A cistern is provided with an overflow and waste pipe. If the cistern is on the roof you would think it the natural thing that the overflow pipe should discharge on to the roof or leads, or into an open head; but, as a matter of fact, it is generally not the case. (By an “overflow” pipe is meant a pipe from the top, and by a “waste” pipe a pipe starting above the level of the water and passing through the bottom of the cistern.) Overflow pipes were not in fashion at all until recently. The fashion was to have a waste pipe, and the most convenient place to take that into was some pipe passing down the house, which might be a rain-water pipe, but more frequently it was the pipe into which the water-closets discharged, which is called the “soil” pipe. When this is the case the waste pipe of the drinking-water cistern becomes the ventilator of the pipe into which the water-closets discharge; and so in nine cases out of ten the ventilator of the house drain and of the sewer under the street, and, indeed, one of the ventilators of the main sewer. So foul air passes continually by means of this ventilator into the drinking-water cistern at the top of the house. Now foul air in sewers and drains contains matters in suspension, and often the poisons of certain diseases, such as typhoid fever; it gains access to the water in the cistern and contaminates it, and the main cause of typhoid fever in London and many other large towns is the connection of the drinking-water cisterns with the drains by means of the waste pipes. Of course the remedy for this--the first remedy--is to put a trap on the waste pipe, as, for instance, connecting it with the trap in one of the closets or sinks. This, of course, is only a palliative, it is not the true remedy. The true remedy is to disconnect this pipe and make it discharge by itself, no matter where, in the open air. Sometimes this pipe is made to discharge into the same pipe that the sink waste-pipe discharges into. It is the practice in London to have a separate pipe for the various wastes and sinks not discharging directly into the drain, and usually carried outside the house. It is also the practice to make the waste pipes of cisterns to discharge into the same pipe. This is entirely wrong. Because, although disconnected at the foot, it is to be regarded as a foul-water pipe, and foul air passes through it up the waste pipe into the cistern. So this practice is to be condemned. Now from the cistern, besides the waste pipe, there are pipes which supply the water to different parts of the house; there are pipes from the cistern to supply water to the taps, which are called “draw-off” pipes; and pipes from the cistern to supply water to the closets; and, as a rule, the same cistern is used for the supply of water to the closets direct and the taps at the upper part of the house. This plan may or may not be very dangerous.