to high temperature, is very different from what they are when cold. One of the best aids to correct valve-setting is a good indicator, as nothing shows the action of the steam in the cylinder so correctly as this instrument. It tells exactly when the steam goes in and out of a cylinder, because it maps down the motions of the steam as determined by the motions of the valve and piston, recording faithfully the times and pressures as they actually are.

To set a slide-valve, place the crank on the dead-centre and the valve centrally on its seat over the ports; then adjust the valve-gear to the right length, and move the eccentric round in the direction in which the engine is intended to run, until the proper lead is attained, as shown in Fig. 1, page 204; then turn the engine on the opposite centre, and, if the lead is exactly the same, the valve ought to travel equally on its seat, and the exhaust appear, as in Fig. 2, page 204. Any difference in the lead at either end must be equalized by lengthening or shortening the valve-gear, as the case may be.

An intelligent engineer can generally tell by observation whether engines exhaust regularly or not; as, if the steam is discharged with long or short puffs, alternately, or shows what is technically termed a long and short leg, it is evident that the valve has an earlier and a freer exhaust at one end than at the other; nevertheless, one exhaust may be heavier than the other, and yet the intervals between them may be equal. In such cases the exhaust is equal as to time, but not as to amount. The difference in amount may be caused by unequal degrees of expansion, and this in turn may be caused by unequal cut-off, or unequal clearance, or both. Such inequality cannot be cured by mere adjustment, since the lap requires to be changed; but in most cases an improvement may be effected by a compromise between equalized cut-off and exhaust, so that the effects of the inequality of both would not be noticeable.

In the case of fast-running engines, or where the exhaust has to pass through long pipes, this inequality is not easily determined from the appearance of the exhaust; but it may be done more

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accurately by holding the ear close to the exhaust-pipe. This latter method may also be resorted to in the case of low-pressure engines exhausting into a condenser.

Valves and Valve-Gear.

The term valve-gear embraces all intermediate connections between the eccentric on the driving-shaft and the valves, and is applicable to all mechanical arrangements employed for working the valves of steam-engines.

The valves most generally employed for the admission of steam to the cylinders of steam-engines, are the slide, poppet, Corliss or semirotary, and rotary; plug- or piston-valves are also used, but most generally for steam-pumps. All valves, whether used for the admission or escape of steam to or from the cylinders of steamengines, receive their motion from cams, eccentrics, or cranks; the movements of the former being indefinite as to character, and of the two latter, definite. Whatever the device employed to give motion to the valves may be termed, whether cams, eccentrics, cranks, gearing, rockers, wrist-plates, toes, lifters, trips, links, rods, levers, etc., they may be placed under the head of valve-gear.

There are engines without valves, such as the Wardwell, which was on exhibition at the Centennial Exposition at Philadelphia, and some kinds of oscillating engines, in which faces on the cylinder fit against faces on stationary steam-chests, through which the steam enters and escapes from the cylinder. Such arrangements may be called stationary valves, but they possess inherent defects, which render them useless for the most important purposes for which the steam-engine is employed.

A "releasing" valve-gear is an arrangement in which the valve is liberated from the control of its moving agent, and allowed to close in obedience to the action of a spring, weight, or other force independent of that which opened it. The agent which determines the time of release may be the governor, or it may be, and often is, some device adjustable by hand.

An automatic cut-off valve-gear is one in which the movement of the cut-off valve is so controlled by the governor, as to cut off the steam as early or as late in the stroke as may be required, to maintain the desired uniformity of speed, under variations of load and pressure.

A positive cut-off is an arrangement of valve-gear by which the expansion of the steam is effected by what is known as lap on the valve, the steam being cut off at the same point in each stroke, independent of load or pressure.

An "adjustable" cut-off is an arrangement of valve-gear, in which the point of cut-off can be adjusted by the hand of the engineer, outside of the steam-chest, by means of a screw, hand-wheel, or other mechanical arrangement, to meet the requirements of work and pressure. The link, in its application to the steam-engine, belongs to this class of cut-offs, as it effects the adjustment of the cut-off by means of coincident variations in the travel and angular advance using a single valve.

