[Sidenote: In the underground caverns of the Selom, Dr. Bird once again locks wills with the subversive genius, Saranoff.]

So that's the "Port of Missing Planes," mused d.i.c.k Purdy as he looked down over the side of his c.o.c.kpit. "It looks wild and desolate all right, but at that I can't fancy a bus cracking up here and not being found p.r.o.nto. Gosh, Wilder cracked in the wildest part of Arizona and he was found in a week."

The mail plane droned monotonously on through perfect flying weather.

Purdy continued to study the ground. Recently transferred from a western run, he was getting his first glimpse of that section of ill repute. Below him stretched a desolate, almost uninhabited stretch of country. By looking back he could see Bellefonte a few miles behind him, but Philipsburg, the next spot marked on his map, was not yet visible. Twelve hundred feet below him ran a silver line of water which his map told him was Little Moshannon Run. As he watched he suddenly realized that the ground was not slipping by under him as rapidly as it should. He glanced at his air-speed meter.

"What the d.i.c.kens?" he cried in surprise. For an hour his speed had remained almost constant at one hundred miles an hour. Without apparent cause it had dropped to forty, less than flying speed. He realized that he was falling. A glance at his altimeter confirmed the impression. The needle had dropped four hundred feet and was slowly moving toward sea-level.

With an exclamation of alarm, Purdy advanced his throttle until the three motors of his plane roared at full capacity. For a moment his air-speed picked up, but the gain was only momentary. As he watched, the meter dropped to zero, although the propellers still whirled at top speed. His altimeter showed that he was gradually losing elevation.

He stood up and looked over the side of his plane. The ground below him was stationary as far as forward progress was concerned, but it was slowly rising to meet him. He fumbled at the release ring of his parachute but another glance at the ground made him hesitate. It was not more than three hundred feet below him.

"I must be dreaming!" he cried. The ground was no longer stationary.

For some unexplained reason he was going backward. The motors were still roaring at top speed. Purdy dropped back into his seat in the c.o.c.kpit. With his ailerons set for maximum lift he coaxed every possible revolution from his laboring motors. For several minutes he strained at the controls before he cast a quick glance over the side.

His backward speed had accelerated and the ground was less than fifty feet below him. It was too close for a parachute jump.

"As slow as I'm falling, I won't crack much, anyway," he consoled himself. He reached for his switch and the roar of the motors died away in silence. The plane gave a sickening lurch backwards and down for an instant. Purdy again leaned over the side. He was no longer going either forward or back but was sinking slowly down. He looked at the ground directly under him. A cry of horror came from his lips. He sat back mopping his brow. Another glance over the side brought an expression of terror to his white face and he reached for the heavy automatic pistol which hung by the side of the control seat.

"He cleared Bellefonte at nine in the morning, Dr. Bird" said Inspector Dolan of the Post Office Department, "and headed toward Philipsburg. He never arrived. By ten we were alarmed and by eleven we had planes out searching for him. They reported nothing. He must have come to grief within a rather restricted area, so we sent search parties out at once. That was two weeks ago yesterday. No trace of either him or his plane has been found."

"The flying conditions were good?"

"Perfect. Also, Purdy is above suspicion. He has been flying the mail on the western runs for three years. This is his first accident. He was carrying nothing of unusual value."

"Are there any local conditions unfavorable to flying?"

"None at all. It is much uninhabited country, but there is no reason why it shouldn't be safe country to fly over."

"There are some d.a.m.nably unfavorable local conditions, Doctor, although I can't tell you what they are," broke in Operative Carnes of the United States Secret Service. "d.i.c.k Purdy was rather more than an acquaintance of mine. After he was lost I looked into the record of that section a little. It is known among aviators as 'The Port of Missing Planes.'"

"How did it get a name like that?"

"From the number of unexplained and unexplainable accidents that happen right there. Dugan of the air mail, was lost there last May.

They found the mailbags where he had dropped them before he crashed, but they never found a trace of him or his plane."

"They didn't?"

"Not a trace. The same thing happened when Mayfield cracked in August.

He made a jump and broke his neck in landing. He was found all right, but his ship wasn't. Trierson of the army, dropped there and _his_ plane was never found. Neither was he. He was seen to go down in a forced landing. He was flying last in a formation. As soon as he went down the other ships turned back and circled over the ground where he should have fallen. They saw nothing. Search parties found no trace of either him or his ship. Those are the best known cases, but I have heard rumors of several private ships which have gone down in that district and have never been seen or heard of since."

