“S O S. S O S. S O S.” Three short, three long, three short, the flashes winked from the dark headland. Dan McNally, master and owner of the small and ancient trading schooner, Virginia, caught the feeble flickering light from the island as he strode across the fore-deck. He stopped, stared at the looming black line of land beneath the tropical stars. Again light flashed from a point of rock far above the dim white line of phosphorescent surf, spelling out the signal of distress.

“Somebody bane callin’ with a flashlight, I t’ank,” the big Swede, Larsen, rumbled from the wheel.

Dan thought suddenly of a reply. He rushed into the charthouse, to return in a moment with a lighted lantern and a copy of the _Nautical Almanac_ which would serve to hide the flame between flashes. He flashed an answer.

Again the pale light flickered from the dark mass of land, spelling words out rather slowly, as if the sender were uncertain in his knowledge of Morse. Surprised as Dan had been by the signal from an island marked on the charts as uninhabited, he was astonished at the message that now came to him.

“You are in terrible danger,” he read in the flashes. “Dreadful thing here. Hurry away. Radio for warships. I am--”

The winking light suddenly went out. Dan strained his eyes to watch the point where it had been, and a few seconds later he saw a curious thing. A darting, stabbing lance of green fire flashed out across the barren, rocky cliff, lighting it fleetingly with pale green radiance. It leapt out and was gone in an instant, leaving the shoulder of the island dark as before.

Dan watched for long minutes, but he saw nothing more brilliant than the pale gleam of phosphorescence where the waves dashed against the sheer granite wall of the island.

“What you t’ank?” Larsen broke in upon him.

Dan started, then answered slowly. “I don’t know. First I thought there must be a lunatic at large. But that green light! I didn’t like it.”

He stared again at the looming mass of the island. Davis Island is one of the innumerable tiny islets that dot the South Pacific; merely the summit of a dead volcano, projecting above the sea. Nominally claimed by Great Britain, it is marked on the charts as uninhabited.

“Radio for warships, eh?” he muttered. A wireless transmitter was one of many modern innovations that the Virginia did not boast. She had been gathering copra and shell among the islands long before such things came into common use, though Dan had invested his modest savings in her only a year before.

“What would anyone want with warships on Davis Island?” The name roused a vague memory. “Davis Island?” he repeated, staring in concentration at the black sea. “Of course!” It came to him suddenly. A newspaper article that he had read five years before, at about the time he had abandoned college in the middle of his junior year, to follow the call of adventure.

The account had dealt with an eclipse of the sun, visible only from certain points on the Pacific. One Dr. Hunter, under the auspices of a Western university, had sailed with his instruments and assistants to Davis Island, to study the solar corona during the few precious moments when the shadow covered the sun, and to observe the displacement of certain stars as a test of Einstein’s theory of relativity.

The reporter had interviewed the party at San Francisco, on the eve of sailing. There had been photographs of the chartered vessel, of Dr. Hunter and his instruments, and of his daughter, Helen, who acted as his secretary. She looked not at all like a scientist, Dan recalled. In fact, her face had seemed rather pretty, even in the blurred newspaper half-tone.

But the memory cast no light upon the present puzzle. In the rambling years that had led him to this spot upon the old Virginia, he had lost touch with the science that had interested him during his college days. He had heard nothing of the results of the Hunter expedition. But this island had been its destination.

He turned decisively to the man at the wheel. “Larsen, we’ll stand well offshore till daylight,” he said. “Then, unless we see something unusual, we can sail in and land a boat to--”

The sentence was never finished. Through the corner of his eye, Dan saw a ray of green light darting toward them from the island. A line of green fire seemed to reach out and strike him a physical blow. Green flame flared around him; and somehow he was hurled from the bridge, clear of the rail and into the sea.

His impression of the incident was very confused. He seemed to have struck the water with such force that his breath was knocked out. He struggled back to the surface, strangling, and coughing the bitter brine from his lungs. It was several minutes before he was comfortably treading water, and able to see what had happened.

The old schooner was then a hundred yards away, careening crazily, and drifting aimlessly before the light breeze. The strange green fire had vanished. Parts of the ship apparently had been carried away or disintegrated by the ray or the force of which it was a visible effect. The mainmast was down, and was hanging over the side in a tangle of rigging.

Bright yellow flames were dancing at a dozen points about the wreckage on the listing deck. A grotesque broken thing, queerly illuminated by the growing fires, was hanging over the wheel--the body of Larsen. No living thing was visible; and Dan, after a second look at the wreck of the bow, knew that he must be the sole survivor of the catastrophe.

“Too bad about the boys,” he muttered through teeth that chattered, for the cold water had already chilled him. “And poor old Larsen.”

