_Iron Pipe_ is joined to _Iron Pipe_ by lead-calked joints. These joints are made as follows: the spigot end of one pipe is inserted into the enlarged end, or the "hub," of the next pipe. The space between the spigot and hub is half filled with oakum or dry hemp. The remaining space is filled with hot molten lead, which, on cooling, is well rammed and calked in by special tools made for the purpose. To make a good, gas-tight, lead-calked joint, experience and skill are necessary. The ring of lead joining the two lengths of pipe must be from 1 to 2 inches deep, and from 1/2 to 3/4 of an inch thick; 12 ounces of lead must be used at each joint for each inch in the diameter of the pipe. Iron pipes are sometimes connected by means of so-called rust joints. Instead of lead, the space between the socket and spigot is filled in with an iron cement consisting of 98 parts of cast-iron borings, 1 part of flowers of sulphur, and 1 part of sal ammoniac. [Illustration: FIG. 17.] All connections between _Lead Pipes_ and between _Lead_ and _Brass_ or _Copper_ pipes must be made by means of "wiped" solder joints. A wiped joint is made by solder being poured on two ends of the two pipes, the solder being worked about the joint, shaped into an oval lump, and wiped around with a cloth, giving the joint a bulbous form. All connections between _Lead Pipes_ and _Iron Pipes_ are made by means of brass ferrules. Lead cannot be soldered to iron, so a brass fitting or ferrule is used; it is jointed to the lead pipe by a wiped joint, and to the iron pipe by an ordinary lead-calked joint. _Putty_, _Cement_, and _Slip_ joints should not be tolerated on any pipes. [Illustration: FIG. 18.] =Traps.=--We have seen that a trap is a bend in a pipe so constructed as to hold a quantity of water sufficient to interpose a barrier between the sewer and the fixture. There are many and various kinds of traps, some depending on water alone as their "seal," others employing mechanical means, such as balls, valves, lips, also mercury, etc., to assist in the disconnection between the house and sewer ends of the pipe system. The value of a trap depends: (1) on the depth of its water seal; (2) on the strengths and permanency of the seal; (3) on the diameter and uniformity of the trap; (4) on its simplicity; (5) on its accessibility; and (6) on its self-cleansing character. The depth of a trap should be about three inches for water-closet traps, and about two inches for sink and other traps. Traps must not be larger in diameter than the pipe to which they are attached. The simpler the trap, the better it is. Traps should be provided with cleanout screw openings, caps, etc., to facilitate cleaning. The shapes of traps vary, and the number of the various kinds of traps manufactured is very great.