All the unanalyzed or new details of the Tuscan Order used in this exercise are shown at a larger scale on this plate. The interior entablature of this problem is the same as the exterior.
This concludes the required Examination; the remaining plates are given as a guide for students desiring to do further work by themselves.
155. The entrance pavilion in the Ionic Order, shown in Plate XXVIII, is a problem similar to the one that has just been taken up. The student is to reproduce this plate at the large size to which lie has already drawn Plate 0, with border line of 13 X 18 inches.
The small edifice is such as might be used at the entrance to certain public buildings, its plan—the same as that of the guard house—being composed of a porch with a room upon One of these rooms might be the lodge for a porter, the other might be a ticket office. One quarter of the plan only is given as the arrangement is the same on the other sides of the axes.
156. The front is composed of three divisions separated by columns or pilasters. In the center is the archway of the porch and at each side is a window whose sill is supported by consoles, and surmounting the outside frame are consoles of a different char. acter which support the cornice and pediment. These details are shown in A, B and C on this plate. The same details may be applied to the door of the porch.
The windows at the side are similar to those on the principal elevation. The entablature is surmounted by a balustrade divided by pedestals carrying vases; the details of these balusters and of the vases are shown on this plate.
157. The porch is square in plan but has a ceiling or "cupola" in the form of a dome or spherical vault; that is, the ceiling has the shape of a segment of a sphere, whose radius is 2-En and 20 parts, as shown in the sectional elevation in Plate XXVIII. This kind of ceiling requires explan ation. The ceiling must be sup ported on the walls of the porch, which is square in plan, but the domical ceiling is circular in plan; therefore a horizontal see tion of the porch at the point where the walls end and the ceiling begins will show a square for the section of the walls and a circle for the section of the ceiling. These two geometrical figures must be joinod in some way so that the walls will support the ceiling and the ceiling cover all the space enclosed by the walls.
Whenever a square space is to be covered by a dome, the semi. diagonal of the square may be taken as the radius for the circle which forms the base or springing line of the dome. Fig. 28 shows at A B C D such a square and circle. If the four walls which form the sides of the square building are now continued upward, they will cut into the spherical segment whose base is represented by the circle, since this circle overhangs the square on all four sides. The figures cut from the domical surface by the walls will be segments of circles,—the intersection of a plane with a segment of a sphere. These segments of circles are shown in plate XXVIII as the semi-circular arches of radius 1-En and 50 parts, which cover the doorways. A horizontal section taken through the dome at the elevation of the crowns of these circular segments will show a circle which (in plan) will be inscribed in the square formed by the four walls, as shown by the smaller circle E F G H in Fig. 27. This circle is also shown dotted in the plan in Plate XXVIII.
The spherical surface which forms the ceiling of the porch has now been cut into, first by the four walls as they are continued upward from the springing line (A B C D) of the dome, and second by a horizontal plane (E F G H) passing through the crowns of the four arches cut from the sphere by the walls. All that is left of the spherical surface is a triangular` segment E D H in each corner. This portion of the ceiling is called the pendentive. In Plate XXVIII an elevation of the pendentive is shown at P.
158. The horizontal plane at the crowns of the arches cuts out from the spherical surface a circle (E F G H), which may now. be covered over by a dome, or segment of a sphere, which may spring directly from it. In Plate XXVIII this circle is represented in elevation by the first horizontal line of mouldings above the arches. In this particular case, the domical ceiling or cupola does not spring directly from this circle but a small cylindrical band, or entablature, is built up above it for a height of 90 parts, from the top of which the ceiling springs.