He thought again of the warning flashed from the shore. “Guess there must be something hellish afoot after all,” he muttered again. “The flicker of green that stopped the signals, and the green fire that got us--what can they mean?” He looked toward the looming black shadow of the island, and began divesting himself of his clinging, sodden garments. “I don’t wonder somebody wanted battleships. But even a battleship, if that green ray hit it--”

He drew a deep breath and ducked his head while he unlaced his shoes and kicked out of them. Then, with a final look at the burning wreck of the Virginia, he tore off the last bit of his underclothing and swam for the shore in an easy crawl.

The rocky ramparts of Davis Island were three or four miles away. But there was no wind; the black sea was calm save for a long, hardly perceptible swell. A strong swimmer and in superb condition, Dan felt no anxiety about being able to make the distance. There was danger, however, that a shark would run across him, or that he could not find a landing place upon the rocky shore.

Four bells had rung when he had seen the first flash; it had been just ten o’clock. And it was some four hours later that Dan touched bottom and waded wearily up a bit of smooth hard beach, through palely glittering phosphorescent foam.

He rubbed the brine from his tired limbs, and sat down for a time, in a spot where a fallen boulder sheltered his naked body from the cool morning wind. In a few moments he rose, flexed his muscles and peered through the starlit darkness for a way up the cliff behind the beach. He found it impossible to distinguish anything.

“Got to keep moving, or find some clothes,” he muttered. “And I may stumble onto what made the green light. Darn lucky I’ve been so far, anyhow. Larsen and the others--but I shan’t think of them. Wonder who was flashing the signals from the island. And did the green fire get him?”

He turned to look out over the black plain of the sea. Far out, the Virginia lay low in the water, a pillar of yellow flame rising from her hull. As he watched, the flame flickered and vanished: the old schooner, he supposed, had sunk. Then he noticed a pale glow come into being among the stars on the eastern horizon.

“Hello,” he muttered again. “So we’re going to have a moon? In the last quarter, but still it ought to light me up from this beach.”

A moment later the horns of the crescent had come above the black rim of the sea. Dan waited, swinging his arms and tramping up and down on the sand, until the silvery moon had cleared the horizon and illuminated the rugged face of the cliff with pale white radiance.

He chose a path to the top of the cliff and clambered up, emerging in a jungle-like thicket of brush. Picking his way with the greatest caution, yet scratching his naked skin most painfully, he made his way for a few yards through the brush to a point of vantage from which he could look about.

He was, he perceived, in a narrow valley or ravine, with rugged black walls rising sheer on either side. The silvery light of the crescent moon fell upon the rank jungle that covered the narrow floor of the canyon, which rose and dwindled as it penetrated inland.

Gazing up the canyon, Dan gasped in amazement at what he saw.

Mars, the red planet, hung bright and motionless, low in the western sky, gleaming with deep bloody radiance. Directly beneath it, bathed in the white light of the moon, was a bare, rocky peak that seemed the highest point of the island. And upon that highest pinnacle, that chanced to be just below the ruddy star, was an astounding machine.

Three slender towers, of a white metal that gleamed in the moonlight with the silvery luster of aluminum, rose from the rocky peak. They supported, in a horizontal position, an enormous metal ring. It must be, Dan reckoned swiftly, at least a hundred feet in diameter, and held a hundred feet above the summit of the mountain.

The huge ring gleamed with a strange purple radiance. A shimmering mist of red-violet light surrounded it. An unknown force seemed to throb within the mighty ring, drawing the mantle of purple haze about it.

And suspended inside the ring and below it was a long, slender needle of dazzling white light. To Dan, from where he stood in the canyon, it seemed a fine, sharp line, though he knew it must be some kind of pointer, luminous with the strange force pulsing through it.

The strange needle wavered a little, with quick, uncertain motions. The brilliance of its light varied oddly; it seemed to throb with a queer, irregular rhythm.

And the gleaming needle pointed straight at the planet Mars!

Dan stood a long time, watching the purple ring upon the silver towers, with the shining white needle hanging below it. He stared at Mars, glowing like a red and sinister eye above the incredible mechanism.

His mind was in a wild storm of wonder shot with fear. What was the meaning of the gleaming ring and needle? What connection did this great device have with the signal of distress from the cliff, and the green fire that had destroyed the Virginia? And why did the glowing needle point at Mars?

He did not know when he first began to hear the sound. For a time it was merely part of the strange mystery of the island, only another element in the atmosphere of fear and wonder that surrounded him. Then it rose a little, and he became suddenly sharply conscious of it as an additional menace. The sound was not loud, but deep and vibrant. A whir or hum, like that of a powerful, muffled motor, but deeper than the sound of any motor man has ever made. It came down the gorge, from the direction of the machine on the mountain.

