198 HOW CROPS FEEi). 



the effect that may be iiroduced by means within human 

 control, it may be stated that previous to the year 1821, 

 the Frencli district Provence was a fertile and well-water- 

 ed region. In 1822, the olive trees which were largely 

 cultivated there were injured l)y 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 Ehine 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, (iui-ing a series of observations lasting seven days 

 (April, 1852), Avas 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 attaia 



