To-day was the third successive Friday, and Constans had made preparations for the careful noting of the phenomenon should it reoccur. He waited with a lively sense of expectation, and he was not disappointed. At high noon the humming began again, and it seemed to be louder than when he had listened to it on the two former occasions--the air was full of the vibrant droning. There was a sinister quality, too, in its monotone, and Constans for the moment felt himself swayed by a gust of superstitious terror. He recalled the traditions current among the House-dwellers, the belief that Doom was inhabited not only by the outlaws but by demons of many a grewsome sort and kind. There were strange tales of lights that lured the wanderer onward, only to vanish as the victim sank into some frightful abyss; of invisible hands that plucked at the rash intruder's skirts; of monstrous shapes that leered and gabbled behind the traveller's back and were only blocks of stone when he turned to face them; of bloodless creatures that one might meet in the full flood of day, and whose unearthly character was only to be proved by observing that they cast no shadow in even the brightest sunlight; of vampires and ghouls and fair women with enchanting voices, who enticed their victims into blind passageways and then changed suddenly to foul, harpy-like monsters. But in this latter case the foolish one had only himself to blame, for if he kept on the lookout he could always detect the masquerade by observing the creature's hands. The harpies could transform themselves in every other way, but their claws remained unchanged, and they were, consequently, obliged to cover them with gloves. "Beware the gloved hand," was a familiar aphorism among the wise women of the West Inch, and Constans, shaken in spite of himself by the remembrance of these old fables, felt the sweat break out upon his forehead, for all that the wind blew shrewdly cold. Yet as he waited and listened and still nothing happened his natural good-sense reasserted itself. Overhead a glorious winter sun was shining; as everybody knew, the sirens never sang until after dark, and assuredly they were accustomed to give a much more artistic performance. His courage re-established, curiosity asserted her rights; he must discover the source and nature of this mystery, and so he proceeded cautiously in the direction from whence it now appeared to come, a course that led him south by east for perhaps ten of the city blocks. Constans found himself a short distance below the Citadel Square and in a quarter of the city that he had never yet explored. Suddenly he came upon a large building of brick covering a full square in area but only two stories in height. As he approached the humming noise grew louder and louder; the secret, whatever it was, lay concealed behind those common-place-looking walls. Constans held his breath and went forward slowly.