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 breakfast. I'm going to keep at this swamp till I solve that mystery." Crossing the board walk, they made a detour and reached the house. It was a fine country residence with many acres of ground, part of which was formed by the swamp. They Saw a big negro in overalls standing at the gate and Harry asked him: "Say--who lives here?" "Oliver Dalton, the Broad street broker, sir," replied the colored man. This reply startled the detectives, for the owner was the very man whose case of mail robbery had been placed in their hands. They glanced significantly at each other, and that look spoke volumes. CHAPTER IV. THE DISAPPEARANCE OF MR. DALTON. "See here, my friend," said Old King Brady to the darky, "do you know anything about that little hut standing out there in the swamp?" "Dat hut? Sho'. Nobuddy nebber go in no mo'. Useter be fo' Massa Dalton when he go out shootin' reed birds." "Then it isn't in use any more?" "No, sah." "Does Mr. Dalton live here all the year round?" "Golly, no. Only in de winter. Comes heah wif Missy Lizzy an' his nephew, Ronald. Me an' my ole gal keep de house fo' dem de rest ob de time." "I see. Then you don't expect them here for a long time, do you?"