It was the regular Saturday night dance at the club, a brilliant spectacle, faces that radiated pleasure, gowns that for startling combinations of color would have shamed a Futurist, music that set the feet tapping irresistibly—a scene which I shall pass over because it really has no part in the story.

The fascination of the ballroom was utterly lost on Craig. "Think of all the houses only half guarded about here to-night," he mused, as we joined Armand and McNeill on the end of the dock. I could not help noting that that was the only idea which the gay, variegated, sparkling tango throng conveyed to him.

In front of the club was strung out a long line of cars, and at the dock several speed boats of national and international reputation, among them the famous Streamline II, at our instant beck and call. In it Craig had already placed some rather bulky pieces of apparatus, as well as a brass case containing a second triple mirror like that which he had left with Armand.

With McNeill, I walked back along the pier, leaving Kennedy with Armand, until we came to the wide porch, where we joined the wallflowers and the rocking-chair fleet. Mrs. Verplanck, I observed, was a beautiful dancer. I picked her out in the throng immediately, dancing with Carter.

McNeill tugged at my sleeve. Without a word I saw what he meant me to see. Verplanck and Mrs. Hollingsworth were dancing together. Just then, across the porch I caught sight of Kennedy at one of the wide windows. He was trying to attract Verplanck's attention, and as he did so I worked my way through the throng of chatting couples leaving the floor until I reached him. Verplanck, oblivious, finished the dance; then, seeming to recollect that he had something to attend to, caught sight of us, and ran off during the intermission from the gay crowd to which he resigned Mrs. Hollingsworth.

"What is it?" he asked.

"There's that light down the bay," whispered Kennedy.

Instantly Verplanck forgot about the dance.

"Where?" he asked.

"In the same place."

I had not noticed, but Mrs. Verplanck, woman-like, had been able to watch several things at once. She had seen us and had joined us.

"Would you like to run down there in the Streamline?" he asked. "It will only take a few minutes."

"Very much."

"What is it—that light again?" she asked, as she joined us in walking down the dock.

"Yes," answered her husband, pausing to look for a moment at the stuff Kennedy had left with Armand. Mrs. Verplanck leaned over the Streamline, turned as she saw me, and said: "I wish I could go with you. But evening dress is not the thing for a shivery night in a speed boat. I think I know as much about it as Mr. Verplanck. Are you going to leave Armand?"

"Yes," replied Kennedy, taking his place beside Verplanck, who was seated at the steering wheel. "Walter and McNeill, if you two will sit back there, we're ready. All right."

Armand had cast us off and Mrs. Verplanck waved from the end of the float as the Streamline quickly shot out into the night, a buzzing, throbbing shape of mahogany and brass, with her exhausts sticking out like funnels and booming like a pipe organ. It took her only seconds to eat into the miles.

"A little more to port," said Kennedy, as Verplanck swung her around.

Just then the steady droning of the engine seemed a bit less rhythmical. Verplanck throttled her down, but it had no effect. He shut her off. Something was wrong. As he crawled out into the space forward of us where the engine was, it seemed as if the Streamline had broken down suddenly and completely.

Here we were floundering around in the middle of the bay.

"Chuck-chuck-chuck," came in quick staccato out of the night. It was Montgomery Carter, alone, on his way across the bay from the club, in his own boat.

"Hello—Carter," called Verplanck.

"Hello, Verplanck. What's the matter?"

"Don't know. Engine trouble of some kind. Can you give us a line?"

"I've got to go down to the house," he said, ranging up near us. "Then I can take you back. Perhaps I'd better get you out of the way of any other boats first. You don't mind going over and then back?"

Verplanck looked at Craig. "On the contrary," muttered Craig, as he made fast the welcome line.

The Carter dock was some three miles from the club on the other side of the bay. As we came up to it, Carter shut off his engine, bent over it a moment, made fast, and left us with a hurried, "Wait here."

Suddenly, overhead, we heard a peculiar whirring noise that seemed to vibrate through the air. Something huge, black, monster-like, slid down a board runway into the water, traveled a few feet, in white suds and spray, rose in the darkness—and was gone!

As the thing disappeared, I thought I could hear a mocking laugh flung back at us.

"What is it?" I asked, straining my eyes at what had seemed for an instant like a great flying fish with finny tail and huge fins at the sides and above.

"'Aquaero,'" quoted Kennedy quickly. "Don't you understand—a hydroaeroplane—a flying boat. There are hundreds of privately owned flying boats now wherever there is navigable water. That was the secret of Carter's boathouse, of the light we saw in the air."

"But this Aquaero—who is he?" persisted McNeill.
"Carter—Wickham—Australia Mac?"

We looked at each other blankly. No one said a word. We were captured, just as effectively as if we were ironed in a dungeon. There were the black water, the distant lights, which at any other time I should have said would have been beautiful.

Kennedy had sprung into Carter's boat.

"The deuce," he exclaimed. "He's put her out of business."

Verplanck, chagrined, had been going over his own engine feverishly. "Do you see that?" he asked suddenly, holding up in the light of a lantern a little nut which he had picked out of the complicated machinery. "It never belonged to this engine. Some one placed it there, knowing it would work its way into a vital part with the vibration."

Who was the person, the only one who could have done it? The answer was on my lips, but I repressed it. Mrs. Verplanck herself had been bending over the engine when last I saw her. All at once it flashed over me that she knew more about the phantom bandit than she had admitted. Yet what possible object could she have had in putting the Streamline out of commission?

My mind was working rapidly, piecing together the fragmentary facts. The remark of Kennedy, long before, instantly assumed new significance. What were the possibilities of blackmail in the right sort of evidence? The yeggman had been after what was more valuable than jewels—letters! Whose? Suddenly I saw the situation. Carter had not been robbed at all. He was in league with the robber. That much was a blind to divert suspicion. He was a lawyer—some one's lawyer. I recalled the message about letters and evidence, and as I did so there came to mind a picture of Carter and the woman he had been dancing with. In return for his inside information about the jewels of the wealthy homes of Bluffwood, the yeggman was to get something of interest and importance to his client.

The situation called for instant action. Yet what could we do, marooned on the other side of the bay?

From the Club dock a long finger of light swept out into the night, plainly enough near the dock, but diffused and disclosing nothing in the distance. Armand had trained it down the bay in the direction we had taken, but by the time the beam reached us it was so weak that it was lost.

Craig had leaped up on the Carter dock and was capping and uncapping with the brass cover the package which contained the triple mirror.

Still in the distance I could see the wide path of light, aimed toward us, but of no avail.

"What are you doing?" I asked.

"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 balls 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 shore 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 passengers 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 cockpit 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 shore 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 shore he stood, without a hat, his hair blown pompadour by the wind.

As we approached Carter turned superciliously, unbuttoning his bulky khaki life preserver jacket.

"Well?" he asked coolly.

Not for a moment did Kennedy allow the assumed 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."