"Using the triple mirror to signal to Armand. It is something better than wireless. Wireless requires heavy and complicated apparatus. This is portable, heatless, almost weightless, a source of light depending for its power on another source of light at a great distance."

I wondered how Armand could ever detect its feeble ray.

"Even in the case of a rolling ship," Kennedy continued, alternately covering and uncovering the mirror, "the beam of light which this mirror reflects always goes back, unerring, to its source. It would do so from an aeroplane, so high in the air that it could not be located.

The returning beam is invisible to anyone not immediately in the path of the ray, and the ray always goes to the observer. It is simply a matter of pure mathematics practically applied. The angle of incidence equals the angle of reflection. There is not a variation of a foot in two miles."

"What message are you sending him?" asked Verplanck.

"To tell Mrs. Hollingsworth to hurry home immediately," Kennedy replied, still flashing the letters according to his code.

"Mrs. Hollingsworth?" repeated Verplanck, looking up.

"Yes. This hydroaeroplane yeggman is after something besides jewels to-night. Were those letters that were stolen from you the only ones you had in the safe?"

Verplanck looked up quickly. "Yes, yes. Of course."

"You had none from a woman--"

"No," he almost shouted. Of a sudden it seemed to dawn on him what Kennedy was driving at--the robbery of his own house with no loss except of a packet of letters on business, followed by the attempt on Mrs. Hollingsworth. "Do you think I'd keep dynamite, even in the safe?"

To hide his confusion he had turned and was bending again over the engine.

"How is it?" asked Kennedy, his signaling over.

"Able to run on four cylinders and one propeller," replied Verplanck.

"Then let's try her. Watch the engine. I'll take the wheel."

Limping along, the engine skipping and missing, the once peerless Streamline started back across the bay. Instead of heading toward the club, Kennedy pointed her bow somewhere between that and Verplanck's.

"I wish Armand would get busy," he remarked, after glancing now and then in the direction of the club. "What can be the matter?"

"What do you mean?" I asked.

There came the boom as if of a gun far away in the direction in which he was looking, then another.

"Oh, there it is. Good fellow. I suppose he had to deliver my message to Mrs. Hollingsworth himself first."

From every quarter showed huge b.a.l.l.s of fire, rising from the sea, as it were, with a brilliantly luminous flame.

"What is it?" I asked, somewhat startled.

"A German invention for use at night against torpedo and aeroplane attacks. From that mortar Armand has shot half a dozen bombs of phosphide of calcium which are hurled far into the darkness. They are so constructed that they float after a short plunge and are ignited on contact by the action of the salt water itself."

It was a beautiful pyrotechnic display, lighting up the sh.o.r.e and hills of the bay as if by an unearthly flare.

"There's that thing now!" exclaimed Kennedy.

In the glow we could see a peculiar, birdlike figure flying through the air over toward the Hollingsworth house. It was the hydroaeroplane.

Out from the little stretch of lawn under the accentuated shadow of the trees, she streaked into the air, swaying from side to side as the pilot operated the stabilizers on the ends of the planes to counteract the puffs of wind off the land.

How could she ever be stopped?

The Streamline, halting and limping, though she was, had almost crossed the bay before the light bombs had been fired by Armand. Every moment brought the flying boat nearer.

She swerved. Evidently the pilot had seen us at last and realized who we were. I was so engrossed watching the thing that I had not noticed that Kennedy had given the wheel to Verplanck and was standing in the bow, endeavoring to sight what looked like a huge gun.

In rapid succession half a dozen shots rang out. I fancied I could almost hear the ripping and tearing of the tough rubber-coated silken wings of the hydroaeroplane as the wind widened the perforation the gun had made.

She had not been flying high, but now she swooped down almost like a gull, seeking to rest on the water. We were headed toward her now, and as the flying boat sank I saw one of the pa.s.sengers rise in his seat, swing his arm, and far out something splashed in the bay.

On the water, with wings helpless, the flying boat was no match for the Streamline now. She struck at an acute angle, rebounded in the air for a moment, and with a hiss skittered along over the waves, planing with the help of her exhaust under the step of the boat.

There she was, a hull, narrow, scow-bowed, like a hydroplane, with a long pointed stern and a c.o.c.kpit for two men, near the bow. There were two wide, winglike planes, on a light latticework of wood covered with silk, trussed and wired like a kite frame, the upper plane about five feet above the lower, which was level with the boat deck. We could see the eight-cylindered engine which drove a two-bladed wooden propeller, and over the stern were the air rudder and the horizontal planes. There she was, the hobbled steed now of the phantom bandit who had accomplished the seemingly impossible.

In spite of everything, however, the flying boat reached the sh.o.r.e a trifle ahead of us. As she did so both figures in her jumped, and one disappeared quickly up the bank, leaving the other alone.

"Verplanck, McNeill--get him," cried Kennedy, as our own boat grated on the beach. "Come, Walter, we'll take the other one."

The man had seen that there was no safety in flight. Down the sh.o.r.e he stood, without a hat, his hair blown pompadour by the wind.

As we approached Carter turned superciliously, unb.u.t.toning his bulky khaki life preserver jacket.

"Well?" he asked coolly.

Not for a moment did Kennedy allow the a.s.sumed coolness to take him back, knowing that Carter's delay did not cover the retreat of the other man.

"So," Craig exclaimed, "you are the--the air pirate?"

Carter disdained to reply.

"It was you who suggested the millionaire households, full of jewels, silver and gold, only half guarded; you, who knew the habits of the people; you, who traded that information in return for another piece of thievery by your partner, Australia Mac--Wickham he called himself here in Bluffwood. It was you---"

A car drove up hastily, and I noted that we were still on the Hollingsworth estate. Mrs. Hollingsworth had seen us and had driven over toward us.

"Montgomery!" she cried, startled.

"Yes," said Kennedy quickly, "air pirate and lawyer for Mrs. Verplanck in the suit which she contemplated bringing--"

Mrs. Hollingsworth grew pale under the ghastly, flickering light from the bay.

"Oh!" she cried, realizing at what Kennedy hinted, "the letters!"

"At the bottom of the harbor, now," said Kennedy. "Mr. Verplanck tells me he has destroyed his. The past is blotted out as far as that is concerned. The future is--for you three to determine. For the present I've caught a yeggman and a blackmailer."