_Answer._ They are used to exhaust the water which collects in the cylinders. When the engine is not working the cylinders and steam-pipes are all cooled off, so that when steam is first introduced into them a great deal of it is condensed until they become warmed. Water is also frequently carried over from the boiler with the steam. When this occurs the boiler is said to _prime_, or to “_work water_.” This water and that produced by the condensation of steam collect in the bottom of the cylinder and will not escape through the exhaust-pipes until the piston moves up so near to the end of the cylinder that the water will fill the whole space between it and the cylinder-head. As has already been stated, it will then escape so slowly that the momentum of the piston and other machinery is liable to “knock out” the cylinder-heads or even break the cylinder itself. The cocks _C_, _C_, called _cylinder-cocks_, are therefore placed in the under side of the cylinder, so that when they are open if there is any water in the cylinder it will escape through the cocks. They are therefore always opened when the engine is starting, or at any other time when there is any indication that there is water in the cylinders. QUESTION 166. _How are these cocks opened and closed?_ _Answer._ A shaft, _R R_, figs. 92 and 94, which extends across the frames, has an arm, _R S_, fig. 92, at each end. These arms are connected by rods, _S T_, with the handles of the cylinder-cocks. The shaft also has a vertical arm, _R U_, the upper end of which is connected by a rod with the cab. At the end of the rod is a suitable handle, _f_, fig. 71, by which the cocks can be either opened or closed at pleasure by the locomotive runner. QUESTION 167. _How is the piston-rod fastened to the piston?_ _Answer._ It fits into a straight or tapered hole in the piston-head, in which it is fastened either with a key, _k k_, as shown in figs. 96 and 97,[44] or by a nut on the front side of the piston. [44] Fig. 96 is an end view of the piston with the follower-plate removed. Fig. 97 is a section through the centre. [Illustration: Fig. 96.] [Illustration: Fig. 97. Scale 1¹⁄₂ in. = 1 foot.] QUESTION 168. _How is the piston constructed?_ _Answer._ It is made of two cast-iron pieces, _B_ and _C_, fig. 97, the one, _B_, called the _piston-head_ or _spider_, to which the piston-rod _D_ is attached. The other part, _C_, called the _follower-plate_, is bolted to the piston-head by the bolts _c_, _c_, called _follower-bolts_. The piston-head has lugs or projections, _d_, _d_, _d_, fig. 96, cast on the inside, to which the follower plate is bolted. Hollow spaces are thus left between these lugs. QUESTION 169. _How is the piston made to work steam-tight in the cylinder?_