At once he knew that it came from the earth-borer's disintegrators.

The sphere had started down without him.

He stood stock still, petrified with surprise, facing the sound, while his attacker melted farther and farther into the night. And then, suddenly, Phil Holmes was sprinting desperately back towards the Guinness camp.

He ran until he was exhausted; walked for a little while his legs gathered more strength, and his laboring lungs more air; and then ran again. As the minutes pa.s.sed, the thunder lessened rapidly into a m.u.f.fled drone; and by the time Phil had panted up to the brink of the hole that gaped where but a little time before the sphere was standing, it had become but a distant purr. He leaned far over and peered into the hot blackness below, but could see nothing.

Phil knelt there silently for some minutes, shocked by his strange attack, bewildered by the unexpected descent of the borer. For a time his mind would not work; he had no idea what to do. But gradually his thoughts came to order and made certain things clear.

He had been deliberately ambushed. Only by luck had he escaped, he told himself. If it hadn't been for the water jug, he'd now be out of the picture. And on the heels of the ambush had came the surprising descent of the earth-borer. The two incidents coincided too well: the same mind had planned them. And two, men, at least, were in on the plot.... It suddenly became very clear to him that the answer to the puzzle lay with the man who had ambushed him. He would have to get that man. Track him down.

Phil acted with decision. He got to his feet and strode rapidly to the deserted Guinness shack, horribly quiet and lonely now in the bright moonlight. In a minute he emerged with a flashlight at his belt and a rifle across his arm.

Once again he went over to the new black hole in the desert and looked down. From far below still came the purr, now fainter than ever. His friend, the girl he loved, were down there, he reflected bitterly, and he was helpless to reach them. Well, there was one thing he could do--go man-hunting. Turning, he started off at a long lope for the water-hole.

Ten minutes later he was there, and off to the side he found the marks of their scuffle--and small black blotches that could be nothing but blood. The other was wounded: could probably not get far. But he might still have his gun, so Phil kept his rifle handy, and tempered his impatience with caution as he set out on the trail of the widely s.p.a.ced footprints.

They led off towards the nearby hills, and in the bright moonlight Phil did not use his flashlight at all, except to investigate other round black blotches that made a line parallel to the prints. As he went on he found his quarry's steps coming more closely together: becoming erratic. Soon they showed as painful drags in the sand, a laborious hauling of one foot after the other.... Phil put away his light and advanced very cautiously.

He wondered, as he went, who in the devil was behind it all. The radium-finding project had been kept strictly secret. Not another soul was supposed to know of the earth-borer and its daring mission into the heart of the earth. Yet, obviously, someone had found out, and whoever it was had laid at least part of his scheme cunningly. An old man and a girl cannot offer much resistance: he, Phil, would have been well taken care of had it not been for the water jug. So far, there were at least two in the plot: the man who had ambushed him and the unknown who had evidently kidnapped both Professor and Sue Guinness.

But there might be still more.

There might be friends, nearby, of the man he was tracking. The fellow might have reached them, and warned them that the scheme hadn't gone through, that Phil was loose. They could very easily conceal themselves alongside their partner's tracks and train their rifles on the tracker....

The trail was leading up into one of the canons in the cl.u.s.ter of hills to the west. For some distance he followed it up through a slash of black below the steep moonlit heights of the hills to each side--and then, suddenly, he vaguely made out the forms of two huts just ahead.

Immediately he stooped low, and went skirting widely off up one side.

He proceeded slowly, with great caution, his rifle at the ready. At any moment, he knew, the hush might be split by the cracks of waylaying guns. Warily he advanced along the narrow canon wall above the huts. No lights were lit, and the place seemed unoccupied. He was debating what to do next when his attention was attracted to a large dark object lying in the canon trail some twenty yards from the nearest hut. Straining his eyes in the inadequate moonlight, he saw that it was the outstretched figure of a man. His quarry--his ambusher!

Phil dropped flat, fearful of being seen. Keeping as best he could in the shadows, fearing every moment to hear the sharp bark of a gun, he crawled forward. It took him a long time to approach the sprawled figure, but he wasn't taking chances. When within twenty feet, he rose suddenly and darted forward to the man's side.

His rapid glance showed him that the fellow was completely out: and another quick look around failed to show that anyone else was watching, so he returned to his examination of the man. It was the ambusher, all right: a Mexican. He was still breathing, though his face was drawn and white from the loss of blood from a wound under the blood-soaked clothing near his upper right arm. A hasty search showed that he no longer had his gun, so Phil, satisfied that he was powerless for some time to come, cautiously wormed his way towards the two shacks.

