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MODERN STEAM PRACTICE.

ward or backward motions, as the case may be. In some instancesthe lifting or reversing lever only describes a small arc of a circle,having the radius of the lever much longer than by the formerarrangement; then the point of Suspension is at some intermediatepart of the half circle; in fact, just similar to the examples givenfor the locomotive engine in the preceding pages. In all examplesthe most convenient method of setting out the link is by placingthe valve and adjuncts at half stroke, A being the centre of thecrank shaft, and B the centre of the pin on the valve spindle fortaking the block on which the link slides. With the radius A Bdescribe the arc DD; this is the centre line of the curve of thelink; make the distance between the pins for taking the eccentricrods, as at E E, equal to three times the throw of the eccentricor diameter of the path. Then from the point B, with a convenientlength of lifting rod as the radius, describe the arc C F; it gives thePosition at the half lift of the link of the reversing lever, or, as inthe main connecting rod for the engine, the vertical distance fromB to C is the length of the lifting rod, the radius of the lifting armC F being half of the distance between the centres of the pins onthe link for the eccentric rods. Thus we have given the leadingpoints to attend to in setting out the double eccentrics and linkmotion.

THE LAP OF THE VALVE VARIES AS THE CUT-OFF ANDLENGTH OF CONNECTING ROD.

With the opening of port by valve, and the lead remaining thesame, the lap of the valve must vary as the cut-off. The less thechord and versed sine of the arc of supply becomes on the crankpath the greater is the chord of the arc of supply on the eccentric.The figure shows the chord of the arc of supply on the crank path inplain lines, while the eccentric circles are delineated by dotted lines.The line A A is the opening of port by valve, while the curved linerepresents the laps for the various points of cut-off. It will be seenthat when the steam is cut off at five-eighths of the stroke of thepiston that the valve at B has less cover than at C ; or when the steamis cut off at five-eighths of the stroke the valve requires less lap thanfor cutting off at the half stroke of the piston, and so on increasingthe diameter of the eccentric path. The diameter of the sheave like-wise increases rapidly, more especially when cutting off at one-eighth