bottom, and with an outlet or
delivery valve, as it is termed, at the top. The single acting air pump
requires to be provided with a valve or valves in the piston or bucket of
the pump, to enable the air and water lying below the bucket when it begins
to descend, and which have entered from the condenser during the upward
stroke, to pass through the bucket into the space above it during the
downward stroke, from whence they are expelled into the atmosphere on the
upward stroke succeeding. But in the double acting air pump no valve is
required in the piston or bucket of the pump, and all that is necessary is
an inlet and outlet valve at each end.

337. _Q_--What are the dimensions of the foot and discharge valves of the
air pump?

_A._--The area through the foot and discharge valves is usually made equal
to one fourth of the area of the air pump, and the diameter of the waste
water pipe is made one fourth of the diameter of the cylinder, which gives
an area somewhat less than that of the foot and discharge valve passages.
But this proportion only applies in slow engines. In fast engines, with the
air pump bucket moving as fast as the piston, the area through the foot and
discharge valves should be equal to the area of the pump itself, and the
waste water pipe should be of about the same dimensions.

328. _Q._--You have stated that double acting air pumps need only be of
half the size of single acting ones. Does that relation hold at all speeds?

_A._--It holds at all speeds if the velocity of the pump buckets are in
each case the same; but it does not hold if the engine with the single
acting pump works slowly, and the engine with the double acting pump moves
rapidly, as in the case of direct acting screw engines. All pumps moving at
a high rate of speed lose part of their efficiency, and such pumps should
therefore be of extra size.

329. _Q._--How do you estimate the quantity of water requisite for
condensation?

_A._--Mr. Watt found that the most beneficial temperature of the hot well
of his engines was 100 degrees. If, therefore, the temperature of the steam
be 212 deg., and the latent heat 1,000 deg., then 1,212 deg. may be taken to represent
the heat contained in the steam, or 1,112 deg. if we deduct the temperature of
the hot well. If the temperature of the injection water be 50 deg., then 50
degrees of cold are available for the abstraction of heat; and as the total
quantity of heat to be abstracted is that