The four of them, with Sally, went along the empty corridors to the major's office. He was waiting for them. He shook hands all around. But it was not possible for Major Holt to give an impression of cordiality. "I'm very glad to see all of you back," he said curtly. "It didn't look like you'd make it. Joe, you will be able to reach your father by long-distance telephone as soon as you finish here. I--ah--thought it would not be indiscreet to tell him you had landed safely, though I did ask him to keep the fact to himself." "Thank you, sir," said Joe. "You answered most of the questions you needed to answer on the plane," added the major, grimly, "and now you may want to ask some. You know there is no ship for you. You know that the enemies of the Platform copied our rocket fuel. You know they've made rockets with it. You've met them! And Intelligence says they're building a fleet of space ships--not for space exploration, but simply to smash the Platform and get set for an ultimatum to the United States to backwater or be bombarded from space." Mike said crisply: "How long before they can do it?" Major Holt turned uncordial eyes upon him. "It's anybody's guess. Why?" "We've been working something out," said Mike, firmly but in part untruthfully. He stood sturdily before the major's desk, which he barely topped. "The four of us have been working it out. Joe says they've done powder metallurgy welds, back at his father's plant. Joe and Haney and the Chief and me, we've been working out an idea." Major Holt waited. His hands moved nervously on his desk. Joe looked at Mike. Haney and the Chief regarded him warily. The Chief cocked his head on one side. "It'll take a minute to get it across," said Mike. "You have to think of concrete first. When you want to make a cubic yard of concrete, you take a cubic yard of gravel. Then you add some sand--just enough to fill in the cracks between the gravel. Then you put in some cement. It goes in the cracks between the grains of sand. A little bit of cement makes a lot of concrete. See?" Major Holt frowned. But he knew these four. "I see, but I don't understand." "You can weld metals together with powder-metallurgy powder at less than red heat. You can take steel filings for sand and steel turnings for gravel and powdered steel for cement--" Joe jolted erect. He looked startledly at Haney and the Chief. And Haney's mouth was dropping open. A great, dreamy light seemed to be bursting upon him. The Chief regarded Mike with very bright eyes. And Mike sturdily, forcefully, coldly, made a sort of speech in his small and brittle voice.