"You think the Mayan angle is important, Doc?"

"I sure do, Johnny."

The telephone bell jangled.

"That's my long-distance call to England," Doc guessed. "They took their time getting it through!"

Lifting the phone, he spoke, got an answer, then rapidly gave the model of the double-barreled elephant rifle, and the number of the weapon.

"Who was it sold to?" he asked.

In a few minutes, he got his answer.

Doc rung off. His bronze face was inscrutable; golden gleamings were in his eyes.

"The English factory says they sold that gun to the government of Hidalgo," Doc said thoughtfully. "It was a part of a large lot of weapons sold to Hidalgo some months ago."

Johnny adjusted his glasses which had the magnifying lens. "We've got to be careful, Doc," he said. "If this enemy of ours persists in making trouble, he may try to tamper with our plane."

"I have a scheme that will prevent danger from that angle," Doc assured him.

Johnny blinked, then started to ask what the scheme was. But he was too slow. Doc had already quitted the office.

With a grin, Johnny went about his own part of the preparations. He felt supreme confidence in Doc Savage.

Whatever villainous moves the enemy made against them, Doc was capable of checkmating. Already, Doc was undoubtedly putting into operation some plan which would guarantee them safety in their flight southward.

The plan to protect their plane would be one worthy of Doc's vast ingenuity.

Chapter 7. DANGER TRAIL

The rain had stopped.

A bilious dawn, full of fog, shot through with a chill wind, was crawling along the north shore of Long Island. The big hangars at North Beach airport, just within the boundary line of Mew York City, were like pale-gray, roundbacked boxes in the mist. Electric lights made a futile effort to dispel the sodden gloom.

A giant tri-motored, all-metal plane stood on the tarmac of the flying field near by. On the fuselage, just back of the bow engine, was emblazoned in firm black letters:

Clark Savage, Jr.

One of Doc's crates!

Airport attendants, in uniforms made very untidy by mud, grease, and dampness, were busy transferring boxes from a truck to the interior of the big plane. These boxes were of light, but stout, construction, arid on each was imprinted, after the manner of exploration expeditions, the words:

Clark Savage, Jr., Hidalgo Expedition.

"What's a Hidalgo?" a thick-necked mechanic wanted to know.

"Dunno — a country, I reckon," a companion greaseball told him.

The conversation was unimportant, except in that it showed what a little-known country Hidalgo was. Yet the Central American republic was of no inconsiderable size.

The last box was finally in the plane. An airport worker closed the plane door. Because of the murky dawn and moisture on the windows, it was impossible to see into the pilot's compartment of the great tri-motor plane.

A mechanic climbed atop the tin pants over the big wheels, and standing there, cranked the inertia starter of first one motor, then the other. All three big radial engines thundered into life. More than a thousand throbbing horsepower.

The big plane trembled to the tune of the hammering exhaust stacks. It was not an especially new ship, being about five years old.

Perhaps one or two attendants about the tarmac heard the sound of another plane which had arrived overhead. Looking up, maybe they saw a huge gray bat of a shape go slicing through the mist. But that was all, and the noise of its great, muffled exhaust was hardly audible above the bawl of the stacks of the old-fashioned tri-motor.

The tri-motor was moving now. The tail was up, preliminary to taking off. Faster and faster it raced across the tarmac. It slowly took the air.

Without banking to either side, climbing gently, the big all-metal plane flew possibly a mile.

An astounding thing happened then.

The tri-motor ship seemed to turn instantaneously into a gigantic sheet of white-hot flame. This resolved into a monster ball of villainous smoke. Then flipped fragments of the plane and its contents rained downward upon the roofs of Jackson Heights, a conservative residential suburb of New York City.

So terrific was the explosion that windows were broken in the houses underneath, and shingles even torn off roofs.

No piece more than a few yards in area remained of the great plane. Indeed, the authorities could never have identified it, had not the airport men known it had just taken off from there.

No human life could have survived aboard the tri-motor aircraft.

Doc Savage merely blinked his golden eyes once after the blinding flash which marked the blast that annihilated the tri-motor ship.

"That was what I was afraid of!" he said dryly.

The rush of air thrown by the explosion caused his plane to reel. Doc stirred the controls expertly to right it.

For Doc and his men had not been in the ill-fated tri-motor plane. They were in the other craft which had flown over the airport a moment before the tri-motor took off. Indeed, Doc himself had maneuvered the take-off of the tri-motor, using remote radio control to direct it.

Doc's radio remote control apparatus was exactly the same type used by the army and navy in extensive experiments, employing changing frequencies and sensitive relays for its operation.

Doc did not know how their mysterious enemy had managed to blow up the tri-motor. But thanks to his foresight, Doc's men had escaped the devilish blast. Doc had used the tri-motor plane for a decoy. It was one of his old ships, almost ready to be discarded, anyway.

"They must have managed to slip high explosive into one of our boxes," Doc concluded aloud. "It is too bad we lost the equipment in the destroyed plane. But we can get along without it."

"What dizzies me," Renny muttered, "is how they fixed their bomb to explode in the air, and not on the ground."

Doc banked his plane, set a course directly for the city of Washington, using not only the gyroscopic compass with which the craft was fitted, but calculating wind drift expertly.

"How they made the bomb explode in the air can be simply explained," he told Renny at last. "They probably put an altimeter or barometer in the bomb. The altimeter would register a change in height. All they had to do was fix an electrical contact to be closed at a given height, and — bang!"

"Bang, is right!" Monk put in, grinning.

Their plane flashed past the upraised arm of the Statue of Liberty, and sang its song of speed southward over the Jersey marshes.

Unlike the tri-motor which had been destroyed, this plane was of the latest design. It was a tri-motor craft also, but the great engines were in eggs built directly into the wings. It was what pilots call a low-wing job, with the wings attached well down on the fuselage, instead of at the top. The landing gear was retractible — folded up into the wings so as not to offer a trace of wind resistance.

It was the ultra in an airman's steed, this supercraft. And two hundred miles an hour was only its cruising speed.

No small point was the fact that the cabin was soundproof, enabling Doc and his friends to converse in ordinary tones.

The really essential portion of their equipment was loaded into the rear of the speed-ship cabin. Packed compactly in light metal containers, an alloy metal that was lighter even than wood, each carton was fitted with straps for carrying.

In a surprisingly short time they picked up the clustered buildings of Philadelphia. Doc whipped the plane past a little east of the city hall — the center of the downtown business districts.

Onward they swept, to zoom down on an airport at the outskirts of Washington.