"Guy Fawkes himself would shudder in that mill. Think of it—five explosions on five successive days, and not a clue!"

Our visitor had presented a card bearing the name of Donald MacLeod, chief of the Nitropolis Powder Company's Secret Service. It was plain that he was greatly worried over the case about which he had at last been forced to consult Kennedy.

As he spoke, I remembered having read in the despatches about the explosions, but the accounts had been so meager that I had not realized that there was anything especially unusual about them, for it was at the time when accidents in and attacks on the munitions-plants were of common occurrence.

"Why," went on MacLeod, "the whole business is as mysterious as if there were some phantom destroyer at work! The men are so frightened that they threaten to quit. Several have been killed. There's something strange about that, too. There are ugly rumors of poisonous gases being responsible, quite as much as the explosions, though, so far, I've been able to find nothing in that notion."

"What sort of place is it?" asked Kennedy, interested at once.

"Well, you see," explained MacLeod, "since the company's business has
increased so fast lately, it has been forced to erect a new plant.
Perhaps you have heard of the Old Grove Amusement Park, which failed?
It's not far from that."

MacLeod looked at us inquiringly, and Kennedy nodded to go on, though I am sure neither of us was familiar with the place.

"They've called the new plant Nitropolis—rather a neat name for a powder-works, don't you think?" resumed MacLeod. "Everything went along all right until a few days ago. Then one of the buildings, a storehouse, was blown up. We couldn't be sure that it was an accident, so we redoubled our precautions. It was of no use. That started it. The very next day another building was blown up, then another, until now there have been five of them. What may happen to-day Heaven only knows! I want to get back as soon as I can."

"Rather too frequent, I must admit, to be coincidences," remarked
Kennedy.

"No; they can't all be accidents," asserted MacLeod, confidently. "There's too great regularity for that. I think I've considered almost everything. I don't see how they can be from bombs placed by workmen. At least, it's not a bit likely. Besides, the explosions all occur in broad daylight, not at night. We're very careful about the men we employ, and they're watched all the time. The company has a guard of its own, twenty-five picked men, under me—all honorably discharged United States army men."

"You have formed no theory of your own?" queried Kennedy.

MacLeod paused, then drew from his pocket the clipping of a despatch from the front in which one of the war correspondents reported the destruction of wire entanglements with heat supposed to have been applied by the use of reflecting mirrors.

"I'm reduced to pure speculation," he remarked. "To-day they seem to be reviving all the ancient practices. Maybe some one is going at it like Archimedes."

"Not impossible," returned Craig, handing back the clipping. "Buffon tested the probability of the achievement of Archimedes in setting fire to the ships of Marcellus with mirrors and the sun's rays. He constructed a composite mirror of a hundred and twenty-eight plane mirrors, and with it he was able to ignite wood at two hundred and ten feet. However, I shrewdly suspect that, even if this story is true, they are using hydrogen or acetylene flares over there. But none of these things would be feasible in your case. You'd know it."

"Could it be some one who is projecting a deadly wireless force which causes the explosions?" I put in, mindful of a previous case of Kennedy's. "We all know that inventors have been working for years on the idea of making explosives obsolete and guns junk. If some one has hit on a way of guiding an electric wave through the air and concentrating power at a point, munitions-plants could be wiped out."

MacLeod looked anxiously from me to Kennedy, but Craig betrayed nothing by his face except his interest.

"Sometimes I have imagined I heard a peculiar, faint, whirring noise in the air," he remarked, thoughtfully. "I thought of having the men on the watch for air-ships, but they've never seen a trace of one. It might be some power either like this," he added, shaking the clipping, "or like that which Mr. Jameson suggests."

"It's something like that you meant, I presume, when you called it a 'phantom destroyer' a moment ago?" asked Kennedy.

MacLeod nodded.

"If you're interested," he pursued, hastily, "and feel like going down there to look things over, I think the best place for you to go would be to the Sneddens'. They're some people who have seen a chance to make a little money out of the boom. Many visitors are now coming and going on business connected with the new works. They have started a boarding-house—or, rather, Mrs. Snedden has. There's a daughter, too, who seems to be very popular." Kennedy glanced whimsically at me.

