that plate; GI, GI a plane surface forming an inner seat on which that plate slides; KL, KL the plane

surface of the face of the slide valve. The movable plate derives motion from a cam or eccentric, and so

cuts off the steam. But arrangements very similar had been employed thirty years before this contrivance was brought out.

LINK MOTION.

This contrivance for giving motion to the valves is now employed in nearly every class of engines. It was brought out and applied to locomotives by Mr. Robert Stephenson in 1843, and was first applied to marine engines by Mr. Edward Humphrys. In a form of the link motion constructed by me in 1836, only one eccentric was employed, which gave motion to a double-ended lever in which a slot was made nearly from end to end. In this slot the pin was placed which communicated with the valve rod; and by moving this pin along in the slot, which was done by appropriate mechanism, the engine was stopped or reversed, and any desired amount of expansion was accomplished. In another form of this apparatus a solid link was employed that turned down at the ends like a staple, and connected with the eyes of a doubleended lever, so that the link partook of the motion of the lever. The link was encircled by an eye within which was a globular brass to permit the angulation of the link, and a rod proceeding from this link joined the valve rod. This arrangement is shown in fig. 22, where a is the double-ended lever, B the connecting rod which joins the valve rod, and c the starting handle, by moving which the rod в is

raised up on the link either to the mid-position when the engine stops, or the top-position when it is reversed. Intermediate positions will affect the degree of expansion in the same way as in the case of the common link motion. The defect of this arrange

ment was, that it did not reverse the lead in reversing the engine. But in most cases an imperfect action of the valve when the engine is moving backward is not very material, seeing that it is only in rare cases that the engine is required to work backward for

any considerable length of time; and the arrangement is simpler than the species of link motion now in common use.

Examples of the forms of link motion now commonly employed in marine engines are shown in figs. 23 and 24. In these cases two eccentrics are employed-a combination accidental to the use of a forward and backing eccentric in the old class of locomotives; and the ends of each pair of eccentric rods were in these cases connected by a link so planned that when one eccentric was in gear the other was out of gear. formed sprung from the of the valve might be derived from this link by a sliding eye or pin instead of from the ends of the eccentric rods, whereby the necessity of frequent connections and disconnections would be averted; whereas the idea of the arrangement I proposed in 1836 was taken from the double-ended lever, before that time sometimes employed in steam engines, and by which the motion of the valve was reversed when the eccentric rod was shifted from the pin at one end of the lever to the pin at the other end of the lever. In Messrs. Humphrys and Tennant's link motion, shown in fig. 23, the link is solid and passes through an eye in connexion with the valve rod, within which eye there is a swivelling brass to reconcile the discrepant motions of the link and rod. In Messrs. Penn and Son's link motion, shown in fig. 24, the same end is attained by the pin which swivels in the brass which

The link motion as now perception that the motion