between the ditches being two or sometimes three feet in width, as shown in Fig. 5. In this case crops are grown on the ridges and the ditch is used exactly as a sewage filter.

Intermittent filtration is intensified irrigation. It is, in fact, a compromise between irrigation and the adoption of sewage filters. It is true that crops are grown on the ridges between the ditches, the sewage percolating laterally to their roots. On a properly laid out sewage farm there should be part of the land prepared for irrigation and part for intermittent downward filtration, and it is a good plan to alternate the land which is used for these two purposes, as on the Berlin, Birmingham, and other large sewage farms.

Irrigation Proper.-The usual method of carrying out irrigation is for the sewage to be brought to the highest level of the surrounding land, generally through an underground pipe; from this point it is conveyed through an open earthenware or concrete carrier along a contour; branch distributing carriers being given off in its course. These branch distributing carriers may be allowed to overflow on to the surface of the soil which should be laid out for a uniform fall of about 1 in 100. The actual fall should depend upon the impermeability of the soil. With impermeable soil the fall being even less than this. The final distributing carriers may be merely furrows in the soil itself. With pervious soils far more distributing carriers are necessary to spread the sewage

evenly over the land than are necessary with retentive soils.

With regard to stiff clay lands, the best results will be obtained by irrigating twice over, or even more times, as at Leicester, by a catch-water system, as in Fig. 5.

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PLAN

FIG. 6.-METHOD OF IRRIGATING STIFF CLAY LAND.

The arrangement of the distributing carriers must to some extent depend upon the conformation of the land. If it has a gentle slope or is irregular a catchwater system should be adopted, the distributing carriers running along the slope at various levels,

the usual distance allowed between two carriers being about a chain; but this distance must depend purely upon the slope and the permeability of the soil, with a rapid slope it being a greater distance, and with a very pervious soil less. When the ground is level, it must be prepared in broad ridges on the ridge and furrow system, the beds being about forty feet wide with the feeder along the top of the ridge.

If the sewage is first clarified by precipitation, from half to one-third of the land otherwise necessary will be sufficient. The actual population the sewage of which can be purified upon an acre of land depends upon the nature of the soil. The author has under his supervision a farm consisting of about forty acres of stiff clay land irrigated with the sewage of a population of 600, the greatest care being used to apply the sewage intermittently; but he has rarely met with a satisfactory effluent therefrom.

When land is irrigated with raw sewage the solid matters in suspension are arrested mechanically on the upper layers of the soil. If the sewage is applied for longer than three or four hours to the same land, the surface nearest to the distributing carrier becomes covered with a layer of slime, which, when the sewage ceases to be applied to the land, dries and forms an impervious film. This not only kills the crops, but also prevents the air from entering the soil and stops nitrification. Where irrigation is adopted, the system used at Berlin-of passing the