That deep, throbbing noise frightened Dan as none of his previous experiences had done. Shivering from fear as much as from cold, he crouched down beside a huge boulder in the edge of the tangle of brush that covered the bottom of the ravine. His heart pounded wildly. He was in the clutches of an unreasoning fear that some terrible Thing had seen him, and was about to seek him out. For a moment he had to use all his will to keep himself from panic flight through the brush. The unknown is always terrible, and he had invaded the domain of a force he could not understand.

In a moment, however, he recovered himself. He would be as safe there in the jungle, he thought, as anywhere on the island. He thought of starting a fire, then realized that he had no matches, and that he would not dare to make a light if he were able. He pulled a few handfuls of dry grass to make a sort of bed, upon which he huddled up, thanking his lucky stars that the island was in semi-tropical latitudes.

His mind returned again to the riddles that confronted him: the green flash and the strange mechanism on the peak. He recalled fantastic stories he had read, of hermit scientists conducting amazing experiments in isolated parts of the world. Presently he decided that something of the kind must be on foot here.

“The green flash is a sort of a death ray,” he summed up, aloud. “And they shoot it from that bright needle. No wonder they don’t want to be bothered! Somebody may be fixing to upset civilization!

“But it’s queer that the needle points at Mars...”

Of this last fact, which might have been a clue to the most reasonable solution of the mystery, if a rather astounding one, he was able to make nothing. In fact, huddled up on his pile of grass in some degree of comfort, he presently went to sleep, still pondering in vain upon this last clue.

He was awakened by a soft, insistent purring sound, rather like that of a small electric motor run without load at very high speed. Recollection of the night’s events came abruptly to him, and he sprang to his feet in alarm, finding his muscles sore and stiff from his cramped position.

From one side Dan heard the rumble of thunder, and, glancing up, saw that the sky above the sea was overcast with a rolling mass of dark, menacing clouds. There was a strange portentous blackness about these storm clouds that filled him with a nameless fear.

Suddenly he was struck with the thought that it was not thunder that had wakened him. The noise he had heard had not the rumbling or booming quality of thunder. As he stood there he again became conscious of the low, whirring sound, behind him. He whirled around to face it. The shock of what he saw left him momentarily dizzy and trembling--though undoubtedly his surroundings had much to do with its effect upon him.

The sound came from a glistening metal machine which stood half-hidden in the brush a dozen yards away looking at him!

The thing was made of a lustrous, silvery metal, which Dan afterwards supposed to be aluminum, or some alloy of that metal. Its gleaming case was shaped more like a coffin, or an Egyptian mummy-case, than any other object with which he was familiar, though rather larger than either.

That is, it was an oblong metal box, tapering toward the ends, with the greatest width forward of the middle. Twin tubes projected from the end of it, lenses in them glistening like eyes. Just below them sprang out steely, glistening tentacles several feet long, writhing and twitching as if they were alive. The tangle of green brush hid the thing’s legs, so that Dan could not see them.

Suddenly it sprang toward him, rising ten feet high and covering half the distance between them. It alighted easily upon the two long, jointed metal limbs upon which it had leapt, and continued to keep the lens-tubes turned toward Dan, so he knew that the grotesque metal thing was watching him.

The limbs, he observed, were similar to the hind legs of a grasshopper, both in shape and position. And evidently the thing leapt upon them in about the same way. Then he noticed another curious thing about it.

Three little bars of metal projected above the thickest part of its case, on the upper side. Their ends were joined by a little ring, three inches across. The tiny metal ring glowed with purple luminosity. A purple haze seemed to cling about it, as to the huge ring Dan had seen on the towers above the peak. And suspended inside this ring was a tiny metal needle, shimmering with pulsating white fire.

On the back of this metal monster was a miniature replica of the strange mechanism upon the pinnacle. The little needle pointed up the canyon. A glance that way showed Dan that it pointed at the great device upon the mountain, which looked even more brilliant on this gloomy morning than in the uncertain radiance of the moon. The colossal ring was shrouded in a splendid mantle of purple flame; and the long, slender needle, which seemed to have swung on down to follow Mars below the horizon, still throbbed with scintillating white fire.

For several minutes the two stood there, studying each other. A naked man, tense and bewildered in the presence of mysterious forces--and a grotesque machine, cased in gleaming white metal, whose parts seemed to duplicate most of the functions of a living creature.

Then one of the writhing tentacles that shot from the “head” of the machine reached back under the metal case, and reappeared grasping what appeared to be a flat disk of emerald, two inches across and half an inch thick.

This green disk it held up, with a flat side toward Dan. There was no sound, but a flash of green light came from it, cutting a wide swath into the jungle, and littering its path with smoking and flaming debris.

But Dan, expecting something of the kind, had flung himself sidewise into the shelter of the boulder beside which he had slept. Behind it, he gathered his feet under him, picked up a rock of convenient size for throwing, and waited, ready and alert.

