The conception of entropy came from the theoretical study of the steam-engine, and, neglecting refinements of mathematical treatment, can easily be illustrated. The motive power of the engine comes from the transformation of the heat generated by the combustion of the fuel. The mechanism transforms the kinetic energy of the molecules of heated steam into the kinetic energy of the parts of the engine. Consider a steamship under way: for a brief period of time this can be regarded as an automatic physical system. Some of the heat conveyed by the steam transforms directly into mechanical energy and some passes out into the ocean in the condenser water. We take the latter fraction: in the boiler there is a certain quantity, Q1, at a temperature of, say, 200°C, that is, 473° absolute, and its entropy is simply Q1/473° ab. The same quantity passes out into the ocean where its temperature falls to, say, 10°C, or 283° ab., and its entropy is now Q1/283° ab. Plainly the former quantity of entropy is less than the latter, so that in the transformation of heat into mechanical energy entropy has increased. The conception has been generalized so as to include all forms of energy and all physical transformations, and it can be shown that in all physical events that occur of themselves entropy increases. The conception includes these notions: (1) Available and unavailable energy; (2) greater or lesser improbability of the elements of the systems contemplated; and (3) the physical meanings of before and after.