Divide the inside of a dome into coffers that shrink in step with the meridians as they rise, so that their perspective deepens the dome and draws the eye to the light of the lantern.
The sweating room: as high as it is wide
Make the sweating room of a bath circular, as high to the springing of its dome as it is wide, with an opening at the crown closed by a bronze disc that can be raised and lowered on chains to regulate the heat.
height to springing = width; oculus with a movable bronze shield
The laconicum was the dry sweating room of a Roman bath. Vitruvius wants it round so that the heat of the fire spreads evenly from the centre to the walls, and vaulted with a dome pierced at the top. The domed rotundas of bath buildings, and the so-called Temple of Mercury at Baiae (a bath hall with a concrete dome of over 21 m in the first century BC), were the laboratories in which Roman builders learned to cover round rooms.
The Pantheon applies the same recipe on a colossal scale, with the proportions reversed: its drum is half, not the whole, of its width, and its oculus is always open. Rain falls through it onto a floor that slopes gently to drains.
Related rules
Raise the central nave above the lean-to roofs of the aisles and pierce the upper walls with a row of windows, the clerestory, so that the middle of a wide hall is lit from high on both sides.
nave height ≈ 1½–2 × nave width; one window over each bay of the arcade
For a hall of assembly use the “D” or tribune plan, a half-circle closed by a straight side: it gives ample room to the audience, a central place to the speaker and scope for display in the architecture.
Set the columns of a round temple on a circle over a stepped base, carry the entablature round the circle, and cover the cella with a dome whose height is half the diameter of the whole.
dome height ≈ ½ diameter (Vitruvius IV.8, for the peripteral round temple)
Make the height of the rotunda to the top of its dome equal to its diameter, so that the interior would exactly contain a sphere resting on the floor: the drum rises to the centre of the sphere and the dome is its upper half.
Pantheon: Ø = height = 43.3 m (150 Roman feet); drum = ½ Ø
Divide the inner surface of the dome into sunk panels arranged in rings and meridians; let them diminish as they rise with the converging meridians, and stop them short of the crown so that a smooth zone surrounds the eye.
Pantheon: 5 rings × 28 coffers, each stepped in several recessions
Cite this page Chris Paton, “The sweating room: as high as it is wide”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/laconicum-dome. Text, drawings and models licensed under CC BY-NC 4.0.