"Oh, I'll promise," cried Sidney anxiously. A warning to Aunt Achsa would most likely curtail their precious freedom. But she could not resist the temptation of questioning. "What mischief?" she asked, eagerly. Cap'n Davies hesitated. Then he drew a letter from his pocket and tapped it with his finger. "That's from the Custom House in Boston. Come last week. They're sending secret service men down to comb the Cape. Been huntin' the hul coast for a year and a half and they sort o' suspicion these parts because a lot of 'em was shipped into Boston that--" "Oh, _what_! You haven't said _what_--" broke in Sidney, aquiver. "So I didn't. I'm sailin' stern first, I cal'late. Well, there's always smuggling and smuggling and I guess there always will be, but when it comes to _diamonds_ Uncle Sam sets up and takes notice. And they're suspicionin' that they're comin' in somewheres along the Cape, and this part of the Cape, too. And _this_--" he shook another sheet in Sidney's face, "this is a notice of a reward offered by Wellfleet and Truro counties for findin' the dog that's givin' this part of the Cape a bad name! Five thousand dollars. In two weeks it'll be stuck on every post hereabouts 's far as Provincetown. And Phin Davies ain't goin' to lay to 'till I've found out whether it's someone on the Cape that's doing it or not. Cape Cod's brung up a race of honest men who could sleep with their doors wide open and if anybody is hurtin' the good name of the Cape I want to know it. 'Taint the money I want." [Illustration: CAPTAIN DAVIES DREW A LETTER FROM HIS POCKET AND TAPPED IT WITH HIS FINGER] Sidney was scarcely drawing a breath for excitement. The Captain, suddenly subsiding, observed her tenseness. He laughed embarrassedly. "Now there I go spillin' everything _I_ know like a ship that's sprung a leak. I'll have to ask you to keep mum 'bout what I've told you, mate, and remember your word to keep off the beach come night. Ain't no place for a gal like you." And without another word he rose and led Sidney down the narrow stairs. On the homeward ride Miss Letty found Sidney an abstracted companion. After a few attempts to keep up conversation she subsided into silence herself. "It's good to find a young one who can keep her tongue still a spell and enjoy her own thoughts." But Sidney was not enjoying her thoughts, not at all. With the realization that she could not share with Mart and Lavender the astounding revelations Cap'n Phin Davies had made all joy in them had fled. Had not she and Mart and Lavender agreed solemnly to tell one another anything any one of them discovered? It would be so perfectly thrilling to greet them the instant she reached home with "Hook!" They would be so surprised. They wouldn't laugh if she told them what she knew! But she couldn't. Cap'n Phin Davies had said: "I'll have to ask you to keep mum" and that was quite enough to seal Sidney's lips. CHAPTER XVI