures changed as fol- 

 lows: 



Temperature above the horse-chestnut 17°. 5 (C.) 



Temperature around 15 .2 



Difference 2 .3 



Here, within an interval of half an hour, the atmosphere that sur- 

 rounded the tree had therefore cooled but l^.O, while at a little distance 

 away it had cooled 3°.l, so that the tree must have radiated heat in 

 order to have warmed the surrounding air. The sun having reappeared 

 some moments after, the temperature arose at both points of observa- 

 tion, but a little less over the horse-chestnut than at some distance from 

 it, and at three o'clock these temperatures were as follows : 



Above the tree 20°. 8 (C.) 



At a little distance , 19 .2 



Difference - 1 .6 



To get an idea of the warming of the air from the presence of the 

 leaves, we will take as an example the temperature of the atmosphere 

 in July, 1863, at nine o'clock in the morning, at three o'clock p. m., and 

 at nine in the evening. They gave as monthly means as follows : 



At 9 o'clock in the morning , 21°. 56 (C.) 



At 3 o'clock in the afternoon 26 .76 



At 9 o'clock in the evening 19 .20 



We observe that the temperature of the air was at its maximum at three 

 o'clock, and that it diminished about a quarter of this number of degrees 

 by nine o'clock in the evening. The diminution of the internal heat of 

 the trunks and branches continued to restore by radiation the losses sus- 

 tained by the leaves in radiating their heat during the night, until 6 

 o'clock in the morning, when the temperature is the same at 1.33 meters 

 from the ground on the north and at 16 meters above the ground 

 on the south and at 21.25 meters at the tree-tops. At this time the 

 celestial radiation ceased to predominate, and there was an equilibrium 



