the vacuum was 134 at 75 revolutions of the
engine, and went down to 132 at 40 revolutions, but rose again to 135,
equal to about 28-1/4 inches of mercury, at 20 revolutions.

467. _Q._--To what do you attribute the circumstance of a better vacuum
being got at low speeds than at high speeds?

_A._--It is difficult to assign the precise reason, but it appears to be a
consequence of the largeness of the vacant space between the valve plates.
When the piston of the air pump is drawn back, the air contained in this
large collection of water will cause it to boil up like soda water; and
when the piston of the pump is forced forward, this air, instead of being
expelled, will be again driven into the water. There will consequently be a
quantity of air in the pump which cannot be got rid of at all, and which
will impair the vacuum as a matter of course.

468. _Q._--What expedient did you adopt to improve the vacuum in the engine
to which you have referred?

_A._--I put blocks of wood on the air pump piston, which at the end of its
stroke projected between the valve plates and forced the water out. I also
introduced a cock of water at each end of the pump between the valve
plates, to insure the presence of water at each end of the pump to force
the air out. With these ameliorations the pump worked steadily, and the
vacuum obtained became as good as in the old pump. I had previously
introduced an injection cock into each end of the air pump in steam
vessels, from which I had obtained advantageous results; and in all
horizontal air pumps I would recommend the piston and valve plates to be so
constructed that the whole of the water will be expressed by the piston. I
would also recommend an injection cock to be introduced at each end of the
pump.

PUMPS, COCKS, AND PIPES.

469. _Q._--Will you explain the arrangement of the feed pump?

_A._--In steam vessels, the feed pump plunger is generally of brass, and
the barrel of the pump is sometimes of brass, but generally of cast iron.
There should be a considerable clearance between the bottom of the plunger
and the bottom of the barrel, as otherwise the bottom of the barrel may be
knocked out, should coal dust or any other foreign substance gain
admission, as it probably would do if the injection water were drawn at any
time from the bilge of the vessel, as is usually done if the vessel springs
a leak. The valves of the feed pump in marine engines are generally of the
spindl