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



between the effects of radiation between the earth and sky. In July, 

 18G4, we found : 



At 9 o'clock in the morning 21O.04 (C.) 



At 3 o'clock in the afternoon 25 .94 



At 9 o'clock in the evening 19 .00 



The cooling continued until 6 o'clock in the morning, when the tem- 

 perature was the same at 1.33 meters above the ground on the north, at 

 16 metres on the south, where it was 15o.5 (C). 



Taking the month of January in 1863 and in 1864, we had : 



1863. 1864. 



At 9 o'clock in the morning 4o.57 (C.) — 0".0.S (C.) 



At 3 o'clock in the afternoon 7 .41 +3 .30 



At 9 o'clock in the evening 5 .13 .00 



At 6 o'clock in the morning 3 .19 — 1 ,08 



We see, therefore, that whether we take trees covered with foliage, or 

 deprived of their leaves, the heat acquired during the day diminishes 

 till six o'clock in the morning. 



It is now well proved that the radiations of heat acquired from the 

 sun's rays warm, and that celestial radiations cool, the adjacent air, a 

 faculty which we did not suppose that they possessed, because we sup- 

 posed that the evaporation from the leaves was always a cooling process. 

 It may intervene to modify the effect, but it is not a dominating cause. 

 Tbis question will be taken up for further consideration in a future 

 paper. 



The experiments above mentioned were made upon isolated trees, but 

 the results were the same in groups of trees which sheltered one another, 

 so as to present an obstacle to the direct action of the sun, with the ex- 

 ception that the temperature of the trunk did not increase so much, 

 other things being equal, as when the tree stood alone. 



Forests, groves, and groups of trees ought practically to show the 

 same results as the horse-chestnut, excepting that the effects of the heat, 

 of which we have spoken, should vary according to the height of the 

 trees, the spread of their branches, and the amount of leaves with which 

 they were covered. What conclusions can we draw from these facts, 

 as to the influence of forests upon the local climate 1 We will refer to 

 this subject in a future memoir, but will here remark, that we should 

 take into account the nature of the soil, as whether dry or moist — the 

 greater or less facility with which the air circulates, the exposure, and 

 other circumstances not now well understood, and which may vary ac- 

 cording to locality. But as the wcods by radiating the heat acquired 

 from the sun may warm the ambient air, and as this air may be cooled 

 by nocturnal radiations, ought we not to believe that the air they 

 warmed should cause during the night a double current — the former of 

 the warmed air upward, and the other of the cooled air toward the 

 ground "? The warm air, being carried by lateral currents, should 

 ameliorate the temperature of the surrounding parts. 



Within the tropics, and especially under the equator, where the sun's 

 rays act with greatest power, and are least inclined, the trees ought to 

 produce in high degree, the effects of which we come to speak, and which 

 should be felt in the strata of air of neighboring parts. On the other 

 hand, the radiations of the night, which are very great under a sky 

 almost always clear, should act powerfully in hastening the cooling of 

 the leaves. 



The following fact illustrates, to a certain degree, the heat that may 

 be emitted by the woods when warmed by solar radiation. Every one 

 knows that during the intense heat of summer, in the middle of the day, 