"Well, Walter," he remarked, tentatively, "entirely aside from the young lady, this ought to make a good story for the Star."

"Indeed it ought!" I replied, enthusiastically.

"Then you'll go down to Nitropolis?" queried MacLeod, eagerly. "You can catch a train that will get you there about noon. And the company will pay you well."

"MacLeod, with the mystery, Miss Snedden, and the remuneration, you are irresistible," smiled Kennedy.

"Thank you," returned the detective. "You won't regret it. I can't tell you how much relieved I feel to have some one else, and, above all, yourself, on the case. You can get a train in half an hour. I think it would be best for you to go as though you had no connection with me—at least for the present."

Kennedy agreed, and MacLeod excused himself, promising to be on the train, although not to ride with us, in case we should be the target of too inquisitive eyes.

For a few moments, while our taxicab was coming, Kennedy considered thoughtfully what the company detective had said. By the time the vehicle arrived he had hurriedly packed up some apparatus in two large grips, one of which it fell to my lot to carry.

The trip down to Nitropolis was uninteresting, and we arrived at the little station shortly after noon. MacLeod was on the train, but did not speak to us, and it was perhaps just as well, for the cabmen and others hanging about the station were keenly watching new arrivals, and any one with MacLeod must have attracted attention. We selected or were, rather, selected by one of the cabmen and driven immediately to the Snedden house. Our cover was, as Craig and I had decided, to pose as two newspaper men from New York, that being the easiest way to account for any undue interest we might show in things.

The powder-company's plant was situated on a large tract of land which was surrounded by a barbed-wire fence, six feet high and constructed in a manner very similar to the fences used in protecting prison-camps in war-times. At various places along the several miles of fence gates were placed, with armed guards. Many other features were suggestive of war-times. One that impressed us most was that each workman had to carry a pass similar, almost, to a passport. This entire fence, we learned, was patrolled day and night by armed guards.

A mile or so from the plant, or just outside the main gate, quite a settlement had grown up, like a mushroom, almost overnight—the product of a flood of new money. Originally, there had been only one house for some distance about—that of the Sneddens. But now there were scores of houses, mostly those of officials and managers, some of them really pretentious affairs. MacLeod himself lived in one of them, and we could see him ahead of us, being driven home.

The workmen lived farther along the line, in a sort of company town, which at present greatly resembled a Western mining-camp, though ultimately it was to be a bungalow town.

Just at present, however, it was the Snedden house that interested us most, for we felt the need of getting ourselves established in this strange community. It was an old-fashioned farm-house and had been purchased very cheaply by Snedden several years before. He had altered it and brought it up to date, and the combination of old and new proved to be typical of the owner as well as of the house.

Kennedy carried off well the critical situation of our introduction, and we found ourselves welcomed rather than scrutinized as intruders.

Garfield Snedden was much older than his second wife, Ida. In fact, she did not seem to be much older than Snedden's daughter Gertrude, whom MacLeod had already mentioned—a dashing young lady, never intended by nature to vegetate in the rural seclusion that her father had sought before the advent of the powder-works. Mrs. Snedden was one of those capable women who can manage a man without his knowing it. Indeed, one felt that Snedden, who was somewhat of both student and dreamer, needed a manager.

"I'm glad your train was on time," bustled Mrs. Snedden. "Luncheon will be ready in a few moments now."

We had barely time to look about before Gertrude led us into the dining-room and introduced us to the other boarders.

Knowing human nature, Kennedy was careful to be struck with admiration and amazement at everything we had seen in our brief whirl through Nitropolis. It was not a difficult or entirely assumed feeling, either, when one realized that, only a few short months before, the region had been nothing better than an almost hopeless wilderness of scrub-pines.

We did not have to wait long before the subject uppermost in our minds was brought up—the explosions.

Among the boarders there were at least two who, from the start, promised to be interesting as well as important. One was a tall, slender chap named Garretson, whose connection with the company, I gathered from the conversation, took him often on important matters to New York. The other was an older man, Jackson, who seemed to be connected with the management of the works, a reticent fellow, more given to listening to others than to talking himself.