He heard the soft humming sound on the other side of the boulder. A glittering object flashed above him. Crashing through the brush the metal monster came to earth on the same side of the boulder with him.

But the metal thing had not turned in its flight: consequently its rear end was toward Dan. As it began cumberously to turn about, he hurled his rock with an accuracy that came of a boyhood on the farm. Instinct had made him try for the little ring and needle on the back of the monster, apparently its most vulnerable part.

Whether by luck or skill, the rock struck the gleaming ring, crushing it against the needle--and instant paralysis overtook the metal thing. Its tentacles and limbs became fixed and rigid, and it toppled over in the brush.

Dan walked over to it, and examined it briefly. The green disk had fallen on the ground, and he picked it up. It was made of emerald crystal, it had a little knob of glistening metal set in one side. Rather afraid of it, Dan forebore to twist the knob. But he still clutched it in his hand a few moments later, when, partly for fear that others of its kind would come to succor the fallen monster, and partly to secure shelter from the threatening rain, he retired into the shadows of the tangled jungle.

He spent perhaps half an hour in creeping back to what he supposed a place of comparative safety. For some time he lay there in the cool gloom, brushing occasional insects off his bare skin, wishing by turns that he had a cup of coffee and a good beefsteak, and that he could puzzle out a logical solution of all the astounding things he had met in the island. After the encounter with the metal monster, he felt his theory of the hermit scientists a bit inadequate.

Presently his attention was attracted by the unmistakable mew of a kitten. Then he heard the padding sound of cautious human footsteps, and a clear feminine voice calling “Kitty, kitty,” in low tones. The steps and the voice seemed coming toward him; since there was no sound of crackling brush, he supposed there was a trail which he had not found.

“Hello,” he ventured, when the voice seemed only a few yards away through the green tangle.

At the same instant a gray kitten appeared out of the underbrush, and frisked trustfully across to him. He put out a hand, caressed it, picked it up. In a moment the feminine voice replied, “Hello yourself. Who are you?”

A crackling sound came from the brush, as if the speaker were starting toward him. Dan, abruptly conscious of his lack of attire, said quickly, “Wait a minute! I haven’t anything on, you see. I’m Dan McNally. I owned the schooner that something happened to off the island last night.”

A delicious, trilling laugh greeted the panic of his first words. Then the clear, sweet voice, serious again, replied, “So you swam ashore from the boat I signaled?”

“Yes.”

“Gee, but I’m glad to find you! And you say you haven’t any clothes? I wonder what...” The voice paused reflectively, then resumed, “Here’s a sheet that I got to signal with in the daytime, if I had a chance. You might wrap it around you until we find something better.”

The low, liquid laugh rang out again; again there was a rustling in the brush, and presently an arm appeared, holding a rolled-up sheet.

“All right,” he called. “Throw it this way.”

In a moment, with the sheet draped around him like a Roman toga, and the kitten on his arm, he advanced to meet the owner of the beautiful voice.

At the trail he met a trim, active-looking young woman, clad in out-of-door attire and with a canvas knapsack on her back. Bareheaded, she wore her brown hair closely shingled. Her face, Dan recognized from the photograph he had seen five years before, though it was more lovely than the splotched newspaper picture had hinted. Her brown eyes were filled with laughter at his predicament and his present unusual garb.

He bowed with mock gravity and said, “How do you do, Miss Helen Hunter?”

Brown eyes widened in surprise. “You know me?” she asked.

“Not half so well as I hope to,” he grinned.

Then, handing her the kitten, he spoke seriously. “What about this island? The green flashes? The big machine on the mountain? The metal thing that jumps about like a grasshopper? What’s it all about? You know anything about it?”

“Yes, I know a good deal about it,” she told him soberly. “It’s rather a terrible story. And one you may not believe--no, you’ve seen them! But the kitten is hungry, and you must be, too, if you swam ashore.”

“Well, yes, I am.” Dan admitted.

The storm clouds were drifting out to sea; the sun was beginning to assert itself, and it now lighted up the scene with a cheerful brightness. She slung off her pack and sat down cross-legged at the side of the trail. Dan sat down opposite her as she opened the knapsack and produced a can of condensed milk, one of sardines, a can-opener, and half a loaf of bread.

“I had to select my supplies rather at random,” she said, “and you’ll have to make the best of them.”

She started to open the sardines. “You’d better give it to me,” Dan advised. “You might cut your hand.”

“You think so?” she asked, deftly lifting the lid, fishing out a fish for the kitten, and presenting the can to Dan. Then with capable hands she broke off a large chunk of bread, which she handed him.

“Go ahead and finish this up,” she said. “I’ve already had breakfast.” She punched two holes in the end of the milk can, and poured some of the thick yellow fluid into the palm of her left hand, from which she let the kitten lap it.