to the cylinder, as a valve on this construction is unable to
leave the face. In locomotive engines, the valve universally employed is
the common three ported valve.

450. _Q._--Might not an equilibrium valve be so constructed by the
interposition of springs, as to enable it to leave the cylinder face when
an internal force is applied?

_A._--That can no doubt be done, and in some engines has been done. In the
screw steamer Azof, the valve is of the equilibrium construction, but the
plate which carries the packing on which the top ring rests, is an octagon,
and fits into an octagonal recess on the back of the valve. Below each side
of the octagon there is a bent flat spring, which lifts up the octagonal
plate, and with it the packing ring against the back of the valve casing;
and should water get into the cylinder, it escapes by lifting the valve,
which is rendered possible by the compressibility of the springs. An
equivalent arrangement is shown in figs. 39 and 40, where the ring is
lifted by spiral springs.

[Illustration: Fig. 39. EQUILIBRIUM GRIDIRON SLIDE VALVE. Longitudinal
Section. Scale 3/4 inch = 1 foot.]

451. _Q._--What species of valve is that shown in figs. 39 and 40?

[Illustration: Fig. 40. EQUILIBRIUM GRIDIRON SLIDE VALVE. Back View with
Ring removed. Scale 3/4 inch = 1 foot.]

_A._--It is an equilibrium gridiron valve; so called because it lets the
steam in and out by more than one port. A A are the ordinary steam
passages to the top and bottom of the cylinder; B B is the ring which rubs
against the back of the valve casing, and D is the eduction passage, S S S
S shows the limits of the steam space, for the steam penetrates to the
central chamber S S by the sides of the valve. When the valve is opened
upon the steam side, the cylinder receives steam through both ports at that
end of the cylinder, and both ports at the other end of the cylinder are at
the same time open to the eduction. The benefit of this species of valve
is, that it gives the same opening of the valve that is given in ordinary
engines, with half the amount of travel; or if three ports were made
instead of two, then it would give the same area of opening that is given
in common engines with one third the amount of travel. For direct acting
screw engines this species of valve is now extensively used.

452. _Q._--Will you describe the configuration and mode of attachment of
the eccentric by which the valve is moved?

_A._--In