In the Session of 1867-8 I communicated to this Society a paper on the Dissipation of Energy, of which only a very brief abstract was published in the Proceedings. The main feature of that paper was the suggestion, as at least a valuable working hypothesis, that even in cases of the steady motion of heat, electricity, &c., the unexhausted energy is probably as small as possible, consistently with the conditions of each form of experiment.