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 the roof, from which a brazen shield is suspended by chains. By raising or lowering this shield, which opens or shuts the aperture, the heat of the caldarium is regulated[1]."
Secondly. "For heating the water to supply the baths, there are to be three caldrons, one for hot water, another for tepid, and a third for cold; arranged so that as the hot water runs out of the lower vessel, it may be replaced from the tepid vessel, and that in like manner replenished from the cold vessels[2]."
A third use of the hypocaust, viz. for heating domestic apartments, is stated by Seneca to have come into fashion within his memory. For this purpose, "The hypocaust being constructed in the under story of a building, and in the manner described by Vitruvius, several pipes of baked clay are then built into walls, having their lower ends left open to the hypocaust. These pipes were carried to the height of the first or second story, and had their upper orifices made to open into the chamber that was to be heated. They were closed by moveable covers."
It is clear that this system must have been subject to many of the evils attendant on the use of the simple charcoal brazier, and it appears from Seneca that they were considered as unwholesome, as similar methods of heating are now found to be.
The author then enters more fully into the details of the construction of the heating apparatus, and gives several woodcuts which illustrate admirably his statement of the case. The first of these illustrations enables us to present to our readers the representation of the caldarium resting on its pillars. [3]footnote of unknown location (Wikisource contributor note)
The next woodcut gives a plan of the arrangement of the pillars, which rested upon a thick stratum of cement, composed of lime and pounded bricks. The floor of the caldarium itself was made of a stratum of cement nine inches thick, ornamented by mosaics. The sides were hollow, so as to permit the warm air from the hypocaust to ascend to the cornice of the room.
