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?"

"Why not?" said Norrich. "It makes no difference to me whether it's on or not, so it might as well be on as long as I'm awake for the convenience of those who come visiting, like you."

"All right," said Bigman. "That shows how he can think up an explanation for everything-how he can play chess, how he can identify the pieces, everything. Once he almost forgot himself. He dropped one of his chess pieces and bent to pick it up when he remembered just in time and asked me to do it for him."

"Usually," said Norrich, "I can tell where something drops by the sound. This piece rolled."

"Go on, explain," said Bigman. "It won't help you because there's one thing you can't explain. Lucky, I was going to test him. I was going to put out the light, then flash my pocketlight in his eyes at full intensity. If he weren't blind, he'd be bound to jump or blink his eyes anyway. I was sure I'd get him. But I didn't even have to go that far. As soon as I put out the light, the poor cobber forgets himself and says, 'Why did you put out the light?'... How did he know I put out the light, Lucky? How did he know?"

"But-" Norrich began.

Bigman drove on. "He can feel chess pieces and threedee puzzles and all that but he can't feel light going out. He had to see that."

Lucky said, "I think it's time to let Mr. Norrich say something."

Norrich said, "Thank you. I may be blind, Councilman, but my dog is not. When I put out the light at night, it makes no difference to me, as I said before, but to Mutt it signals bedtime and he goes to his own corner. Now I heard Bigman tiptoe to the wall in the direction of the light switch. He was trying to move without sound, but a man who has been blind for five years can hear the lightest tiptoe. A moment after he stopped walking I heard Mutt jump into his corner. It didn't take much brain power to figure out what had happened. Bigman was standing at the light switch and Mutt was turning in for the night. Obviously he had put out the light."

The engineer turned his sightless face in the direction first of Bigman, then of Lucky, as though straining his ears for an answer.

Lucky said, "Yes, I see. It seems we owe you an apology."

Bigman's gnomelike face screwed up unhappily. "But Lucky-"

Lucky shook his head. "Let go, Bigman! Never hang on to a theory after it's been exploded. I hope you understand, Mr. Norrich, that Bigman was only doing what he felt to be his duty."

"I wish he had asked a few questions before acting,'' said Norrich, coldly, "Now may I go? Do you mind?"

"You may go. As an official request, however, please make no mention of what has occurred to anyone. That's quite important."

Norrich said, "It comes under the heading of false arrest, I imagine, but we'll let it go. I won't mention this." He walked to the door, reached the signal patch with a minimum of fumbling, and walked out.

Bigman turned almost at once to Lucky. "It was a trick. You shouldn't have let him go."

Lucky, rested his chin on the palm of his right hand, and his calm, brown eyes were thoughtful. "No, Bigman, he isn't the man we're after."

"But he's got to be, Lucky. Even if he's blind, really blind, it's an argument against him. Sure, Lucky," Bigman grew excited again, his small hands clumping into fists, "he could get close to the V-frog without seeing it. He could kill it."

Lucky shook his head. "No, Bigman. The V-frog's mental influence doesn't depend on its being seen. It's direct mental contact. That's the one fact we can't get around." He said slowly, "It had to be a robot who did that. It had to be, and Norrich is no robot."

"Well, how do you know he-?" But Bigman stopped.

"I see you've answered your own question. We sensed his emotion during our first meeting, when the V-frog was still with us. He has emotions, so he's not the robot and he's not the man we're looking for."

But even as he said so, there was a look of deep trouble on his face and he tossed the book-film on advanced robotics to one side as though despairing of help from it.

The first Agrav ship ever to be built was named Jovian Moon and it was not like any ship Lucky had ever seen. It was large enough to be a luxury liner of space, but the crew and passenger quarters were abnormally crowded forward,. since nine tenths of the ship's volume consisted of the Agrav converter and the hyperatomic force-field condensers. From the midsec-tion, curved vanes, ridged into a vague resemblance to bat's wings, extended on either side. Five to one side, five to the other, ten in all.

Lucky had been told that these vanes, in cutting the lines of force of the gravitational field, converted the gravity into hyperatomic energy. It was as prosaic as that, and yet they gave the ship an almost sinister appearance.

The ship rested now in a gigantic pit dug into Jupiter Nine. The lid, of reinforced concrete, had been retracted, and the whole area was under normal Jupiter Nine gravity and exposed to the normal airlessness of Jupiter Nine's surface.

Nevertheless the entire personnel of the project, nearly a thousand men, were gathered in this natural amphitheater. Lucky had never seen so many men in space suits at one time. There was a certain natural excitement because of the occasion; a certain almost hysterical restlessness that manifested itself in horseplay made possible by the low gravity.

Lucky thought grimly: And one of those men in space suits is no man at all.

But which one? And how could he tell?