Riding cut-off.-A term applied to cut-off valves which ride on the back of the main steam-valve.

An independent cut-off is one in which the expansion is effected by an independent or auxiliary valve riding on the back of the main valve, and receiving its motion from an independent eccentric.

An "expansion" valve-gear is one that cuts off the supply of steam at any required point of the stroke. It embraces all the foregoing arrangements.

A "whole" stroke valve-gear is one that admits steam through the whole length of the stroke.

A "reversing" valve-gear is an arrangement employed for reversing the motion of engines. It is effected in different ways: in some cases with a single eccentric, while in others with two eccentrics, as in the case of the link; and in others, still, by means of a loose eccentric which revolves on the shaft, but is prevented from making a complete revolution by two stops so placed that one arrests it in the proper position for the forward, and the other

for the backward motion. This arrangement is peculiarly adapted to tug-boats and ferries, owing to the ease and quickness with which the engine can be reversed.

Double-beat valves are poppet-valves so arranged, that the pressure of steam is nearly equal on both sides, thus rendering the motion of the valve much easier than in the case of an ordinary single-beat valve. (See cut, page 223.)

Throttle-valves are valves located in the steam-pipe, through which steam is admitted to the steam-chest. At present their use is confined to locomotives and old-fashioned stationary engines.

Relief-valves are used on the cylinders of large engines, particularly marine, to prevent fracture of the cylinder-head and cylinder, in consequence of an accumulation of water in the latter. When a greater pressure is exerted in the cylinder than would result from the ordinary pressure of the steam, the relief-valve will open and admit of the discharge of the water, thus averting an accident. They are used on fire-engines for the purpose of preventing the hose from bursting when the escape of the water is obstructed.

Balance-valves.-Arrangements by which the weight on the back of slide-valves, induced by the pressure of the steam, is relieved by the action of the steam in the steam-chest.

Rotary-valves. A term applied to any valve that describes a revolution in working.

Semirotary-valves.-A term applied to all valves similar to the Corliss that have a vibratory or rocking motion.

Starting-valve gear.-A mechanical arrangement employed in connection with a small engine, called the starting-engine, for moving the valves of large engines when stopping or starting.

Gridiron-valves.-A modification of the slide-valve, containing a number of openings for the steam, by which means its travel and friction are materially diminished.

Dash-pot.-An arrangement employed for closing the valves of engines of the Corliss type, and in many instances for arresting the closing movement when it is sudden and violent. The dash

pots contain usually either water or oil, though in many instances they are cushioned with air.

Spring-levers.-Arrangements employed for closing semirotary- and poppet-valves. They are a substitute for the dash-pot, which has many advantages over them, on account of the disagreeable noise induced by their workings.

Lifters. A term applied to the toes on the lifting-rods, which open and close the valves of steam-engines, particularly those constructed with what is called a Stevens' front.

Wrist-plate. An arrangement employed in engines of the Corliss type for transmitting the motion of the eccentric to the valves, and in many instances for modifying their throw or movement.

Trips. A term applied to the pawls which liberate the valves of engines having what is termed a releasing-valve gear.

Crab-claw. A term applied to the pawls, which liberate the valve-gear of engines of the Corliss type from the influence of the eccentric, when the point of cut-off is reached.

Valves and Cocks Connected with Engines and Boilers.

The valves and cocks on a ship's side, in the engine, boiler-room, and hold of a steamship, are the injection-, main-, bilge-, discharge-, and water-service valves, and the blow-off-, scum-, and ash-cocks.

The valves on a marine engine that can be worked by hand are the stop-, safety-, slide-, throttle-, starting-, feed-, and suctionvalves.

The valves that are worked by the motion of the engine are the slide, cut-off, or expansion, feed, and bilge-pump, check, and discharge valves.

The valves and pipes, through which the steam passes from the boiler to the condenser, are the steam stop-valve on the boilerdome, the steam-pipe, the throttle-valve, the slide- or poppet-valve in the steam-chest, and the eduction-pipe between the cylinder and the condenser.

The cocks and valves through which the injection and boiler