se 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?

_A._--The main cause of th