Commander Donahue made his short speech of dedication to a group of men grown silent, impressed despite themselves; while Lucky, looking up at Jupiter, glanced at a small object near it that was not a star but a tiny sliver of light, curved like the paring of a small fingernail, almost too small for the curve to be seen. If there had been any air in the way, instead of Jupiter Nine's airless vacuum, that small curve would have been blurred into a formless spot of light.

Lucky knew the tiny crescent to be Ganymede, Jupiter Three, Jupiter's largest satellite and worthy moon of the giant planet. It was nearly three times the size of Earth's moon; it was larger than the planet Mercury. It was almost as large as Mars. With the Agrav fleet completed, Ganymede would quickly become a major world of the solar system.

Commander Donahue christened the ship at last in a voice husky with emotion, and then the assembled audience, in groups of five and six, entered the air-filled interior of the satellite through the various locks.

Only those who were to be aboard the Jovian Moon remained. One by one they climbed the ramp to the entrance lock, Commander Donahue first.

Lucky and Bigman were last to board. Commander Donahue turned away from the air lock as they entered, stiffly unfriendly.

Bigman leaned toward Lucky, to say tightly, "Did you notice, Lucky, that Red Summers is on board?"

"I know."

"He's the cobber who tried to kill you."

"I know, Bigman."

The ship was lifting now in what was at first a majestic creep. The surface gravity of Jupiter Nine was only one eightieth of Earth, and though the weight of the ship was still in the hundreds of tons, that was not the cause of the initial slowness. Even were gravity absent altogether, the ship would still retain its full content of matter and all the inertia that went with it. It would still be just as hard to put all that matter into motion, or, if it came to that, to stop it or change its direction of travel, once it had begun moving.

But first slowly, then more and more rapidly, the pit was left behind. Jupiter Nine shrank beneath them and became visible in the visiplatps as a rugged gray rock. The constellations powdered the black sky and Jupiter was a bright marble.

James Panner approached them and placed an arm on the shoulder of each man. "Would you two gentlemen care to join me in my cabin for a meal? There'll be nothing to watch here in the viewing room for a while." His wide mouth pulled back in a grin that swelled the cords of his thick neck and made it seem no neck at all but a mere continuation of head.

"Thank you," Lucky said. "It's kind of you to invite us."

"Well," said Panner, "the commander isn't going to and the men are a little leery of you, too. I don't want you to get too lonely. It will be a long trip."

"Aren't you leery of me, Dr. Panner?" Lucky asked dryly.

"Of course not. You tested me, remember, and I passed."

Panner's cabin was a small one in which the three barely fitted. It was obvious that the quarters in this, the first Agrav ship, were as cramped as engineering ingenuity could make them. Panner broke out three cans of ship-ration, the concentrated food that was universally eaten on space ships. It was almost home to Lucky and Bigman; the smell of heating rations, the feeling of crowding walls, outside of which was the infinite emptiness of space, and, sounding through those walls, the steady vibrating hum of hyperatomic motors converting field energies into a directional thrust or, at the very least, powering the energy-consuming innards of the ship.

If ever the ancient belief of the "music of the spheres" could be said to have come literally true, it was in that hum of hyperatomics that was the very essential of space flight.

Panner said, "We're past Jupiter Nine's escape velocity now, which means we can coast without danger of falling back to its surface."

Lucky said, "That means we're in free fall down to Jupiter."

"With fifteen million miles to fall, yes. Once we've piled up enough velocity to make it worthwhile, we'll shift to Agrav."

, He took a watch out of his pocket as he spoke. It was a large disc of gleaming, featureless metal. He pressed a small catch, and luminous figures appeared upon its face. A glowing line of white encircled it, turning red in a sweeping arc until the redness closed in upon itself and the arc turned white again.

Lucky said, "Are we scheduled to enter Agrav so soon?"

"Not very long," said Panner. He placed the watch on the table, and they ate silently.

Panner lifted the watch again. "A little under a minute. It should be completely automatic." Although the chief engineer spoke calmly enough, the hand that held the watch trembled very slightly.

Panner said, "Now," and there was silence. Complete silence.

The hum of the hyperatomics had stopped. The very power to keep the ship's lights on and its pseudo-grav field in operation were now coming from Jupiter's gravitational field.

Panner said, "On the nose! Perfect!" He put away his watch, and though the smile on his broad, homely face was a restrained one, it virtually shouted relief. "We're actually on an Agrav ship now in full Agrav operation."

Lucky was smiling, too. "Congratulations. I'm pleased to be on board."

"I imagine you are. You worked hard enough for it. Poor Donahue."

Lucky said gravely, ''I'm sorry I had to push the commander so hard, but I had no choice. One way or another, I had to be on board."

Panner's eyes narrowed at the sudden gravity in Lucky's voice. "Had to be?"

"Had to be! It seems almost certain to me that on board this ship at the present moment is the spy we're looking for."