The point C, which is the lowest, is the principal one, wherewith he turned the first half-circle of the tribune, with which he gave the form, height and breadth of this vault, which he ordered to be built entirely of bricks well baked and fired, laid herring-bone fashion. This shell he makes four palms and a half thick, and as thick at the top as at the foot, and leaving beside it, in the centre, a space four palms and a half wide at the foot, which is to serve for the ascent of the stairs that are to lead to the lantern, rising from the platform of the cornice where there are balusters. The arch of the interior of the other shell, which is to be wider at the foot and narrower at the top, is turned on the point marked B, and the thickness of the shell at the foot is four palms and a half. And the last arch, which is to be turned in order to make the exterior of the cupola, wider at the foot and narrowing towards the top, is to be raised on the point marked A, which arch turned, there remains at the top all the hollow space of the interior for the ascent of the stairs, which are eight palms high, so that one may climb them upright; and the thickness of that shell comes to diminish little by little, insomuch that, being as before four palms and a half at the foot, it decreases at the top to three palms and a half. And the outer shell comes to be so well bound to the inner shell with bonds and with the stairs, that the one supports the other; while of the eight parts into which the fabric is divided at the base, the four over the arches are left hollow, in order to put less weight upon the arches, and the other four are bound and chained together with bonds upon the piers, so that the structure may have everlasting life. The stairs in the centre between one shell and the other are constructed in this form; from the level where the springing of the vault begins they rise in each of the four sections, and each ascends from two entrances, the stairs intersecting one another in the form of an X, until they have covered the half of the arch marked C, on the upper side of the shell, when, having ascended straight up the half of that arch, the remaining space is then easily climbed circle after circle and step after step in a direct line, until finally one arrives at the eye of the cupola, where the rise of the lantern begins, around which, in accord with the diminution of the compartments that spring above the piers, there is a smaller range of double pilasters and windows similar to those that are constructed in the interior, as will be described below.