A.—­They are almost invariably so made if the engines are travelling fast, as in the case of direct acting screw engines, and they are very often made of large discs or rings of india rubber, even when the engines travel slowly.  A very usual and eligible arrangement for many purposes is that shown in fig. 41, where both foot and delivery valves are situated in the ends of the pump, and they, as well as the valve in the bucket are made of india rubber rings closing on a grating.  The trunk in the air pump enables guide rods to be dispensed with.

[Illustration:  Fig. 42.  PENN’S DISK VALVE FOR AIR PUMP.  Section.]

[Illustration:  Fig. 43.  PENN’S DISK VALVE FOR AIR PUMP.  Ground Plan.]

[Illustration:  Fig. 44.  MAUDSLAY’S DISC VALVE FOR AIR PUMP.  Section.]

458. Q.—­The air pump, when double acting, has of course inlet and outlet valves at each end?

A.—­Yes; and the general arrangement of the valves of double acting air pumps, such as are usual in direct acting screw engines, is that represented in the figure of Penn’s trunk engine already described in Chapter I. Each inlet and outlet valve consists of a number of india rubber discs set over a perforated brass plate, and each disc is bound down by a bolt in the middle, which bolt also secures a brass guard set above the disc to prevent it from rising too high.  The usual configuration of those valves is that represented in figs. 42, 43, and 44; figs. 42 and 43 being a section and ground plan of the species of valve used by Messrs. Penn, and fig. 44 being a section of that used by Messrs. Maudslay.  It is important in these valves to have the india rubber thick,—­say about an inch thick for valves eight inches in diameter.  It is also advisable to make the central bolts with a nut above and a nut below, and to form the bolt with a counter sunk neck, so that it will not fall down when the top nut is removed.  The lower point of the bolt should be riveted over on the nut to prevent it from unscrewing, and the top end should have a split pin through the point for the same purpose.  The hole through which the bolt passes should be tapped, though the bolt is not screwed into it, so that if a bolt breaks, a temporary stud may be screwed into the hole without the necessity of taking out the whole plate.  The guard should be large, else the disc may stretch in the central hole until it comes over it; but the guard should not permit too much lift of the valve, else a good deal of the water and air will return into the pump at the return stroke before the valve shuts.  Penn’s guard is rather small, and Maudslay’s permits too much lift.

459. Q.—­What is the proper area through the valve gratings?

A.—­The collective area should be at least equal to the area of the pump piston, and the lower edges of the perforations should be rounded off to afford more free ingress or egress to the water.