FORESTS AND THEIR CLIMATIC INFLUENCE. 407 



the leaves subjected to the influence of the nocturnal radiation until G 

 o'clock in the morning', when the temperature was found to be the same 

 at l"'.oo (4 feet) al)ove the soil to the north and south, at IG metres (53 

 feet) and at liPM*") (G9 feet) on the summit of the tree. This is the 

 period of the day when the celestial radiation has ceased to be prepon- 

 derant, and when there is an equilibrium between the effects of the ter- 

 restrial radiation and those of the celestial radiation. 

 In July, 18G4, the results obtained were: 



At 9 o'clock in the morning 21O.04 (70^ F.) 



At 3 o'clock in tlio evening 250.94 (78^ F.) 



At 9 o'clock in the evening 19^^.00 (70° F.) 



The in-ogressive reduction again continued till G o'clock in the morn- 

 ing, when the temperature was the same at 1"\33 (4 feet) above the soil 

 to the north and south, and also at IG metres (53 feet) above the soil, 

 and was equal to 15^.50, (G0° F.) If the months of January, 18G3 and 

 1184, be taken we have — 



1863. 1864. 



At 9 o'clock in the morning 4o.57 (40*^ F.) — 0o.05 (31" F.) 



At 3 o'clock in the evening 7°A1 (4.5° F.) -f3o.30 (38^ F.) 



At 9 o'clock in the evening 5^.13 (41° F.) O^.OO (32" F.) 



At 6 o'clock in the morning 3^^.19 (38° F.) —1°X)8 (303 F.) 



It is thus seen that, whether we take the trees with or without leaves, 

 the heat acquired during- the day diminishes till G o'clock in the morn- 

 ing. 



We see now^ that it may be assumed as an ascertained fact that trees, 

 exposed to the solar and celestial radiation, impart heat or cold to the 

 contiguous strata of air, a property which had not previously been sus- 

 pected ; it was supposed, on the contrary, that the evaporation which 

 takes place by means of the leaves was always a source of refrigeration; 

 this may indeed exert an influence, but it is not the predominant cause. 

 This question, however, will be resumed in another memoir. 



The experiments above spoken of were made on isolated trees, but 

 the results have been the same on groups of trees sheltering one another, 

 so as to form an obstacle to the direct action of the sun; only the ele- 

 vation of temperature in the trunk has been found to be less, all else 

 being equal, than when the tree was isolated. In fact, forests, coppices, 

 and groups of trees must observe the same laws as the single chestnut 

 tree; but the effects of heat, of which we have been speaking, vary ac- 

 cording to the height of the trees, the extent of their branches, and the 

 mass of leaves with w^hich they are charged. What consequences 

 shoidd be inferred in relation to the influence exerted by forests on 

 the local climated This question we shall answer on another occasion. 

 We shall content ourselves here with saying that it is necessary to 

 have regard to the nature of the soil, as to whether it be dry or humid, 

 to the gTeater or less facility with which the air circulates, to the ex- 

 posure and other indeterminate causes which vary according to locali- 

 ties. But from the fact that the wood, under the influence of the solar 

 radiation raises the teuiperature of the ambient atmosphere, and lowers 

 it under the effects of the nocturnal radiation, must we not infer that 

 the stratum of air which has been heated gives rise during the night to 

 a double current ; an ui)per one of warm air and a lower one of cool air 

 which descends toward the ground? It may be that the warm air, 

 being driven by lateral currents, has a tendency to ameliorate the tem- 

 perature of surrounding i)arts. 



Under the tropics, and especially under the equator, where the solar 

 rays act with the more force as they are less inclined, trees must pro- 



