pty.

Staring around, Old King Brady observed that there was only one door
and the two windows they had been watching--one at the rear and the
other at the side of the little building.

The door was at another side.

"Gone!" ejaculated the old detective.

"Where?" asked the astonished boy.

"Heaven only knows."

"Search outside."

Old King Brady dashed out the door.

He saw by the moonlight that the little island on which the hut stood
was in the midst of the swamp.

If any one left it, he was bound to see them.

But not a soul met his view.

No one could leave the hut, cross the swamp and reach the mainland so
quick that he could not see them escaping in such a short space of
time.

Yet he failed to see any one in the swamp.

That convinced him that no one left the oasis.

And nobody was on the solid island of earth.

He could not have failed to observe them if they were there.

"What the deuce does this mean?" he gasped in bewilderment.

Then he finally returned to his partner.

"See any one?" eagerly asked the boy.

"Not a soul. And you?"

"I've searched this place, but no one is here."

"Then where did they disappear to so mysteriously?"

"Blessed if I know."

"You heard two human voices here, didn't you?"

"Of course I did."

"Have you thoroughly examined this place?"

"Every inch of it, and couldn't find them."

Old King Brady made a round of the room and came back.

He plainly saw that the walls were not double, and that the floor was
merely covered with common dirt.

There was not a piece of furniture nor a lamp in the place.

Yet the detectives had seen a light distinctly.

"They ain't here, sure enough," said the old detective, "and they
didn't leave here. Now, how could they vanish, and where did they go?"

"I'm completely rattled. Beyond my depth entirely."

"So am I. This mystery is too much for me to solve."

"See! Daylight is breaking."

"Let's search the place again."

They went at it with renewed zest, and spent two hours vainly searching
for the means those speakers employed to drop out of sight.

Finally they desisted.

The great swamp mystery was too much for them.

By this time the sun had risen and flooded the scene, dispelled the
vapors that hung over the bog and lighted up the surrounding country.

"We'd better get out of here," said Old King Brady. "I see a fine old
residence over there on the mainland. Let's get over there and get our
breakf