Figure 3 represents four different contours of domes, upon a square plan, No. 5; No. I, shows a semicircle contour ; No. 2, a pointed contour; No. 3, a Turkish, or Mahometan contour; No. 4, a bell-formed contour, as used iu the reigns of Elizabeth and James I.
Figure 4 represents five di&rent contours, upon an octa gonal plan, No. 6; No. 1, the contour of a dome, the vertical section of which being a semi-ellipsis, with the lesser axis placed horizontal ; No. 2, the contour of a semicircular sec tion ; No. 3, the contour of a dome, the vertical section of which is a semi-ellipsis, placed upon the greater axis; No. 4, the contour of a dome, the vertical section parallel to either side being the segment of a circle : No. 5, a pointed contour, formed similar to the dome of the cathedral church of S'' Maria del Fiore, at Florence, which was both octangular upon the plan, and pointed in its vertical section.
Figure 5 represents five contours upon a circular plan, No. 6 ; No. 1, is formed by having its vertical section the segment of a circle, which was the general practice of the Romans, in the exterior form of their domes, as in the Pan theon at Rome ; No. 2, a dome with a semicircular axal section, a form very frequently used in modern times; No. 3, au ellipsoidal dome, whose axis is the greater semi-axis of the ellipsis; No. 4, represents a dome with a parabolical contour, the vertical section being that of a curve, and the axis of tile parabojeal section being the same as that of the dome. This contour is more pointed than any part of an ellipsis; No. 5, represents a dome with an hyperbolical con tour, the vertical section being an hyperbola ; this is still more pointed than the parabolical dome.
Figure 6 shows tau• variations of contours : No. 1, pointed like the dome of St Paul's cathedra] church, London, which is composed of two arcs of a circle; No. 2, a dome with a small concavity upwards, springing perpendicularly from its base ; No. 3, another contour of a dome, with a small con cavity at the top, but spreading outwards as it rises from the bottom, with a convexity in its vertical section ; No. 4, a dome of the same nature as the last, but dissimilar in its form, the convex part being much quicker. Such forms as
the last three may be denominated Moresque, Turkish, or as they are practised by the Moors, Turks, and Ilindoos. This form was introduced into England in the reign of Henry and in constant use in the time of Ilenry VIII. Its use was in the crowning of turrets, as in the octagonal buttresses of Henry the Seventh's chapel, and the towers of King's College chapel, Cambridge : the turrets at the entrance of Christ's College, Oxford, executed by Sir Christopher Wren, are surmounted with domes of this form. The bell-funned dome, Figure 3, No. 4, succeeded Figure 6, No. 2 ; examples of it may be seen at Audley-End, in Essex, built in the reign of James I., and in the Tower of London.
It is a property of all domes to have their horizontal sec tions similar to each other, and to the base of the dome.
What may be properly denominated a Roman dome, is one whose axal section is a semicircle, or the segment of a circle less than a semicircle. Among the ancients, domes were only used in covering whole buildings of a circular plan; but among moderns they are used in covering any apartment, or distinct portion of a building, which too frequently has a very insignificant appearance, and, for want of magnitude, destroys the genera] contour of the whole, producing a kind of mixed outline at the finish of the edifice ; where in most points of view it is completely lost.
In modern architecture, the dome, when used, is gene rally raised upon a tower, or turret, and by this means the figure is shown more completely than if it were immediately raised from the walls; but care should be taken, that if the place over which it is to be erected be too small, it should not be adopted ; and if admitted in an edifice, it should bear a good proportion to the whole mass: when a dome is properly managed, nothing adds more grace or dignity to the te•mina tion of an edifice than the domic contour.
Plate Ill. Figure 1.—Given the plan of a square dome, and one of the axal ribs, at right angles to one of the sides ; the curie of the angle rib, and the covering.