and
furnace surface per horse power, reckoning the total amount as effective;
but in the boilers of the Retribution, by the same makers, but of larger
size, a somewhat smaller proportion of heating surface was adopted. Boulton
and Watt have found that in their marine flue boilers, 9 square feet of
flue and furnace surface are requisite to boil off a cubic foot of water
per hour, which is the proportion of heating surface that is allowed in
their land boilers per horse power; but inasmuch as in most modern engines,
and especially in marine engines, the nominal considerably exceeds the
actual power, they allow 11 or 12 square feet of heating surface per
nominal horse power in their marine boilers, and they reckon as effective
heating surface the tops of the flues, and the whole of the sides of the
flues, but hot the bottoms. For their land engines they still retain Mr.
Watt's standard of power, which makes the actual and the nominal power
identical; and an actual horse power is the equivalent of a cubic foot of
water raised into steam every hour.

256. _Q._--What is the proper proportion of fire grate per horse power?

_A._--Boulton and Watt allow 0.64 of a square foot area of grate bars per
nominal horse power in their marine boilers, and a good effect arises from
this proportion; but sometimes so large an area of fire grate cannot be
conveniently got, and the proportion of half a square foot per horse power,
which is the proportion adopted in the original boiler of the Great
Western, seems to answer very well in engines working with a moderate
pressure, and with some expansion; and this proportion is now very widely
adopted. With this allowance, there will be 22 to 24 square feet of heating
surface per square foot of fire grate; and if the consumption of fuel be
taken at 6 lbs. per nominal horse power per hour, there will be about 12
lbs. of coal consumed per hour on each square foot of grate. The furnaces
should not be more than 6 ft. long, as, if much longer than this, it will
be impossible to work them properly for any considerable length of time, as
they will become choked with clinker at the back ends.

257. _Q._--What quantity of fuel is usually consumed per hour on each
square foot of fire grate?

_A._--The quantity of fuel burned on each square foot of fire grate per
hour, varies very much in different boilers; in wagon boilers it is from 10
to 13 lbs.; in Cornish boilers from 3-1/2 to 4 lbs.; and in locomoti