(_b._) =Joints.=—=Cement-joints= are pervious to gases. =Putty-joints= crack and so become pervious. A =lead= pipe is sometimes connected with an =iron= one by means of an iron ferrule, the lead pipe passing inside the ferrule and being turned over its lower edge, extending up on the outside, the ferrule then being calked into the hub. When paper is used for the gasket or packing, it rots away in time, and a passage is left through which gas can escape as follows: up between the hub and the turned-over lead pipe, over its edge, between it and the calking, down between the lead pipe and the ferrule, across the lower edge of the ferrule, and out between the inner surface of the ferrule and the outer surface of the lead pipe. (See Fig. 27.) Sometimes a connection of lead with iron is made by means of a =lead flange= fastened around the iron pipe with wire, and sometimes the lead pipe is simply stuck into a hole in the iron pipe and fastened with cement or putty. Such work should be replaced by properly made joints as described in the above regulations. (Or in “Bad Odors,” 2, _c_.) [Illustration: Fig. 27.—Iron ferrule improperly calked.] (_c._) =Pipes.=—If the =leaders= are not trapped at the bottom, offensive gases from them may enter the nearest windows. A leader is never of the same length as the soil-pipe, and, if there is direct communication between them through the house-drain, there will always be a current of air through them in one direction or the other. If the external air is colder than that inside the pipes, then the heavier column of air will be over the shorter pipe, and the current will be down that one and up the other. If the external air is warmer than that in the pipes, the heavier column will be that which includes the longer pipe, and the current will be down that one and up the shorter. So, as a rule, the current in winter is down the short pipe and up the long one, while in summer it is reversed, and, as the short pipe often ends near windows, the nuisance is greatest when these windows are open. Of course, in such cases, the leader must be trapped. =Ventilating-pipes= sometimes end in chimney-flues. This is a bad plan, for, if the flue is in use, the open end of the pipe will become choked with soot and finally be rendered useless. If the flue is not used, there will often be a down draught in it, and the offensive gases may be conveyed through stove-pipes or fireplaces into the rooms of the house. Such gases may even penetrate the walls of the flue and so enter the house. If such a pipe is ever run into a flue, it should be extended at least two feet above the chimney-top. Ventilating-pipes are sometimes badly arranged, so that they actually neutralize a trap and render it useless. (See Fig. 28.) In this figure _a_ is the soil-pipe and _b_ the ventilating-pipe. [Illustration: Fig. 28.—Faulty arrangement of ventilating pipes.]