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 spindle kind, and are most conveniently arranged in a chest, which may be attached in any accessible position to the side of the hot well.  There are two nozzles upon this chest, of which the lower one leads to the pump, and the upper one to the boiler.  The pipe leading to the pump is a suction pipe when the plunger ascends, and a forcing pipe when the plunger descends.  The plunger in ascending draws the water out of the hot well through the lowest of the valves, and in descending forces it through the centre valve into the space above it, which communicates with the feed pipe.  Should the feed cock be shut so as to prevent any feed water from passing through it, the water will raise the topmost valve, which is loaded to a pressure considerably above the pressure of the steam, and escape into the hot well.  This arrangement is neater and less expensive than that of having a separate loaded valve on the feed pipe with an overflow through the ship’s side, as is the more usual practice.