“Nordheim!” I echoed, involuntarily.  I had expected an Oriental name.

“Yes, a German.  I have been looking up his record, and I find that once he was connected in some way with the famous Titan Iron Works, at Kiel, Germany.  We began watching him day before yesterday, but suddenly he disappeared.  Then, there is a society woman in Washington, a Mrs. Bayard Brainard, who was at the Department that night.  We have been trying to find her.  To-day I got word that she was summering in the cottage colony across the bay from Lookout Hill.  At any rate, I had to go up there to see the captain, and I thought I’d kill a whole flock of birds with one stone.  The chief thought, too, that if you’d take the case with us you had best start on it up there.  Next, you will no doubt want to go back to Washington with me.”

Lookout Hill was the name of the famous old estate of the Shirleys, on a point of land jutting out into Long Island Sound and with a neighbouring point enclosing a large, deep, safe harbour.  On the highest ground of the estate, with a perfect view of both harbour and sound, stood a large stone house, the home of Captain Shirley, of the United States navy, retired.

Captain Shirley, a man of sixty-two or three, bronzed and wiry, met us eagerly.

“So this is Professor Kennedy; I’m glad to meet you, sir,” he welcomed, clasping Craig’s hand in both of his—­a fine figure as he stood erect in the light of the portecochere.  “What’s the news from Washington, Burke?  Any clues?”

“I can hardly tell,” replied the secret service man. with assumed cheerfulness.  “By the way, you’ll have to excuse me for a few minutes while I run back into town on a little errand.  Meanwhile, Captain, will you explain to Professor Kennedy just how things are?  Perhaps he’d better begin by seeing the Turtle herself.”

Burke had not waited longer than to take leave.

“The Turtle,” repeated the captain, leading the way into the house.  “Well, I did call it that at first.  But I prefer to call it the Z99.  You know the first submarines, abroad at least, were sometimes called Al, A2, A3, and so on.  They were of the diving, plunging type, that is, they submerged on an inclined keel, nose down, like the Hollands.  Then came the B type, in which the hydroplane appeared; the C type, in which it was more prominent, and a D type, where submergence is on a perfectly even keel, somewhat like our Lakes.  Well, this boat of mine is a last word—­ the Z99.  Call it the Turtle, if you like.”

We were standing for a moment in a wide Colonial hall in which a fire was crackling in a huge brick fireplace, taking the chill off the night air.

“Let me give you a demonstration, first,” added the captain.  “Perhaps Z99 will work—­perhaps not.”

There was an air of disappointment about the old veteran as he spoke, uncertainly now, of what a short time ago he had known to be a certainty and one of the greatest it had ever been given the inventive mind of man to know.