Various modifications of this form of tank-filter will suggest themselves to readers possessing any mechanical genius. The great point is to prevent contamination from the soil by using good material and making sound work. Further, the overflow pipe of the tank must not communicate with any drain or sewer. [Illustration: 4. Rain-water Separator.] Recently several inventors have introduced apparatus for separating rain-water from impurities. One of these, bearing the name of Roberts, is illustrated in Fig. 4. Its principle of action is to reject the first portion of the rain which falls (as it is this which chiefly washes the dirt off the roof), and only to collect the latter portion of the rain. The water from the roof first runs on to a strainer, that intercepts rubbish; it then passes through one of two channels in the upper part of the canter, balanced upon a pivot. At the commencement of a shower, the canter is raised in the position shown in Fig. 4, “running to waste,” and the bulk of the water passes through a channel which directs it into the lower or wastewater outlet. Meanwhile, a very small proportion of the water is accumulating in the lower part of the canter, very slowly in light rain but more rapidly in heavy rain, so that it is filled up by the time the roof has become clean. Then the weight of water causes it to fall down as shown in Fig. 4A “running to storage,” so that the clean water may run through the upper storage outlet pipe. This very useful little apparatus is made and sold by C. G. Roberts, Collards, Haslemere, Surrey. [Illustration: 4A. Rain-water Separator.] Perhaps this affords as good an opportunity as any of drawing attention to the highly artistic rain-water heads that have lately been introduced by Thomas Elsley, of 32 Great Portland Street, W. These are made to suit every style of architecture and every variety of roof and guttering, and practically without limit as to size. Their quality is beyond praise. It is essential to bear in mind that rain-water is liable to exert considerable solvent action on lead, consequently pipes and cisterns of this metal must be avoided. The pipes may be of iron, or of specially lead-encased block-tin, and the cisterns of “galvanised” iron or slate.