One of the design problems that confronted Wren was to create a landmark dome, tall enough to visually replace the lost tower of St Paul’s, while at the same time appearing visually satisfying when viewed from inside the building. Wren planned a double-shelled dome, as at St Peter’s Basilica. His solution to the visual problem was to separate the heights of the inner and outer dome to a much greater extent than had been done by Michelangelo at St Peter’s, drafting both as catenary curves, rather than as hemispheres. Between the inner and outer domes, Wren inserted a brick cone which supports both the timbers of the outer, lead covered dome and the weight of the ornate stone lantern that rises above it. Both the cone and the inner dome are 18 inches thick and are supported by wrought iron chains at intervals in the brick cone and around the cornice of the peristyle of the inner dome to prevent spreading and cracking.