How did the brick cone inside St. Paul’s Cathedral support its dome?

The dome of St. Paul’s Cathedral is a complex structure composed of three nested shells, with the middle shell serving as the primary structural support.12

This middle shell is a brick cone built in a nearly conical catenary form, which was chosen to effectively manage the large, concentrated load of the heavy stone lantern positioned at the apex. To ensure stability, the brick cone is constrained by iron chains. This concealed cone acts as the main load-bearing element, supporting both the exterior lead-covered dome and the lantern above it. Within this structure, a shallower inner dome is suspended to serve as the interior ceiling, carrying only its own weight.12

By utilizing this hidden conical support, Christopher Wren was able to independently design the internal and external forms of the cathedral to achieve his desired aesthetic and structural goals.1