"Nothing has happened so far to-day, anyhow," remarked Garretson, tapping the back of his chair with his knuckle, as a token of respect for that evil spirit who seems to be exorcised by knocking wood.

"Oh," exclaimed Gertrude, with a little half-suppressed shudder, "I do hope those terrible explosions are at last over!"

"If I had my way," asserted Garretson, savagely, "I'd put this town under martial law until they WERE over."

"It may come to that," put in Jackson, quietly.

"Quite in keeping with the present tendency of the age," agreed
Snedden, in a tone of philosophical disagreement.

"I don't think it makes much difference how you accomplish the result, Garfield," chimed in his wife, "as long as you accomplish it, and it is one that should be accomplished."

Snedden retreated into the refuge of silence. Though this was only a bit of the conversation, we soon found out that he was an avowed pacifist. Garretson, on the other hand, was an ardent militarist, a good deal of a fire-eater. I wondered whether there might not be a good deal of the poseur about him, too.

It needed no second sight to discover that both he and Gertrude were deeply interested in each other. Garretson was what Broadway would call "a live one," and, though there is nothing essentially wrong in that, I fancied that I detected, now and then, an almost maternal solicitude on the part of her stepmother, who seemed to be watching both the young man and her husband alternately. Once Jackson and Mrs. Snedden exchanged glances. There seemed to be some understanding between them.

The time to return to the works was approaching, and we all rose. Somehow, Gertrude and Garretson seemed naturally to gravitate toward the door together.

Some distance from the house there was a large barn. Part of it had been turned into a garage, where Garretson kept a fast car. Jackson, also, had a roadster. In fact, in this new community, with its superabundant new wealth, everybody had a car.

Kennedy and I sauntered out after the rest. As we turned an angle of the house we came suddenly upon Garretson in his racer, talking to Gertrude. The crunch of the gravel under our feet warned them before we saw them, but not before we could catch a glimpse of a warning finger on the rosy lips of Gertrude. As she saw us she blushed ever so slightly.

"You'll be late!" she cried, hastily. "Mr. Jackson has been gone five minutes."

"On foot," returned Garretson, nonchalantly. "I'll overtake him in thirty seconds." Nevertheless, he did not wait longer, but swung up the road at a pace which was the admiration of all speed-loving Nitropolitans.

Craig had ordered our taxicab driver to stop for us after lunch, and, without exciting suspicion, managed to stow away the larger part of the contents of our grips in his car.

Still without openly showing our connection with MacLeod, Kennedy sought out the manager of the works, and, though scores of correspondents and reporters from various newspapers had vainly applied for permission to inspect the plant, somehow we seemed to receive the freedom of the place and without exciting suspicion. Craig's first move was to look the plant over. As we approached it our attention was instantly attracted to the numerous one-story galvanized-iron buildings that appeared to stretch endlessly in every direction. They seemed to be of a temporary nature, though the power-plants, offices, and other necessary buildings were very substantially built. The framework of the factory-buildings was nothing but wood, covered by iron sheathing, and even the sides seemed to be removable. The floors, however, were of concrete.

"They serve their purpose well," observed Kennedy, as we picked our way about. "Explosions at powder-mills are frequent, anyhow. After an explosion there is very little debris to clear away, as you may imagine. These buildings are easily repaired or replaced, and they keep a large force of men for these purposes, as well as materials for any emergency."

One felt instinctively the hazard of the employment. Everywhere were signs telling what not and what to do. One that stuck in my mind was, "It is better to be careful than sorry." Throughout the plant at frequent intervals were first-aid stations with kits for all sorts of accidents, including respirators, for workmen were often overcome by ether or alcohol fumes. Everything was done to minimize the hazard, yet one could not escape the conviction that human life and limb were as much a cost of production in this industry as fuel and raw material.

Once, in our wanderings about the plant, I recall we ran across both Garretson and Jackson in one of the offices. They did not see us, but seemed to be talking very earnestly about something. What it was we could not guess, but this time it seemed to be Jackson who was doing most of the talking. Kennedy watched them as they parted.

"There's something peculiar under the surface with those people at the boarding-house," was all he observed. "Come; over there, about an eighth of a mile, I think I see evidences of the latest of the explosions. Let's look at it."

