was let down carefully on the end of a cable. Before it could touch the ground, Russ Steele rushed over and grabbed it, bringing the fragile package gently to earth. Quickly, he unhooked the cable and waved up at the helicopter. The cable was reeled in smoothly, then with a roar of its engines, the copter leaped into the air. Minutes later it disappeared over the treetops.

The boys watched with interest as Russ Steele unpacked the carton and removed two oblong black Bakelite boxes from the packing. They had a very unscientific, unprepossessing appearance.

“Is that all a Geiger counter is?” Jerry said with a trace of disappointment. “The transformer on my old electric trains looks more complicated.”

Russ smiled. “The Geiger counter is very simple, Jerry—especially when you consider how delicate it is and what it can accomplish.”

“How does it work?” Sandy asked.

“We made one in the science lab once,” Quiz said eagerly. “It’s just two electrodes, really. One of the electrodes is a thin metal cylinder; the other is a metal wire enclosed in a glass tube filled with gas—like a neon light. When the counter is brought near any radioactive substance, the rays given off ionize the gas—so it can conduct electricity—allowing the current to jump the gap and close the circuit, the same way it does when you switch on a light—”

“Only instead of a light, it activates an audible indicator,” Russ said. “That’s the clack-clack you hear when the counter detects radioactivity. Look how sensitive it is.” He held one of the black boxes near his wrist watch, and it began to chatter vigorously.

“Holy cow!” Jerry exclaimed, leaping backward.

Russ laughed. “That’s the infinitesimal grain of radium in the luminous dial. So, you can feel secure that it will warn us if we enter an area where there’s any unusual radioactivity.”

He rummaged around in the carton and pulled out two canvas straps. “These hook on the ends so the counter can be slung across your shoulder like a camera.”

Prince came over and sniffed suspiciously at the plastic boxes. “Nothing to eat there, feller,” Sandy told him.

“Eat! That’s a good idea,” Jerry said. “I’m famished.”

Quiz was disgusted. “Only Jerry could think of food at a time like this. Who cares about eating when there’s an A-bomb lying right at your door-step?”

“I hope not,” Jerry said, looking around with an expression of exaggerated horror.

“Jerry’s right,” Russ said firmly. “The first order of the day is to pack away a substantial breakfast. We may be tramping through the woods until dark. Let’s go upstairs and see what Ranger Fellows has cooking.” He gathered up the two Geiger counters and walked to the tower.

Prince whined reprovingly as they left him at the foot of the steps. “I’ll bring you down a bowl of chow right away,” Sandy promised.

They were halfway up the stairs when a sudden thought struck Jerry. “Say, Mr. Steele, what would happen if one of those big lightning bolts hit that atomic bomb square on the nose?”

Russ Steele’s face contracted in a sour grimace. “I don’t know. And stop trying to spoil my appetite.”