tian purpose."
The puzzled silence that fell on the group was broken suddenly by a hoarse scream from Richardson. He had wandered from the others to inspect the next steel ball. Simultaneously with his scream everybody present saw a light wink momentarily with blinding brilliance high up in the lofty ceiling of the chasm. Richardson, dead in line with it, collapsed his length on the cavern floor.
"My God!" Cliff ejaculated, startled—then he rushed forward with the others beside him.
That they were too late was obvious the moment they turned the engineer over. His face was charred to ashes, the upper part of his neck and chest were burned away horribly.
Cliff raised a grim face and stared round on the now inscrutable roof and its galleries of rock and pumice stone.
"Something mechanical that killed him," he whispered, standing up again. "Boys, we're facing something deadly around here. It's got to be located . . ."
He stood watching bitterly as the unfortunate Richardson was carried to the borer. The solemn ticking of the giant ball followed the stunned party.
ONCE back at the upper levels in the base camp Cliff summoned his engineering chiefs from their different tasks and put the position to them.
". . . and so we face a mystery," he concluded. "Down there in the core of Mars is a mechanical system of destruction controlled by God knows what. It's taken Richardson. We know neither the extent nor the nature of the thing we're fighting—but we do know that we are going to stop it. We've found anilum too, though not exactly in the way we had hoped. That makes searching in other parts of this planet unnecessary. What you have got to do is get every available flame gun machine and transport it down to the lower cavern. We're going to try and liquefy those anilum balls, and the big one which ticks. We have one or two portable furnaces and since the balls are on tripods we can shove the furnaces under them.
"You, Townshend, are our chief scientist." Cliff looked at the squat, broad-shouldered man standing before him. "You'll go to work to try and figure out why those balls tick, and what they are supposed to be. It won't be easy, and you may never solve the mystery—but there's no harm in trying."
"I've got the instruments; maybe I'll find something." Townshend nodded his gray head.
"We others will go to work to find out exactly what it was that struck Richardson down," Cliff concluded grimly. "Sparks, you stay here in case we have to radio to Earth for help."
He turned to the door of the base with the others beside him. Then they paused and glanced at each other quickly at a sudden alien sound. It was a noise such as they had never heard before on Mars, a noise other than that of their own work.
From remote distances came clanging concussions, the rattle of metal flanges slamming against each other and followed by the sharper note of locks snapping into position. Four times it was repeated. Twice from high over their heads and twice from below their feet. Then all was quiet again.
"You know something," Val said in the ensuing calm, "I dare to think that that was the locks to the outer surface closing! There are four of them you know . . ."
Puzzled, loath to believe the startling possibility of Val's assertion, Cliff led the way to the shaft cage. Once the men were gathered—fifty in all—the
