"And Agrav?" asked Bigman.

"Ah! Now that's a different thing. Once you use an Agrav converter, potential energy can be converted into forms of energy other than kinetic energy. In the Agrav corridor, for instance, the force of gravity in one direction is used to charge the gravitational field in the other direction as you fall. People falling in one direction provide the energy for people falling in the other. By bleeding off the energy that way, you yourself, while falling, need never speed up. You can fall at any velocity less than the natural falling velocity. You see?"

Bigman wasn't quite sure he did but he said, "Go on."

"In space it's different. There's no second gravitational field to shift the energy to. Instead, it is con-, verted to hyperatomic field energy and stored so. By doing this, a space ship can drop from Jupiter Nine to Io at any speed less than the natural falling speed without having to use any energy to decelerate. Virtually no energy is expended except in the final adjustment to Io's orbital speed. And safety is complete, since the ship is always under perfect control. Jupiter's gravity could be completely blanketed, if necessary.

"Going back to Jupiter Nine still requires energy. There is no getting around that. But now you can use the energy you had previously stored in the hyperatomic field condenser to get you back. The energy of Jupiter's own gravitational field is used to kick you back."

Bigman said, "It sounds good." He squirmed in his seat. He wasn't getting anywhere. Suddenly he said, "What's that you're fooling with on your desk?"

"Chess," said Norrich. "Do you play?"

"A little," Bigman confessed. "Lucky taught me, but it's no fun playing with him. He always wins." Then he asked, offhand, "How can you play chess?"

"You mean because I'm blind?"

"Uh-"

"It's all right. I'm not sensitive about being blind… It's easy enough to explain. This board is magnetized and the pieces are made of a light magnetic alloy so that they stick where they're put and don't go tumbling if I move my arm about carelessly. Here, try it, Bigman."

Bigman reached for one of the pieces. It came up as though stuck in syrup for a quarter of an inch or so, then was free.

"And you see," said Norrich, "they're not ordinary chess pieces."

"More like checkers," grunted Bigman.

"Again so I don't knock them over. They're not completely flat, though. They've got raised designs which I can identify easily enough by touch and which resemble the ordinary pieces closely enough so that other people can learn them in a moment and play with me. See for yourself."

Bigman had no trouble. The circle of raised points was obviously the queen, while the little cross in the center of another piece signified the king. The pieces with grooves slanting across were the bishops, the raised circle of squares the rooks, the pointed horse's ears the knights, and the simple round knobs the pawns.

Bigman felt stymied. He said, "What are you doing now? Playing a game by yourself?"

"No, solving a problem. The pieces are arranged just so, you see, and there's one way and only one in which white can win the game in exactly three moves and I'm trying to find that way."

Bigman said suddenly, "How can you tell white from black?"

Norrich laughed. "If you'll look closely, you'll see the white pieces are grooved along the rims and the black pieces aren't."

"Oh. Then you have to remember where all the pieces are, don't you?"

"That's not hard," Norrich said. "It sounds as though you would need a photographic memory, but actually all I have to do is pass my hand over the board and check the pieces any time. You'll notice the squares are marked off by little grooves, too."

Bigman found himself breathing hard. He had forgotten about the squares on the checkerboard, and they were grooved off. He felt as though he were playing a kind of a chess game of Ms own, one in which he was being badly beaten.

"Mind if I watch?" he said sharply. "Maybe I can figure out the right moves."

"By all means," said Norrich. "I wish you could. I've been at this for half an hour and I'm getting frustrated."

There was silence for a minute or more, and then Bigman rose, his body tense and catlike in its effort to make no noise. He drew a small flashlight from one pocket and stepped toward the wall in little motions. Norrich never moved from his bowed position over the chessboard. Bigman threw a quick glance toward Mutt, but the dog made no move, either.

Bigman reached the wall and, hardly breathing, put one hand lightly and noiselessly over the light patch. At once, the light in the room went out and a profound darkness rested everywhere.

Bigman remembered the direction in which Norrich's chair was. He raised the flashlight.

He heard a muted thump and then Norrich's voice calling out in surprise and a little displeasure, "Why did you put out the light, Bigman?"

"That does it," yelled Bigman in triumph. He let the flashlight's beam shine full on Norrich's broad face. "You're not blind at all, you spy."

Norrich cried out, "I don't know what you're doing, but Space, man, don't do anything sudden or Mutt will jump you!"

"You know exactly what I'm doing," said Bigman, "because you can see well enough I'm drawing my needle-gun, and I think you've heard I'm a dead shot. If your dog moves in my direction, it's the end for him."

"Don't hurt Mutt. Please!"

Bigman was taken aback by the sudden anguish in the other's voice. He said, "Just keep him quiet then and come with me and no one will be hurt. We'll go see Lucky. And if we pass anyone in the corridor, don't you say anything but 'Good day.' I'll be right beside you, you know."

Norrich said, "I can't go without Mutt."

"Sure you can," said Bigman. "It's only five steps down the corridor. Even if you were really blind, you could manage that-a fellow who can do threedees and all."

Lucky lifted the viewer from his head at the sound of the door opening and said, "Good day, Norrich. Where's Mutt?"

Bigman spoke before the other had a chance to answer. "Mutt's in Norrich's room, and Norrich doesn't need him. Sands of Mars. Lucky, Norrich isn't any blinder than we are!"

"What?"

Norrich began, "Your friend is quite mistaken, Mr. Starr. I want to say-"

Bigman snapped. "Quiet, you! I'll talk, and then when you're invited, you can make some remarks."

Lucky folded his arms. "If you don't mind, Mr. Norrich, I'd like to hear what Bigman has on his mind. And meanwhile, Bigman, suppose you put away the needle-gun."

Bigman did so with a grimace. He said, "Look, Lucky, I suspected this cobber from the beginning. Those threedee puzzles of his set me to thinking. He was just a little too good. I got to wondering right away that he might be the spy."

"That's the second time you've called me a spy," Norrich cried. "I won't stand for that."

"Look, Lucky," said Bigman, ignoring Norrich's outcry, "it would be a clever move to have a spy a supposed blind man. He could see an awful lot no one would think he was seeing. People wouldn't cover up. They wouldn't hide things. He could be staring right at some vital document and they'd think, 'It's only poor Norrich. He can't see.' More likely they wouldn't give it a thought at all. Sands of Mars, it would be a perfect setup!"

Norrich was looking more astonished with every moment. "But I am blind. If it's the threedee puzzles or the chess, I've explained-"

"Oh, sure, you've explained," Bigman said scornfully. "You've been practicing explanations for years. How come you sit in the privacy of your room with the lights on, though? When I walked in, Lucky, about half an hour ago, the light was on. He hadn't just put it on for me. The switch was too far away from where he was sitting. Why?"