The Pantheon, Rome

The rules it illustrates

Proportion each order for its real size. Do not enlarge or reduce an order by one set of ratios: design its members for the size at which it will be built, the material and the distance from which it is seen. In a large order make the parts relatively finer, in a small one relatively bolder.

Steady a great rotunda with projecting features. Do not leave a large circular building as a bare cylinder: steady it with features that break the curve and fix its axes: a portico, stairs, a projecting bay or a recess on the axis.

A sphere within a cylinder. 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.

Coffers in rings that shrink toward the crown. 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.

Thick and heavy below, thin and light above. Build a great concrete dome thickest and heaviest at its haunches and thinnest and lightest toward the crown, grading the aggregate from dense stone at the base to light volcanic pumice at the top.

The lower dome pulls apart. Expect a hemispherical dome to be in compression along its meridians everywhere but in tension round its horizontal rings below about 52° from the crown; restrain that lower zone with weight, thickness, rings or chains, or it will crack along the meridians.

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.

Where it appears in the book

In the styles Ancient Roman.

In the chapters Domes, The Classical World.