box, and the piston prevents the steam from passing through it; but when the edge of the cylinder is raised from the bottom of the box, the steam then gains an exit, and it is clear that the cylinder can be raised without any considerable exertion of force, as it is pressed equally in all directions. Instead of the rubbing surface of a piston, however, two ground valve-faces are employed in practice; and the moving part of the valve is not a perfect cylinder."

In working expansively with the slide-valve, the steam is cut off by what is termed the "lap" of the valve. In engines not working expansively the steam is allowed to enter the cylinder during its whole stroke; in this case the length of the valve-face is just equal to the breadth of the steam-port. In working expansively the valve-face is lengthened, so that when the valve is at the middle of its stroke, there will be an excess of width of valve-face over the width of port. Thus in fig. 94, let d be the upper and f the lower port of a long D slide valve: in the method of allowing the steam to enter the cylinder during the whole stroke of the piston, the valve-face would just be equal to the breadth of port; but where the expansive system is adopted, the valve-face, as above stated, is lengthened at the parts e e, the dotted lines giving the portion a a where no lap is used. Generally the lap is given in what is called the "steam

side" of the port, that is towards tt; sometimes, however, the lap is made on the eduction side, as well as towards uu, shown by the dotted lines. The effect of the "lap" on the valve is to cut off the steam at an earlier portion of the stroke than would otherwise be the case. An inspection of the diagram in fig. 94 will explain how this is effected: suppose the part u to be equal to the face of valve a without any "lap" to it, that is, equal to breadth of port d,-when the lower edge of valve at u arrived at the upper edge of the port d the port would be fully open; but if the valve happened to be lengthened to an amount equal to the part c below the dotted line, the port would in this case be closed a space equal to width of port c, instead of being fully open if the valve-face had been equal to a. In fig. 95 we give a diagram showing the lap of a locomotive or three

ported slide: ab the steam ports, c the exhaust; the lap is towards the steam side of the valves, as ee. In working engines, it is considered to promote their efficiency by giving what is called "lead" to the valve; this is done by making the port open a little before the termination of the foregoing stroke: thus, suppose the piston just about to terminate the upstroke, the upper port is opened a little, thus admitting steam before the other stroke is quite finished, and in consequence of the lap in the valve the exhaust port is opened sooner; so that by the time the piston begins its down-stroke, the steam from below the piston is escaping freely. If this arrangement was not adopted, we can easily conceive of the engine in its down-stroke having to press against the steam below the piston, which would get slowly out, inasmuch as the valve opens but slowly at the beginning of its stroke. To have the full perfection of working in the cylinder, the escape or release of the steam should be instantaneous if possible; the less steam the piston has to encounter in its motion either up or down the cylinder the better. By the arrangement of the valves now described, the "lap" and the "lead," the speed of locomotives has been much increased.

To obtain the full efficiency of the expansive method of working, it is considered best to have the cut-off instantaneously effected-this the slidevalve cannot do: at the beginning of the throw of the eccentric the motion is slow, and is gradually accelerated; the valve is therefore both opened and closed slowly. In some cases, therefore, expansion-valves and gearing are adopted. One species of expansion-valve is identical in principle and construction with the "throttle-valve;" another form is shown in fig. 96,

α

fig. 96.

it is a species of double beat valve; when this is raised, the steam from the boiler passing down d goes past the upper and lower valves bc, and through e to the cylinder. The supply of steam from the boiler to the pipe d is regulated by the ordinary throttle-valve; the upper valve b is made somewhat larger than the lower c; by this arrangement the pressure is greater on the upper valve, and tends to keep it in its seat; it will be observed that little opening of this valve will admit a large supply of steam, and that it is easily worked. The expansion-valve is worked by a series of levers and a cam. In p. 81., fig. 156, Mechanics and Mechanism, a diagram is given explaining how the revolution of a cam gives a reciprocating motion to the rod g: now suppose this rod to be connected with the expansionvalve, in such a way that it can lift it and depress it at intervals, and that these intervals are so timed as to close the expansion-valve at the exact period when the cut-off is to be effected, the system of expansive-working with an instantaneous cut-off will be carried out. It only remains for us to describe how the rod g is actuated on at the intervals required. Suppose the pulley c, attached to the end of the lever a, fig. 156, Mechanics and Mechanism, p. 81, to be in contact with the circular part a of the cam; it is obvious that no motion of a reciprocating kind would result. But supposing the circular part to be only continued for a certain distance of its circum

ference, and that at one part the face of a swells abruptly out, when the pulley c came in contact with this portion it would be forced out, and the lever with which it is connected would have a reciprocating motion; this would act on the expansion-valve and raise it, admitting a certain portion of steam to work the piston. As soon, however, as the swelled portion of 'the cam, by its revolution, passed the line of the pulley c, the lever would be put into another position, and the valve would be instantly closed, thus effecting the cut-off. By having along the face or breadth of the cam a variety of swells or projections, and by mechanism by which the pulley can he brought in contact with one or other of these "steps" or "grades" as they are termed, any amount of expansion can be effected. The steps are so arranged as to cut off the steam at a certain period of the stroke; and thus the engineer can command any degree of expansion required, by making the desired step come in contact with the pulley: this is done by means of a screw. The pulley can also be disconnected from the cam.

