ers seem to have attributed too
much importance to the influence of forests, particularly those of
America, as if they were the primary cause of the moisture of the
climate.

The theory of a modern author on this subject "that forests exist in
those parts of America only where the predominant winds carry with them
a considerable quantity of moisture from the ocean," seems far more
rational. In all countries, he says, "having a summer heat exceeding
70 degrees, the presence or absence of natural woods, and their greater
or less luxuriance, may be taken as a measure of the amount of humidity,
and of the fertility of the soil. Short and heavy rains in a warm
country will produce grass, which, having its roots near to the surface,
springs up in a few days, and withers when the moisture is exhausted;
but transitory rains, however heavy, will not nourish trees; because,
after the surface is saturated, the remainder of the water runs off, and
the moisture lodged in the soil neither sinks deep enough, nor is in
sufficient quantity, to furnish the giants of the forests with the
necessary sustenance. It may be assumed that twenty inches of rain
falling moderately or at intervals, will leave a greater permanent
supply in the soil than forty inches falling, as it sometimes does in
the torrid zone, in as many hours."[999]

"In all regions," he continues, "where ranges of mountains intercept the
course of the constant or predominant winds, the country on the windward
side of the mountains will be moist, and that on the leeward dry; and
hence parched deserts will generally be found on the west side of
countries within the tropics, and on the east side of those beyond them,
the prevailing winds in these cases being generally in opposite
directions. On this principle, the position of forests in North and
South America may be explained. Thus, for example, in the region within
the thirtieth parallel, the moisture swept up by the trade-wind from the
Atlantic is precipitated in part upon the mountains of Brazil, which are
but low, and so distributed as to extend far into the interior. The
portion which remains is borne westward, and, losing a little as it
proceeds, is at length arrested by the Andes, where it falls down in
showers on their summits. The aarial current, now deprived of all the
humidity with which it can part, arrives in a state of complete
exsiccation at Peru, where consequently no rain falls. But in the region
of America,