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 experi-
