Generations of rats had used that run. Its sides were smooth and polished as a metal tube. Here it was narrower, there wider, but throughout its length it was free and unimpeded. For the most part it lay between wall and wainscot. At times it seemed to pierce the masonry itself. Midway in the ascent the path of least resistance had been towards the outer wall. Two round holes pierced its surface--a brick's length dividing them. One can understand the making of the first hole, but the making of the second? Fifteen feet below resounded the busy traffic of the city. Did two tunnels converge by chance? did they converge by design? or was the second made by some colossal rat, stretched at full length, and trusting his life to his superhuman hearing? I can only state the facts. I do not pretend to explain them. From the second hole the run passed into the masonry once more, zigzagged upwards into the storeroom, and ended. From the storeroom there were countless exists--down the gutter into the courtyard (a short cut to the shambles), beneath the flooring to the scullery, and thence along the drain-pipe to the great sewer, through the ventilator on to the roof--anywhere, everywhere. * * * * * The scratching was certainly louder. The black rat was stepping very delicately, but a slippery corn-husk shot from underneath his foot, and with the rustle of the corn-husk the scratching ceased. Nothing but a rat could have heard that; it was certainly a rat, but who? For ten minutes he waited, listening. Then he stole forward, until the points of his whiskers brushed lightly against the door. Instantly there was a movement on the far side--a four-footed movement. Caution against such cunning seemed superfluous. He boldly forced his nose between door and flooring and sniffed; but only for a second, for his nose had gone farther than he meant; the bottom of the woodwork had been gnawed through until it was a bare half-inch thick all along its length. He drew back with a jerk, and waited another ten minutes, staring at the door and thinking. The silence on the far side grew unendurable. The black rat whisked round, and rushed madly for the run. He gained the shelf by a beautiful swinging leap, easy and silent as a cat's. [Illustration: HE STOLE FORWARD UNTIL THE POINTS OF HIS WHISKERS BRUSHED LIGHTLY AGAINST THE DOOR.] For the first few yards, between brickwork and wainscot, the run was clear enough; but, as it turned upwards to the floor above, something seemed unfamiliar. The light, which had always faintly shimmered from the hole in the outer wall, was gone. As he drove forward headlong, he bruised his nose against the cause of its disappearance. The wall had been repaired with concrete. It was utterly ungnawable, and he slowly retraced his steps to the cellar. He was just in time to hear the scratching recommence. [Illustration: HE TRIED TO STEADY HIMSELF BY NIBBLING AT A STRAY CORN-EAR.]