"Caw! Caw!" sounded with startling distinctness in the still, dark wooded depths of Ross island. For a moment the silence was intense; then it was broken again by the familiar, long-drawn out, guttural cry, "Caw! caw!" of the black scavenger bird. And silence once more settled down upon the scene, and seemed deeper, thicker and more profound than before. It may have been a half a minute after the second cry when an answer, faint, though clearly audible, was echoed from a neighboring part of the woods. "Come on!" quietly exclaimed Sam Harris, who, with John Thorpe, stood beside the trunk of a fir that grew midway on the island near its north end. "An uncanny signal!" remarked Mr. Thorpe, in the same low tones. "Yes, somehow I feel as though it betokens serious business," softly replied Sam. "Be careful. A thick vine here. Step clear," he whispered, as they moved cautiously along. They had proceeded in silence some distance, part of the time groping their way by the aid of a match, lighted now and again, but artfully concealed, for the darkness was very deep, when through a rift in the wild growth of underbrush a man's form was seen to move. "Wait!" suddenly whispered Sam, in a warning tone. "There is a man ahead of us." There was no mistaking it, for as they stood stock-still in their tracks, they saw a man's form occasionally obtruding between them and an electric light that shed its rays from afar off, across the water. "Do you think he is the detective?" asked Thorpe, in a low voice. "Wait!" and Sam placed his two hands over his mouth so as to form a hollow, and called out in moderate tones: "Caw! caw!" It was answered by a single "caw," low, but seemingly so near that they were startled, and for a moment felt that they were being deceived. They remained motionless and silent--Sam with his hand grasping the butt of a revolver. The "caw" was repeated low, but with reassuring effect, for they now discovered that while the sound was apparently near, due to atmospheric conditions, it was in reality fully two hundred feet away. "Detective Simms," whispered Sam. "He is waiting for us." "Then let's hurry," urged his companion. The words had scarcely left his lips when Thorpe's boot caught in a vine and down he went, making considerable noise as he stumbled and fell on his hip. "You must be more careful," enjoined Sam, in a low tone, as he helped Thorpe to his feet. "Much haste, less speed, and then a little noise may endanger our success, I fear. Are you hurt?" "No, thanks. Let's go on," impatiently replied Thorpe. As they drew near the detective, in order to make doubly sure of avoiding a trap, Sam uttered in a low voice the word "Hope!" It was a watchword previously arranged and provided as an additional precaution against a possible contingency of deep darkness rendering prompt recognition difficult. It was answered by the word "Good," uttered in equally low and cautious tones, and which at once put them at their ease.