r hour is a common consumption;
but such engines exert nearly twice their nominal power, so that the
consumption per actual horse power per hour may be taken at from 5 to 6
lbs. Engines working very expansively, however, attain an economy much
superior to this. The average duty of the pumping engines in Cornwall is
about 60,000,000 lbs. raised 1 ft. high by a bushel of Welsh coals, which
weighs 94 lbs. This is equivalent to a consumption of 3.1 lbs. of coal per
actual horse power per hour; but some engines reach a duty of above
100,000,000, or 1.74 lbs. of coal per actual horse power per hour.
Locomotives consume from 8 to 10 lbs. of coke in evaporating a cubic foot
of water, and the evaporation of a cubic foot of water per hour may be set
down as representing an actual horse power in locomotives as well as in
condensing engines, if expansion be not employed. When the locomotive is
worked expansively, however, there is of course a less consumption of water
and fuel per horse power, or per ton per mile, than when the full pressure
is used throughout the stroke; and most locomotives now operate with as
much expansion as can be conveniently given by the slide valves.

230. _Q._--But is not the evaporative power of locomotives affected
materially by the proportions of the boiler?

_A._--Yes, but this may be said of all boilers; but in locomotive boilers,
perhaps, the effect of any misproportion becomes more speedily manifest. A
high temperature of the fire box is found to be conducive to economy of
fuel; and this condition, in its turn, involves a small area of grate bars.
The heating surface of locomotive boilers should be about 80 square feet
for each square foot of grate bars, and upon each foot of grate bars about
1 cwt. of coke should be burnt in the hour.

231. _Q._--Probably the heat is more rapidly absorbed when the temperature
of the furnace is high?

_A._--That seems to be the explanation. The rapidity with which a hot body
imparts heat to a colder, varies as the square of the difference of
temperature; so that if the temperature of the furnace be very high, the
larger part of the heat passes into the water at the furnace, thereby
leaving little to be transmitted by the tubes. If, on the contrary, the
temperature of the furnace be low, a large part of the heat will pass into
the tubes, and more tube surface will be required to absorb it. About 16
cubic feet of water should be evaporated by a locomotive boiler fo