woodcut of that design in Serlio's book, or in that of Palladio. Wren has, of course, altered and amplified the scheme in adaptation to his vast scale and lofty proportions, but the general composition of the two is substantially the same, though the internal structure is entirely different. The leading features of the exterior, the encircling order crowned with the balustrade and the dome rising over it surmounted by the lantern, are those of Bramante's design.
Fig. 136.—Section of the dome of St. Paul's.
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