MacLeod had evidently reasoned that, sooner of later, Kennedy would appear in this part of the grounds, and as we passed one of the shops he joined us.

"You mentioned something about rumors of poisonous gases," hinted
Craig, as we walked along.

"Yes," assented MacLeod; "I don't know what there is in it. I suppose you know that there is a very poisonous gas, carbon monoxide, or carbonic oxide, formed in considerable quantity by the explosion of several of the powders commonly used in shells. The gas has the curious power of combining with the blood and refusing to let go, thus keeping out the oxygen necessary for life. It may be that that is what accounts for what we've seen—that it is actual poisoning to death of men not killed by the immediate explosion."

We had reached the scene of the previous day's disaster. No effort had yet been made to clear it up. Kennedy went over it carefully. What it was he found I do not know, but he had not spent much time before he turned to me.

"Walter," he directed, "I wish you would go back to the office near the gate, where I left that paraphernalia we brought down. Carry it over—let me see—there's an open space there on that knoll. I'll join you there."

Whatever was in the packages was both bulky and heavy, and I was glad to reach the hillside he had indicated.

Craig was waiting for me there with MacLeod, and at once opened the packages. From them he took a thin steel rod, which he set up in the center of the open space. To it he attached a frame and to the frame what looked like four reversed megaphones. Attached to the frame, which was tubular, was an oak box with a little arrangement of hard rubber and metal which fitted into the ears. For some time Kennedy's face wore a set, far-away expression, as if he were studying something.

"The explosions seem always to occur in the middle of the afternoon," observed MacLeod, fidgeting apprehensively.

Kennedy motioned petulantly for silence. Then suddenly he pulled the tubes out of his ears and gazed about sharply.

"There's something in the air!" he cried. "I can hear it!"

MacLeod and I strained our eyes. There was nothing visible.

"This is an anti-aircraft listening-post, such as the French use," explained Craig, hurriedly. "Between the horns and the microphone in the box you can catch the hum of an engine, even when it is muffled. If there's an aeroplane or a Zeppelin about, this thing would locate it."

Still, there was nothing that we could see, though now the sound was just perceptible to the ear if one strained his attention a bit. I listened. It was plain in the detector; yet nothing was visible. What strange power could it be that we could not see or feel in broad daylight?

Just then came a low rumbling, and then a terrific roar from the direction of the plant. We swung about in time to see a huge cloud of debris lifted literally into the air above the tree-tops and dropped to earth again. The silence that succeeded the explosion was eloquent. The phantom destroyer had delivered his blow again.

"The distillery—where we make the denatured alcohol!" cried MacLeod, gazing with tense face as from other buildings, we could see men pouring forth, panic-stricken, and the silence was punctured by shouts. Kennedy bent over his detector.

"That same mysterious buzzing," he muttered, "only fainter."

Together we hastened now toward the distillery, another of those corrugated-iron buildings. It had been completely demolished. Here and there lay a dark, still mass. I shuddered. They were men!

As we ran toward the ruin we crossed a baseball-field which the company had given the men. I looked back for Kennedy. He had paused at the wire backstop behind the catcher. Something caught in the wires interested him. By the time I reached him he had secured it—a long, slender metal tube, cleverly weighted so as to fall straight.

"Not a hundred per cent. of hits, evidently," he muttered. "Still, one was enough."

"What is it?" asked MacLeod.

"An incendiary pastille. On contact, the nose burns away anything it hits, goes right through corrugated iron. It carries a charge of thermit ignited by this piece of magnesium ribbon. You know what thermit will penetrate with its thousands of degrees of heat. Only the nose of this went through the netting and never touched a thing. This didn't explode anything, but another one did. Thousands of gallons of alcohol did the rest."

Kennedy had picked up his other package as we ran, and was now busily unwrapping it. I looked about at the crowd that had collected, and saw that there was nothing we could do to help. Once I caught sight of Gertrude's face. She was pale, and seemed eagerly searching for some one. Then, in the crowd, I lost her. I turned to MacLeod. He was plainly overwhelmed. Kennedy was grimly silent and at work on something he had jammed into the ground.

