Cargraves let them experiment, while seeing to it that as many field tests as possible were made to supplement the manufacturer's laboratory tests. Then the suits were turned over to Art for installation of walky-talky equipment.

The following day the doctor turned all the boys to work on the conversion of the drive mechanism. He was expecting delivery of the atomic fission element thorium; the anti-radiation shield had to be ready. This shield was constructed of lead, steel, and organic plastic, in an arrangement which his calculations indicated would be most effective in screening the alpha, beta, and gamma radiations and the slippery neutrons, from the forward part of the rocket.

Of these radiations, the gamma are the most penetrating and are much like X-rays. Alpha particles are identical with the nuclei of helium atoms; beta particles are simply electrons moving at extremely high speeds. Neutrons are the electrically uncharged particles which make up much of the mass of most atomic nuclei and are the particles which set off or trigger the mighty explosions of atomic bombs.

All of these radiations are dangerous to health and life.

The thorium drive unit was to be shielded only on the forward side, as radiations escaping to outer space could be ignored. Morrie had landed the rocket with one side facing the cabin, inside the corral. It was now necessary to jack the rocket around until the tubes pointed away from the cabin, so that radiations, after the thorium was in place, would go harmlessly out across the crater of the Doomsday Bomb and, also, so that the rocket would be in position for a captive test run with the exhaust directed away from the cabin.

The jacking-around process was done with hydraulic jacks, muscle, and sweat, in sharp contrast to the easy-appearing, powered manipulation of rockets by dolly and cradle and mobile sling, so familiar a sight on any rocket field. It took all of them until late afternoon. When it was over Cargraves declared a holiday and took them on a long-promised trip into the Doomsday Crater.

This bomb site has been pictured and described so much and the boys were so used to seeing it in the distance that the thrill of being in it was limited. Nevertheless the desolation, the utter deadness, of those miles and miles of frozen, glassy waste made their flesh creep. Cargraves marched ahead, carrying a Geiger radiation counter, of the sort used to prospect for uranium in Canada during the war. This was largely to impress the boys with the necessity for unsleeping watchfulness in dealing with radioactive elements. He did not really expect to hear the warning rattle of danger in the ear phones; the test had been made so long before that the grim lake was almost certainly as harmless as the dead streets of Hiroshima.

But it put them in the mood for the lecture he had in mind. "Now, listen, sports," he started in when they got back, "day after tomorrow the thorium arrives. From then on the holiday is over. This stuff is poison. You've got to remember that all the time."

"Sure," agreed Morrie. "We all know that."

"You know it at the tops of your minds. I want you to know it every minute, way down in your guts. We'll stake out the unshielded area between the ship and the fence. If your hat blows into that stretch, let it stay there, let it rot—but don't go after it."

Ross looked perturbed. "Wait a second, Doc. Would it really hurt anything to expose yourself for just a few seconds?"

"Probably not," Cargraves agreed, "provided that were all the dosage you ever got. But we will all get some dosage all the time, even through the shield. Radioactivity accumulates its poisonous effect. Any exposure you can possibly avoid, you must avoid. It makes your chances better when you get a dose of it accidentally. Art!"

"Uh? Yes, sir!"

"From now on you are the medical officer. You must see to it that everybody wears his X-ray film all the time- and I mean all the time -and his electroscope. I want you to change the films and develop them and check the electroscopes according to the dose in the manual. Complete charts on everything, and report to me each Friday morning—oftener if you find anything outside the limits. Got me?"

"Got you, Doc."

"Besides that, you arrange for blood counts once a week for everybody, over in town."

"I think I could learn to do a blood count myself," Art offered.

"You let the regular medic do it. You've got enough to worry about to keep all the electronic equipment purring along properly. One more thing." He looked around him, waiting to get their full attention. "If any one shows the possibility of overdosage of radiation, by film or by blood count or whatever, I will have to send him home for treatment. It won't be a case of ‘just one more chance.' You are dealing with hard facts herd—not me, but natural laws. If you make a mistake, out you go and we'll have to find somebody to take your place."

They all nodded solemnly. Art said, "Doc?"

"Suppose it's your film that shows the overdosage?"

"Me? Not likely! If it does you can kick me all the way to the gate—I'm afraid of that stuff!

"Just the same," he went on more seriously, "you run the same checks on me as on everybody else. Now let's have supper. I want you and Morrie to do the KP tonight, so that Ross can start his study period right after supper. Ross, you and I are getting up at five, so let's hit the sack early." "Okay. What's cookin'?"

"Trip into Albuquerque—shopping." He was reluctant to explain. The place had no firearms. They had seemed a useless expense—many a man has spent years in the desert without shooting off anything but his mouth, he had reasoned. As for the dreamed of trip, what could one shoot on the moon? But signs of prowlers, even in this fenced and forbidding area, had him nervous. Art's watch-dog fence was tested each night and Art slept with the low power-hum of the hot circuit in his ears; thus far there had been no new alarm. Still he was nervous.

Cargraves was awakened about three A.M. to find Art shaking his shoulder and light pouring in his eyes. "Doc! Doc! Wake up!"

"Huh? Wassamatter?"

"I got a squawk over the loudspeaker."

Cargraves was out of bed at once. They bent over the speaker. "I don't hear anything."

"I've got the volume low, but you'd hear it. There it is again—get it?" There had been an unmistakable squawk from the box. "Shall I wake the others?"

"Mmmm... no. Not now. Why did you turn on the light?"

"I guess I wanted it," Art admitted.

"I see." Cargraves hauled on trousers and fumbled with his shoes. "I want you to turn out the lights for ten seconds. I'm going out that window. If I'm not back in twenty minutes, or if you hear anything that sounds bad, wake the boys and come get me. But stay together. Don't separate for any reason." He slipped a torch in his pocket. "Okay."

"You ought not to go by yourself."

"Now, Art. I thought we had settled such matters."

"Yes, but—oh, well !" Art posted himself at the switch.

Cargraves was out the window and had cat-footed it around behind the machine shop before the light came on again. He lurked in the shadow and let his eyes get used to the darkness.

It was a moonless night, clear and desert sharp. Orion blazed in the eastern sky. Cargraves soon was able to pick out the sage bushes, the fence posts, the gloomy bulk of the ship a hundred yards away.

The padlock on the machine shop was undisturbed and the shop's windows were locked. Doing his best to take advantage of the scanty cover, he worked his way down to the ship.

The door was ajar. He could not remember whether he or Ross had been last man out. Even if it had been Ross, it was not like Ross to fail to lock the door.

He found that he was reluctant to enter the craft. He wished that he had not put off buying guns; a forty-five in his hand would have comforted him. He swung the door open and scrambled in fast, ducking quickly away from the door, where his silhouette would make a target. He crouched in the darkness, listening and trying to slow his pounding heart. When he was sure he could hear nothing, he took the flashlight, held it at arm's length away from him and switched it on.