10First, as warm a spot as possible is to be selected, that is to say, one sheltered from the north and north-east. The hot and tepid baths are to receive their light from the winter west; but, if the nature of the place prevent that, at all events from the south, because the hours of bathing are principally from noon to evening. Care must be taken that the warm baths of the women and men adjoin, and have the same aspect; in which case the same furnace and vessels will serve both. The caldrons over the furnaces are to be three in number, one for hot water, another for tepid water, and a third for cold water: and they must be so arranged, that the hot water which runs out of the heated vessel, may be replaced by an equal quantity from the tepid vessel, which in like manner is supplied from the cold vessel, and that the arched cavities in which they stand may be heated by one fire.
2The floors of the hot baths are to be made as follows. First, the bottom is paved with tiles of a foot and a half inclining towards the furnace, so that if a ball be thrown into it, it will not remain therein, but roll back to the mouth of the furnace; thus the flame will better spread under the floor. Upon this, piers of eight inch bricks are raised, at such a distance from each other, that tiles of two feet may form their covering. The piers are to be two feet in height, and are to be laid in clay mixed with hair, on which the above-mentioned two feet tiles are placed, which carry the pavement.
3The ceilings, if of masonry, will be preferable; if, however, they are of timber, they should be plastered on the under side, which must be done as follows. Iron rods, or arcs, are prepared and suspended by iron hooks to the floor as close as possible. These rods or arcs are at such distances from each other, that tiles, without knees, may rest on and be borne by every two ranges, and thus the whole vaulting depending on the iron may be perfected. The upper parts of the joints are stopped with clay and hair. The under side towards the pavement is first plastered with pounded tiles and lime, and then finished with stucco or fine plastering. If the vaulting of hot baths is made double it will be better, because the moisture of the steam cannot then affect the timber, but will be condensed between the two arches.
4The size of baths must depend on the number of persons who frequent them. Their proportions are as follow: their width is to be two thirds of their length, exclusive of the space round the bathing vessel (schola labri) and the gutter round it (alveus). The bathing vessel (labrum) should be lighted from above, so that the bye standers may not cast any shadow thereon, and thereby obstruct the light. The schola labri ought to be spacious, so that those who are waiting for their turn may be properly accommodated. The width of the alveus between the wall of the labrum and the parapet must not be less than six feet, so that it may be commodious after the reduction of two feet, which are allotted to the lower step and the cushion.
5The laconicum and sudatories are to adjoin the tepid apartment, and their height to the springing of the curve of the hemisphere is to be equal to their width. An opening is left in the middle of the dome from which a brazen shield is suspended by chains, capable of being so lowered and raised as to regulate the temperature. It should be circular, that the intensity of the flame and heat may be equally diffused from the centre throughout.