Back of, or in front of the Carter house, according as you fancied it faced the bay or not, was the boathouse, built by Carter’s father, who had been a great yachtsman in his day and commodore of the club.  His son had not gone in much for water sports and had converted the corner underneath a sort of observation tower into a sort of country law office.

“There has always seemed to me to be something strange about that boathouse since the old man died,” remarked McNeill in a half whisper as we left Carter.  “He always keeps it locked and never lets anyone go in there, although they say he has it fitted beautifully with hundreds of volumes of law books, too.”

Kennedy had been climbing the hill back of the house and now paused to look about.  Below was the Carter garage.

“By the way,” exclaimed McNeill, as if he had at last hit on a great discovery, “Carter has a new chauffeur, a fellow named Wickham.  I just saw him driving down to the village.  He’s a chap that it might pay us to watch—­a newcomer, smart as a steel trap, they say, but not much of a talker.”  “Suppose you take that job—­ watch him,” encouraged Kennedy.  “We can’t know too much about strangers here, McNeill.”

“That’s right,” agreed the detective.  “I’ll follow him back to the village and get a line on him.”

“Don’t be easily discouraged,” added Kennedy, as McNeill started down the hill to the garage.  “If he is a fox he’ll try to throw you off the trail.  Hang on.”

“What was that for?” I asked as the detective disappeared.  “Did you want to get rid of him?”

“Partly,” replied Craig, descending slowly, after a long survey of the surrounding country.

We had reached the garage, deserted now except for our own car.

“I’d like to investigate that tower,” remarked Kennedy with a keen look at me, “if it could be done without seeming to violate Mr. Carter’s hospitality.”

“Well,” I observed, my eye catching a ladder beside the garage, “there’s a ladder.  We can do no more than try.”

He walked over to the automobile, took a little package out, slipped it into his pocket, and a few minutes later we had set the ladder up against the side of the boathouse farthest away from the house.  It was the work of only a moment for Kennedy to scale it and prowl across the roof to the tower, while I stood guard at the foot.

“No one has been up there recently,” he panted breathlessly as he rejoined me.  “There isn’t a sign.”

We took the ladder quietly back to the garage, then Kennedy led the way down the shore to a sort of little summerhouse cut off from the boathouse and garage by the trees, though over the top of a hedge one could still see the boathouse tower.

We sat down, and Craig filled his lungs with the good salt air, sweeping his eye about the blue and green panorama as though this were a holiday and not a mystery case.

“Walter,” he said at length, “I wish you’d take the car and go around to Verplanck’s.  I don’t think you can see the tower through the trees, but I should like to be sure.”