"Stand back!" he cautioned, as he touched a match to the thing. With a muffled explosion, something whizzed and shrieked up into the air like a sky-rocket.

Far above, I could now see a thing open out like a parachute, while below it trailed something that might have been the stick of the rocket. Eagerly Kennedy followed the parachute as the wind wafted it along and it sank slowly to the earth. When, at last, he recovered it I saw that between the parachute and the stick was fastened a small, peculiar camera.

"A Scheimpflug multiple camera," he explained as he seized it almost ravenously. "Is there a place in town where I can get the films in this developed quickly?"

MacLeod, himself excited now, hurried us from the scene of the explosion to a local drug-store, which combined most of the functions of a general store, even being able to improvise a dark-room in which Kennedy could work.

It was some time after the excitement over the explosion had quieted down that MacLeod and I, standing impatiently before the drug-store, saw Snedden wildly tearing down the street in his car. He saw us and pulled up at the curb with a jerk.

"Where's Gertrude?" he shouted, wildly. "Has any one seen my daughter?"

Breathlessly he explained that he had been out, had returned to find his house deserted, Gertrude gone, his wife gone, even Jackson's car gone from the barn. He had been to the works. Neither Garretson nor Jackson had been seen since the excitement of the explosion, they told him. Garretson's racer was gone, too. There seemed to have been a sort of family explosion, also.

Kennedy had heard the loud talking and had left his work to the druggist to carry on and joined us. There was no concealment now of our connection with MacLeod, for it was to him that every one in town came when in trouble.

In almost no time, so accurately did he keep his fingers on the fevered pulse of Nitropolis, MacLeod had found out that Gertrude had been seen driving away from the company's grounds with some one in Garretson's car, probably Garretson himself. Jackson had been seen hurrying down the street. Some one else had seen Ida Snedden in Jackson's car, alone.

Meanwhile, over the wire, MacLeod had sent out descriptions of the four people and the two cars, in the hope of intercepting them before they could be plunged into the obscurity of any near-by city. Not content with that, MacLeod and Kennedy started out in the former's car, while I climbed in with Snedden, and we began a systematic search of the roads out of Nitropolis.

As we sped along, I could not help feeling, though I said nothing, that, somehow, the strange disappearances must have something to do with the mysterious phantom destroyer. I did not tell even Snedden about the little that Kennedy had discovered, for I had learned that it was best to let Craig himself tell, at his own time and in his own way. But the man seemed frantic in his search, and I could not help the impression that there was something, perhaps only a suspicion, that he knew which might shed some light.

We were coming down the river, or, rather, the bay, after a fruitless search of unfrequented roads and were approaching the deserted Old Grove Amusement Park, to which excursions used, years ago, to come in boats. No one could make it pay, and it was closed and going to ruin. There had been some hint that Garretson's racer might have disappeared down this unfrequented river road.

As we came to a turn in the road, we could see Kennedy and MacLeod in their car, coming up. Instead of keeping on, however, they turned into the grove, Kennedy leaning far over the running-board as MacLeod drove slowly, following his directions, as though Craig were tracing something.

With a hurried exclamation of surprise, Snedden gave our car the gas and shot ahead, swinging around after them. They were headed, following some kind of tire-tracks, toward an old merry-go-round that was dismantled and all boarded up. They heard us coming and stopped.

"Has any one told you that Garretson's car went down the river road, too?" called Snedden, anxiously.

"No; but some one thought he saw Jackson's car come down here," called back MacLeod.

"Jackson's?" exclaimed Snedden.

"Maybe both are right," I ventured, as we came closer. "What made you turn in here?'"

"Kennedy thought he saw fresh tire-tracks running into the grove."

We were all out of our cars by this time, and examining the soft roadway with Craig. It was evident to any one that a car had been run in, and not so very long ago, in the direction of the merry-go-round.

We followed the tracks on foot, bending about the huge circle of a building until we came to the side away from the road. The tracks seemed to run right in under the boards.

Kennedy approached and touched the boards. They were loose. Some one had evidently been there, had taken them down, and put them up. In fact, by the marks on them, it seemed as though he had made a practice of doing so.

