"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."