TREES AS SHELTER TO GROUND TO THE LEEWARD. 159 
events, well known that the great swamps of Virginia and the 
Carolinas, in climates nearly similar to that of Italy, are healthy 
even to the white man, so long as the forests in and around 
them remain, but become very insalubrious when the woods 
are felled.* 
Trees as Shelter to Ground to the Leeward. 
As a mechanical obstruction, trees impede the -passage of 
air-currents over the ground, which, as is well known, is one 
of the most efficient agents in promoting evaporation and the 
refrigeration resulting from it.+ In the forest, the air is almost 
the result. These influences, if we may believe certain able writers on medi- 
cal subjects, are telluric rather than meteoric ; and they regard it as probable 
that the uniform moisture of soil in forests may be the immediate cause of 
the immunity enjoyed by such localities. See an article by PETTENKOFER 
in the Siid-Deutsche Presse, August, 1869; and the observations of EBuR- 
MAYER in the work above quoted, pp. 246 et seg. 
In Australia and New Zealand, as well as generally in the Southern Hemis- 
phere, the indigenous trees are all evergreens, and even deciduous trees intro- 
duced from the other side of the equator become evergreen. In those 
regions, even in the most swampy localities, malarious diseases are nearly, if 
not altogether, unknown. Is this most important fact due to the persistence 
of the foliage ? 
Mossman, Origin of Climates, pp. 874, 393, 410, 425, ef seq. 
* Except in the seething marshes of northern tropical and subtropical 
regions, where vegetable decay is extremely rapid, the uniformity of tempera- 
ture and of atmospheric humidity renders all forests eminently healthful. 
See HOmENSTEIN’s observations on this subject, Der Wald, p. 41; also A. 
Maury, Les Foréts dela Gaule, p. 7. 
The flat and marshy district of the Sologne in France was salubrious until 
its woods were felled. It then became pestilential, but within the last few 
years its healthfulness has been restored by forest plantations. JULES CLAVE 
in Revue des Deux Mondes for 1st March, 1866, p. 209. 
There is no question that open squares and parks conduce to the salubrity 
of cities, and many observers are of opinion that the trees and other vegeta- 
bles with which such grounds are planted contribute essentially to their bene- 
ficial influence. See an article in Aws der Natur, xxii., p. 813. 
{+ It is perhaps tco much to say that the influence of trees upon the 
wind is strictly limited to the mechanical resistance of their trunks, branches, 
and foliage. So far as the forest, by dead or by living action, raises or 
x=) 
