The plunger in the Cameron pump is reversed by means of two plain tappet valves, shown in the accompanying engraving, Fig. 261, and the entire valve mechanism consists of four pieces, all of which work in a direct line with the main piston. This pump is simple and has no delicate parts. [Illustration: FIG. 261.] As here represented—_A_ is the steam cylinder; _C_, the main piston; _L_, the steam chest; _F_, the valve piston, the right-hand end of which is shown in section; _G_, the slide valve; _H_, a lever, by means of which the valve piston, _F_, may be moved by hand when expedient; _I_, _I_ are reversing valves; _K_, _K_ are the reversing valve chamber bonnets, and _E_, _E_ are exhaust ports leading from the ends of steam chest direct to the main exhaust. The passages, _M_, _M_, lead directly into the exhaust (although the connection is not shown, being cut away in the sectional view), and closed by the reversing valves, _I_, _I_. The piston, _C_, is driven by steam admitted under the ^B^ slide valve, _G_, which, as it travels backward and forward, alternately supplies steam to opposite ends of the cylinder, _A_. The slide valve, _G_, is operated by the valve piston, _F_; _F_ hollow at the ends, which are filled with steam, which, issuing through a hole in each end, fills the spaces between it and the heads of the steam chest in which it works. The pressure being equal at each end, the valve piston, _F_, under ordinary conditions, is balanced and motionless; but when the main piston, _C_, has traveled far enough to the left to strike and open the reverse valve, _I_, the steam exhausts from behind the valve piston through the port, _E_. The pressure being now unequal behind the two ends, the valve piston immediately shifts accordingly and carries with it the slide valve, _G_, reversing the piston. No matter how fast the piston may be traveling, it must instantly reverse when it touches the valve, _I_. In its movement the valve piston, _F_, acts as a slide valve to close the port, _E_, and is cushioned on the confined steam between the ports and steam chest cover. The reverse valves, _I_, _I_, as soon as the piston, _C_, leaves them, close by the constant pressure of steam behind supplied direct from the steam chest through the ports, _N_, _N_, shown by dotted lines. _To set the valve_, it is only necessary to keep valves tight by occasional grinding. The piston as it nears the end of each stroke strikes the stem and lifts the valve off its seat; this allows the exhaust steam behind the piston valve to escape. The live steam pushes the piston towards the exhausted end carrying the main slide valve along with it. THE MASON. The Mason pump has a valve piston, a main valve, a preliminary valve and a yoke connected directly to the valve stem, as shown in Fig. 262. The valve piston is contained in a cylinder above the steam chest, and moves the main valve by means of a pin, which projects into a pocket in the top.