THE FOREST AS A SHELTER. 
151 
stratum of air, will retard the passage of the strata above and 
below it. To this effect may often be added that of an ascend¬ 
ing current from the forest itself, w T hich 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. 
Experience, in fact, has shown that mere rows of trees, and 
even much lower obstructions, are of essential service in de¬ 
fending 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 expe¬ 
rience 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 defence against the mechanical force of the wind, but its uses 
are by no means limited to this effect. If the currrent 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 sur¬ 
face, still a considerable extent of ground would be protected 
by a forest to the windward of it. If we suppose the trees of 
a wood to have a mean height of only twenty yards, they 
would often beneficially affect the temperature or the moisture 
of a belt of land two or three hundred yards in width, and thus 
perhaps rescue valuable crops from destruction.^: 
* Becquerel, Des Glimats , etc., p. 179. t Ibid., p. 116. 
J The following well-attested instance of a local change of climate is 
probably to be referred to the influence of the forest as a shelter against 
cold winds. To supply the extraordinary demand for Italian iron occa¬ 
sioned by the exclusion of English iron in the time of Napoleon I, the 
furnaces of the valleys of Bergamo were stimulated to great activity. 
“ The ordinary production of charcoal not sufficing to feed the furnaces 
and the forges, the woods were felled, the copses cut before their time, and 
the whole economy of the forest was deranged. At Piazzatorre there was 
