670 
Report on the Trials of Portable 
equivalent of heat." It is now generally admitted that all 
forms of energy, due to the physical forces, are mutually con- 
vertible, with a definite quantivalence ; and Joule showed 
what that equivalence is whenever heat-energy is converted into 
mechanical energy, or vice versa. 
He proved, experimentally, that the quantity of heat required 
to increase the temperature of water by one degree needs for 
its production the expenditure of a force measured by the fall 
of 772 pounds from a height of one foot. This is "Joule's 
Equivalent," which may be conveniently stated as follows : " A 
(British) unit of heat equals 772 foot-pounds of work." 
Now a pound of pure carbon is capable of liberating, by its 
perfect combustion, 14,500 (British) heat units, equivalent to 
(14,500 X 772 =) 11,194,000 foot-pounds of work; or, if 
burned in one hour, equal to 5*6 horse-power. In other words, 
if it were possible perfectly to utilize all the heat-energy con- 
tained in a pound of carbon, only one-sixth of that pound 
would be required for the production of mechanical energy 
equal to one horse-power for one hour. 
But the best coal is not all carbon, and is usually rated as 
yielding ten millions of foot-pounds of work per pound of coal 
consumed, while, again, coal burnt in a steam-boiler is not all 
utilized — more than a fourth of it being lost, even in the best 
boilers : so that the amount of work which a pound of coal is 
capable of doing in the furnace of a boiler becomes reduced to 
7^ million foot-pounds ; roughly equalling, if the coal were 
burnt in an hour, four horse-power, or a quarter of a pound of 
coal per horse-power per hour. 
Excellent as they were, the best engines tried at Newcastle 
burnt rather more than seven times this amount of fuel, and the 
worst, more than twenty-four times as much ; while the records 
of the Cardiff trials immortalize, at least, one engine which burnt 
forty times more coal than theory demands per horse-power per 
hour. There is, then, evidently room and to spare for further 
economy in the best modern steam-engines, which, indeed, may 
be said, speaking generally, to consume ten times more coal 
than they would do if all the heat-energy of the fuel could be 
converted, without loss, into mechanical energ}' ; a fact which 
compels the further question, " How is all this energy lost ? " 
Heat-energy, which cannot itself be conveniently applied 
directly to the production of mechanical effort, suffers a certain 
loss in being primarily converted into steam ; but, since this 
loss is inherent in the steam-engine, its story (" fairy-tale of 
science " though it be) need not be told here ; it is enough 
to point out how loss of energy occurs in the steam-engine, 
after heat has been converted into steam pressure. ^ 
