This section is from the "The elements of drawing & the elements of perspective" book, by J. M. Dent & Sons. Also see Amazon: The Elements Of Drawing & The Elements Of Perspective.
If it happens that the vanishing-points of the diagonals are awkwardly placed for use, bisect the nearest base line of the pyramid in B, as in Fig. 59.
Erect the vertical D B and join G B and D G (G being the apex of pyramid).
Find the vanishing-point of D G, and use D B for division, carrying the measurements to the line G B.
In Fig. 59., if we join A D and D c, A D C is the vertical
profile of the whole pyramid, and B D C of the half pyramid, corresponding to F G B in Fig. 57.
We may now proceed to an architectural example.
Let A H, Fig. 60., be the vertical profile of the capital of a pillar, A B the semi-diameter of its head or abacus, and F D the semi-diameter of its shaft.
Let the shaft be circular, and the abacus square, down to the level E.
Join B D, E F, and produce them to meet in G.
Therefore E C G is the semi-profile of a reversed pyramid containing the capital.
Construct this pyramid, with the square of the abacus, in the required perspective, as in Fig. 61.; making A E equal to A E in Fig. 60., and A K, the side of the square, equal to twice A B in Fig. 60. Make E G equal to C G, and E D equal to C D. Draw D F to the vanishing-point of the diagonal D v (the figure is too small to include this vanishing-point), and F is the level of the point F in Fig. 60., on the side of the pyramid.
Draw F m, F n, to the vanishing-points of A H and A K. Then F n and F m are horizontal lines across the pyramid at the level F, forming at that level two sides of a square.
Complete the square, and within it inscribe a circle, as in Fig. 62., which is left unlettered that its construction may be clear. At the extremities of this draw vertical lines, which will be the sides of the shaft in its right place. It will be found to be somewhat smaller in diameter than the entire shaft in Fig. 6a, because at the centre of the square it is more distant than the nearest edge of the square abacus. The curves of the capital may then be drawn approximately by the eye. They are not quite accurate in Fig. 62., there being a subtlety in their junction with the shaft which could not be shown on so small a scale without confusing the student; the curve on the left springing from a point a little way round the circle behind the shaft, and that on the right from a point on this side of the circle a little way within the edge of the shaft. But for their more accurate construction see Notes on Problem XIV.