184. Q.—­The steam is admitted to and from the cylinder by means of a slide or sluice valve?

[Illustration:  Fig. 33.]

A.—­Yes; and of the slide valve there are many varieties; but the kinds most in use are the D valve,—­so called from its resemblance to a half cylinder or D in its cross section—­and the three ported valve, shown in fig. 33, which consists of a brass or iron box set over the two ports or openings into the cylinder, and a central port which conducts away the steam to the atmosphere or condenser; but the length of the box is so adjusted that it can only cover one of the cylinder ports and the central or eduction port at the same time.  The effect, therefore, of moving the valve up and down, as is done by the eccentric, is to establish a connection alternately between each cylinder port and the central passage whereby the steam escapes; and while the steam is escaping from beneath the piston, the position of the valve is such, that a free communication exists between the space above the piston and the steam in the boiler.  The piston is thus urged alternately up and down—­the valve so changing its position before the piston arrives at the end of the stroke, that the pressure is by that time thrown on the reverse side of the piston, so as to urge it into motion in the opposite direction.

185. Q.—­Is the motion of the valve, then, the reverse of that of the piston?

A.—­No.  The valve does not move down when the piston moves down, nor does it move down when the piston moves up; but it moves from its mid position, to the extremity of its throw, and back again to its mid position, while the piston makes an upward or downward movement, so that the motion is as it were at right angles to the motion of the piston; or it is the same motion that the piston of another engine, the crank of which is set at right angles with that of the first engine, would acquire.

186. Q.—­Then in a steam vessel the valve of one engine may be worked from the piston of the other?

A.—­Yes, it may; or it may be worked from its own connecting rod; and in the case of locomotive engines, this has sometimes been done.

187. Q.—­What is meant by the lead of the valve?

A.—­The amount of opening which the valve presents for the admission of the steam, when the piston is just beginning its stroke.  It is found expedient that the valve should have opened a little to admit steam on the reverse side of the piston before the stroke terminates; and the amount of this opening, which is given by turning the eccentric more or less round upon the shaft, is what is termed the lead.

188. Q.—­And what is meant by the lap of the valve?