arrangement of the valves of double acting air
pumps, such as are usual in direct acting screw engines, is that
represented in the figure of Penn's trunk engine already described in
Chapter I. Each inlet and outlet valve consists of a number of india rubber
discs set over a perforated brass plate, and each disc is bound down by a
bolt in the middle, which bolt also secures a brass guard set above the
disc to prevent it from rising too high. The usual configuration of those
valves is that represented in figs. 42, 43, and 44; figs. 42 and 43 being a
section and ground plan of the species of valve used by Messrs. Penn, and
fig. 44 being a section of that used by Messrs. Maudslay. It is important
in these valves to have the india rubber thick,--say about an inch thick
for valves eight inches in diameter. It is also advisable to make the
central bolts with a nut above and a nut below, and to form the bolt with a
counter sunk neck, so that it will not fall down when the top nut is
removed. The lower point of the bolt should be riveted over on the nut to
prevent it from unscrewing, and the top end should have a split pin through
the point for the same purpose. The hole through which the bolt passes
should be tapped, though the bolt is not screwed into it, so that if a bolt
breaks, a temporary stud may be screwed into the hole without the necessity
of taking out the whole plate. The guard should be large, else the disc may
stretch in the central hole until it comes over it; but the guard should
not permit too much lift of the valve, else a good deal of the water and
air will return into the pump at the return stroke before the valve shuts.
Penn's guard is rather small, and Maudslay's permits too much lift.

459. _Q._--What is the proper area through the valve gratings?

_A._--The collective area should be at least equal to the area of the pump
piston, and the lower edges of the perforations should be rounded off to
afford more free ingress or egress to the water.

460. _Q._--Is there much strain thrown on the plates in which the valves
are set?

_A._--A good deal of strain; and in the earlier direct acting screw engines
these plates were nearly in every case made too light. They should be made
thick, have strong feathers upon them, and be very securely bolted down
with split pins at the points of the bolts, to prevent them from
unscrewing. The plate will be very apt to be broken should some of the
bolts become loose. Of cour