The Thinking Machine examined the work that had been done, grunted his satisfaction, and together they went to the skylight, leaving a thin, insulated wire behind them, stringing along to mark their path. They passed down through the roof and into the darkness of the hall of the upper story. Here the light was extinguished. From far below came the faint echo of a man's footsteps as the watchman passed through the silent, deserted building.