the men I know best are searching the living quarters, under command of Professor Erickson. If you will come with me now, we will go to the observation room, where the rest of the men are loafing while off duty."

As they passed down the central hall in the section where the cabins were located, a man ran out from a side passage, saw them, and turned at full speed for the bow.

"Stop him," came a shout. Holden recognized the voice as Erickson's. The man heard it, too, for he whirled in his tracks, whipped an old-fashioned automatic pistol from his pocket, leveled it at Holden, and took careful aim. The fraction of a second during which his eye rested along the sights was his undoing.

Captain Linet's hand, hidden under the loose jacket he was wearing, pressed the release on his short-range ray pistol, a light bluish streak touched the man's breast, and he fell forward, his heart literally shattered by the energy of the ray.

Holden reached him first, and rolled him over. His face was faintly familiar, and doubt changed to recognition as Captain Linet exclaimed, "It's Chambers, a former petty officer on my airliner."

It was the man who had come up to the Captain while Holden and Erickson were conversing with him on the bridge.

"What on earth could the man have been up to? He must have been mad to attack me on this ship, with no chance of escape," exclaimed Holden. "Do you know anything of his record, Captain?"

"Nothing whatsoever, except that he seemed honest enough, and hard working. I was the one responsible for his presence on board here, as he had mentioned some knowledge of interplanetary travel, and we needed men."

Erickson had come up by that time.

"We found nothing in this man's cabin except some tools that he had evidently stolen from the machine shop, and a code book of the type used by commercial companies for interplanetary messages. He entered the room while we were searching it, and bolted when he saw us."

The thing was puzzling, but most of the men on board accepted the explanation that the man was mad, and had for some reason resorted to desperate measures to assure the safety of the moon.

"You know," explained Captain Linet, "back a few hundred years ago, there was the expression 'moonstruck' applied to people who were mentally deranged."

At any rate, the incident was closed, as no one could be found who might possibly have been an accomplice. Minor damage caused by the cloud of meteorites was repaired, and the three ships swung in close together, heading for the satellite which they were commissioned to destroy.

The men spent as much time as they could in their bunks, for there was hard dangerous work ahead of them. Huge cartridges had to be filled with hexoxen, caps of Europium placed on top, and adjustments made so that, after a certain time had elapsed, the catalyst would come into contact with the hexoxen, causing a reaction to take place which would continue almost as long as there was solid material present to be vaporized. One slip of tired hands, one miscalculation and many men, perhaps the entire party, would suffer a terrible fate.

Holden was busy with one of the latest and best maps of the moon, looking for places where landing could be made, and charting the spots where the cartridges would be buried. The exact time for which every charge was to be set had to be worked out in advance.

CHAPTER III

A Sudden Encounter

The map of the moon was not as complete as it could have been, either. No particular interest had been taken in our satellite since the first exploratory expeditions nearly fifty years before, when it had been determined that the moon was of no value to Earthmen, either as an outpost for colonization or a station for the production of power from the sun's rays. Jack did the best he could, however, and the little dots he placed on the map were close enough together to assure complete vaporization of the solid material in less than the allotted time.

At the end of the second day out, by earth-time, the dead satellite loomed immense, only five thousand miles ahead. Holden was in the pilot house when Edwards began turning on the deceleration tubes.

"I flashed your message to the other ships," he said, as his quick fingers touched the buttons which sent messages to the tube-room, "telling them to stand by and land with us. I understand that the plan is to use these ships to travel over the surface of the moon, making landings in such positions that expeditions can be sent out in four directions to plant cartridges. That will certainly give us plenty of time, if nothing goes wrong."

"I don't see what could go wrong," replied Holden, "since that madman is out of the way."

Eagerly he watched the dead, dust-covered surface approach, marveling at the huge craters and precipitous peaks.

In two hours the five thousand miles had been reduced to less than that many yards, and in a few more minutes the three great ships were settling softly on the smooth surface of the plain at the foot of Mount Julian.

Space suits were rapidly donned, the air-locks set in operation, and the men hastily began unloading the first four charges of hexoxen and Europium. Holden called a meeting of the ship commanders in the pilot room of the San Francisco.

"Commander Huges," he addressed the man in charge of the Los Angeles, "you will proceed toward Mount Locke, and continue in that line until you reach the spot marked on this chart, which is directly opposite our present position. Rogers, you take the Ganymede, and go at an angle of 120 degrees to Huges' course, toward Mount Zoga. I will continue over the Crater of Aristotle. We will keep in constant communication with each other by means of the space phone. Time the charges so that they will commence to react on the afternoon of the twenty-eighth, thus giving a sufficient margin of time in case of delays due to parties getting lost. That's all."

The Ganymede and the Los Angeles left almost immediately, while men from the San Francisco set out to plant the first charges. There were four men to each cartridge, since it was necessary that they travel fast.

Holden smiled as the lean figure of Professor Erickson, almost lost in his space-suit, bounded away in great leaps at the head of his party. In five hours they returned, having had no trouble at all. Edwards manipulated the controls, and the ship rose quickly to an altitude of about five thousand feet and headed for the rim of the Crater of Aristotle, barely visible in the distance. As they neared the rim, they rose higher and higher. The mammoth cliffs of black rock towered above them, and the meters registered a height of five miles as they passed through a crack in the cliffs and looked down on the level floor beneath them.

Suddenly Holden, who had been inspecting the country from one of the bow ports, uttered an exclamation of astonishment.

"A tiny ship is rising toward us from the floor of the crater, near the cliffs!"

There it was, a speck rapidly growing larger, headed straight for them, and gaining velocity with every foot it covered.

Edwards worked frantically with the controls, diving in a zig-zag path toward the strange craft. Captain Linet rushed in, carrying one of the light hexoxen guns. Holden hurried to help him place it in a specially designed aperture in the bow, while Erickson and the regular radio man endeavored to establish communications with the intruder. A voice suddenly spoke from their instrument.

"You will consider yourselves our captives. Land at once as close as possible to the white spot you see at the base of the cliff. If you do not obey instructions, we will ram you immediately."

"Don't reply for a moment," Holden commanded, focusing his glasses in the direction indicated. As the powerful lenses brought out every detail of the scene below, he paled