1. The presence of heat may be recognized in every natural object; and
there is scarcely an operation in nature which is not more or less
affected by its all-pervading influence. An evolution and subsequent
absorption of heat generally give rise to a variety of effects; among
which may be enumerated, chemical combinations or decompositions; the
fusion of solid substances; the vaporization of solids or liquids;
alterations in the dimensions of bodies, or in the statical pressure by
which their dimensions may be modified; mechanical resistance overcome;
electrical currents generated. In many of the actual phenomena of nature
several or all of these effects are produced together; and their
complication will, if we attempt to trace the agency of heat in
producing any individual effect, give rise to much perplexity. It will,
therefore, be desirable, in laying the foundation of a physical theory
of any of the effects of heat, to discover or to imagine phenomena free
from all such complication, and depending on a definite thermal agency;
in which the relation between the cause and effect, traced through the
medium of certain simple operations, may be clearly appreciated. Thus it
is that Carnot, in accordance with the strictest principles of
philosophy, enters upon the investigation of the theory of the motive
power of heat.
2. The sole effect to be contemplated in investigating the motive power
of heat is _resistance overcome_, or, as it is frequently called, “_work
performed_,” or “_mechanical effect_.” The questions to be resolved by a
complete theory of the subject are the following:
(1) What is the precise nature of the thermal agency by means of which
_mechanical effect_ is to be produced, without effects of any other
kind?
(2) How may the amount of this thermal agency necessary for performing a
given quantity of work be estimated?
3. In the following paper I shall commence by giving a short abstract of
the reasoning by which Carnot is led to an answer to the first of these
questions; I shall then explain the investigation by which, in
accordance with his theory, the experimental elements necessary for
answering the second question are indicated; and, in conclusion, I shall
state the _data_ supplied by Regnault’s recent observations on steam,
and apply them to obtain, as approximately as the present state of
experimental science enables us to do, a complete solution of the
question.