The influence of Hero's invention on history is not direct, because his engine has never been employed for any industrial purpose. But Hero's engine has had an enormous influence on history, nevertheless, because it supplied the basis on which the steam engine of the last two centuries has rested. The influence of Hero's invention was not realized until two thousand years after he had died, and until after all those men had died whose names have just been mentioned. It is inconceivable that any of those men could really have expected that their work was to have even a small fraction of the influence on mankind that it actually has had. The influence of Watt's work became visible to some degree before he died, and became clearly visible not very long after he had died; so clearly visible that by many men Watt is credited with the invention of the steam engine. But his good work was built on the good work of his predecessors, whose main work was in making Watt's work possible. The successive feats of all, like the successive layers in the foundations of any building, were to support, in time, the whole superstructure of the great and beneficent science of steam engineering. But the work done by these men was not all the work that had to be done, to make Watt's steam engine the efficient machine it was. These men were the men who are directly to be credited, but they were not the only men engaged. Neither did they belong to the only class of men engaged. There was another class of men whose labors were equally arduous, and equally important, though not so clearly in evidence--the physicists, as we now call them. It was by the knowledge which they gleaned regarding the properties of steam and air and water and iron, regarding the laws of motion and heat and work and force and weight and mass, that the inventors' experiments were guided. It is true that the science of physics was then in its infancy, as we realize with the knowledge of the science today; but Aristotle in the days of Greece, and Archimedes and Hero later, and Galileo and many others in Italy--as well as Guericke in Germany, Newton and Gilbert in England, and others of less note, had evolved a good deal of order out of what had been chaos, and had given inventors a great deal of firm ground on which to stand themselves and raise their structures. And reciprocally, the inventors found themselves confronted with problems of a kind that gave opportunities for the physicists to show their skill and knowledge.