boathouse and I screwed it on the outside so's I could lock it up besides, and covered the padlock with vines and sand. Nobody'd ever dream there was such a place here, and I guess nobody ever has, either. That's my secret!"

"But, Sally," exclaimed Doris, "how did it ever come here to begin with? Who made it? It must have some sort of history."

"There you've got me!" answered Sally.

"Some one must have stayed here," mused Doris, half to herself. "And, what's more, they must have hidden here, or why should they have taken such trouble to keep it from being discovered?"

"Yes, they've hidden here, right enough," agreed Sally. "It's the best hiding place any one ever had, I should say. But the question is, what did they hide here for?"

"And also," added Doris, "if they were hiding, how could they make such a room as this, all finished with wooden walls, without being seen doing it? Where did they get the planks?"

"Do you know what that timber is?" asked Sally.

"Why, of course not," laughed Doris. "How should I?"

"Well, I do," said her companion. "I know something about lumber because Dad builds boats and he's shown me. I scratched the mold off one place,—here it is,—and I discovered that this planking is real seasoned cedar like they build the best boats of. And do you know where I think it was got? It came from some wrecked vessel down on the beach. There are plenty of them cast up, off and on, and always have been."

"But gracious!" cried Doris, "how was it got here?"

"Don't ask me!" declared Sally. "The beach is miles away."

They stood for some moments in silence, each striving to piece together the story of this strange little retreat from the meagre facts they saw about them. At last Doris spoke.

"Sally," she asked, "was this all you ever found here? Was there absolutely nothing else?" Sally started, as if surprised at the question and hesitated a moment.

"No," she acknowledged finally. "There was something else. I wasn't going to tell you right away, but I might as well now. I found this under the mattress of the bed."

She went over to the straw pallet, lifted it, searched a moment and, turning, placed something in Doris's hands.

CHAPTER V

MYSTERY

Doris received the object from Sally and stood looking at it as it lay in her hands. It was a small, square, very flat tin receptacle of some kind, rusted and moldy, and about six inches long and wide. Its thickness was probably not more than a quarter of an inch.

"What in the world is it?" she questioned wonderingly.

"Open it and see!" answered Sally. Doris pried it open with some difficulty. It contained only a scrap of paper which fitted exactly into its space. The paper was brown with age and stained beyond belief. But on its surface could be dimly discerned a strange and inexplicable design.

"Of all things!" breathed Doris in an awestruck voice. "This certainly is a mystery, Sally. What do you make of it?"

"I don't make anything of it," Sally averred. "That's just the trouble. I can't imagine what it means. I've studied and studied over it all winter, and it doesn't seem to mean a single thing."

It was indeed a curious thing, this scrap of stained, worn paper, hidden for who knew how many years in a tin box far underground. For the riddle on the paper was this: