"The phenomenon of the production of motion by heat has not been considered from a sufficiently general point of view. We have considered it only in machines... [for which] the phenomenon is... incomplete. It becomes difficult to recognize its principles and study its laws. ...[T]he principle of the production of motion by heat... must be considered independently of any mechanism or... particular agent. It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated."