We returned to Verplanck’s in the Streamline in record time, dined, and then found McNeill, a local detective, waiting to add his quota of information.  McNeill was of the square-toed, double-chinned, bull-necked variety, just the man to take along if there was any fighting.  He had, however, very little to add to the solution of the mystery, apparently believing in the chauffeur-and-maid theory.

It was too late to do anything more that night, and we sat on the Verplanck porch, overlooking the beautiful harbor.  It was a black, inky night, with no moon, one of those nights when the myriad lights on the boats were mere points in the darkness.  As we looked out over the water, considering the case which as yet we had hardly started on, Kennedy seemed engrossed in the study in black.

“I thought I saw a moving light for an instant across the bay, above the boats, and as though it were in the darkness of the hills on the other side.  Is there a road over there, above the Carter house?” he asked suddenly.

“There is a road part of the way on the crest of the hill,” replied Mrs. Verplanck.  “You can see a car on it, now and then, through the trees, like a moving light.”

“Over there, I mean,” reiterated Kennedy, indicating the light as it flashed now faintly, then disappeared, to reappear further along, like a gigantic firefly in the night.

“N-no,” said Verplanck.  “I don’t think the road runs down as far as that.  It is further up the bay.”

“What is it then?” asked Kennedy, half to himself.  “It seems to be traveling rapidly.  Now it must be about opposite the Carter house.  There—­it has gone.”

We continued to watch for several minutes, but it did not reappear.  Could it have been a light on the mast of a boat moving rapidly up the bay and perhaps nearer to us than we suspected?  Nothing further happened, however, and we retired early, expecting to start with fresh minds on the case in the morning.  Several watchmen whom Verplanck employed both on the shore and along the driveways were left guarding every possible entrance to the estate.

Yet the next morning as we met in the cheery east breakfast room, Verplanck’s gardener came in, hat in hand, with much suppressed excitement.

In his hand he held an orange which he had found in the shrubbery underneath the windows of the house.  In it was stuck a long nail and to the nail was fastened a tag.

Kennedy read it quickly.

“If this had been a bomb, you and your detectives would never have known what struck you.

“Aquaero.”

CHAPTER V

THE ULTRA-VIOLET RAY

“Good Gad, man!” exclaimed Verplanck, who had read it over Craig’s shoulder.  “What do you make of that?”

Kennedy merely shook his head.  Mrs. Verplanck was the calmest of all.

“The light,” I cried.  “You remember the light?  Could it have been a signal to some one on this side of the bay, a signal light in the woods?”