MacLeod and Kennedy unhooked the boarding, while Snedden looked on in a sort of daze. They had taken down only two or three sections, which indicated that that whole side might similarly be removed, when I heard a low, startled exclamation from Snedden.

We peered in. There, in the half-light of the gloomy interior, we could see a car. Before we knew it Snedden had darted past us. An instant later I distinguished what his more sensitive eye had seen—a woman, all alone in the car, motionless.

"Ida!" he cried.

There was no answer.

"She—she's dead!" he shouted.

It was only too true. There was Ida Snedden, seated in Jackson's car in the old deserted building, all shut up—dead.

Yet her face was as pink as if she were alive and the blood had been whipped into her cheeks by a walk in the cold wind.

We looked at one another, at a loss. How did she get there—and why? She must have come there voluntarily. No one had seen any one else with her in the car.

Snedden was now almost beside himself.

"Misfortunes never come singly," he wailed. "My daughter Gertrude gone—now my wife dead. Confound that young fellow Garretson—and Jackson, too! Where are they? Why have they fled? The scoundrels—they have stolen my whole family. Oh, what shall I do? what shall I do?"

Trying to quiet Snedden, at the same time we began to look about the building. On one side was a small stove, in which were still the dying coals of a fire. Near by were a work-bench, some tools, pieces of wire, and other material. Scattered about were pieces of material that looked like celluloid. Some one evidently used the place as a secret workshop. Kennedy picked up a piece of the celluloid-like stuff and carefully touched a match to it. It did not burn rapidly as celluloid does, and Craig seemed more than ever interested. MacLeod himself was no mean detective. Accustomed to action, he had an idea of what to do.

"Wait here!" he called back, dashing out. "I'm going to the nearest house up the road for help. I'll be back in a moment."

We heard him back and turn his car and shoot away. Meanwhile, Kennedy was looking over carefully Jackson's roadster. He tapped the gas-tank in the rear, then opened it. There was not a drop of gas in it. He lifted up the hood and looked inside at the motor. Whatever he saw there, he said nothing. Finally, by siphoning some gas from Snedden's tank and making some adjustments, he seemed to have the car in a condition again for it to run. He was just about to start it when MacLeod returned, carrying a canary-bird in a cage.

"I've telephoned to town," he announced. "Some one will be here soon now. Meanwhile, an idea occurred to me, and I borrowed this bird. Let me see whether the idea is any good."

Kennedy, by this time, had started the engine. MacLeod placed the bright little songster near the stove on the work-bench and began to watch it narrowly.

More than ever up in the air over the mystery, I could only watch
Kennedy and MacLeod, each following his own lines.

It might, perhaps, have been ten minutes after MacLeod returned, and during that time he had never taken his eyes off the bird, when I began to feel a little drowsy. A word from MacLeod roused me.

"There's carbon monoxide in the air, Kennedy!" he exclaimed. "You know how this gas affects birds."

Kennedy looked over intently. The canary had begun to show evident signs of distress over something.

"It must be that this stove is defective," pursued MacLeod, picking up the poor little bird and carrying it quickly into the fresh air, where it could regain its former liveliness. Then, when he returned, he added, "There must be some defect in the stove or the draught that makes it send out the poisonous gas."

"There's some gas," agreed Kennedy. "It must have cleared away mostly, though, or we couldn't stand it ourselves."

Craig continued to look about the car and the building, in the vain hope of discovering some other clue. Had Mrs. Snedden been killed by the carbonic oxide? Was it a case of gas poisoning? Then, too, why had she been here at all? Who had shut her up? Had she been overcome first and, in a stupor, been unable to move to save herself? Above all, what had this to do with the mysterious phantom slayer that had wrecked so much of the works in less than a week?

It was quite late in the afternoon when, at last, people came from the town and took away both the body of Mrs. Snedden and Jackson's car. Snedden could only stare and work his fingers, and after we had seen him safely in the care of some one we could trust Kennedy, MacLeod, and I climbed into MacLeod's car silently.

"It's too deep for me," acknowledged MacLeod. "What shall we do next?"

"Surely that fellow must have my pictures developed by this time," considered Kennedy. "Shoot back there."

