57 



necessary to accomplish this, and that, too, to a greater deofree if the 

 soil l)e covered with vegetation. The ])henomena of autunni geniu-ally 

 resemble closely those of sunnner. 



(28) In general the fluctuations in the temperature of the earth 

 are not less dependent on the precipitation than on the variations in 

 the temperature of the air. 



SOIL, TEMPERATURES AS AFFECTED BY SURFACE SLOPE AND 

 COVERING (WOLLNY). 



In reference to the effect of the slope of the earth's surface on the 

 temperature of the soil, "Wollny (1888, p. 364) has made an extensive 

 series of measurements at Munich from which he draws the following 

 conclusions in continuation of those published by him in 1883. His 

 temperatures were measured bihourly at a depth of 15 centimeters 

 under both fallow soil and grass sod; the differences referred to 

 amounted to 3° and 4° F. in individual cases, but on the average to 

 scarcelj^ 1° F. 



(1) That soil whose exposure is toward the south is the warmest, 

 then comes the east, then the west, and finally the north exposure. 



(2) The southern exposure is warmer in proportion as the inclina-/^ 

 tion to the horizon is greater. 



(3) The difference of temperature betw^een the north and south 

 exposure is much greater than between east and west. 



(4) The difference in the Avarming of the soil for north and south 

 exposures is greater in proportion as the surfaces have a greater 

 inclination. 



AVollny (1888, p. 415) has also investigated the influence of the 

 covering of straw and chaff on tlie temperature and moisture of the 

 soil. He finds the following conclusions: 



(1) That at a depth of 10 centimeters the naked soil is warmed 

 more with rising air temperatures and is cooled more with falling 

 air temperatures than under any one of the different forms of straw 

 covering. 



(2) That the variations in the temperature within the straw litter 

 are very much less than in the earth. 



(3) That the earth is in general somewhat colder than the material 

 of which the litter is made, except when the latter is moss. 



(4) That among the various materials forming a litter the pine^ 

 needles are w^armed the most, the oak leaves and the fir-tree needles 

 are less warm, while the litter of moss is the coldest. 



The different temperatures observed were as follows, on the average 

 of the months April to September: Pine needles, lf).i)3'' C: oak leaves. 

 16.62° C; fir needles, 16.34° C. ; the naked soil at a depth of 10 

 centimeters, 16.18° C; moss, 15.95° C. 



