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?