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-gla.s.s, 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 s.p.a.ce 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, pa.s.sing 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 m.u.f.fler-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 unantic.i.p.ated 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 quant.i.ties.

"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 circ.u.mstances 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 d.a.m.ning 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-gla.s.s," 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.