"They moved from caves into structures of wood and stone--and engineering had its beginning. With the buildings came little chemical engines to destroy them; warfare was developing. Then came the first crude flying-machines, using clumsy, inefficient engines. Chemical engines! Engines so crude that one could watch the flow of their fuel! One part in one hundred thousand million of the energy of their propellents they released to run the engines, and they carried fuel in such vast quantities that they staggered under its load as they left the ground! And warfare became world-wide. After flight came other machines and other ages. Other scientists began to have visions of the realms beyond, and they sought to tap the vast reservoirs of Nature's energies, the energies of matter. "Other ages saw it done--a few thousand years later there passed out into space a machine that forced its way across the void to another planet! And the races of the three living worlds became as one--but there was no peace. "Swiftly now, science grew upon itself, building with ever faster steps, like a crystal which, once started, forms with incalculable speed. "And while that science grew swiftly greater, other changes took place, changes in our universe itself. Ten million years passed before the first of those changes became important. But slowly, steadily our atmosphere was drifting into space. Through ages this gradually became apparent. Our worlds were losing their air and their water. One planet, less favored than another, fought for its life, and space itself was ablaze with the struggles of wars for survival. "Again science helped us. Thousands of years before, men had learned how to change the mass of matter into energy, but now at last the process was reversed, and those ancestors of ours could change energy into matter, any kind of matter they wished. Rock they took, and changed it to energy, then that energy they transmuted to air, to water, to the necessary metals. Their planets took a new lease of life! "But even this could not continue forever. They must stop that loss of air. The process they had developed for reformation of matter admitted of a new use. Creation! They were now able to make new elements, elements that had never existed in nature! They designed atoms as, long before, their fathers had designed molecules. At last their problem was solved. They made a new form of matter that was clearer than any crystal, and yet stronger and tougher than any metal known. Since it held out none of the sun's radiations, they could roof their worlds with it and keep their air within! "This was a task that could not be done in a year, nor a decade, but all time stretched out unending before them. One by one the three planets became tremendous, roofed-in cities. Only their vast powers, their mighty machines made the task possible, but it was done."