at into mechanical work or electricity, electricity into heat, and so
forth, but the relationship between these convertible forms is fixed and
invariable. From a given quantity of chemical energy represented, let us
say, by a certain weight of coal, we can get a certain fixed amount of
heat and no more. We can employ that heat to work a steam-engine, which we
can in turn use as a source of electricity by causing it to drive a
dynamo-machine. Then this doctrine of science teaches us that our given
weight of coal in burning evolves a quantity of heat which is the
equivalent of the chemical energy which it contains, and that this
quantity of heat has also its equivalent in mechanical work or in
electricity. This great principle--known as the Conservation of
Energy--has been gradually established by the joint labours of many
philosophers from the time of Newton downwards, and foremost among these
must be ranked the late James Prescott Joule, who was the first to measure
accurately the exact amount of work corresponding to a given quantity of
heat.

In measuring heat (as distinguished from temperature) it is customary to
take as a unit the quantity necessary to raise a given weight of water
from one specified temperature to another. In measuring work, it is
customary to take as a unit the amount necessary to raise a certain weight
at a specified place to a certain height against the force of gravity at
that place. Joule's unit of heat is the quantity necessary to raise one
pound of water from 60 deg. to 61 deg. F., and his unit of work is the foot-pound,
_i.e._ the quantity necessary to raise a weight of one pound to a height
of one foot. Now the quantitative relationship between heat and work
measured by Joule is expressed by saying that the mechanical equivalent of
heat is about 772 foot-pounds, which means that the quantity of heat that
would raise one pound of water 1 deg. F. would, if converted into work, be
capable of raising a one-pound weight to a height of 772 feet, or a weight
of 772 lbs. to a height of one foot.

This mechanical equivalent ought to tell us exactly how much power is
obtainable from a certain weight of coal if we measure the quantity of
heat given out when it is completely burnt. Thus an average Lancashire
coal is said to have a calorific power of 13,890, which means that 1 lb.
of such coal on complete combustion would raise 13,890 lbs. of water
through a temperature of 1 deg. F., if we could collec