THE PHYSICAL PROPERTIES OF THE SOIL. 961 



and niiniraiun soil temperatures? are not so far apart as in regions 

 remote from the coast. If we take into consideration also the action of 

 numerous local influences (mechanical structure, exposure, inclination, 

 covering of the soil, etc.), it is not surprising that the " isogeothermals" 

 {i. e., lines connecting places having the same average soil temperature) 

 should diverge very considerably from the parallels of latitude. 



The yearly, monthly, and seasonal temperature of the soil at the 

 same latitude decreases as the altitude above the sea increases. This 

 lowering of temperature with higher altitudes is greater in the soil 

 than in the air, and is more noticeable in spring and summer than in 

 autumn and winter. In spite of the greater exposure to the sun's rays 

 in high altitudes during the summer the soil temperature is lower than 

 on the plains, because the action of the sun's heat is shorter in duration 

 and radiation of heat and consequent cooling during the night prepon- 

 derates. Local influences (exposure and inclination, massiveuess of 

 the mountains, etc.) give rise under such circumstances to many modifi- 

 cations of the soil heat. 



Eegarding local influences, it should be noted that the condition of 

 the atmosphere (precipitation and winds), the position and covering of 

 the soil, the presence of bodies of water, etc., may cause numerous 

 changes in the temperature of the soil. 



The condition of the atmosphere is of importance inasmuch as the 

 absorption of rays diminishes and the emission of rays increases as the 

 air becomes rarefied and contains dust and water i^articles. Clouds 

 and mist diminish the absorption and emission. Eain usually lowers 

 the temperature of the soil, because the water is generally cooler than 

 the soil, and also because the evaporation from the surface is increased 

 by wetting. The opposite effect may be brought about in summer when 

 the soil is covered with vegetation and is dried up by previous drought. 

 In this case the precipitation may cause a rise in temperature owing to 

 the improved conductivity for heat due to the moistening. Winds 

 generally produce a lowering of temperature proportional to their veloc- 

 ity because they increase evaporation; and this effect is the stronger 

 in proportion to the moistness of the soil and its capacity for replacing 

 by capillarity the water lost by evaporation. Under some circum- 

 stances the temperature of the wind may be very important. Bodies 

 of water may lower tlie temperature considerably during spring and 

 summer without raising it in autumn and winter, and may therefore 

 cause a moderate lowering of the average temperature of the land in 

 the surrounding region. The influence of position and covering on the 

 temperature of the soil has already been noted. The influence of veg- 

 etation, especially of forest growth, on the neighborhood is analogous 

 to that of a body of water.^ 



1 C. S. Cornelius, Meteorologie, 1863, p. 113, J. R. Lorenz von Liburnan, Lehrbuch 

 der Klimatologie, Wien, 1874, p. 230. J. Soyka, Der Bodcn, Leipzig, 1887, p. 135. 

 E. Ebemiayer, Die pliysikalischen Einwirkinigen des Waldes, etc., 1873; Forsch. 

 Geb. agr. Phys., 15, p. 385. J. A. Heusele, Ibid., 16, p. 357. 



