Concerning the depth of soil—evaporation zone—that is necessary for the successful retention, absorption and decomposition of sewage, no generally applicable rule can be laid down, as this will depend on a variety of factors, amongst which may be mentioned: the character and thickness of the top soil (humus), the nature and cultivation of the top soil; the character of the subsoil—its permeability for air and its retaining and absorbing powers; the surface slopes of the land and the level of the subsoil water. [Sidenote: Greater depths than 4 ft. will be rarely necessary.] On some farms a depth of 3 feet on an average has proved sufficient, and on others the drains have been laid at depths ranging between 3 and 6 feet, but very special reasons ought to be shown for all depths over 4 feet. [Sidenote: Soil best suited for sewage farms.] Whilst practically no soil is entirely useless for sewage farming, with the exception perhaps of peat, owing to the quantity of moisture it contains, a soil that combines great permeability for air with high retaining and absorbing powers—such as a loamy sand with fairly large grains—is probably the best. [Sidenote: Clay soil not unsuitable for sewage farms, but it necessitates a greater area of land.] It has been maintained that clay, owing to its impervious character, is totally unsuitable for sewage farming, but the experience of such farms as South Norwood, Wimbledon, Warwick and Leicester disproves this. It is true, however, that as the purifying powers of the soil are restricted in a vertical sense to the upper layers, it may become necessary in places to extend the area of the farm beyond what would be necessary with a more pervious soil. [Sidenote: Changes observed in the heavy clay land at Leicester since sewage treatment was commenced.] It may not be without interest to draw attention here to some of the changes that have taken place on the Leicester sewage farm since the land has received regular dressings of sewage. When I was engaged in laying it out in 1888 my powers of locomotion over the land were greatly impeded during wet seasons by the inordinate amount of clay that adhered to the boots; but when engaged again for some considerable time on the land during the winter 1900 to 1901 this unpleasant peculiarity had completely disappeared even on land that had recently been sewaged. Through the action of the sewage the very dense clay had been disintegrated and become so pliable that, when trod upon, it crumbled to pieces. The colour of the soil had been changed from a yellowish-brown to a greyish-black, and altogether the land had been greatly improved by the application of the sewage. [Sidenote: Movement of liquid through the passage and capillary zones to the impervious layer.]