But the next invention, the discovery of the use of fire, was a departure without a parallel to anything in the kingdoms of Nature. It placed mankind by a single act in a position of god-like authority over the living world. It is not surprising that the discovery of fire is surrounded by countless legends. In Greek mythology the first use of fire is attributed to Prometheus, the Fore-Thinker, the man who thinks ahead. He stole it from the heavens, and was punished by Zeus with terrible torment for having dared to endow mankind with a divine privilege. How great the privilege was we cannot even yet realize. For the uses of fire are by no means exhausted, and are multiplying from day to day. But the essential element of the change was that something came into the hands of man which did not exist in “Nature” at all, not, at least, in a manner ordinarily accessible to organic being. It was quite “unnatural” to use fire. Fire represented a state in which no organism could survive, in which all its functions were ruthlessly stopped, and its living tissues destroyed. No animal except man has ever attained to the use of fire. Its use represents the transcendence of man from the ordinary scheme of Nature and his ascent into a sort of supernatural, or at least super-organic realm. From this step there has been no retreat, and there can be no retreat until the sun itself grows cold. Whatever our “back-to-Nature” cranks may say, mankind cannot repudiate and renounce its most precious acquisition and all that it involves in the present and the future. We may dislike the smoke of blast-furnaces, but the remedy is not to do away with them, but to stop their smoking. The bellows of Hephæstus are blowing and his fires are burning. The age of machinery, begun in far-off Palæolithic days, but only established within the last hundred years or so, has now gripped us in a scheme from which none may escape. Taken in its narrower sense, the word “machine” means a contrivance for increasing the force we can bring to bear upon objects until it exceeds the limits imposed upon the tension of our muscles. As most of the 400 voluntary muscles of the human body are attached to the bones in a manner which diminishes the force exerted below the actual tension of the muscles, the adoption of machinery constitutes a reversal of the “natural” use of the system of levers which we call our skeleton. The Lever, the Inclined Plane, the Wedge, the Pulley, the Wheel and Axle, and the Screw, are all contrivances for slowing down the rate of work until the force required to perform it comes within the compass of our muscles. Our bones, on the other hand, are mostly levers “of the third order,” in which the force is applied between the fulcrum and the object to be moved. Thus it happens that in order to lift a weight of one pound, our muscles are strained by a force which may amount to as much as six pounds.