THE PHYSICAL PROPERTIES OF THE SOIL. 863 



soil and its poorness in clayey and bumus materials. In periods of 

 drought the variations in the level of the surface of the ground water 

 increase with the thinness of the permeable layer in which the water 

 occurs, and with the ease with which the water is raised by capillarity. 

 In bare soils and in those covered with lifeless materials the ground 

 water rises and falls with the quantity of precipitation so long as the 

 water table does not reach the surface. In soils covered with living 

 plants no ground water collects during the period of growth even when 

 the soil is very deep (1.2 meters or more), or at least only temx)orarily.^ 



THK DISTRIBUTION OF WATER IN Tl/K SOIL. 



From what has been already said it is readily understood that the 

 moisture, as a rule, increases with the depth. This is true of all soils as 

 long as there is no precipitation and no downward movement of water. 

 In case of a soil of low permeability or one covered with strongly 

 transpiring plants a thorough moistening of the ujiper layers occurs 

 only when atmospheric moisture is supplied. Under these circum- 

 stances more water is contained in the upper layer temporarily than 

 in the lower strata, and a dry layer may occur between the upper and 

 lower moist layers if the rain is not sufficient for complete saturation — 

 an occurrence observed quite frequently in soils covered with plants 

 during the summer. 



According to Hofmann^ we may conveniently distinguish three soil 

 layers. First there is a superficial zone of evaporation, in which the 

 degree of moisture varies from complete saturation to great dryness. 

 In this zone in case of a dry, hot summer the whole of the summer and 

 late autumn precipitation may often be absorbed without filling up the 

 capillary pores down to the lower limit of the zone. In such cases an 

 intermediate dry layer always exists between the upper layer, tempo- 

 rarily moistened by precipitation, and the deeper water-conducting 

 layer. Below this superficial zone, then, there lies the so-called transi- 

 tion zone, which is that part which is never dried up, but which always 

 has its capillary pores filled with water. If this second zone receives 

 water from above after the superficial layer has been completely satu- 

 rated with water it does not retain it, but allows the excess to sink into 

 the third zone — that of the ground water. 



• Hofmanii, Arcli. Hyg., 1. J. Soyka, Der Boden. Pettenkofer and Ziemssen, 

 Handbuch der Hygiene, 1887, p. 251; Die Schwankungen des Grundwassers. A. 

 Penck, Geographische Abhandlungen, 1888, 2, No. 3. E. Wollny, Forscli. Geb. agr. 

 Phys., 14, p. 332. 



2Arch Hyg., 1 and 2, No. 2. 



