sure remaining the same, may be found by the following
rule:--To each of the temperatures before and after expansion, add the
constant number 458: divide the greater sum by the less, and multiply the
quotient by the volume at the lower temperature; the product will give the
expanded volume.

174. _Q._--If the relative volumes of steam and water are known, is it
possible to tell the quantity of water which should be supplied to a
boiler, when the quantity of steam expended is specified?

_A._--Yes; at the atmospheric pressure, about a cubic inch of water has to
be supplied to the boiler for every cubic foot of steam abstracted; at
other pressures, the relative bulk of water and steam may be determined as
follows:--To the temperature of steam in degrees of Fahrenheit, add the
constant number 458, multiply the sum by 37.3, and divide the product by
the elastic force of the steam in pounds per square inch; the quotient will
give the volume required.

175. _Q._--Will this rule give the proper dimensions of the pump for
feeding the boiler with water?

_A._--No; it is necessary in practice that
the feed pump should be able to supply the boiler with a much larger
quantity of water than what is indicated by these proportions, from the
risk of leaks, priming, or other disarrangements, and the feed pump is
usually made capable of raising 3-1/2 times the water evaporated by the
boiler. About 1/240th of the capacity of the cylinder answers very well for
the capacity of the feed pump in the case of low pressure engines,
supposing the cylinder to be double acting, and the pump single acting; but
it is better to exceed this size.

176. _Q._--Is this rule for the size of the feed pump applicable to the
case of high pressure engines?

_A._--Clearly not; for since a cylinder full of high pressure steam,
contains more water than the same cylinder full of low pressure steam, the
size of the feed must vary in the same proportion as the density of the
steam. In all pumps a good deal of the effect is lost from the imperfect
action of the valves; and in engines travelling at a high rate of speed, in
particular, a large part of the water is apt to return, through the suction
valve of the pump, especially if much lift be permitted to that valve. In
steam vessels moreover, where the boiler is fed with salt water, and where
a certain quantity of supersalted water has to be blown out of the boiler
from time to time, to prevent the water from r