§ XVI. I have also supposed Yb, when expanded from beneath Xb, as always expanded into a square, and four spurs only to be added at the angles. But Yb may be expanded into a pentagon, hexagon, or polygon; and Xb then may have five, six, or many spurs. In proportion, however, as the sides increase in number, the spurs become shorter and less energetic in their effect, and the square is in most cases the best form. § XVII. We have hitherto conducted the argument entirely on the supposition of the pillars being numerous, and in a range. Suppose, however, that we require only a single pillar: as we have free space round it, there is no need to fill up the first ranges of its foundations; nor need we do so in order to equalise pressure, since the pressure to be met is its own alone. Under such circumstances, it is well to exhibit the lower tiers of the foundation as well as Yb and Xb. The noble bases of the two granite pillars of the Piazzetta at Venice are formed by the entire series of members given in Fig. X., the lower courses expanding into steps, with a superb breadth of proportion to the shaft. The member Xb is of course circular, having its proper decorative mouldings, not here considered; Yb is octagonal, but filled up into a square by certain curious groups of figures representing the trades of Venice. The three courses below are octagonal, with their sides set across the angles of the innermost octagon, Yb. The shafts are 15 feet in circumference, and the lowest octagons of the base 56 (7 feet each side).