Dr. Bird sat forward with a glitter in his piercing black eyes. Carnes gave a grunt of satisfaction. He knew the meaning of that glitter. The Doctor's interest had been fully aroused.

"Inspector Dolan," said Dr. Bird sharply, "why didn't you tell me those things?"

"Well, Doctor, we don't like to talk about mail wrecks any more than we have to. Of course, the loss of so many planes in one area is merely a coincidence. Probably the wrecked planes were stolen as souvenirs. Such things happen, you know."

"Fiddlesticks!" said Dr. Bird sharply. He raised one long slender hand with beautifully modeled tapering fingers and threw back his unruly mop of black hair. His square, almost rugged jaw, protruded and the glitter in his eyes grew in intensity. "No souvenir hunting vandals could cart away whole planes without leaving a trace. In that case, what became of the bodies? No, Inspector, this has gone beyond the range of coincidence. There is some mystery here and it needs looking into. Fortunately, my work at the Bureau of Standards is in such shape that I can safely leave it. I intend to devote my entire time to clearing this matter up. The ramifications may run deeper than either you or I suspect. Please have all of your records dealing with plane disappearances or wrecks in that locality sent to my office at once."

The Post Office inspector stiffened.

"Of course, Dr. Bird," he said formally, "we are very glad to hear any suggestion that you may care to offer. When it comes, however, to a matter of surrendering control of a Post Office matter to the Department of Commerce or to the Treasury Department, I doubt the propriety. Our records are confidential ones and are not open to everyone who is curious. I will inform the proper authorities of your desire to help, but I doubt seriously if they will avail themselves of your offer."

Dr. Bird's black eyes shot fire. "Idiot!" he said. "If you're a specimen of the Post Office Department, I'll have the entire case taken out of your hands. Do you mean to cooperate with me or not?"

"I fail to see what interest the Bureau of Standards can have in the affair."

"The Bureau isn't mixed up in it; Dr. Bird is. If necessary, I will go direct to the President. Oh, thunder! What's the use of talking to you? Who's your chief?"

"Chief Inspector Watkins is in charge of all investigations."

"Carnes, get him on the telephone. Tell him we are taking charge of the investigation. If he balks, have Bolton go over his head. Then get the chief of the Air Corps on the wire and arrange for an army plane to-morrow. There is something more than a mail robbery back of this or I'm badly fooled."

"Do you suspect--"

"I suspect nothing and no one, Carnes--yet! I'll get a few instruments together to take with us to-morrow. We'll fly over that section until something happens if it takes us until this time next year."

A three-seated scout plane rose from Langley Field at eight the next morning. Captain Garland was at the controls. In the rear c.o.c.kpit sat Dr. Bird and Carnes. Inside his flying helmet, the doctor wore a pair of headphones which were connected to a box on the floor before him.

Carnes carried no apparatus but his hand rested carelessly on the grip of a machine-gun.

The plane cleared Bellefonte at nine-thirty and bore east toward Philipsburg. Captain Garland kept his eyes on his instrument board and on a map. Less than six hundred feet above the ground, he was following the air-mail route as exactly as possible. Overhead a mail plane winged its way east, three thousand feet above them.

Fifteen minutes brought them to Philipsburg. Captain Garland shot his plane upward a few hundred feet.

"Turn back, Captain," said Dr. Bird into the speaking tube. "Retrace your course a quarter of a mile farther north. At Bellefonte, turn back and go over the same ground another quarter of a mile north. Keep flying back and forth, working your way north, until I tell you to stop."

The plane swung around and headed back toward Bellefonte.

"Of course, we can't tell exactly what route he followed," said the doctor to Carnes, "but he was new on this run and it is safe to a.s.sume that he didn't stray far. We'll quarter the whole area before we stop."

Carnes watched the ground below them carefully. There was nothing about it to distinguish it from any other wooded mountainous country and his interest waned. He glanced aloft. The mail plane had disappeared in the distance and the sky was clear of aircraft. He turned again to the ground. It looked closer than it had before. He turned and looked at the duplicate altimeter. The plane had lost nearly a hundred feet elevation.

"There's something wrong about this plane, Doctor," came Captain Garland's voice through the speaking tube. "It doesn't behave like it should."

"I guess we've found what we were looking for, Carnes," said Dr. Bird grimly. "What seems to be the matter, Captain?"