[Illustration: FIG. 184.] [Illustration: FIG. 185.] The valves shown in Fig. 181 are carried by two plates or decks, the suction valves being attached to the lower plate and the delivery valves to the upper one. The upper deck, and sometimes both decks, are removable. The valves are secured to the plates by means of bolts or long machine screws, which, in turn, are screwed into the bridge across the board in the plate, as shown in Figs. 185 and 186 or capped as in Fig. 187. The valves in all pumps except the large sizes, which may properly be classed with pumping engines, are of _the flat rubber disc type_, with a hole in the center to enable the valve to rise easily on the bolt, the latter serving as a guide. _A conical spring_ is employed to hold the valve firmly to its seat, the spring being held in position by the head of the bolt, or cap, as shown. Certain improvements in pump-valves have been made which tend to increase the durability and to prevent the liability of sticking, which is not an uncommon occurrence after the valves have become badly worn. The improved forms of pump valves are shown in Figs. 186 and 187. [Illustration: FIG. 186.] [Illustration: FIG. 187.] When these valves leak through wear the disc may be reversed, using the upper side of the disc next to the valve seat. This can be done with ordinary valves also, provided the spring has not injured the upper surface of the disc. Valve seats are generally pressed into the plates, although instances may be found where they are screwed. When pressed in they may be withdrawn by substituting a bolt having longer screw threads than the regular bolt, and provided with a nut, as shown in Fig. 188. The bolt is slipped through a yoke and screwed into the bridge. By turning the nut the seat can generally be started without difficulty. [Illustration: FIG. 188.] Fig. 189 represents the customary _gland and stuffing-box_ in which the gland is adjusted by the nuts C and D upon two studs. After the adjustment has been properly made lock-nuts are tightened which leaves the gland free yet preserves the alignment. It has been proven by practice—after long and costly experiments—that a number of small valves instead of one large one are far the most durable; _durability_ being the question. Corliss, Leavitt, Holly and other leading pump builders had occasion to find the truth of this statement early in their careers. The “slamming” of large valves under moderate speeds proved itself an almost insurmountable difficulty until the principle of keeping the valve area as low as possible within reasonable limits had been fully demonstrated. [Illustration: FIG. 189.]