In a form of expansion-valve adopted in locomotives, a supplementary valve to effect the cut-off is placed above the ordinary three-ported slide. This arrangement is shown

ordinary valve-casing; the travel of the valve mm pleasure, so as to cut off the steam at any desired point.

can be altered at

ported slide: a b the steam ports, c the exhaust; the lap is towards the steam side of the valves, as ee. In working engines, it is considered to promote their efficiency by giving what is called "lead" to the valve; this is done by making the port open a little before the termination of the foregoing stroke: thus, suppose the piston just about to terminate the upstroke, the upper port is opened a little, thus admitting steam before the other stroke is quite finished, and in consequence of the lap in the valve the exhaust port is opened sooner; so that by the time the piston begins its down-stroke, the steam from below the piston is escaping freely. If this arrangement was not adopted, we can easily conceive of the engine in its down-stroke having to press against the steam below the piston, which would get slowly out, inasmuch as the valve opens but slowly at the beginning of its stroke. To have the full perfection of working in the cylinder, the escape or release of the steam should be instantaneous if possible; the less steam the piston has to encounter in its motion either up or down the cylinder the better. By the arrangement of the valves now described, the "lap" and the "lead," the speed of locomotives has been much increased.

To obtain the full efficiency of the expansive method of working, it is considered best to have the cut-off instantaneously effected--this the slidevalve cannot do: at the beginning of the throw of the eccentric the motion is slow, and is gradually accelerated; the valve is therefore both opened and closed slowly. In some cases, therefore, expansion-valves and gearing are adopted. One species of expansion-valve is identical in principle and construction with the "throttle-valve;" another form is shown in fig. 96,

α

fig. 96.

it is a species of double beat valve; when this is raised, the steam from the boiler passing down d goes past the upper and lower valves bc, and through e to the cylinder. The supply of steam from the boiler to the pipe d is regulated by the ordinary throttle-valve; the upper valve b is made somewhat larger than the lower c; by this arrangement the pressure is greater on the upper valve, and tends to keep it in its seat; it will be observed that little opening of this valve will admit a large supply of steam, and that it is easily worked. The expansion-valve is worked by a series of levers and a cam. In p. 81., fig. 156, Mechanics and Mechanism, a diagram is given explaining how the revolution of a cam gives a reciprocating motion to the rod g: now suppose this rod to be connected with the expansionvalve, in such a way that it can lift it and depress it at intervals, and that these intervals are so timed as to close the expansion-valve at the exact period when the cut-off is to be effected, the system of expansive-working with an instantaneous cut-off will be carried out. It only remains for us to describe how the rod g is actuated on at the intervals required. Suppose the pulley c, attached to the end of the lever a, fig. 156, Mechanics and Mechanism, p. 81, to be in contact with the circular part a of the cam; it is obvious that no motion of a reciprocating kind would result. But supposing the circular part to be only continued for a certain distance of its circum

T

́ference, and that at one part the face of a swells abruptly out, when the pulley c came in contact with this portion it would be forced out, and the lever with which it is connected would have a reciprocating motion; this would act on the expansion-valve and raise it, admitting a certain portion of steam to work the piston. As soon, however, as the swelled portion of the cam, by its revolution, passed the line of the pulley c, the lever would be put into another position, and the valve would be instantly closed, thus effecting the cut-off. By having along the face or breadth of the cam a variety of swells or projections, and by mechanism by which the pulley can he brought in contact with one or other of these "steps" or "grades" as they are termed, any amount of expansion can be effected. The steps are so arranged as to cut off the steam at a certain period of the stroke; and thus the engineer can command any degree of expansion required, by making the desired step come in contact with the pulley: this is done by means of a screw. The pulley can also be disconnected from the cam.

In a form of expansion-valve adopted in locomotives, a supplementary valve to effect the cut-off is placed above the ordinary three-ported slide. This arrangement is shown

ordinary valve-casing; the travel of the valve mm pleasure, so as to cut off the steam at any desired point.

can be altered at