The evaporation zone reaches from the surface of the soil to that point below, which marks the extent of the drying influence of the atmospheric air. In the same the quantity of water stored in the pores may at times sink below the retentive power of the soil, i.e. below that quantity which can be retained in the pores owing to the mechanical powers of adhesion, etc. When it has become very dry through evaporation and other causes the zone, especially when it extends some way down, may retain large quantities of water. In a depth of 10 inches, 1 square yard of soil, with fine pores, may retain about 10 gallons of water, and as a rainfall of ½ inch produces only 2·3 gallons per square yard, it is clear that subsoil of this nature may retain a number of successive showers. During the height of summer fine porous soil may become so dry that practically no water finds its way into deeper zones; in this state the evaporation zone can be compared to a large sponge. [Sidenote: Passage zone.] The next zone traversed by the water in its downward movement is the passage zone, which lies beyond the drying influence of atmospheric air. When too far removed from the level of the subsoil water, its pores will not be completely filled with water, but will only contain that amount which is due to the retentive powers of the soil. By direct measurement it has been found that on an average a cubic yard of fine porous soil will retain from 30 to 80 gallons of water, and it can easily be calculated that in a layer from 1 to 2 yards in thickness the rainfall of a whole year may be retained. The passage zone, especially if it is of considerable thickness, represents a very large storage reservoir. [Sidenote: Capillary zone.] The last zone before the level of the subsoil water is reached is the capillary zone, in which the pores are partially or wholly filled by the upward movement—due to capillary attraction—from the subsoil water. The extent of this filling will depend on the size of the pores. [Sidenote: Springs.] When the descending water has finally reached the subsoil water it either comes to a standstill altogether on the impervious layer or moves along the same, if the latter is not horizontal, until it may eventually leave the subsoil again by issuing therefrom in the form of visible or invisible springs. [Sidenote: Rate of downward movement governed by pores.] The rate of movement of any liquid—rain-water, sewage or other polluting liquid—is largely governed by the size of the pores. Where these are large, as for instance in coarse gravel, the descent of the water will be comparatively rapid, but when they are small it may take a very long time before the water reaches the level of the subsoil water, and in that case it will have undergone material changes as regards its chemical or bacterial composition. [Sidenote: With a high level of subsoil water zones become indistinguishable.]