FORESTS AXD THEIR CLIMATIC INFLUENCE. 411 



it appears liow important it is, in the examination of the eah^ritic effects 

 resnltin«>- from disboscation, to have rciSiarcl to the ]>hysical i)roperties of 

 the soil, when it is once denuded. Here probably is to be found the 

 reason why the conclusions which llundjoldt has drawn from the ther- 

 mometric observations made at stations in !N"orth America, no attention 

 having been paid to the nature of the soil of the denuded surface, are 

 not the same with those at which M. Boussiugault has arrived by taking 

 that condition into consideration. 



It has been competently proved, then, that the disboscation of a soil 

 formed of a siliceous, pebbly sand, must raise the mean temperature of 

 the air more than any other formation, at the same time that it causes 

 the disappearance of a source of humidity: while, if the soil is argilla- 

 ceous, whether dry or humid, the capacity of warming the air and re- 

 taining heat is, relatively to the former, in the ratio of 08.4 to 100. 

 The calorific effect must be considerably less from the denudation of a 

 dry formation. 



We see now in what manner we should consider the influence of disbos- 

 cation on the temperature of the air. The effects, liowever, are so com- 

 plex that we can only determine the resultant by the help of diurnal 

 observations of temperature ; it is necessary besides to collate the max- 

 imum and minimum temperatures, which play a very important i)art in 

 the constitution of climates, and to have regard to the nature of the soil. 

 We shall resume this question on an early occasion. 



The following illustration is of a nature to give an idea of the influ- 

 ence Avhich forests may exert on the climate of a vast region. The 

 presence of extensive forests in the tropical portions of the African con- 

 tinent, situated under the meridians of the western part of Europe, 

 would probably modify the ascending current of warm air w hich at 

 present results from the heating of a sandy surface, and which descends 

 upon the middle latitudes of Europe. If, in the lapse of centuries, the 

 sands of the Sahara should become covered with woods, these sands 

 woidd not be heated to so high a degree as at the present epoch ; con- 

 sequently the winds of the south, which now mollify our climate, hav- 

 ing no longer so high a temperature, w^ould render it more rigid. To be 

 satisfied of this it is sufficient to consider the state of things on the 

 American continent, where the tropical regions are occupied by vast 

 forests, immense savannas, or great water-courses ; the descending cur- 

 rents of Avarm air cannot moderate the climate of countries situated in 

 the middle latitudes of North America as much as the w^arm currents 

 coming from the Sahara mitigate the countries of the eastern conti- 

 nent situated under the same latitudes. And here is precisely the rea- 

 son why the western continent, under corresponding latitudes, is colder 

 than ours, judging from the objects of culture and the course of the 

 isothermal lines in each. 



Nor does it suffice- to study the calorific influence of the extirpation of 

 woods upon climate; it is further necessary to inquire into the action 

 which it exerts on the sources of streams, and the physical effects pro- 

 duced in mountainous countries on a denuded soil, as well as those re- 

 sulting from such denudation in argillaceous and humid formations. 

 Another observation we will make, as not being without some inqiort- 

 ance: It has been previously seen that a tree becomes warm or cool 

 like an unorganized body, and that in proportion as the leaves are cooled 

 at night by the effects of the nocturnal radiation, the loss of heat is 

 repaired by a radiation transmitted by the trunk and branches ; this 

 state of things, which has not been hitherto noticed by physicists, hin- 

 ders the air from being chilled as much as if the calorific radiation oi 