"They came out beautifully—all except one," reported the druggist, who was somewhat of a camera fiend himself. "That's a wonderful system, sir."

Kennedy thanked him for his trouble and took the prints. With care he pieced them together, until he had several successive panoramas of the country taken from various elevations of the parachute. Then, with a magnifying-glass, he went over each section minutely.

"Look at that!" he pointed out at last with the sharp tip of a pencil on one picture.

In what looked like an open space among some trees was a tiny figure of a man. It seemed as if he were hacking at something with an ax. What the something was did not appear in the picture.

"I should say that it was half a mile, perhaps a mile, farther away than that grove," commented Kennedy, making a rough calculation.

"On the old Davis farm," considered MacLeod. "Look and see if you can't make out the ruins of a house somewhere near-by. It was burned many years ago."

"Yes, yes," returned Kennedy, excitedly; "there's the place! Do you think we can get there in a car before it's dark?"

"Easily," replied MacLeod.

It was only a matter of minutes before we three were poking about in a tangle of wood and field, seeking to locate the spot where Kennedy's apparatus had photographed the lone axman.

At last, in a large, cleared field, we came upon a most peculiar heap of debris. As nearly as I could make out, it was a pile of junk, but most interesting junk. Practically all of it consisted in broken bits of the celluloid-like stuff we had seen in the abandoned building. Twisted inextricably about were steel wires and bits of all sorts of material. In the midst of the wreckage was something that looked for all the world like the remains of a gas-motor. It was not rusted, either, which indicated that it had been put there recently.

As he looked at it, Craig's face displayed a smile of satisfaction.

"Looks as though it might have been an aeroplane of the tractor type," he vouchsafed, finally.

"Surely there couldn't have been an accident," objected MacLeod. "No aviator could have lived through it, and there's no body."

"No; it was purposely destroyed," continued Craig. "It was landed here from somewhere else for that purpose. That was what the man in the picture was doing with the ax. After the last explosion something happened. He brought the machine here to destroy the evidence."

"But," persisted MacLeod, "if there had been an aeroplane hovering about we should have seen it in the air, passing over the works at the time of the explosion."

Kennedy picked the pieces, significantly.

"Some one about here has kept abreast of the times, if not ahead. See; the planes were of this non-inflammable celluloid that made it virtually transparent and visible only at a few hundred feet in the air. The aviator could fly low and so drop those pastilles accurately—and unseen. The engine had one of those new muffler-boxes. He would have been unheard, too, except for that delicate air-ship detector."

MacLeod and I could but stare at each other, aghast. Without a doubt it was in the old merry-go-round building that the phantom aviator had established his hangar. What the connection was between the tragedy in the Snedden family and the tragedy in the powder-works we did not know, but, at least, now we knew that there was some connection.

It was growing dark rapidly, and, with some difficulty, we retraced our steps to the point where we had left the car. We whirled back to the town, and, of course, to the Snedden house.

Snedden was sitting in the parlor when we arrived, by the body of his wife, staring, speechless, straight before him, while several neighbors were gathered about, trying to console him. We had scarcely entered when a messenger-boy came up the path from the gate. Both Kennedy and MacLeod turned toward him, expecting some reply to the numerous messages of alarm sent out earlier in the afternoon.

"Telegram for Mrs. Snedden," announced the boy.

"MRS. Snedden?" queried Kennedy, surprised, then quickly: "Oh yes, that's all right. I'll take care of it."

He signed for the message, tore it open, and read it. For a moment his face, which had been clouded, smoothed out, and he took a couple of turns up and down the hall, as though undecided. Finally he crumpled the telegram abstractedly and shoved it into his pocket. We followed him as he went into the parlor and stood for several moments, looking fixedly on the strangely flushed face of Mrs. Snedden. "MacLeod," he said, finally, turning gravely toward us, and, for the present, seeming to ignore the presence of the others, "this amazing series of crimes has brought home to me forcibly the alarming possibilities of applying modern scientific devices to criminal uses. New modes and processes seem to bring new menaces."

