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