which operated on the principle of external cold, and which
consisted of a faggot of small copper tubes surrounded by water; but the
use of those condensers has not been persisted in, and most of the vessels
fitted with them have returned to the ordinary plan.

336. _Q._--You stated that the capacity of the feed pump was 1/240th of the
capacity of the cylinder in the case of condensing engines,--the engine
being double acting and the pump single acting,--and that in high pressure
engines the capacity of the pump should be greater in proportion to the
pressure of the steam. Can you give any rule that will express the proper
capacity for the feed pump at all pressures?

_A._--That will not be difficult. In low pressure engines the pressure in
the boiler may be taken at 5 lbs. above the atmospheric pressure, or 20
lbs. altogether; and as high pressure steam is merely low pressure steam
compressed into a smaller compass, the size of the feed pump in relation to
the size of the cylinder must obviously vary in the direct proportion of
the pressure; and if it be 1/240th of the capacity of the cylinder when the
total pressure of the steam is 20 lbs., it must be 1/120th of the capacity
of the cylinder when the pressure is 40 lbs. per square inch, or 25 lbs.
per square inch above the atmospheric pressure. This law of variation is
expressed by the following rule:--multiply the capacity of the cylinder in
cubic inches by the total pressure of the steam in lbs. per square inch, or
the pressure per square inch on the safety valve plus 15, and divide the
product by 4,800; the quotient is the capacity of the feed pump in cubic
inches, when the feed pump is single acting and the engine double acting.
If the feed pump be double acting, or the engine single acting, the
capacity of the pump must just be one half of what is given by this rule.

337. _Q._--But should not some addition be made to the size of pump thus
obtained if the pump works at a high rate of speed?

_A._--No; this rule makes allowance for defective action. All pumps lift
much less water than is due to the size of their barrels and the number of
their strokes. Moderately good pumps lose 50 per cent. of their theoretical
effect, and bad pumps 80 per cent.

338. _Q._--To what is this loss of effect to be chiefly ascribed?

_A._--Mainly to the inertia of the water, which, if the pump piston be
drawn up very rapidly, cannot follow it with sufficient rapidity; so that
there