But the skipper's voice boomed at him from the wall.

"Mr. Baird! What have you to add to the information you sent back?"

"Three items, sir," said Baird. He drew a deep breath. "For the first, sir, the Plumie ship is unarmed. They've tractor and pressor beams for handling material. They probably use them to build their cairns. But they weren't meant for weapons. The Plumies, sir, hadn't a thing to fight with when they drove for us after we detected them."

The skipper blinked hard.

"Are you sure of that, Mr. Baird?"

"Yes, sir," said Baird uncomfortably. "The Plumie ship is an exploring ship—a survey ship, sir. You saw their mapping equipment. But when they spotted us, and we spotted them—they bluffed! When we fired rockets at them, they turned them back with tractor and pressor beams. They drove for us, sir, to try to destroy us with our own bombs, because they didn't have any of their own."

The skipper's mouth opened and closed.

"Another item, sir," said Baird more uncomfortably still. "They don't use iron or steel. Every metal object I saw was either a bronze or a light metal. I suspect some of their equipment's made of potassium, and I'm fairly sure they use sodium in the place of aluminum. Their atmosphere's quite different from ours—obviously! They'd use bronze for their ship's hull because they can venture into an oxygen atmosphere in a bronze ship. A sodium-hulled ship would be lighter, but it would burn in oxygen. Where there was moisture—"

The skipper blinked.

"But they couldn't drive in a non-magnetic hull!" he protested. "A ship has to be magnetic to drive!"

"Sir," said Baird, his voice still shaken, "they don't use a magnetronic drive. I once saw a picture of the drive they use, in a stereo on the history of space travel. The principle's very old. We've practically forgotten it. It's a Dirac pusher-drive, sir. Among us humans, it came right after rockets. The planets of Sol were first reached by ships using Dirac pushers. But—" He paused. "They won't operate in a magnetic field above seventy Gauss, sir. It's a static-charge reaction, sir, and in a magnetic field it simply stops working."

The skipper regarded Baird unwinkingly for a long time.

"I think you are telling me," he said at long last, "that the Plumies' drive would work if they were cut free of the Niccola."

"Yes, sir," said Baird. "Their engineers were opening up the drive-elements and checking them, and then closing them up again. They couldn't seem to find anything wrong. I don't think they know what the trouble is. It's the Niccola's magnetic field. I think it was our field that caused the collision by stopping their drive and killing all their controls when they came close enough."