"Gravity," said Burke, evenly. "Artificial gravity. We don't know how to make it, but the people who built this fortress did, and the Enemy does. So they've made artificial-gravity fields to give their ships the seeming mass of suns, and they've set them in close orbits around each other. They'll come spinning into this solar system. What will happen when objects with the mass of suns--artificial or otherwise--come riding through between our sun and its planets? There'll be tidal stresses to crack the planets and let out their internal fires. There'll be no stability left in the sun. Maybe it'll be a low-grade nova when they've gone, surrounded by trash that once was worlds. Anyhow there'll be no humans left! And then the Enemy will go driving on toward the other solar systems that the builders of this fortress own. They can't conquer anything with a weapon like that, but they can surely destroy!" Keller nodded distressedly. He gave Pam a number of sheets of paper, filled with his neat handwriting. He said sorrowfully, "For Earth. In code." Sandy served the meal she had prepared. "It's a matter of days," said Burke curtly. "Not weeks. Just days." He picked up a fork and began his meal. "So," he said after a moment, with a sort of unnatural calm, "we've got to get the thing licked fast. Up in the instrument-room there are some theory-cubes--lectures on theories with which the operators of the room were probably required to be familiar. They were intended to figure out what the Enemy might come up with, so it could at least be reported before the fortress was destroyed. The trick of sun-gravity fields was suggested as possible, but it seemed preposterously difficult. Apparently, it was. It took the Enemy some thousands of years to get it. But they've got it, all right!" "How do you know?" demanded Holmes. "The disk with the red sparks in it," said Burke, "is a detector of gravity-fields. It sees by gravity, which is not radiation. Keller's sending instructions back to Earth telling how to make such detectors." He busied himself with his food once more. After a moment he spoke again. "We're going to try to get some help," he observed. "At least we'll try to find out if there's any help to be had. I think there's a chance. There was a civilization which built this fortress. Something happened to it. Perhaps it simply collapsed, like Rome and Greece and Egypt and Babylonia back on Earth. But on Earth when an old civilization died a new, young one rose in its place. If the one that built this fort collapsed, maybe a new one has risen in its stead. If so, it will need to defend itself against the Enemy just like the old culture did. It might prefer to do its fighting here, instead of in its own land. I think we may be able to contact it." "How'll you look for them?" Burke shrugged.