of uniform diameter is nearly similar in its mode of execution to wire drawing. The sheet brass is bent round and soldered so as to form a hollow cylinder; and if the diameter outside is that which is required to be uniform, it is drawn through a succession of holes, as in wire-drawing. If the inside diameter is to be uniform, a succession of steel cylinders, called triblets, are drawn through the brass tube. In making tubes for telescopes, it is necessary that both the inside and outside should be uniform. A steel triblet, therefore, is first passed into the tube, which is then drawn through a succession of holes, until the outside diameter is reduced to the required size. The metal of which the tube is formed is condensed between these holes and the steel cylinder within; and when the latter is withdrawn the internal surface appears polished. The brass tube is considerably extended by this process, sometimes even to double its first length.
(143.) Leaden pipes.—Leaden pipes for the conveyance of water were formerly made by casting; but it has been found that they can be made both cheaper and better by drawing them through holes in the manner last described. A cylinder of lead, of five or six inches in diameter and about two feet long, is cast with a small hole through its axis, and in iron triblet of about fifteen feet in length is forced into the hole. It is then drawn through a series of holes, until the lead is extended upon the triblet from one end to the other, and is of the proper thickness in proportion to the size of the pipe.
(144.) Iron rolling.—When cylinders of iron of greater thickness than wire are required, they are formed by passing wrought iron between rollers, each
