One of the early influences of the art of printing was to bring to the notice of some restless minds the writings of Hero and Archimedes. In Hero's _Pneumatics_, published more than 120 years before Christ, he gives such a clear account of an invention of his own, in which the expansive force of steam was used to give and maintain motion, as to establish thoroughly his right to the basic invention of the steam engine. He described three apparatus that he devised. In one, the currents of air and aqueous vapor rising through a tube from a hollow sphere, containing water, under which a fire is burning, support a ball placed immediately above the tube, and make it seem to dance. In another apparatus, a hollow sphere into which steam has arisen from what we now call a boiler, is supported on a horizontal or vertical axis, and provided with tubes that protrude from the sphere, and are bent at right angles to the radius and also to the pivot. The inner ends of these tubes lie within the sphere, so that the steam passes from the sphere through the tubes. As soon as this happens, the sphere takes up a rapid rotation, that continue so long as the steam continues to escape from the nozzles of the tubes, which point rearwardly. A third apparatus was merely an elaboration of the second, in that the sphere was connected with an altar which supported a large drum on which were figures representing human beings. The fire being lighted, the sphere would soon begin to revolve, and with it the drum; and the figures on it would seem to dance around, above the altar. The invention was probably to impress the people with the idea that the priests were exerting supernatural power. [Illustration: Hero's Altar Engine] Hero's wonderful invention remained unused and unappreciated for nearly 2,000 years. About 1601, an Italian named Della Porta, published a book that seems to show acquaintance with it, also with the fact that if water be heated it is converted into a gas that can raise water to a height. In 1615, a Frenchman named de Caus published a book in which he showed a hollow sphere into which water could be introduced through an orifice that could then be closed; the sphere carrying a vertical tube that dipped into the water at its lower end, and ending in a small nozzle at its upper end. When a fire was started under the sphere, the air in the upper part expanded, and forced down the water that occupied the lower part, so that a jet of water would soon issue from the upper end of the tube. Of course, this was really less than Hero had done, because the appliance described did not constitute a machine, in any real sense of the word.