b.  Second Class.

Composite plans of this class are generally obtained by combining two types of the first class—­the one worked out on the principal axes, the other on the diagonal ones.

MS. B. 22 shows an elementary combination, without any additions on the diagonal axes, but with the dimensions of the squares on the two principal axes exceeding those of the sides of the octagon.

In the drawing W. P. 5b (see page 44 Fig. 1) the exterior only of the edifice is octagonal, the interior being formed by a circular colonnade; round chapels are placed against the four sides of the principal axes.

The elevation, drawn on the same sheet (see page 47 Fig. 3), shows the whole arrangement which is closely related with the one on Pl.  LXXXVI No. 1, 2.

MS. B. 21a shows: 

a) four sides with rectangular chapels crowned by pediments Pl.  LXXXVII No. 3 (plan and elevation);

b) four sides with square chapels crowned by octagonal domes.  Pl.  LXXXVII No. 4; the plan underneath.

MS. B. 18a shows a variation obtained by replacing the round chapels in the principal axes of the sketch MS. B. l8a by square ones, with an apse.  Leonardo repeated both ideas for better comparison side by side, see page 47.  Fig. 2.

Pl.  LXXXIX (MS. B. 17b).  Elevation for the preceding figure.  The comparison of the drawing marked M with the plan on page 47 Fig. 2, bearing the same mark, and of the elevation on Pl.  LXXXIX below (marked A) with the corresponding plan on page 47 is highly instructive, as illustrating the spirit in which Leonardo pursued these studies.

Pl.  LXXXIV No. 12 shows the design Pl.  LXXXVII No. 3 combined with apses, with the addition of round chapels on the diagonal sides.

Pl.  LXXXIV No. 13 is a variation of the preceding sketch.

Pl.  XC No. 3.  MS. B. 25b.  The round chapels of the preceding sketch are replaced by octagonal chapels, above which rise campaniles.

Pl.  XC No. 4 is the elevation for the preceding plan.

Pl.  XCII No. 1. (MS. B. 39b.); the plan below.  On the principal as well as on the diagonal axes are diagonal chapels, but the latter are separated from the dome by semicircular recesses.  The communication between these eight chapels forms a square aisle round the central dome.

Above this figure is the elevation, showing four campaniles on the angles. [Footnote 1:  The note accompanying this drawing is reproduced under No. 753.]

Pl.  LXXXIV No. 3.  On the principal axes are square chapels with three niches; on the diagonals octagonal chapels with niches.  Cod.  Atl. 340b gives a somewhat similar arrangement.

MS. B. 30.  The principal development is thrown on the diagonal axes by square chapels with three niches; on the principal axes are inner recesses communicating with outer ones.

The plan Pl.  XCIII No. 2 (MS. B. 22) differs from this only in so far as the outer semicircles have become circular chapels, projecting from the external square as apses; one of them serves as the entrance by a semicircular portico.