The importance of fire to mankind has been understood from the earliest times and is indicated by the inclusion of fire in the four elements recognized by the Greeks and by mediæval scientists. Its significance was augmented during the Industrial Revolution and its aftermath, the Communication Revolution, when heat began to be used to generate power for driving machinery and for transporting people and materials over ever-increasing distances at higher and still higher speeds. It is not too much to say that modern civilization is largely based on heat, as energy derived from other sources forms a very small fraction of the world's consumption. In 1936, it was estimated that, of the world's supply of energy, coal contributed 63 per cent, oil 18 per cent, wood 12 per cent, natural gas 5 per cent, and water power 2 per cent. Many attempts have been made to obtain power from the sun, to tap the heat of the earth, to use wind and tidal power but, though some of these projects have worked satisfactorily, the total amount of power produced by them is negligible. About 40 per cent of the energy derived from fuel is used as power, the remaining 60 per cent being employed for heating purposes.