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.
66. The abacus of the Ionic capital is square; it projects six parts from the lower face of the architrave or from the upper diameter of the shaft of the column, is four parts in height and is composed of a fillet of two parts and a cyma-reversa of two. The fillet also has a projection of two. The upper face of the abacus forms a square of fifty-one on each side, and the lower face a square of forty-seven; the volutes grow from beneath the abacus on opposite sides; the catheti, which are the vertical axes or center lines of the volutes, are placed a distance of twenty-one from the axis of the column, or project one and five-tenths be -yond its upper diameter. The height of the volute being twenty, the three following dimensions may be laid out on the catheti below the abacus; ten for the volute above the eye, two and five-tenths for the diameter of the eye, and seven and five-tenths for the lower part of the volute. The volute may then be drawn.
67. The spiral or volute is composed of twelve quarter circles drawn from twelve different centres, which may be located in the following manner. Having established, on a vertical line called the "cathetus," the height of the volute, twenty parts, it is divided into eight equal portions. The divisions are marked 1, 2, 3, 4, 5, 6, 7 and 8, commencing at the lower edge. Mark the middle of the space included between the points 3 and 4, and draw through this central point a horizontal line. Taking this same point for the center, draw with a radius of one-half part, a circle which will be the eye of the volute. This eye is shown enlarged in Fig. 11. Divide into two equal parts the two radii of the eye which coincide with the cathetus, C-D giving the points 1 and 4, and here construct a square of 1,2, 3, and 4, in the direction in which the mass of the volute is to be drawn, in this case on the left of the cathetus. The side of this square which coincides with the cathetus being divided into six equal parts, the other two squares -five, six, seven, eight, and nine, ten, eleven, twelve-may be drawn. In this manner are obtained twelve center points at the corners of the squares, numbered from 1 to 12 from which are drawn the twelve quarter circles that constitute the exterior spiral. Horizontal and vertical lines from these twelve centres determine the limits of the twelve quarter circles.
SIDE VIEW OF THE MODERN BANK BUILDING OF THE AMERICAN SECURITY AND TRUST COMPANY, WASHINGTON, D. C.
68. In order to trace the second spiral which forms the inner edge of the fillet of the volute, divide into three parts on the cathetus (Plate X) the space included between the first and the second revolution, that is to say, the distance between the points six and eight. One-third of this distance 6-8 will be the width of the fillet. To find the twelve centers for the second spiral, draw three new squares of which the height and position are determined by dividing into thirds the space between the squares of the first spiral so that the new square 1', 2', 3', 4', (Fig. 11) shall be within the square 1, 2, 3, 4, by just 1/3 the distance from 1 to 5 and from 4 to 8. The new squares 5', 6', 7', 8', and 9', 10', 11', 12', will have corresponding relations to squares 5, 6, 7, 8, and 9, 10, 11, 12, respectively. From the points 1' to 12' inclusive, the second spiral may be drawn in the same manner as the first.
69. For the outer fillet, which appears below the abacus and beyond the cathetus, (Plate X) find four center points by constructing a new square larger than the square 1, 2, 3, 4, (Fig. 11). This is determined by taking on the cathetus C-D, half of the distance from the point 1 to the point 1', and measuring this distance outside of the point 1 to the point 1", from which 2", 3", 4", etc., can be readily drawn.
70. The space included between the lower part of the abacus and the first complete revolution of the volute forms a flat band which ties together the two volute faces of the capital, and this band is set back two and five-tenths from the projection of the abacus. (See section through side of capital.) The fillet disappears in this face by a quarter of a circle drawn from the point six on the cath-etus. The space between the lower line of this face and the horizontal line passing through the center of the volute eye is taken up by a quarter-round drawn with a radius of six and projecting four and five-tenths from the face of the volutes or eight from the outside of the shaft, as may be seen at B in the section on the right of the drawing of the "Side of the Capital." This moulding follows the circular plan of the shaft and is ordinarily decorated with eggs and darts. Below this quarter-round is found an astragal which unites the capital with the shaft; this astragal is three and five-tenths parts in height, of which two and five-tenths are for the bead and one for the fillet, the projection is two and five-tenths of which one and five-tenths is for the bead, and one for the conge
71. The side face of the capital, called the "roll," unites the volutes of the two faces. It is forty-six parts in width and is divided in the center by a sunken band of six (or seven) parts in width which is ornamented with two bead mouldings of two parts each spaced one part apart. The height of this band below the abacus is fourteen, as shown in the section; the space included between it and the return or inner edge of the face of the volute is sixteen or sixteen and five-tenths. This part is bell-shaped, and its outline is obtained on the side of the capital as follows: Having prolonged the horizontal line marking the lowest point of the volutes, find on it two points, the one, two and five-tenths from the band at the center, the other five and five-tenths from the inner edge of the volute, and here erect two perpendiculars; on the first of which mark heights of four and five-tenths, and of eight and five-tenths, and on the second three and five-tenths, and nine and five-tenths. Four points a, b, c and d will be obtained by this means through which the curves may be readily drawn.