reduced to very nearly that of the feed water. Under such
circumstances the proportion of heat which can be realized is

5,150 deg. - 560 deg.
= --------------- = 0.891;
5,150 deg.

that is to say, under the extremely favorable if not impracticable
conditions assumed, there must be a loss of 11 per cent. Next, to give
a numerical value to the potential energy, H, to be derived from a
pound of carbon, calculating from absolute zero, the specific heat of
carbon being 0.25, and absolute temperature of air 520 deg.:

Units.
1 lb. of carbon X 0.25 X 520 = 130
12.2 of air X 0.238 X 520 = 1,485
Heat of combustion = 14,544
------
16,159
Deduct heat equivalent to work of \
displacing atmosphere by products of }
combustion raised from 60 deg. to 100 deg., } 32
or from 149.8 cubic feet to 161.3 }
cubic feet, /
------
Total units of heat available 16,127

Equal to 16.69 lb. of water evaporated from and at 212 deg.. Hence the
greatest possible evaporation from and at 212 deg. from a lb. of carbon--

16,159 u. X 0.891 - 32 u.
W = --------------------------- = 14.87 lb.
966 u.

I will now take a definite case, and compare the potential energy of a
certain kind of fuel with the results actually obtained. For this
purpose the boiler of the eight-horse portable engine, which gained
the first prize at the Cardiff show of the Royal Agricultural Society
in 1872, will serve very well, because the trials, all the details of
which are set forth very fully in vol. ix. of the _Journal_ of the
Society, were carried out with great care and skill by Sir Frederick
Bramwell and the late Mr. Menelaus; indeed, the only fact left
undetermined was the temperature of the furnace, an omission due to
the want of a trustworthy pyrometer, a want which has not been
satisfied to this day.[2]

[Footnote 2: In the fifty-second volume of the _Proceedings_
(1887-78), page 154, will be found a remarkable experiment on the
evaporative power of a vertical boiler with internal circulating
pipes. The experiment was conducted by Sir Frederick Bramwell and
Dr. Russell, and is r