For ten days I lay in that little cell, prisoned with the big Northman. At my first inspection of the place, I saw that there was no possibility of escape, for the walls were of smooth stone, and the only opening in them was that of a two-inch pipe that served to ventilate the cell. There was no window, as we think of it, yet the room was light enough in the daytime, for as the sun rose, the side of the cell facing on the building’s outer wall became invisible, allowing plenty of light to enter. This explained a fact that had puzzled me, the absence of windows on the exteriors of the cylindrical buildings of the city. Evidently the people of the city treated the outside walls of their buildings in such a manner that in daylight they were invisible from the inside, while perfectly opaque wh
