The influence of man upon rain is seen in the progress of
civilization, the destruction of forests, and the drying-up of meres,
swamps, and water-courses.
Forests undoubtedly affect the distribution of rain, and the supplies
of streams and springs. Their cooling influence precipitates the vapor
passing over them, and the ground beneath them not getting heated does
not readily evaporate moisture. Lands, on the contrary, which are
cleared of forests become sooner heated, give off larger quantities of
rarefied air, and the passing clouds are borne away to localities of
greater atmospheric density.
The Canary Islands, when first discovered, were thickly clothed with
forests. Since these have been destroyed, the climate has been dry. In
Fuerteventura the inhabitants are sometimes obliged to flee to other
islands to avoid perishing from thirst. Similar instances occur in the
Cape Verdes. Parts of Egypt, Syria, and Persia, that once were wooded,
are now arid and sterile deserts.
In the temperate zones these results are not so immediately apparent.
It is now much in doubt whether the climate of our country has changed
its character within the last two hundred years.
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