of the lap and the lead:  the points in which his parallel intersects the circle of the eccentric are the positions of the forward and backward eccentrics.  Through these points draw straight lines from the centre of the circle, and mark the intersection of these lines with the circle of the crank shaft; measure with a pair of compasses the chord of the arc intercepted between either of these points, and the diameter which is at right angles with the crank, and the diameters being first marked on the shaft itself, then by transferring with the compasses the distance found in the diagram, and marking the point, the eccentric may at any time be adjusted without difficulty.

[Illustration:  Fig. 45.]

522. Q.—­Will you describe the structure and arrangement of the feed pumps of locomotive engines?

A.—­The feed pumps of locomotives are generally made of brass, but the plungers are sometimes made of iron, and are generally attached to the piston, cross head, though in Stephenson’s engines they are worked by rods attached to eyes on the eccentric hoops.  There is a ball valve, fig. 45, between the pump and the tender, and two usually in the pipe leading from the pump to the boiler, besides a cock close to the boiler, by which the pump may be shut off from the boiler in case of any accident to the valves.  The ball valves are guided by four branches, which rise vertically, and join together at the top in a hemispherical form.  The shocks of the ball against this cap have in some cases broken it after one week’s work, from the top of the cage having been flat, and the branches not having had their junction at the top properly filleted.  These valve guards are attached in different ways to the pipes; when one occurs at the junction of two pieces of pipe it has a flange, which along with the flanges of the pipes and that of the valve seat are held together by a union joint.  It is sometimes formed with a thread at the under end, and screwed into the pipe.  The balls are cast hollow to lessen the shock, as well as to save the metal.  In some cases where the feed pump plunger has been attached to the cross head, the piston rod has been bent by the strain; and that must in all cases occur, if the communication between the pump and boiler be closed when the engine is started, and there be no escape valve for the water.

523. Q.—­Are none but ball valves used in the feed pump?

A.—­Spindle valves have in some cases been used instead of ball valves, but they are more subject to derangement; but piston valves, so contrived as to shut a portion of water in the cage when about to close, might be adopted with a great diminution of the shock.  Slide valves might be applied, and would probably be found preferable to any of the expedients at present in use.  In all spindle valves opened and shut rapidly, it is advisable to have the lower surface conical, to take off the shock of the water; and a large lift of the valve should be prevented, else much of the water during the return stroke of the pump will flow out before the valve shuts.