=Filters.=--Unfiltered cistern water is not, as a rule, fit for drinking purposes because of pollution from dust and impurities washed from the roof, but for bathing and laundry work filtered rain water is greatly to be desired. As rain water comes from the roofs of buildings, there is washed into the cistern a considerable quantity of dust, leaves, bird droppings and other polluting materials which contaminate and discolor the water. This foreign matter is not injurious for the purposes intended, but to render the water clear it should be filtered before using. Filters for cisterns are quite generally made of soft brick laid in cement mortar, the face of the brick being left uncovered. Fig. 137 illustrates a simple and efficient form of filter made of a single course of brick. A space one-fourth to one-third of the volume of the cistern is left for the filtered water. The opening at the top of the wall must be large enough to admit a man, for some sediment will collect even in the filtered water and the filter must be occasionally cleaned. [Illustration: FIG. 137.--Cross-section of a brick curbed cistern with a brick filter wall.] [Illustration: FIG. 138.--Cross-section of a concrete cistern with a brick dome filter.] The filter shown in Fig. 138 is dome-shaped and built of brick. The water is pumped from inside the filter and the suction of pumping filters the water as it is used. In this case the filtering action is accelerated by reason of the reduced pressure inside the filter as the water is pumped. The chief disadvantage in this form of filter is the small area exposed for the filtering action and the relatively greater amount of work required for pumping the water, due to the partial vacuum formed as the water is pumped. The cistern in Fig. 139 is provided with a catch basin which acts as a strainer for removing leaves, etc., that would stain the water. It is made in the form of a concrete basin and partly filled with gravel. The filter in this case is formed by a depression in the cistern floor. A section of tile is placed on the floor, and around it is filled the filtering material of gravel and sand. Filters of this kind are often filled with charcoal or other materials that are expected to purify the water. They are usually inefficient because their value as absorbers of polluting agents is short-lived and unless the materials are frequently renewed they are valueless and sometimes a detriment to rapid filtration. [Illustration: FIG. 139.--Cross-section of a concrete cistern, containing a sand filter.] THE HYDRAULIC RAM