"Vitruvius... thus describes the construction of the or stove for heating the or sweating room of a bath. The floor is made inclining, so that a ball placed on any part of it would roll towards the fireplace, by which means the heat is more equally diffused in the sweating-chamber. The floor is paved with tiles... and on these are built brick pillars... two feet high, and cemented with clay and hair mixed together. The pillars are placed at such a distance, as will allow tiles to be laid on them to form the ceiling of the hypocaust, and support the pavement of the caldarium. The air to the caldarium, or room over the hypocaust, is admitted through an aperture in the centre of its roof, from which a brazen shield is suspended by chains. By raising and lowering this shield, which opens or shuts the aperture, the heat of the caldarium is regulated. For heating the water to supply the baths, there are to be three caldrons... the arched cavities in which they stand are to be heated by one fire. After such minute instructions how to form the stove in which large quantities of wood were to be consumed, it is singular that he should omit to notice in what way the smoke produced was to be conveyed into the atmosphere. From this silence, it has been inferred that he was ignorant of what Anderson calls "the elegant and commodious tube now known by the name of a chimney.""