Let us apply this principle to the solar system, and endeavour to find out whether in that system we have, in relation to the heat thereof, either a cycle of operations or a reversible cycle. We have again to consider the sun as the source of all light and heat in the solar system, radiating forth on every side, year by year, the countless units of heat which go to form the continuance of all planetary life and existence. One of the problems that has confronted scientific men for many years is this, Where does the sun get its supply of heat from? When we remember the incessant loss of heat which the sun suffers through its radiation of heat into space, we are compelled to ask, How is that supply maintained, and how has it been kept up through the countless ages of the past? Several suggestions have been made, and several theories advanced to account for the fact. Mayer, of Germany, suggested that the heat is partly maintained by the falling into the sun of meteors, which, like comets, pursue a path through the heavens, and are subject to the attractive influence of the sun. In the combustion of these meteorites, or meteors, he contended there were the means by which the light and heat of the sun might be maintained. Whatever theory, however, may be suggested as to the maintenance and the source of the continuity of the sun's heat, I do not think it has been suggested by any scientist that the heat emitted and radiated by the sun is ever returned in any way back to the sun from infinite space, whether by reflection or by any other method. So far as I can learn, there are no facts in connection with the solar system which would lead us to make that assumption. On the contrary, experience and experiment teach us that radiation implies loss of heat, and that the body, which so radiates, ultimately becomes cold, unless its internal heat is kept up by some means or other. So that the terms introduced by Carnot in the second law of thermodynamics, viz. that of a Cycle of Operations and of a Reversible Cycle, do not apply to the solar system, and the solar system, viewed from the standpoint of a machine, with the sun as the source of the heat, does not represent a perfect engine, that is, all the heat is not used up in doing work, some of it being radiated out into space. Wherever, however, the heat, that is the aetherial heat waves generated by the sun, comes into contact with a planet, as Mercury, Venus, or Jupiter, then, in accordance with Carnot's reasoning, work is done. Carnot points out that, in order for work to be done, we must have a source and a condenser, that is, two bodies at different temperatures, a hot body and a cold one. Now these conditions of work are satisfactorily fulfilled in the solar system, and as a result work is performed. We have the sun with its huge fires, and its intensity of heat, representing the source or the hot body, while every planet and every meteor and comet, that come under its influence, represent the