_Answer._ They are made of cast iron. When two nozzles are used they are generally cast together, as shown in fig. 90. When only one is used, the form of the pipes resembles somewhat that of an inverted letter ~⅄~, as shown in fig. 91, so as to cover the two openings which connect with the cylinders. The tops of these pipes have rings or bushings, _a a_, fitted into them, which are held by set screws, _b_, so that they can easily be removed and others with larger or smaller openings be substituted. If the openings in the exhaust-nozzles are small, the steam must be discharged at a higher rate of speed, in order to exhaust all that is in the cylinders, than if the blast orifices are larger. Therefore, if the latter are reduced in size, the draft becomes more violent, but at the same time the _back-pressure_ in the cylinder (which will be explained hereafter) is increased. It therefore becomes necessary to adjust the size of the blast orifices with the greatest care, so as to have them just small enough to produce the required draft and yet leave them as large as possible, so as to reduce the back-pressure. For these reasons what are called _variable exhausts_ are sometimes used. In these the blast orifice can be increased or diminished at pleasure, and thus regulated to suit the conditions under which an engine is working. A great variety of such devices has been used, but now nearly all have been abandoned for the simpler arrangement described, which is not variable when the engine is working. [Illustration: Fig. 91. Scale 1¹⁄₂ in. = 1 foot.] PART X. THE CYLINDERS, PISTONS, GUIDE-RODS AND CONNECTING-RODS. QUESTION 159. _How are the steam cylinders constructed?_ _Answer._ They are made of hard cast iron, and have the steam and exhaust ports and valve-seats cast with them. The harder the iron the better will the cylinders withstand the wear of the pistons and valves, but they must at the same time be made soft enough, so that after they are cast the inside can be bored out perfectly cylindrical, the ends turned off, the bolt-holes drilled, and the valve-seats planed smooth. Fig. 92 represents a longitudinal section through the centre of the cylinder and steam-chest. Fig. 94 is a plan of the same parts with the cover of the steam-chest and the valve removed. The left-hand side of fig. 95 shows a transverse section through the centre _c d_, fig. 92, of the cylinders, and the right side is a section through the steam-pipe _G h_, fig. 94. The same letters indicate like parts in the three different views.