M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 513 



consistence to the soil. The branches covered with leaves, secure it 

 against the force of the rains 5 the trunks, the shoots, and the brush 

 which cover the ground, oppose multiplied resistances to the currents, 

 which, without this check, would wear gullies into the soil. The effect 

 of vegetation is, therefore, to give more solidity to the ground, and to 

 divide the waters over the whole surface. The soil being divided by 

 the roots and covered by a spongy humus, absorbs a part of the 

 waters, and thus hinders them from running off from the surface. The 

 woods, therefore, serve as a shelter against the rains in a mountainous 

 country. 



The action of the forests as a shelter against the winds is not absolute, 

 for these effects depend upon the height at which the wind blows. If 

 this height is less than that of the forest, the wind is stopped at every 

 moment by the trees; it loses its velocity, and if the woodland is of 

 sufficient extent it stops it altogether when it has reached its limit. 

 But when the wind blows at a greater height than the trees, the latter 

 have no effect except upon the lower current, at least if its direction is 

 not declined. Above the reach of the forest the upper mass of air, 

 meeting no obstacle, continues its horizontal course with undiminished 

 velocity. The action of a forest upon the wind is therefore limited as 

 regards the shelter it affords. 



Forests may operate in two other ways. When found in the way of 

 a current of air moving with violence, and at the maximum point of 

 saturation with vapor, a part of it penetrates the mass, and a part 

 is turned off" by the obstacles that it meets in the passage. The higher 

 portion, if it meets a stratum of cool air, has its vapor precipitated and 

 it falls as rain. 



When a current of bad air, laden with pestilential miasms, penetrates 

 a forest of a certain extent, it is wholly deprived of these properties. 

 The effect of this is observed in the Pontine marshes, in which a belt of 

 trees preserve all that is behind them, while the uncovered part is ex- 

 posed to fevers. The trees, therefore, tame the infected air and deprive 

 it of its miasms. This fact has been shown by M. Eigaud de I'lsle in 

 his treatise on foul atmospheres.^ 



M. Hardy, director of the government nursery at Algiers, has given 

 facts that well illustrate the beneficial influence that trees may exert as 

 a shelter. There exist in Algeria three classes of trees : the first, of 

 deciduous trees, such as the poplars, alders, «&c., which grow in the 

 ravines and on the banks of streams ; secondly, the agaves, cactuses, 

 and palms ; and, thirdly, trees with evergreen leaves, such as olives, 

 caroub-trees, laurels, &c. M. Hardy has noticed that trees of the first 

 group, that are natives, grow more in breadth than in height, with con- 

 stantly a broad flat top. If it happens that some of them come to a 

 large size and find conditions more favorable for their development, they 

 grow vigorously for a time, when, on coming to the height of the trees 

 around them, the top dries up, and the branches spread only in a hori- 

 zontal direction. This was seen in some poplars planted at Bouffarich, 

 in the middle of the plain of Mitidja, in humid conditions that left noth- 

 ing to be desired for this species, yet these trees could not get more than 

 10 or 12 meters high. We often, however, observed specimens that, 

 nevertheless, did not appear to suffer at the top ; but they grew at the 

 base of a steep hill, of which the top was much higher than the trees. 



^This author was one of the savaDts who was sent to Rome in 1810 to study the 

 question of drainage of the Pontine marshes. He addressed to the Minister of the 

 Interior an extended report, which was fully discussed in the privy council. The 

 work cited was entitled Memoires sur les Causes de Vlnsalubrit^ de I'Air, published in 

 the Bibliotheque Universelle (1816-1817). 



