But the dominant feature of the interior is the octagon at the crossing, which comprises the width not only of the nave and choir but also of the aisles. It permits four great arches, opening into the nave, choir, and transepts, and four smaller and lower arches, connecting with the ambulatory, which is formed by the aisles. This arrangement is somewhat similar to the octagon of Ely Cathedral and may be compared with the plan of the dome of the Invalides. Surmounting the eight pendentives of St. Paul’s is a circular gallery, known as the “Whispering Gallery,” above which rises a circular peristyle. The latter’s entablature supports the interior dome, which mounts to a height of 281 feet from the floor. In recent years the barrenness of the interior has been considerably relieved by glass mosaic decorations, designed by Sir William Richmond. =The Façades= comprise two stories; the lower embellished with the Corinthian order, the upper with the Composite; the line of division being at the height of the aisles. Thus, on the north and south sides of the building, the upper part of the façade is only a screen, carried up for the purpose of composing with the mass of the dome. The flying buttresses of the latter are concealed behind it, while light penetrates through it to the clerestory windows. Admirable features of the lower story of the side façades are the semi-circular porticoes, of beautiful design, which project from the ends of the transepts. Excellently proportioned, if somewhat bald, is the west façade, which is a double storied portico of coupled columns, supporting a pediment. This is flanked by two towers, which rise above the sky-line in diminishing stories, terminating in bell-shaped cupolas. Not only are they fine compositions in themselves, but they are also designed in fine relation to the dominating feature of the dome. =The Dome.=--The latter, in mass and outline and in the relation achieved between its several parts, can lay claim to being the most majestic dome of the Renaissance. Among the elements that enter into its impressiveness is the emphasis given to the lowest course of masonry, which well suggests the union of the nave, choir, and transepts and forms a substantial stylobate to the peristyle. The latter, again, is exceptionally fine in proportion. In appearance, relatively higher than that of S. Peter’s and related with more freedom to the mass above, it is formed of coupled columns attached to radiating buttress walls; every fourth space between the columns being filled with solid masonry, which is relieved in the way of light and shadow by a decorated niche. The effect is at once strong, stately, and of airy lightness. Very fine also, in its peculiar accent of effectiveness is the proportion of the upper drum to the superincumbent mass of the dome, whose curve is lifted to a culminating springiness by the height and freedom and sensitive proportions of the lantern.