"It appears to me," I said slowly, trying to grasp the concept, "that your explosion at B would have to have happened already if it were going to happen at all." The amazing thing about it is that Wyn, the man who had studied all this thoroughly, apparently didn't understand what I meant. It just goes to show that he must have been right, when he said the future is as fixed as the past. * * * * * It took Wyn four years to get his equipment ready for a test. He explained to me what it was supposed to do, but I never did get more than a general idea of the principle involved. The heart of the thing was a heavily wired chamber in the basement. "The human body can take a lot of electricity, if it's administered in the right way," he said. "If it's administered in the wrong way, you've electrocuted somebody. "I still don't know whether I've probed the secrets of the space-time fabric deeply enough to make this work, but I think it will reverse the charge of every atomic particle in the body of whatever is in that cubicle. I'm going to put a cat in it, as our first time-traveler. "We may turn up with a cat and an anti-cat, the latter traveling backward in time. We may end with no cat at all. If so, maybe we've created an anti-cat in the past or maybe we've just electrocuted a cat." "I don't see how you expect to interpret your results," I commented drily. "If there's no cat, I won't risk it," he answered. "If we double our cats, I think we're on the way to something that may help Summer." We picked our way through the mess of wiring and went upstairs. He had torn my bookcases out of one wall of the den and installed a control board with a television screen where the fireplace had been. "The experiment will be controlled from here," he said. "The energies that are going to run around all over the basement would make it pretty dangerous for anyone down there. I'm sorry you can't watch, but somebody's got to keep the children away from here." When he said "children," he meant Summer and Mark. Summer now looked as much a twin to her nine-year-old son as she had looked to her husband when I first saw them. At the last two Christmases, we had bought toys for both of them, and she played happily with Mark. She called Wyn "daddy" and me "Uncle Don," just as Mark did. Making them look even more like twins as we entered the living room on the day of Wyn's experiment was the fact that they were dressed alike. She wore a pair of Mark's overalls, and both had on T-shirts. At the moment, the two were trying to put doll clothes on Thomas, the stray yellow cat Wyn had picked up for his experiment. We had had Thomas about six months now. Wyn and I had dubbed the animal "Tom," unaware of its sex--it had borne kittens during its stay with us--but the children thought the cat too dignified for the nickname. It was, except when they were trying out their various original ideas on it.