Instead of days required to modify the circuits, the three of them had a hundred of the huge round weapons changed over within an hour. Then Keller went up to the instrument-room and painstakingly studied the launching system. He began the launchings while Holmes and Burke completed the change-over task. They joined him in the instrument-room when the last of the metal spheres rose a foot from the stony floor of the magazine and went lurching unsteadily over to the breech of the launching-tube they hadn't noticed before.
"Three hundred," said Keller in a pleased tone, later. "All going out at full acceleration to meet the Enemy. And there are six observer-globes in the lot."
"Observers," said Burke grimly. "That's right. We can't observe anything because the information would come back at the speed of light. But if we lose, the Enemy will arrive before we can know we've lost."
Keller shook his head reproachfully.
"Oh, no! Oh, no! I just understood. There are transposers of electric energy, too. Very tiny. In the observers."
Burke stared. But it was only logical. If matter could be transposed instead of transmitted between distant places, assuredly miniature energy-transposers were not impossible. The energy would no more travel than transposed matter would move. It would be transposed. The fortress would see what the observer-globes saw, at the instant they saw it, no matter what the distance!
Keller glanced at the ten-foot disk with its many small lights and the writhing bright-red sparks which were the Enemy gravity-ships. There was something like a scale of distances understood, now. The red sparks had been not far from the disk's edge when the first space call went out to Earth. They were nearer the center when the spaceship arrived here. They were very, very near the center now.
"Five days," said Burke in a hard voice. "Where will the globes meet them?"
"They're using full acceleration," Keller reminded him gently. "One hundred sixty gravities."
"A mile a second acceleration," said Burke. Somehow he was not astonished. "In an hour, thirty-six hundred miles per second. In ten hours, thirty-six thousand miles per second. If they hit at that speed, they'd smash a moon! They'll cover half a billion miles in ten hours—but that's not enough! It's only a fifth of the way to Pluto! They won't be halfway to Uranus!"