Concerning the retardation of the temperatures with an increase in depth below the surface, it is interesting to point out that this, according to Fodor, amounts to about three weeks for every yard, so that the yearly maximum at a depth of 1 yard will take place in August, at a depth of 2 yards in the beginning of September, and at a depth of 4 yards in October. This is on the assumption that the maximum temperature of the atmospheric air is reached in July. [Sidenote: Frost depth about 3 ft.] The depth to which frost under ordinary conditions penetrates is about 3 feet, but there are cases on record where water pipes at depths of from 4 to 5 feet have been frozen up during long continued severe frost. 7. SUBSOIL AIR. [Sidenote: Subsoil air is saturated with aqueous vapour and contains large quantities of carbonic acid.] The pores of soil are either partly or wholly filled with air, which as a rule is saturated with aqueous vapour. This air consists very largely of carbonic acid (from 0·2 to 14 per cent., on an average from 2 to 3 per cent.) and to a small extent of oxygen, which has been used up for the formation of carbonic acid. It also contains traces of ammonia and gases of decomposition. The movements of subsoil air need not be considered here, and beyond these few general observations it will not be necessary to deal with the subject. 8. MOVEMENTS OF WATER IN SOIL. [Sidenote: Strata above level of subsoil water.] Two main strata may here be distinguished in subsoil, one above the level of the subsoil water and one below this level. The latter strata do not interest us, and those above the level of the subsoil[7] water may again be subdivided into three zones, which in descending order are as follows:— The evaporation zone; The passage zone; and The capillary zone. [Sidenote: One-third of the rain-water evaporates. One-third flows off the surface. One-third percolates.] All these three zones must be passed by the water in its descent from the surface of the ground to the subsoil water level, and the quantity of water retained by them will depend on their state of dryness. Speaking quite generally and within wide limits, one-third of the rain-water flows off the surface, one-third evaporates, and one-third percolates into the subsoil. [Sidenote: Evaporation zone.]