There was something sinister in the strange silence that hung over them. One was of queer construction--a windowless, square, high box of galvanized iron. The other was obviously a dwelling place.

Carefully Phil sneaked up to the latter. Then, rifle ready, he pushed its door open and sent a beam of light stabbing through the darkness of the interior.

There was no one there. Only two bunks, a table, chair, a pail of water and some cooking utensils met his view. He crept out toward the other building.

Come close, Phil found that a dun-colored canvas had been thrown over the top of it, making an adequate camouflage in daytime. The place was about twenty feet high. He prowled around the metal walls and discovered a rickety door. Again, gun ready, he flung it open. The beam from his flash speared a path through the blackness--and he gasped at sight of what stood revealed.

There, inside, was a long, bullet-like tube of metal, the pointed end upper-most, and the bottom, which was flat, toward the ground. It was held in a wooden cradle, and was slanted at the floor. In the bottom were holes of two shapes--rocket tubes and disintegrating projectors.

It was another earth-borer.

Phil stood frozen with surprise before this totally unlooked-for machine. He could easily have been overcome, had the owner been in the building, for he had forgotten everything but what his eyes were staring at. He started slowly around the borer, found a long narrow door slightly ajar, and stepped inside.

This borer, like Guinness's, had a double sh.e.l.l, and much the same instruments, though the whole job was simpler and cruder. A small instrument board contained inclination, temperature, depth and air-purity indicators, and narrow tubes led to the air rectifiers. But what kept Holmes' attention were the wires running from the magneto to the mixing chambers of the disintegrating tubes.

"The fools!" he exclaimed, "--they didn't know how to wire the thing!

Or else," he added after a moment, "didn't get around to doing it." He noticed that the projectile's interior contained no gyroscope: though, he thought, none would be needed, for the machine, being long and narrow, could not change keel while in the ground. Here he was reminded of something. Stepping outside, he estimated the angle the borer made with the dirt floor. Twenty degrees. "And pointed southwest!" he exclaimed aloud. "This borer would come close to meeting the professor's, four miles under our camp!"

At once he knew what he would do. First he went back to the other shack and got the pail of water he had noticed, and took this out where the Mexican lay outstretched. He bathed the man's face and the still slightly bleeding bullet wound in his shoulder.

Presently the wounded man came to. His eyes opened, and he stared up into a steel mask of a face, in which two level black eyes bored into his. He remembered that face--remembered it all too well. He trembled, cowered away.

"No!" he gasped, as if he had seen a ghost. "No--no!"

"Yes, I'm the man," Holmes told him firmly, menacingly. "The same one you tried to ambush." He paused a moment, then said: "Do you want to live?"

It was a simple question, frightening in its simplicity.

"Because if you don't answer my questions, I'm going to let you lie here," Phil went on coldly. "And that would probably mean your death.

If you do answer, I'll fix you up so you can have a chance."

The Mexican nodded eagerly. "I talk," he said.

"Good," said Phil. "Then tell me who built that machine?"

"Senor Quade. Senor James Quade."

"Quade!" Phil had heard the name before. "Of course!" he said.

"Guinness's old partner!"

"I not know," the Mexican answered. "He hire me with much money. He buy thees machine inside, and we put him together. But he could no make him work--it take too long. We watch, hear old man go down to-night, and--"

The greaser stopped. "And so he sent you to get me, while he kidnapped the old man and his daughter and forced them under the ground in their own borer," Holmes supplied, and the other nodded.

"But I only mean to tie you!" he blurted, gesturing weakly. "I no mean shoot! No, no--"

"All right--forget it," Phil interrupted. "And now tell me what Quade expects to do down there."

"I not know, Senor," came the hesitant reply, "but...."

"But what?" the young man jerked.

Reluctantly the wounded Mexican continued. "Senor Quade--he--I think he don' like thees old man. I think he leave heem an' the girl down below. Then he come up an' say they keeled going down."

Phil nodded grimly. "I see," he said, voicing his thoughts. "Then he would say that he and Professor Guinness are still partners--and the radium ore will belong to him. Very nice. Very nice...."

He snapped back to action, and without another word hoisted the Mexican onto his back and carried him into the shack. There he cleansed the wound, rigged up a tight bandage for it, and tied the man to one of the cots. He tied him in such a fashion that he could reach some food and water he put by the cot.