The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

298 THE STEAM ENGINE.
surface, it is clear that a determinate proportion must exist
between the power of the boiler and the extent of grating
in the fireplace. The quantity of oxygen which combines
with the fuel varies with the quality of that fuel; for different kinds of coal it varies from two to three pounds for
each pound of coal.
We shall take it an average of 22 pounds. Now 2 —
pounds of oxygen will measure 30 cubic feet; also 5 cubic
feet of atmospheric air contain 1 cubic foot of oxygen; and
consequently 150 cubic feet of atmospheric air will be necessary for the combustion of 1 pound of average coals. At
least one-third of the air, which passes through a fire, escapes
uncombined into the chimney. We must, therefore, allow
220 cubic feet of atmospheric air to pass through the grate
bars for every pound of fuel which is consumed. Now since
land boilers will consume 15 pounds, and marine boilers 10
pounds, per hour per horse-power, it follows that the spaces
between the grate bars, and the extent of grate surface, must
be sufficient to allow 3000 cubic feet of air per hour in land
boilers, and 2000 cubic feet in marine boilers, to pass through
them for each horse-power, or, what is the same, for each
foot of water converted into steam per hour. The quantity
of grate surface necessary for this does not seem to be ascertained with precision; but, perhaps, we may take as an approximate estimate for land boilers one square foot of grate
surface per horse-power, and for marine boilers two-thirds
of a square foot, the spaces between the grate bars being
equal to their breadth.
It is evident that the capacity of a boiler for water and
steam must have a determinate relation to the power of the
engine it is intended to supply. For each horse-power of
the engine, it has been shown that a cubic foot of water must
pass from the boiler in the form of steam per hour. Now,
it is evident that the steam could not be supplied of a uniform force, if the quantity of steam contained at any moment in the boiler were not considerably greater than the