[ CXXVIII]
Semicircular Arches on a Retreating Plane
First draw perspective square abcd. Let ae· be the height of the figure. Draw ae·f·b and proceed with the rest of the outline. To draw the arches begin with the one facing us, Eo·F enclosed in the quadrangle Ee·f·F. With centre O describe the semicircle and across it draw the diagonals e·F, Ef·, and through nn, where these lines intersect the semicircle, draw horizontal KK and also KS to point of sight. It will be seen that the half-squares at the side are the same size in perspective as the one facing us, and we carry out in them much the same operation; that is, we draw the diagonals, find the point O, and the points nn, &c., through which to draw our arches. See perspective of the circle (Fig. 165).
Fig. 234.
If more points are required an additional diagonal from O to
K may be used, as shown in the figure, which perhaps explains itself. The method is very old and very simple, and of course can be applied to any kind of arch, pointed or stunted, as in this drawing of a pointed arch (Fig. 235).