FOEESTS AND THEIR CLIMATIC IXFLUENCE. 415 



nlato their discliarge; G. The humidity Avhich prevails iu woods and the 

 function of the roots in making- the soil more pervious, should be taken 

 into consideration ; 7. The clearing- away of forests in mountainous coun- 

 tries exercises an influence on the streams and springs in the lowlands, 

 especially in the latter; 8. Hence the action of forests upon climate is of 

 a highly complex nature. 



With the means of securing salubrity now at our disposal, there is no 

 occasion for apprehending- unhealthy swamps as the result of the extir- 

 pation of forests. ]Sor need it be inferred that the denudation of a 

 country entails sterility. As examples, England and S[)ain nuiy be 

 cited, which present, the one only 2 per 100 of wooded surface, the other 

 3.17 per 100. The former has a marine climate, marked by the frequent 

 prevalence of southwest winds charged with vapor to the point of satu- 

 ration, which produces fogs on the least lowering of the temperature. 

 Spain has not a similar climate, but its most fertile parts are those 

 watered by large rivers, while the great table-lands are absolute deserts. 



From what has been said the question presents itself whether the ex- 

 tirpation of a great forest in the vicinity of a fertile plain possessing 

 only springs of water, might not give reason to fear the desiccation of 

 these springs in whole or in part and the consequent impoverisliment of 

 the country '? The denudation of an arenaceous country may lead to the 

 desolation of the neighboring plains through the incursions of the sand, 

 as may easily be conceived from the explanation given by M. Chevreul 

 of the formation of downs in the Landes of Gascony; the sand is here 

 driven by the winds nntil it encounters an obstacle, when a barrier is 

 formed which arrests the discharge of the waters ; these moisten the 

 base of the heap, and by capillary action en use the particles of sand to 

 cohere and become tixed to the soil ; the winds remove only the upper 

 part, which, being carried forward, continues to form new downs until 

 the i>lain in the end is wholly overwhelmed with sand. 



A forest, interposed in the passage of a current of humid air charged 

 with hurtful miasms, sometimes preserves from their influence any tract 

 which is thus sheltered; while uncovered regions, as is exemplilied in the 

 Pontine nuirshes, are exposed to the baleful influence. Trees, therefore, 

 tend to purify an infected air by absorbing or obstructing- its pestilential 

 constituents. Still another kind of action is exercised upon climate by 

 the presence of forests: the trees of lofty growth which compose them 

 withdraw electricity from the clouds, and thus to some extent neutralize 

 the disastrous effects of storms. 



The restoration of forests upon the mountains is an operation of prime 

 necessity for the preservation of the latter; its advantages result: 1st, 

 from the increased facility with which the rain-waters penetrate into the 

 soil and even the subsoil, being traversed by roots which promote infil- 

 tration; 2d, from the effects produced by the resistance which forests 

 oppose to the passage of masses of air saturated with vapors in motion, 

 which promptly descend in rain on being forced upwards and compressed 

 by the obstacle; 3d, from the humidity which generally prevails in the 

 interior and in the vicinity of woods and which gives place to a precipi- 

 tation of dew when the temperature of the air is lowered. 



Of the transformation of lands from which the forest has been re- 

 moved into marshes, some striking examples may be cited, and those 

 not in Asia Minor, of which mention has been made elsewhere, but in 

 France itself. It is to be observed that when trees are cut down, the 

 roots of course die and the soil becomes more compact. La Brenne, 

 situated between the Indre and the Creuse, presents a circular surface 

 of more than 200 kilometres (125 miles) in circiunference, or nearly 80,000 



