M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 315 



M. Arago sajs with reason, that the forests serve as a shelter against 

 •winds, but he has not said within what limits; yet the whole question 

 rests there, as we will see. The Alps, by reason of their location and 

 height, shelter certain parts of the Mediterranean coast against the 

 cold north winds, especially such places as Nice and Hyeres. This 

 same chain of mountains gives an exceptional clime to Lake Maggiore 

 and Lake Como, and the region about them. Nothing of this would 

 appear, or at least to no great extent, if the Alps were not several thou- 

 sand meters high. Had they been only common mountains, or only as 

 high as common hills, the case would have been different, for the pro- 

 tected places, as we shall see, depends on the heights of the mountains. 

 Well, the action of the forests composed of trees of the first rank, and 

 not less than 30 to 40 meters high at most, ought not to be different 

 from that of simple hills. Their mass is virtually alike. 



Oa the plains of Orange (says M. de Gasparin) '■ the north winds from over the 

 Mountains of Danphiny strike the earth at an angle of about 15*^, — from which it 

 follows, that a height of 200 meters protects a space 2,160 meters wide, a belt always 

 reserved for the choicest harvests, and that has nothing to fear from the cold. Under 

 such a shelter, the mean temperature of the year is one degree higher, so that oranges 

 come to full maturity in the open air at Ollioules, and at Hyeres, while they do not 

 stand the winters of Marseilles ; and in like manner they cultivate the olive, which 

 they dare not attempt on the plains of Lombardy. 



We will cite another instance, that gives an idea of the extent to which 

 a small height may afford protection. In the valley of the Ehoue, where 

 the mistral often blows, a simple hedge 2 meters high will shelter to a 

 distance of 22 metres, which is a limit that should serve as a guide for 

 calculation. It is by means of such shelters, very abundantly grown in 

 this valley, that they are able to cultivate vegetables that could not be 

 raised without this aid. 



In the open plains of Provence they raise higher hedges by planting 

 cypress and laurel. All shelters of little elevation preserve wide spaces, 

 when the cold lower winds blow horizontally. 



We should not forget to mention the different aspects which the two 

 slopes of the Pyrenees present — on the side of Spain exposed to the south 

 winds, arid — while on the north, toward France, covered with pasturage 

 and a fine vegetation. 



The examples that we have cited suffice to show, that the action of 

 forests, even in trees of the first size, is limited, and cannot therefore 

 extend to whole regions of country, as M. Arago has asserted. 



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

 or has given only cb priori his answers without proofs. He asks, for ex- 

 ample, whether the evaporation of water is the same on a naked soil as 

 on a soil covered with vegetation? He also further affirms, that the in- 

 fluence we attribute to forests in the regime of waters pertains also in 

 high degree to herbaceous vegetation. The solution of these questions 

 requires that we should take into consideration the following facts. 



ISchubler has proved that all soils do not possess in the same degree 

 the property of absorption.* In 100 parts of soil dried at 40° or 50° he 

 found the quantity of water absorbed to be as follows: 



Silicious sand 25 



Gypseous soil — 

 Calcareous sand 



Barren clay 



Fertile clay 



Loamy clay 



Pure clay 70 



Fine calcareous soil 85 



Humus 190 



Magnesian soil 156 



Garden soil 89 



1 Cours d'Agriculliirt, by Count de Gasparin, i, 196. 

 «It., i,196. 



