He himself made a short trip to their ship, the Shoot ing Starr. He made the trip via the surface, carrying a field pack, the contents of which he did not discuss with Bigman. He returned without it but carrying two small objects that looked like thick belt buckles, slightly curved, in dull-steel finish and centered by a rectangle in glassy red.

"What's that?" asked Bigman.

"Microergometers," said Lucky. "Experimental. You know, like the ergometers on board ship except that those are bolted to the floor."

"What can these things detect?"

"Nothing at a couple of hundred thousand miles like a ship's ergometer, but it can detect atomic power sources at ten miles, maybe. Look, Bigman, you activate it here. See?"

Lucky's thumbnail exerted pressure against a small slit in one side of the mechanism. A sliver of metal moved in, then out, and instantly the red patch on the surface glowed brightly. Lucky turned the small ergometer in this direction and that. In one particular position, the red patch glowed with the energy of a nova.

"That," said Lucky, "is probably the direction of the Dome's power plant. We can adjust the mechanism to zero that out. It's a little tricky." He worked painstakingly on the adjustment of two small controls so smoothly inset as nearly to be invisible.

He smiled as he worked, his engagingly youthful face lighting with pleasure. "You know, Bigman, there isn't a time I visit Uncle Hector but that he doesn't load me up with the Council's latest gadgets. He claims that with the chances you and I are always taking (you know the way he talks) we need them. Sometimes, though, I think he just wants us to act as field-testers for the gadgets. This one, though, may be useful."

"How, Lucky?"

"For one thing, Bigman, if there are Sirians in the mines, they'll have a small atomic power plant. They'll have to. They'll need power for heat, for electrolyzing water, and so on. This ergometer should detect that at ample distance. And for another thing-- "

He fell silent, and Bigman's lips compressed in chagrin. He knew what that silence meant. Lucky had thoughts which later he would claim had been too vague to talk about

"Is one of the ergometers for me?" he asked.

"You bet," said Lucky, tossing one of the ergometers toward him. Bigman snatched it out of the air.

Hanley Cook was waiting for them when they stepped out of their quarters, wearing their suits but with headpieces tucked under their arms.

He said, "I thought I'd lead you as far as the nearest entrance to the shafts."

"Thank you," said Lucky.

It was the tail end of the sleeping period in the Dome. Human beings always established an Earth-like alternation of waking and sleeping, even where there was no day and night to guide them. Lucky had chosen this time on purpose, since he did not want to enter the mines at the head of a curious procession. In this Dr. Peverale had co-operated.

The corridors of the Dome were empty. The lighting was dimmed. And as they walked, a heavy silence seemed to fall about them while the clank of their footsteps sounded even louder.

Cook stopped. "This is Entry Two."

Lucky nodded. "All right. I hope we'll be seeing one another again soon."

"Right."

Cook operated the lock with his usual gloomy gravity, while Lucky and Bigman put on their headpieces, clamping them firmly along the paramagnetic seams. Lucky took his first breath of canned air with what was almost pleasure, he was so accustomed to it.

Lucky first, then Bigman, stepped into the air lock. The wall closed behind them.

Lucky said, "Ready, Bigman?"

"You bet, Lucky." His words rang in Lucky's radio receiver, and his small form was a shadow in the extremely dim light of the lock.

Then the opposing wall opened. They could feel the puff of air escaping into vacuum, and they stepped forward through the opening once again.

A touch at the outer controls and the wall closed behind them again. This time, light was shut off altogether.

Standing in absolute darkness, they found themselves inside the silent and empty mines of Mercury.