396 FORESTS AND THEIR CLIMATIC INFLUENCE. 



Wlioii a sloping: surface is overnm by vegetation, first by low plants, 

 tUen by trees, the roots entwine with one another and form a net-work, 

 which gives consistency to the soil; while the branches, furnished with 

 their leaves, protect it from the violence of rain-storms. The trunks, 

 with the offshoots and brush-wood which surround them, uuiltiply 

 points of resistance to the currents which would otherwise furrow the 

 earth. The effect of vegetation, therefore, is to give more solidity to 

 the soil, and to distribute the waters over the whole surface. The soil, 

 being divided by the roots and covered with a spongy humus, absorbs 

 a piirt of the waters which cease to tlow over the declivities. It is thus 

 that the woods act as a shelter against the force of rains in mountainous 

 countries. 



The action of forests as a shelter in regard to winds is not absolute ; 

 for the effects depend on the height to which the wind blows. If this 

 height does not attain that of the forest, the wind is arrested at every 

 moment by the trees; loses its velocity; and, if the forest has a sufficient 

 density, will have wholly ceased by the time it reaches its limit. When it 

 blows to a height greater than that of the trees of the forest, the latter has 

 no action except on the lower stratum of the current of air, unless its 

 direction be inclined ; above the forest, the upper mass of air which has 

 encountered no obstacle, and which has a horizontal direction, continues 

 with the same velocity. The action, then, of a forest as a shelter, is 

 limited. 



Forests may act in still two other ways : when they happen to lie in 

 the direction of a violent current of air, at a maximum of saturation 

 with vapor, a part penetrates into the mass, the other part is dis])ersed 

 in all directions by the obstacle presented to its passage; the portion 

 which rises, if it encounters a colder stratum of air, yields its vapor to 

 precipitation, and a fall of rain ensues. Again, when a current of humid 

 air charged with pestilential miasms penetrates into a forest of a certain 

 extent, it is altogether divested of them. It has been observed in the 

 Pontine marshes that the interposition of a screen of trees preserves all 

 that is behind it, while uncovered tracts are exposed to fevers. The trees, 

 therefore, sift the infected air by removing its miasms. This fact has 

 been reported by M. Eigaud de Ldle, in his work on malaria. 



M. Hardy, director of the government nurseries at Algiers, has 

 announced facts which clearly show the influence that trees may exer- 

 cise as a shelter. There exist iu Algiers three groups of frutescent or 

 shrubby plants ; the first being formed of trees with caducous leaves, 

 l)0i)lars, alders, &c., which grow in ravines and on the banks of streams; 

 the second comprising the agave, the cactus, the palms, &g. ; the third 

 consisting of vegetable species with persistent leaves, such as the olive, 

 the carob, the wild laurel, &c. M. Hardy has remarked that the trees 

 of the first group which are indigenous groAv rather iu breadth than in 

 height, and that they constantly present a large and flattened top ; if 

 some species happen to attain considerable altitude under the most 

 favorable conditions for their development, they are observed to grow 

 with vigor for some time, but arrived at the height of the trees of the 

 country, the top becomes dry and the branches then extend themselves 

 horizontally. Effects of this kind are to be seen in the poplars planted 

 at Bouffarich, in the center of the plain of Mitidja, under conditions of 

 humidity of soil which leave nothing to be desired for this species, and 

 yet these trees are incapable of surpassing the height of from 10 to 

 12 metres, (33 to 39 feet.) There are found, it is true, specimens which 

 rise higher and still seem not to suffer at the top, but these are situated 

 at the base of a steep hill whose crest is much more lofty than the 

 trees. 



