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 marine engines, whether paddle or screw, if moving at a slow rate of speed, the eccentric is generally loose upon the shaft, for the purpose of backing, and is furnished with a back balance and catches, so that it may stand either in the position for going ahead, or in that for going astern.  The body of the eccentric is of cast iron, and it is put on the shaft in two pieces.  The halves are put together with rebated joints to keep them from separating laterally, and they are prevented from sliding out by round steel pins, each ground into both halves; square keys would probably be preferable to round pins in this arrangement, as the pins tend to wedge the jaws of the eccentric asunder.  In some cases the halves of the eccentric are bolted together by means of flanges, which is, perhaps, the preferable