No less remarkable is the scientific knowledge expended in the construction of this externally superb masterpiece. It is composed, like the domes of the Invalides and the Panthéon in Paris, of three shells, although the arrangement is different. For the intermediate shell consists of a cone of brickwork, 18 inches thick. It springs from behind the upper drum, and on it bears the stone lantern, ball, and cross; the last being 365 feet above the ground level. It also helps to bear the weight of the timber supports of the outer shell, which is constructed entirely of wood, sheathed with lead. The inner dome, resting on the peristyle, is of brickwork, and of the same thickness as the cone. =Wren’s Churches.=--Between the years 1670 and 1711 were erected some fifty-three London churches, in which Wren displayed remarkable versatility in adapting Renaissance design, not only to the different conditions which the crowded site involved but also to the requirements of Protestant worship, which laid so much stress on preaching and needed chiefly an auditorium. A famous example is that of =S. Stephen’s Walbrook=, in which sixteen columns support a coffered ceiling, interrupted by a pendentive dome. This is the predominating feature, for its diameter is 43 feet in a total width of 60 feet. Wren’s churches, however, are better and more characteristically known by the variety of steeples, which may be considered an invention of his own. From a square tower, which is treated as the main feature of the front façade, they pass into circular or octagonal stories, diminishing in diameter, clothed with Renaissance details, and terminating in a slender spire. Their beauty consists in the variety and proportions given to the several parts, achieving an ensemble of peculiar elegance. Occasionally they suggest a certain mechanicalness of repetition; hence the example which is considered the best is that of =S. Mary-le-Bow=. For here the repetition of the orders is interrupted by a story composed of inverted consoles, the effect of which is to vary not only the character of the embellishment, but also, by introducing the contrast of a curve, the regularity of successive steps. Wren’s inexhaustible activity is represented also, among many other examples, by the =Monument= at =London Bridge=; =The Fountain Court= and =Garden Façade= of =Hampton Court=; =Chelsea Hospital=; =Marlborough House, Pall Mall=; and =Temple Bar=. The last, forming the entrance gate to the City of London proper, has been removed from its old site at the foot of Fleet Street, and set up in Theobald’s Park, Northamptonshire. He lies buried beneath the choir of his masterpiece, a tablet bidding you, “Si monumentum requiris, circumspice.”