He worked his boat round, and drove it steadily towards the great building, shaping his course a little upstream, in order to bring himself above it once more. He watched closely as he sculled, and when he checked his way not ten yards from the bank he was quite certain of two things: he had not seen the light again, and he had not seen any boat leave the front of the warehouse. He let himself drift slowly down, staring and staring, and full of wonder. His eyes were now so used to the starshine on the river that he could see the water in front of the building like a smooth, pale plain, and it was empty--it was perfectly empty. Who had struck that light about the water-level? It was all very strange and mysterious. Chippy let his craft drift. It moved slowly on the slow-running stream, but presently it was under the shadow of the lofty wall, and as it slid along, Chippy looked out more sharply than ever for the source of that strange light. He stood in the stern of the boat drifting down in complete silence, with not even the gurgle of the sweep to betray his presence. And to this complete silence Chippy owed the discovery which he made about midway of the river-front. He was staring straight at the blackness of the wall, when suddenly a light appeared in it. To his immense surprise, he found himself looking up a kind of long, arched tunnel, at whose farther end a man stood in a boat, a light in his hand. Only for an instant did Chippy behold this strange vision. His skiff drifted on, and he was faced once more by the darkness of the solid wall. Chippy drew a deep breath, dug his sweep into the water, and sculled rather more than half a circle. This brought him opposite the mouth of the tunnel, but well out from the wall. 'That's wheer they'd slipped in,' reflected Chippy. 'Theer's the light again. Wot does it all mean? I never heerd o' that hole afore.' Chippy was puzzled because he did not know the history of Elliotts' warehouse. It was a fairly old building, having been erected about the middle of the eighteenth century. Its basement had been pierced by a water-gate, which gave small barges direct entrance to the building, their contents being raised to the floor above through a large trap-door. But in the course of time, and under the influence of great floods, the river scoured out its bed in such fashion as to alter its depth against the wall of the warehouse, and largely to block the water-gate with mud. Sooner than undertake the expense of dredging in order to keep the water-gate open, the owners abandoned its use, and knocked a doorway in the front, and hauled up from the barges as they lay outside. But on a very low tide it was possible yet to pole a small boat up the old water-gate, and gain the trap-door, which still existed, though unused, and almost unknown to the present generation of workers in the warehouse.