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198 HOW CROPS FEED. 
the effect that may be produced by means within human 
control, it may be stated that previous to the year 1821, 
the French district Proveuce was a fertile and well-water- 
ed region. In 1822, the olive trees which were largely 
cultivated there were injured by frost, and the inhabitants 
began to cut them up root and branch. This amounted 
to clearing off a forest, and, in consequence, the streams 
dried up, and the productiveness of the country was seri- 
ously diminished. 
The Angle at which the Sun’s Rays Strike a Soil is 
of great influence on its temperature. The more this ap- 
proaches a right angle the greater the heating effect. In 
the latitude of England the sun’s heat acts most power- 
fully on surfaces having a southern exposure, and which 
are inclined at an angle of 25° and 30°. The best vine- 
yards of the Rhine and Neckar are also on hill-sides, so 
situated. In Lapland and Spitzbergen the southern 
side of hills may be seen.covered with vegetation, while 
lasting or even perpetual snow lies on their northern in- 
clinations. 
The Influence of a Wall or other Reflecting Surface 
upon the warmth of a soil lying to the south of it was 
observed in the case of garden soil by Malaguti and 
Durocher. The highest temperature indicated by a ther- 
mometer placed in this soil at a distance of six inches from 
the wall, during a series of observations lasting seven days 
(April, 1852), was 32° Fahrenheit higher at the surface, 
and 18° higher at a depth of four inches than in the same 
soil on the north side of the wall. The average temper- 
ature of the former during this time was 8° higher than 
that of the latter. In another trial in March the difference 
in average temperature between the southern and north- 
ern exposures was nearly double this amount in favor of 
the former. 
As is well known, fruits which refuse to ripen in cold 
climates under ordinary conditions of exposure may attain 
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