374

MODERN STEAM PRACTICE.

a slot on the throw-out handle, a rod belng attached to it and thepin which passes through the socket on which the eccentric rodslides. This arrangement of starting gear is certainly very compact,although for heavy engines we prefer the plain gab on the eccentricrod. The bracket for carrying the starting wheel, &c., for smallpower, is generally bolted to the columns for carrying the head-stock.

THE LINK MOTION.

The application of double eccentrics and link motion to theoscillating engine affects but little the general arrangement of thevalve gear. The pin on the sector for taking the gab end of theeccentric rod, as for the single-eccentric arrangement, is used forthe block on which the link slides; in fact, this pin and blockmay be compared to the pin and block on the slide-valve rod,in the direct applications of the link motion to the locomotive, or inhorizontal direct-acting marine engines. Indeed, the oscillatingengine may be considered a direct-acting engine, whether set verti-cally, horizontally, or lying at an angle; but as levers are interposedbetween the sector and valve spindle, the motion becomes indirect.

As the sector always moves in a direct line similar to the slide-valve spindle in horizontal arrangements, and as the pin for takingthe block on which the link slides is fitted directly to the sector, itonly remains to consider the application of the double eccentricsand link motion from this point to the centre of the main crankshaft. Thus when the levers are placed horizontally or at rightangles to the valve spindle, the slide valve being at half stroke,.then from the centre of the pin on the sector to the centre of themain crank shaft is the radius for describing the link, to which thedouble eccentrics and rods are fitted in the usual manner. So itwill be understood that the double eccentrics, keyed fast on thecrank shaft, having rods and link working directly in a line with.the slot link or sector for taking the levers for actuating the slide-valve spindle, simply take the place of the single-eccentric rod andgab end, having means of throwing out and also of actuating thevalve by hand, to suit the direction required for the forward orbackward movements. The link, however, being attached to eccen-trics for both the forward and backward motions, the combin-ation of both can never err (with proper mechanism for moving iton the block which oscillates on the pin attached to the sector) in