"Nein, you go ahead, den I keeps you in sight," said the policeman, pushing the boy before him. "You gets not pehind me ef I knows vat I do." Frank made no reply, but plowed his way on through the snow, Officer Schneider following close in his rear. At a word from that individual, upon reaching New Church street, he turned to the right and moved slowly along the wall of Trinity church-yard, here rising high above their heads. To the left rose the structure of the elevated railroad, to the right the wall itself, the iron railing still surmounting it, from which hung, depending, clusters of the snow-covered vines. Not a soul was to be seen moving about them. The street was deserted for as great a distance as the eye could reach. They had hardly proceeded half the length of the wall, when, proceeding apparently from the clustered vines above them, a shrill, piping sound was heard. It was precisely such a sound as bats are sometimes heard to make just at nightfall as they go whizzing through the air. "Vat in tonder ish dat?" exclaimed Officer Schneider, coming to a sudden halt, and looking up into the air. Again the strange sound broke upon the stillness of the night a little ahead of them through the thick mass of falling snow. Both Frank and the policeman had now checked their steps close to the church-yard wall. At the recurrence of the sound, forgetting for the instant that he had a prisoner on his hands, Officer Schneider moved a few steps in advance of the boy, and raising his head, tried to peer in among the clustered vines, all laden with snow, which overhung the iron fence upon the top of the wall, leaving the boy standing behind. "I can't see notings," he muttered, "but dere's something crooked going on up dere. Dis is not de first time nor de second I hear dot sound. Who goes up mit dem valls on top, I'd schust like to know?" Now this was a little private mystery of the officer's own, this self-same bat-like sound. Several times previous to the present occasion had he heard it, and---- Hello! And what ails Officer Schneider now? Enough to make him stare as he does, and shower all the German imprecations known to his vocabulary upon the air around. For the handcuffed prisoner, whom he supposed to be standing so close to him that it was only necessary to reach out and touch him, had during that one moment in which he had raised his head to the top of the wall above him, from which proceeded the bat-like sound, most mysteriously vanished. There were his footprints on the snow-covered walk, nor was there other prints anywhere to be seen upon the smooth surface of the street, save those made by the boy and himself, and pointing up--not down the street. And this was all. His handcuffed prisoner, who could not by any possibility have advanced a dozen steps through the snow in the brief space of time during which the policeman's attention had been withdrawn, had strangely, marvelously disappeared. CHAPTER V.