"Like carbon-monoxide poisoning?" suggested MacLeod. "Of course it has long been known as a harmful gas, but—"

"Let us see," interrupted Kennedy. "Walter, you were there when I examined Jackson's car. There was not a drop of gasolene in the tank, you will recall. Even the water in the radiator was low. I lifted the hood. Some one must have tampered with the carburetor. It was adjusted so that the amount of air in the mixture was reduced. More than that, I don't know whether you noticed it or not, but the spark and gas were set so that, when I did put gasolene in the tank, I had but to turn the engine over and it went. In other words, that car had been standing there, the engine running, until it simply stopped for want of fuel." He paused while we listened intently, then resumed. "The gas-engine and gas-motor have brought with them another of those unanticipated menaces of which I spoke. Whenever the explosion of the combustible mixture is incomplete or of moderated intensity a gas of which little is known may be formed in considerable quantities.

"In this case, as in several others that have come to my attention, vapors arising from the combustion must have emitted certain noxious products. The fumes that caused Ida Snedden's death were not of carbon monoxide from the stove, MacLeod. They were splitting-products of gasolene, which are so new to science that they have not yet been named.

"Mrs. Snedden's death, I may say for the benefit of the coroner, was due to the absorption of some of these unidentified gaseous poisons. They are as deadly as a knife-thrust through the heart, under certain conditions. Due to the non-oxidation of some of the elements of gasolene, they escape from the exhaust of every running gas-engine. In the open air, where only a whiff or two would be inhale now and then, they are not dangerous. But in a closed room they may kill in an incredibly short time. In fact, the condition has given rise to an entirely new phenomenon which some one has named 'petromortis.'"

"Petromortis?" repeated Snedden, who, for the first time, began to show interest in what was going on about him. "Then it was an accident?"

"I did not say it was an accident," corrected Craig. "There is an old adage that murder will out. And this expression of human experience is only repeated in what we modern scientific detectives are doing. No man bent on the commission of a crime can so arrange the circumstances of that crime that it will afterward appear, point by point, as an accident."

Kennedy had us all following him breathlessly now.

"I do not consider it an accident," he went on, rapidly piecing together the facts as we had found them. "Ida Snedden was killed because she was getting too close to some one's secret. Even at luncheon, I could see that she had discovered Gertrude's attachment for Garretson. How she heard that, following the excitement of the explosion this afternoon, Gertrude and Garretson had disappeared, I do not pretend to know. But it is evident that she did hear, that she went out and took Jackson's car, probably to pursue them. If we have heard that they went by the river road, she might have heard it, too.

"In all probability she came along just in time to surprise some one working on the other side of the old merry-go-round structure. There can be no reason to conceal the fact longer. From that deserted building some one was daily launching a newly designed invisible aeroplane. As Mrs. Snedden came along, she must have been just in time to see that person at his secret hangar. What happened I do not know, except that she must have run the car off the river road and into the building. The person whom she found must have suddenly conceived a method of getting her out of the way and making it look like an accident of some kind, perhaps persuaded her to stay in the car with the engine running, while he went off and destroyed the aeroplane which was damning evidence now."

Startling as was the revelation of an actual phantom destroyer, our minds were more aroused as to who might be the criminal who had employed such an engine of death.

Kennedy drew from his pocket the telegram which had just arrived, and spread it out flat before us on a table. It was dated Philadelphia, and read:

MRS. IDA SNEDDEN, Nitropolis:

Garretson and Gertrude were married to-day. Have traced them to the
Wolcott. Try to reconcile Mr. Snedden.

HUNTER JACKSON.

I saw at once that part of the story. It was just a plain love-affair that had ended in an elopement at a convenient time. The fire-eating Garretson had been afraid of the Sneddens and Jackson, who was their friend. Before I could even think further, Kennedy had drawn out the films taken by the rocket-camera.

"With the aid of a magnifying-glass," he was saying, "I can get just enough of the lone figure in this picture to identify it. These are the crimes of a crazed pacifist, one whose mind had so long dwelt on the horrors of—"

"Look out!" shouted MacLeod, leaping in front of Kennedy.

The strain of the revelation had been too much. Snedden—a raving maniac—had reeled forward, wildly and impotently, at the man who had exposed him.