[84] This effect is well illustrated by the behavior of a dry brick laid upon a wet sponge. It will quickly absorb all the liquid moisture contained in the latter, while the sponge will be wholly unable to take any moisture from a fully-soaked brick. The flow of irrigation water produces the same compacting effect, but to a less extent; the more as, unlike rain water, irrigation water usually contains a certain amount of alkaline and earth salts, which tend to prevent the diffusion of clay and of fine sediments, and therefore the disintegration of the soil-floccules into single grains. Nevertheless, it is in some soils as necessary to cultivate after surface-irrigation as after rains, in order to prevent great waste of moisture by evaporation. _Determination of rate of percolation._—When water is allowed to soak into an air-dry soil column without sensible shock or motion, from a constant level, we obtain the nearest approach to a definite determination of the relative permeability of soils to water under the conditions usual in the arid region. A number of determinations thus made is tabulated in the diagram given below, which embodies the observations made by Mr. A. V. Stubenrauch[85] in connection with a more extended investigation. [85] Rep’t Calif. Exp’t Station for 1898 to 1901, p. 165. As these experiments were made with soils not in their field condition, but gently broken up with a rubber pestle, a standard of compactness was established by weighing the quantity which could conveniently be settled into a tube space of 100 centimeters capacity by tapping the sides and bottom of the tube, without touching the soil itself. In this way the following standards were established: For the University Adobe soil, 140 grams; for the Yuba loam soil, 110 grams; for the Stanislaus sandy soil, 170 grams. Tubes 1½ inches wide were used, and the soils were introduced in bulk, inside of a cylinder of stiff paper upon which previously to rolling it up the soils had been thoroughly mixed. After introducing the soil-filled paper roll it was gently withdrawn, leaving the soil column in the tube as uniform as before; a condition almost impossible of fulfilment when the soil is introduced piece-meal. The tubes were, of course, left open at the lower end, using a wire netting to keep the soil column in, so that the air could escape freely before the descending water column.