105. Now, every one knows a case in which there may be an unlimited amount of energy present, no part of which is available for transformation. It is the simple one of heat in a number of bodies, _when all are at the same temperature_. To obtain work from heat we must have hotter and colder bodies, to correspond, as it were, with the boiler and condenser of a heat-engine; and just as we can get no work from still water if it be all at the same level, _i.e._ if no part of it can fall, so in like manner we can get no work from heat unless part of it can fall from a higher to a lower temperature. This is a remark of the very utmost consequence to our argument, and must therefore be fully elucidated. Unfortunately it is not as yet possible to do this without introducing a good many scientific technicalities which are unsuited to the great majority of readers. In the next eight sections we endeavour to explain it as simply as we can. The reader who cannot easily follow us may pass, without break of continuity in the argument, at once to Art. 114. * 106. The first step in the investigation of the transformation of heat into work was taken by Sadi Carnot in 1824: a step which has recently been found of inestimable value in every branch of modern physical science. He devised a method of startling originality for the purpose of attacking this special question of the production of work from heat. His inferences from its application were not all correct; this was due however to no fault of the method, but to the fact that he unfortunately assumed (though with caution, and under a protest almost amounting to an assertion of the opposite) the materiality of heat. His method embraces two perfectly new ideas:— (1.) That, at least with our present knowledge, no inference is possible as to the relation between heat and work, until the heated or working substance is brought back, after a complete _Cycle_ of operations, to its initial physical state. Obvious as this statement, once made, is, it was altogether ignored (twenty years after Carnot) by Séguin and Mayer, whom some authors still persist in setting forth as the founders of the dynamical theory of heat. Their speculations were entirely vitiated by their violation of this principle. (2.) That an engine whose cycle of operations is reversible is a perfect engine, that is to say, gives the greatest possible amount of work from a given quantity of heat with any assigned temperatures of boiler and condenser.