Where regularly-flushed masonry-lined side drains are unavailable, the drainage from houses must be led into masonry-lined cesspools, which will be cleaned out daily by the municipal sweepers, and the contents removed in conservancy carts. These cesspools should be semi-circular in shape, and plastered on the inside with Portland cement. In many cases, where masonry drains exist, it will be found possible to intercept this sullage, and purify it through Biological Agency in small tanks, filled with pieces of vitrified brick, broken so as to pass through a 2–inch ring. The broken bricks will last for about a year, when they should be renewed. [Sidenote: Sullage Filter.] A drawing of a small Sullage Filter is given in Fig. 18, page 36. The estimated cost of which is Rs. 180. [Sidenote: Prompt Disposal.] It is of the very greatest importance to insist on the prompt disposal of night-soil and refuse, and all means which aid this, good roads, proper care and feeding of animals, good carts, and the provision of houses for sweepers, help to a great extent. [Sidenote: Registering Private Latrines.] In order to systematise the removal of night-soil from private latrines, all these should be registered and numbered according to the beats of the attendant, or otherwise definitely indicated and located, and the sweepers told off accordingly. [Sidenote: Wheels and Axles.] When purchasing carts especial attention should be paid to the quality of the wheels and axles. The carts should not be too heavy and must be of a convenient height to facilitate working. [Illustration: SULLAGE FILTER FIG. 18. ] CHAPTER VII. DISPOSAL OF NIGHT-SOIL AND TRENCHING GROUNDS. Night-soil is generally disposed of by being buried in trenches, but these are frequently too deep: purification largely depends on the action of the ærobic bacilli, that is, the group of microbes which live close to the surface of the soil and require air for their existence; these swarm in the top layers of the earth, but are not found at a greater depth than 1 foot and diminish enormously in number as this is approached. In order to obtain the best results, both from a purificative as well as a manurial point of view, the burial should be merely superficial and not exceeding in depth the limit of ordinary cultivation, in order to ensure the ground being thoroughly aërated. [Sidenote: Selection of Site.]