The structural system of this dome (Fig. 136) is peculiar. From eight piers arches and pendentives are turned, forming the circular bed from which the drum rises to a great height, and from a level far below the top of this drum a dome of masonry, of slightly oval form is sprung. The drum is double, and the inner wall, which carries the dome, inclines inward, as in the rejected design, up to the springing level, and above this it rises vertically against the haunch of the dome. From the haunch a hollow cone of masonry is carried up far above the crown of the dome, where it is cut off and covered with a small segmental dome surmounted by a tall lantern of stone. The system is devised with a view to stability. The cone shape of the inner drum gives it resistance to the dome thrusts, and these thrusts are further fortified by a solid filling of masonry between the smaller cone above and the vault reaching more than halfway from the springing to the crown. The outer drum is a solid wall up to a level but little higher than the apex of the timber roof of the nave, where it forms a stylobate for the encircling Corinthian order. But the two drums are connected by heavy abutments across the interval between them, one behind each column of the encircling order, with a heavier buttress filling every fourth intercolumniation (Plate XI). The inner drum rises in diminished thickness above the entablature of the outer one in the form of an attic with an order of pilasters and square openings between. From this attic rises a false dome of timber, surrounding and concealing the great cone which is the real support of the lantern.