INFLUENCE OF LATITUDE AND ASPECT 155 



tion represented by the beams KL and LN. If now we 

 replace this flat surface by a hill having a slope of 20, 

 facing south and north, it is at once evident that the equality 

 of the radiation on the two portions is destroyed, and that, 

 with the sun at the angle assumed, the southern slope will 

 now receive twice as much heat as the northern. 



The greater heat obtained from the sun on a southern slope 

 is, of course, received only in the daytime, and when the 

 sky is clear. The increase of heat is confined to the surface 

 of the ground, which becomes sensibly warmer ; the vegetation 

 upon it does not obtain any increased radiation from the sun, 

 (though it will from the ground), the growth of plants being 

 always perpendicular, whatever the slope of the soil. The 

 general temperature of the air is also unaffected by the 

 inclination of the ground, except near the surface during the 

 hours of sunshine. In addition to the increased intensity of 

 solar radiation, the southern slope has also the advantage 

 over the northern of more hours of sunshine, and of protection 

 from cold winds. As the southern slope gains heat only 

 during sunshine, and falls at other times to the general tem- 

 perature, it follows that the extreme range of temperature 

 is greatest with a southern aspect. 



Wollny (Forsch. der Agrikulturphysik, i. 263) determined 

 the temperature of the soil at 6 inches below the surface on 

 various sides of artificial hills of sandy soil containing humus. 

 The experiment was made at Munich, and extended throughout 

 a whole year. When the slope of the hill was 15, the average 

 temperature of the south side was i-5 F. more than that of 

 the north. When the slope was 30, the average excess of 

 temperature on the south side was 3-! F. From May to 

 August the south-east side was the warmest; in September 



