One man caught Lucky's attention particularly. It was Hanley Cook, Dr. Peverale's second in command. He stared at his finger ends, and there was something like weary disgust about him. When he looked up, his expression had changed and settled into a cautious blankness.

Nevertheless Lucky thought: "I'll have to talk to the man."

And then his attention shifted back to Dr. Peverale.

Dr. Peverale was saying, "The saboteur can't be one of us, of course. Dr. Mindes tells me that he has investigated and is sure of that. Even without investigation, I am sure that none of us is capable of such criminal action. Yet the saboteur must be intelligent, since the destruction is too purposeful, too exclusively directed against Project Light, to be the result of chance or of anything nonintelligent. Therefore... "

Bigman interrupted excitedly. "Hey, you mean Mercury has native life? It's Mercurians doing this?"

There was a sudden buzz of confused comment and some laughter, at which Bigman reddened. "Well," said the small Martian, "isn't that what Dr. Peverale is saying?"

"Not quite," said Dr. Peverale gently.

"There is no life of any kind native to Mercury," said one of the astronomers with emphasis. "That's one thing we're sure of."

Lucky interposed, "How sure? Has anyone looked?"

The astronomer who had spoken seemed taken aback. He said, "There have been exploring parties. Certainly."

Lucky smiled. He had met intelligent beings on Mars that no other man knew of. He had discovered semi-intelligent beings on Venus where none had been thought to exist. He, for one, was not ready to admit that any planet lacked life, or even intelligence.

He said, "How many exploring parties? How thorough was each exploration? Has every square mile been searched?"

The astronomer did not answer. He looked away, raising his eyebrows as though to say: What's the use?

Bigman grinned, his little face wrinkling into a caricature of gnomish good humor.

Dr. Peverale said, "My dear Starr, explorations have uncovered nothing. While we grant that the possibility of Mercurian life is not completely excluded, the probability of its existence is very low. Suppose we assume that the only intelligent life in the Galaxy is the human race. Certainly, it's the only one we know of."

With the Martian mind-beings in his memory, Lucky did not agree with that, but he kept silent and let the old man continue.

It was Urteil, little by little having recovered his self-possession, who intervened. "What do you think you're getting at," he asked, and it was characteristic of the man that he could not resist adding, "if anything?"

Dr. Peverale did not answer Urteil directly. He looked from face to face, deliberately ignoring the Congressional investigator. He said, "The point is, there are humans elsewhere than on Earth. There are humans in many star systems." A queer change came across the astronomer's face. It pinched in, grew white, and his nostrils flared as though he were suddenly overpowered with anger. "For instance, there are humans on the planets of Sirius. What if they are the saboteurs?"

"Why should they be?" asked Lucky at once.

"Why not? They have committed aggression against Earth before."

So much was true. Lucky Starr himself had helped, not too long before, to repel a Sirian invasion flotilla that had landed on Ganymede, but in that case they had left the Solar System without pushing matters to a showdown. Yet, on the other hand, it was a common thing for many Earthmen to blame Sirians for anything that went wrong.

Dr. Peverale was saying with energy, "I've been there. I've been to Sirius only five months ago. It took a great deal of red tape because Sirius welcomes neither immigrants nor visitors, but it was a matter of an interstellar astronomical convention, and I managed to get a visa. I was determined to see for myself, and I must say I wasn't disappointed.

"The planets of Sirius are thinly populated and they are extremely decentralized. They live in isolated individual family units, each with its own energy source and services. Each has its group of mechanical slaves -there's no other word possible-slaves in the shape of positronic robots, which do the labor. The Sirian humans maintain themselves as a fighting aristocracy. Every one of them can handle a space-cruiser. They'll never rest till they destroy the Earth."

Bigman shifted restlessly in his seat. "Sands of Mars, let them try. Let them try, is all I say."

