se power, and allowing a square yard or 9 square feet as the proper
proportion of flue surface to evaporate a cubic foot of water in the hour,
it is clear that I must either give 27 square feet of heating surface in
the boiler to have a trebled power without expansion, or I must cut off the
steam at one seventh of the stroke to obtain a three-fold power without
increasing the quantity of heating surface. By cutting off the steam,
however, at one third of the stroke, a heating surface of 13-1/2 square
feet will give a threefold power, and it will usually be the most judicious
course to carry the expansion as far as possible, and then to add the
proportion of heating surface necessary to make good the deficiency still
found to exist.

272. _Q._--But is it certain that a cubic foot of water evaporated in the
hour is equivalent to an actual horse power?

_A._--An actual horse power as fixed by Watt is 33,000 lbs. raised one foot
high in the minute; and in Watt's 40 horse power engine, with a 31-1/2 inch
cylinder, 7 feet stroke, and making 17-1/2 strokes a minute, the effective
pressure is 6.92 lbs. on the square inch clear of all deductions. Now, as a
horse power is 33,000 lbs. raised one foot high, and as there are 6.92 lbs,
on the square inch, it is clear that 33,000 divided by 6.92, on 4768 square
inches with 6.92 lbs. on each if lifted 1 foot or 12 inches high, will also
be equal to a horse power. But 4768 square inches multiplied by 12 inches
in height is 57224.4 cubic inches, or 33.1 cubic feet, and this is the
quantity of steam which must be expended per minute to produce an actual
horse power.

273. _Q._--But are 33 cubic feet of steam expended per minute equivalent to
a cubic foot of water expended in the hour?

_A._.--Not precisely, but nearly so. A cubic foot of water produces 1669
cubic feet of steam of the atmospheric density of 15 lbs. per square inch,
whereas a consumption of 33 cubic feet of steam in the minute is 1980 cubic
feet in the hour. In Watt's engines about one tenth was reckoned as loss in
filling the waste spaces at the top and bottom of the cylinder, making 1872
cubic feet as the quantity consumed per hour without this waste; and in
modern engines the waste at the ends of the cylinder is inconsiderable.

274. _Q._--What power was generated by a cubic foot of water in the case of
the Albion Mill engines when working without expansion?

_A._--In the Albion Mill engines when working without expa