160 TEEES AS SHELTER TO GEOUND TO THE LEEWAED. 



attraction between them, and, therefore, an obstruction wbicli 

 meclianicallj impedes the movement of a given stratum of air 

 will retard the passage of the strata above and below it. To this 

 effect maj often be added that of an ascending current from the 

 forest itself, which must always exist when the atmosphere within 

 the wood is warmer than the stratum of air above it, and must be 

 of almost constant occurrence in the case of cold winds, from 

 whatever quarter, because the still air in the forest is slow in 

 taking up the temperature of the moving columns and currents 

 around and above it. Experience, in fact, has shown that mere 

 rows of trees, and even much lower obstructions, are of essential 

 service in defending vegetation against the action of the wind. 

 Hardy proposes planting, in Algeria, belts of trees at the distance 

 of one hundred metres from each other, as a shelter which ex- 

 perience had proved to be useful in France.* " In the valley of 

 the Rhone," says Becquerel, "a simple hedge, two metres in 

 height, is a sufficient protection for a distance of twenty-two 

 metres." f The mechanical shelter acts, no doubt, chiefly as a de- 

 fence against the mechanical force of the wind, but its uses are 

 by no means limited to this effect. If the current of air which it 

 resists moves horizontally, it would prevent the access of cold or 

 parching blasts to the ground for a great distance ; and did the 

 wind even descend at a large angle with the surface, still a con- 

 siderable extent of ground would be protected by a forest to the 

 windward of it. 



In the report of a committee appointed in 1836 to examine an 

 article of the forest code of France, Arago observes : " If a curtain 

 of forest on the coast of ITormandy and of Brittany were destroyed, 

 these two provinces would become accessible to the winds from 



* Becqxtebel, Bes Glimats, etc., p. 179. f Ibid., p. 116. 



Becquerel's views have been amply confirmed by recent extensive experi- 

 ments on the bleak, stony and desolate plain of the Crau in the Department 

 of the Bouches-du-Rhone, which had remained a naked waste from the earliest 

 ages of history. Belts of trees prove a secure protection even against the 

 furious and chilly blasts of the Mistral, and in this shelter plantations of fruit- 

 trees and vegetables, fertilized by the waters and the slime of the Durance, 

 which are conducted and distributed over the Crau, thrive with the greatest 

 luxuriance. 



StTRELL, Etude 8ur les Torrents, 2d edition, 1872, ii., p. 35. 



