856 EXPERIMENT STATION RECORD. 



fine sand, and concretions formed by precipitation (carbonate of lime, 

 hydrated oxid of iron) or adhesion of particles (liardpan) are almost 

 completely impervions to water. The high permeability of coarse soils 

 may be considerably diminished by an admixture of fine-grained 

 material. In stratified soils the permeability is controlled by that 

 layer which is made up of the finest particles, even when the layer is 

 thin. In crumbly soils the permeability is considerably greater than 

 in powdery soils, and it decreases in loose soils when they are pressed 

 together. 



The quantity of water passing through the soil increases with the 

 water pressure — not in the same proportion, but to a less degree. How- 

 ever, the differences in the quantity of water for a given material and 

 thickness of layer corresponding to equal intervals of pressure are con- 

 stant. Experiments have shown, further, that in fine-grained soils and 

 at high pressures the quantity of water passing through the soil dimin- 

 ishes as the thickness of the layer increases; while in other soils and 

 under lower pressures the relation of the quantity of water to the thick- 

 ness of the layer is not as great, diminishing as the size of the grains 

 increases.^ 



The power of evaporation of the soil. — This is most conveniently 

 measured by the quantity of water given off by evaporation to the 

 atmosi)here from the unit of surface. The method of referring the 

 evaporation to the weight of the soil is subject to the same objections 

 urged against the determination of the water capacity according to the 

 weight of the soil. 



The quantity of water evaporated, where only the physical structure 

 of the soil is considered, depends on (1) extent to which the factors 

 determining evaporation (temperature, moisture, and movement of the 

 air) exert their influence, and (2) the power of the lower layers of soil 

 to supply the loss at the surface. 



Evaporation increases with the extent of surface. Therefore the 

 quantities of water given up to the atmosphere are greater from uneven 

 and rough than from smooth surfaces. Evaporation is greatest from a 

 southern exposure, next greatest from an eastern, next from a western, 

 and least from a northern exposure. The quantity of water evaporated 

 increases on a southerly and decreases on a northerly exposure with 

 the inclination. For easterly and westerly exposures the influence of 

 inclination on evaporation is hardly perceptible. When very wet, dark 

 soils lose more water by evaporation than light-colored ones, but for 

 dryer soils the reverse is the case.'^ 



'E.Wolff, Anleitung zur Untersnchung landw. wichtiger Stoffe, 1875, p. 74. C. 

 Fliigge, Ztschr. Biol., 13, p. 465. A. R. von Schwarz, Erster Bericht iiber Arbeiten 

 der k. k. landw. -cbcmischen Versuchs-Station in Wien, i878, p. 51. F. Seelbeira, 

 Archives N^erland. Sci. Exact, et Nat., 14, p. 393. D. von Welitschkowsky, Arch 

 Hyg., 2, p. 499. E. Wollny, Forsch. Geb. agr. Phys., 14, p. 1. 



'C. Eser, Forsch. Geb. agr. Phys., 7, pp. 46, 47, 53, 97. 