"They will when they are quite ready," said Dr. Peverale, "and, unless we do something quickly to meet the danger, they will win. What have we got to oppose them? A population in the billions, true, but how many can handle themselves in space? We are six billion rabbits and they are one million wolves. Earth is helpless and grows more helpless every year. We are fed by grain from Mars and yeast from Venus. We get our minerals from the asteroids, and we used to get them from Mercury, too, when the mines here were working.

"Why, Starr, if Project Light succeeds, Earth will be dependent on space stations for the manner in which it gets its very sunshine. Don't you see how vulnerable that makes us? A Sirian raiding party, by attacking the outposts of the System, could panic and starve Earth without ever having to fight us directly.

"And can we do anything to them in return? No matter how many of them we kill, the remaining Sirians are always self-contained and self-sufficient. Any of them could continue the war."

The old man was almost breathless with passion. There was no questioning his sincerity. It was as though he were getting something out of himself that had been stifling him.

Lucky's eye wandered back to Dr. Peverale's second, Hanley Cook. The man was resting his forehead on the bony knuckles of one large hand. His face was flushed, but to Lucky it did not seem like a flush of either anger or indignation. Rather, it seemed one of embarrassment.

Scott Mindes spoke up skeptically. "What would be the point, Dr. Peverale? If they're getting along on Sirius, why should they come to Earth? What would they get out of us? Even supposing they conquer Earth, they would only have to support us- "

"Nonsense!" rapped out the senior astronomer. "Why should they? They would want Earth's resources, not Earth's population. Get that through your head. They'd let us starve. It would be part of their policy."

"Oh, come," said Gardoma. "That's unbelievable."

"Not out of cruelty," said Dr. Peverale, "out of policy. They despise us. They consider us scarcely more than animals. The Sirians themselves are very race-conscious. Since Earthmen first colonized Sirius, they have been breeding themselves carefully until they are free of diseases and of various characteristics which they consider undesirable.

"They are of uniform appearance, while Earthmen are of all shapes, sizes, colors, varieties. The Sirians consider us inferior. That's why they won't let us emigrate to Sinus. They wouldn't let me attend the convention till the government pulled every string possible. Astronomers from other systems were all welcome but not from Earth.

"And human life, any kind of human life, doesn't mean much to them, anyway. They're machine-centered. I've watched them with their metal men. They're more considerate of a Sirian robot, almost, than of a Sirian man. They would regard a robot as worth a hundred men of Earth. They pamper those robots. They love them. Nothing's too good for them."

Lucky murmured, "Robots are expensive. They have to be treated carefully."

"Maybe so," said Dr. Peverale, "but men who become accustomed to worrying about the needs of machines become callous about the needs of men."

Lucky Starr leaned forward, elbows on the table, dark eyes serious and the smooth vertical lines of his handsome, still subtly boyish face set gravely. He said, "Dr. Peverale, if the Sirians are race-conscious and are breeding themselves into uniformity, they will defeat themselves in the long run. It is variety in the human race that brings about progress. It is Earth and not Sirius that is in the forefront of scientific research. Earthmen settled Sirius in the first place, and it is we, not our Sirian cousins, who are advancing in new directions every year. Even the positronic robots you mention were invented and developed on Earth by Earthmen."

"Yes," said the astronomer, "but Earthmen don't make use of the robot. It would upset our economy, and we place the comfort and security of today above" the safety of tomorrow. We use our scientific advance to make ourselves weaker. Sinus uses its to make itself stronger. That's the difference and that's the danger."

Dr. Peverale threw himself back in his chair, looking grim. The mechanical tray-carrier cleared the table.

Lucky pointed at it. "That's a sort of a robot, if you like," he said.

The mechanical tray-carrier went quietly about its task. It was a flat-surfaced thing moving smoothly on a diamagnetic field, so that its gently curved base never actually touched the floor. Its limber tentacles removed dishes with careful delicacy, placing some on its upper surface, others within a cabinet in its side.

"That's a simple automaton," snorted Dr. Peverale. "It has no positronic brain. It cannot adapt itself to any change in its task."

"Well, then," said Lucky, "are you saying that the Sirians are sabotaging Project Light?"

"Yes. I am."

"Why should they?"

