398 FORESTS AND THEIR CLIMATIC INFLUENCE. 



Less proof it would be impossible to bring to the solution of a question. 

 We shall i^roceed to discuss each of these opinions ; not contenting our- 

 selves, like their authors, with adhering to generalities, but by a resort 

 to facts and experiments, the sole means of arriving at a solution. 



M. Arago was right in saying that forests served as a shelter against 

 winds, but he did not say within what limits, and yet therein, as we shall 

 show, lies the whole question. The Alps, by reason of their situation 

 and height, protect certain parts of the coast of tbe Mediterranean, es- 

 ])ecially those about Sice and Hyeres, from the cold winds of the north. 

 The same chain of mountains renders exceptional also the climate of 

 Lake Maggiore, Lake Como, and the neighboring districts. Nothing 

 like this would occur, at least over so great an extent, if, in i)lace of the 

 Alps, which are several thousand metres in height, there were moun- 

 tains of an ordinary altitude or mere hills; for, the protected surface, as 

 will be seen, depends on the elevation of the mountains. Now, the ac- 

 tion of forests, composed of the loftiest trees, having at most a height 

 of from 30 to 40 metres, (100 to 130 feet,) cannot be different from that of 

 simple hills. Their mass supplies in this case the comi)actness in 

 which they are defective. 



"In the' plains of Orange," says M. Gasparin, {Tmife (VAgriculturej t. 

 1, p. 190.) "the north wind which passes over the mountains of Dauphine 

 strikes the earth at an angle of about 15° ; whence it follows that a height 

 of 200 metres (G5G feet) protects a qjace of 2,100 metres (7,087 feet,) a 

 border always reserved ibr the most \'aluable crops, and such as most 

 shun the cold. Under the iniiuence of such a shelter, the mean tem- 

 perature of the year is raised by more than 1° ; whence it is that the 

 orange tree flourishes in the open grounds at Ollioules and Hyeres, 

 while it cannot stand the winters of Marseilles ; hence, too, the temper- 

 ature of the air at the Lakes Como and Garda permits the cultivation 

 of the olive, which dares not show itself in the plains of Lombardy." 



We will cite still another example which furnishes an idea of the ex- 

 tent which may be protected by a shelter of slight elevation. In the 

 valley of the Ehone, where the ^nisfm/ frequently blows, a simple hedge 

 2 metres in height (7 feet) is capable of protecting a space of 22 metres 

 (72 feet) in breadth, a limit which, as M. Gasparin observes, should 

 serve as a guide in the discussion. It is by means of such shelters, 

 which are greatly multiplied in this valley, that the cultivation of le- 

 guminous plants is possible, as it could not be without this expedient. 

 In the open plains of Provence, hedges of still greater height are ob- 

 tained by planting the cypress and the laurel. All these shelters, though of 

 little elevation, protect large spaces when the lower currents of cold wind 

 are horizontal. In this connection, we should not forget to mention the 

 different aspect presented by the two faces of the Pyrenees ; the tract 

 on the side of tSpain, which is exposed to the winds of the south, is 

 arid, while that which looks toward France is covered with pastm-es 

 of fine vegetation. 



The examples which we have just cited suffice to show that the action 

 of forests, even when composed of trees of the tallest growth, is limited 

 and cannot consequently extend to an entire country, as is maintained 

 by M. Arago. 



M. Gay-Lussac is still less explicit, for he only propounds questions, 

 or gives their solution a priori without proofs in support of it. He asks, 

 for example, whether the evaporation of water is the same on a naked 

 soil as on one covered with vegetation ; he asserts, on the other hand, 

 that the influence attributed to trees upon the system of waters per- 

 tains in the highest degree to the herbaceous vegetation. The examin- 



