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 course all the bolts and split pins, as well as the plates and guards, must be of brass.

461. Q.—­How are the plates to be taken out should that become necessary?

A.—­They are usually taken out through a door in the top of the hot well provided for that purpose, which door should be as large as the plates themselves; and it is a good precaution to cast upon this door—­which will be of cast iron—­six or eight stout projecting feet which will press upon the top of the outlet or delivery valve plate when the door is screwed down.  The upper or delivery valve plate and the lower or foot valve plate should have similar feet.  A large part of the strain will thus be transferred from the plates to the door, which can easily be made strong enough to sustain it.  It is advisable that the plates should lie at an angle so that the shock of the water may not come upon the whole surface at once.

462. Q.—­Does the double acting air pump usual in direct acting screw engines, produce as good a vacuum as the single acting air pump usual in paddle engines?

A.—­It will do so if properly constructed; but I do not know of any case of a double acting air pump, with india rubber valves, which has been properly constructed.

463. Q.—­What is the fault of such pumps?

A.—­The pump frequently works by starts, as if at times it did not draw at all, and then again on a sudden gorged itself with water, so as to throw a great strain upon the working parts.  The vacuum, moreover, is by no means so good as it should be, and it is a universal vice of direct acting screw engines that the vacuum is defective.  I have been at some pains to investigate the causes of this imperfection; and in a sugar house engine fitted with pumps like those of a direct acting screw engine to maintain a vacuum in the pans, I found that a better vacuum was produced when the engine was going slowly than when it was going fast; which is quite the reverse of what was to have been expected, as the hot water which had to be removed by the condensation of the steam proceeding from the pan, was a constant quantity.  In this engine, too, which was a high pressure one, the irregularities of the engine consequent upon the fitful catching of the water by the pump, was more conspicuous, as the working of this vacuum pump was the only work that the engine had to perform.

464. Q.—­And were you able to discover the cause of these irregularities?