"For practical purposes all she had to understand was the operation of these keys. The pressure of the light-ray in these coils"--he was standing beside a row of wire coils which in the semidarkness I had not noticed before--"is controlled by the key-switches." He indicated the latter as he spoke. "They send a current to the outer metal plates of the car which makes them repel or attract other masses of matter, as desired. "All that Miela had to understand then was how to operate these keys so as to keep the base of the vehicle headed toward the earth. They took her to the outer edge of the atmosphere of Mercury over the Dark Country and showed her the earth. They have used terrestrial telescopes for generations, and since the invention of this vehicle telescopes for celestial observation have been greatly improved. "All Miela had to do was keep the air in here purified. That is a simple chemical operation. By using this attractive and repellent force she allowed the earth's gravity and the repelling power of the sun and Mercury to drive her here." He paused. "But, doesn't she--don't you understand the thing in detail?" I asked finally. "I think father and I understand it now better than she does," he answered. "We have studied it out here and questioned her as closely as possible. We understand its workings pretty thoroughly. But the exact nature of the light-ray we do not understand, any more than we understand electricity. Nor do we understand this metallic substance which when charged with the current becomes attractive or repellent in varying degrees." "Yes," I said. "That I can appreciate." "Father has a theory about the light-ray," he went on, "which seems rather reasonable from what we can gather from Miela. The thing seems more like electricity than anything else, and father thinks now that it is generated by dynamos on Mercury, similar to those we use here for electricity." "Along that line," I said, "can you explain why this light-ray, which will immediately set anything on fire that is combustible, and which acts through metal, like those artillery shells, for instance, does not seem to raise the temperature of the ground it strikes to any extent?" "Because, like electricity, it is dissipated the instant it strikes the ground. The earth is an inexhaustible storehouse and receptacle for such a force. That is why the broken country around the Shoshone River protected Garland and Mantua from its direct rays." "Tell me about the details of this mechanism," I said, reverting to our original subject. "You say you understand its workings pretty thoroughly now."