"You know how to make the metal required," he said suddenly. "You'd better get busy making it. Plenty of it. We'll need it." Von Holtz stared at him, his weak eyes almost frightened. "You _know_? You know how to combine the right angles?" "I think so," said Tommy. "I've got to find out if I'm right. Will you make the metal?" Von Holtz bit at his too-red lips. "But Herr Reames!" he said stridently, "I wish to know the equation! Tell me the method of pointing a body in a fourth or a fifth direction. It is only fair--" "Denham didn't tell you," said Tommy. Von Holtz's arms jerked wildly. "But I will not make the metal! I insist upon being told the equation! I insist upon it! I will not make the metal if you do not tell me!" Smithers was in the laboratory, of course. He had been surveying the big solenoid-catapult and scratching his chin reflectively. Now he turned. * * * * * But Tommy took Von Holtz by the shoulders. And Tommy's hands were the firm and sinewy hands of a sportsman, if his brain did happen to be the brain of a scientist. Von Holtz writhed in his grip. "There is only one substance which could be the metal I need, Von Holtz," he said gently. "Only one substance is nearly three-dimensional. Metallic ammonium! It's known to exist, because it makes a mercury amalgam, but nobody has been able to isolate it because nobody has been able to give it a fourth dimension--duration in time. Denham did it. You can do it. And I need it, and you'd better set to work at the job. You'll be very sorry if you don't, Von Holtz!" Smithers said with a vast calmness. "I got me a hunch. So if y'want his neck broke...." Tommy released Von Holtz and the lean young man gasped and sputtered and gesticulated wildly in a frenzy of rage. "He'll make it," said Tommy coldly. "Because he doesn't dare not to!" Von Holtz went out of the laboratory, his weak-looking eyes staring and wild, and his mouth working. "He'll be back," said Tommy briefly. "You've got to make a small model of that big catapult, Smithers. Can you do it?" "Sure," said Smithers. "The ring'll be copper tubing, with pin-bearings. Wind a coil on the lathe. It'll be kinda rough, but it'll do. But gears, now...." "I'll attend to them. You know how to work that metallic ammonium?" "If that's what it was," agreed Smithers. "I worked it for the Professor." Tommy leaned close and whispered: "You never made any gears of that. But did you make some springs?" "Uh-huh!" Tommy grinned joyously. "Then we're set and I'm right! Von Holtz wants a mathematical formula, and no one on earth could write one, but we don't need it!" * * * * *