and other circumstances. Mr. Watt reckoned his boilers as capable of
evaporating 10.08 cubic feet of water with a bushel or 84 lbs. of Newcastle
coal, which is equivalent to 7 1/2 lbs. of water evaporated by 1 lb. of
coal, and this may be taken as the performance of common land boilers at
the present time. In some of the Cornish boilers, however, a pound of coal
raises 11.8 lbs. of boiling water into steam, or a cwt. of coal evaporates
about 21 cubic feet of water from 212°.

147. _Q._--What method of firing ordinary furnaces is the best?

_A._--The coals should be broken up into small pieces, and sprinkled thinly
and evenly over the fire a little at a time. The thickness of the stratum
of coal upon the grate should depend upon the intensity of the draught: in
ordinary land or marine boilers it should be thin, whereas in locomotive
boilers it requires to be much thicker. If the stratum of coal be thick
while the draught is sluggish, the carbonic acid resulting from combustion
combines with an additional atom of carbon in passing through the fire, and
is converted into carbonic oxide, which may be defined to be invisible
smoke, as it carries off a portion of the fuel: if, on the contrary, the
stratum of coal be thin while the draught is very rapid, an injurious
refrigeration is occasioned by the excess of air passing through the
furnace. The fire should always be spread of uniform thickness over the
bars of the grate, and should be without any holes or uncovered places,
which greatly diminish the effect of the fuel by the refrigeratory action
of the stream of cold air which enters thereby. A wood fire requires to be
about 6 inches thicker than a coal one, and a turf fire requires to be 3 or
4 inches thicker than a wood one, so that the furnace bars must be placed
lower where wood or turf is burned, to enable the surface of the fire to be
at the same distance from the bottom of the boiler.