162 TREES AS SHELTER TO GROUND TO THE LEEWARD. 
It is evident that the effect of the forest, as a mechanical 
impediment to the passage of the wind, would extend to a very 
considerable distance above its own height, and hence protect 
while standing, or lay open when felled, a much larger surface 
than might at first thought be supposed. The atmosphere, 
movable as are its particles, and light and elastic as are its 
masses, is nevertheless held together as a continuous whole by 
the gravitation of its atoms and their consequent pressure on 
each other, if not by attraction between them, and, therefore, 
an obstruction which mechanically impedes the movement of a 
given stratum of air will retard the passage of the strata above 
and below it. To this effect may 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 occur- 
rence 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. Ex- 
perience, 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 hun- 
dred metres from each other, as a shelter which experience had 
proved to be useful in France.* “In the valley of the Rhone,” 
says Becquerel, “a simple hedge, two metres in height, is a suf- 
ficient protection for‘a distance of twenty-two métres.”+ The 
mechanical shelter acts, no doubt, chiefly as a defence against 
* BECQUEREL, Des Climats, etc., p. 179. + Ibid., p. 116. 
Becquerel’s views have been amply confirmed by recent extensive experiments 
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. 
SURELL, Etude sur les Torrents, 2d edition, 1872, ii., p, 35. 
