_Wrought Iron_ is used in the so-called Durham system of plumbing. Wrought iron is very strong; the sections of pipe are twenty feet long, the connections are made by screw joints, and a system of house plumbing made of this material is very durable, unyielding, strong, and perfectly gas-tight. The objections to wrought iron for plumbing pipes are that the pipes cannot be readily repaired and that it is too expensive. _Cast Iron_ is the material universally used for all vertical and horizontal pipes in the house. There are two kinds of cast-iron pipes manufactured for plumbing uses, the "standard and the extra heavy." The following are the relative weights of each: Standard. Extra Heavy. 2-inch pipe, 4 lbs. per foot 5-1/2 lbs. 3 " " 6 " " " 9-1/2 " 4 " " 9 " " " 13 " 5 " " 12 " " " 17 " 6 " " 15 " " " 20 " 7 " " 20 " " " 27 " 8 " " 25 " " " 33-1/2 " The light-weight pipe, though extensively used by plumbers, is generally prohibited by most municipalities, as it is not strong enough for the purpose, and it is difficult to make a gas-tight joint with these pipes without breaking them. Cast-iron pipes are made in lengths of five feet each, with an enlargement on one end of the pipe, called the "hub" or "socket," into which the other, or "spigot," end is fitted. All cast-iron pipe must be straight, sound, cylindrical and smooth, free from sand holes, cracks, and other defects, and of a uniform thickness. The thickness of cast-iron pipes should be as follows: 2-inch pipe, 5/16 inches thick 3 " " " " " 4 " " 3/8 " " 5 " " 7/16 " " 6 " " 1/2 " " Cast-iron pipes are sometimes coated by dipping into hot tar, or by some other process. Tar coating is, however, not allowed in New York, because it conceals the sand holes and other flaws in the pipes. =Joints and Connections.=--To facilitate connections of cast-iron pipes, short and convenient forms and fittings are cast. Some of these connections are named according to their shape, such as L, T, Y, etc. [Illustration: FIG. 16. DIFFERENT FORMS AND FITTINGS.]