This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
(A reproduction at small size of Portfolio Plate XXXII.)
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 character 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 explanation. The ceiling must be supported on the walls of the porch, which is square in plan, but the domical ceiling is circular in plan; therefore a horizontal section 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 joined in some way so that the walls will support the ceiling and the ceiling cover all the space enclosed by the walls. Fig. 28.
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 EFGH 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 (EFGH) 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.
159. The subject of this exercise (Plate XXIX) is a commemorative chapel of the Denticular Doric Order, and is to be drawn at the size indicated-13"X l8". This is the first of three exercises where a dome plays an important part in the exterior effect of an edifice. In any study, in elevation, of a building employing a dome or cylindrical story, it must be remembered that, in perspective, that portion which is circular in plan looks considerably smaller with reference to the square base from which it springs, than it does in any elevation,-on account of the difference in plan between a square, and a circle which is contained within such a square;-in other words, the circle remains of the same diameter if seen from any point; while an object square in plan, seen from any other position than in direct elevation, has its width considerably increased by the projecting corners.
160. The plan of the chapel is a square, having on the side of the principal facade, a projection formed by two columns placed upon pedestals and enclosing an arch whose proportions are like those of Fig. 25, this projection being crowned by a pediment. The opposite side has a semi-circular projection, in which is located a niche in which the altar may be placed.
161. The entablature surrounds the entire building, but the triglyphs are found only beneath the projecting pediment of the main facade. The building itself is surmounted by a low attic in the form of a plain parapet, above which are two steps forming a base for the domical roof.
162. The interior of the chapel is a square with its floor raised three steps above the exterior level. In the corners are pilasters forty parts in width and fifteen in projection; these pilasters, and also the entablature which surmounts them, are repetitions of the exterior order. The ceiling is a semi-circular vault or dome.
163. At the side of the facade is indicated the commencement of a retaining wall, with a grille, which might be continued to enclose a plot of land.
164. Exercise R is a circular temple (Plate XXX, and plan Fig. 29) with a pedimented porch or portico, showing the use of the order set upon a dado around the interior walls. The ceiling is domical, with an opening in the center, and is ornamented on the under side by a series of recessed panels called caissons or coffers. This plate, like the one preceding, is to be drawn at the size of 13 x 18 inches.