Dr. Peverale shrugged. "Perhaps it's just part of a larger plan. I don't know what trouble there is elsewhere in the Solar System. These may be the first random probings to prepare for ultimate invasion and conquest Project Light in itself means nothing, the Sirian danger everything. I wish I could rouse the Council of Science and the government and the people to that truth."

Hanley Cook coughed, then spoke for the first time. "The Sirians are human like the rest of us. If they're on the planet, where are they?"

Dr. Peverale said coldly, "That's for an exploring expedition to find out. A well-prepared, well-equipped expedition."

"Wait a minute," said Mindes, his eyes glinting with excitement, "I've been out on the Sun-side, and I'll swear... "

"A well-prepared, well-equipped expedition," repeated the old astronomer firmly. "Your one-man flights mean nothing, Mindes."

The engineer stuttered a moment and slumped into an embarrassed silence.

Lucky said suddenly, "You seem to be unhappy about this, Urteil. What is your opinion of Dr. Pever-ale's view?"

The investigator lifted his eyes and met those of Lucky for a long moment in hatred and open defiance. It was obvious he had not forgotten, nor would forget, the earlier exchange at this table.

He said, "I'm keeping my opinion to myself. But I will say this, I'm not fooled by anything that's going on here tonight."

His mouth clamped shut and Lucky, having waited a moment for further remarks, turned to Peverale and said, "I wonder if we do need a complete expedition, sir. If we suppose that the Sirians are here on Mercury, can we perhaps deduce where they might be?"

"Go ahead, Lucky," crowed Bigman at once. "Show them how."

Dr. Peverale said, "How do you mean?"

"Well, what would be the best for the Sirians? If they've been sabotaging Project Light at frequent intervals over a period of months, it would be most convenient for them to have a base near the project. Yet at the same time, the base must not be easily detected. They've certainly been successful in the second requirement, anyway. Now where could such a handy, but secret, base be?

"Let's divide up Mercury into two parts, Sun-side and dark-side. It seems to me that they would be foolish to set up a base on Sun-side. Too hot, too much radiation, too inhospitable."

Cook grunted. "No more inhospitable than the dark-side."

"No, no," said Lucky at once, "you're wrong there. The Sun-side presents an environment which is quite unusual. Humans aren't accustomed it it at all. The dark-side is something very familiar. It is simply ground which is exposed to space, and the conditions of space are very familiar. The dark-side is cold but no colder than space. It is dark and airless but no darker than any portion of space not in direct sunlight and certainly no more airless. Men have learned to live comfortably in space, and they can live on the dark-side."

"Go on," said Dr. Peverale, his old eyes gleaming with interest. "Go on, Mr. Starr."

"But establishing a base that would serve over a period of months is not a simple thing. They must have a ship or ships to get back to Sirius someday. Or if they're to be picked up by a ship from outside they must still have ample stores of food and water, as well as an energy source. All this takes up room, and yet they must be certain they will not be detected. It leaves only one place where they can be."

"Where, Lucky?" asked Bigman, nearly jumping up and down in his eagerness. He, at least, had no doubts that whatever Lucky said was so. "Where?"

"Well," said Lucky, "when I first arrived here, Dr.Mindes made mention of mines on Mercury which had failed. Just a few moments ago, Dr. Peverale spoke of mines on Mercury that were once working. From that I gather that there must be empty mine shafts and corridors on the planet, and they must be either here or at the South Pole, since the polar regions are the only places where the temperature extremes are not too great. Am I right?"

Cook faltered. "Yes, there are mines here. Before the Observatory was established, the Dome was the mining center."

"Then we're sitting on top of a large empty hole in Mercury. If the Sirians are successfully hiding a large base, where else would it be? There is the direction of danger."

A murmur of appreciation passed around the table, but it was shattered abruptly by Urteil's guttural tones. "All very pretty," he said, "but what does it all come to? What are you going to do about it?"

"Bigman and I," said Lucky, "intend to enter the mines just as soon as we can get ready. If there's anything there, we'll find it."