kness 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.

Over the first great cornice within the tribune there begin at the
foot the compartments for the recesses that are in the vault of the
tribune, which are formed by sixteen projecting ribs. These at the
foot are as broad as the breadth of the two pilasters which at the
lower end border each window below the vault of the tribune, and they
rise, diminishing pyramidally, as far as the eye of the lantern; at
the foot they rest on pedestals of the same breadth and twelve palms
high, and these pedestals rest on the level platform of the cornice
which goes in a circle right round the tribune. Above this, in the
recessed spaces between the ribs, there are eight large ovals, each
twenty-nine palms high, and over them a number of straight-sided
compartments that are wider at the foot and narrower at the top, and
twenty-four palms high, and then, the ribs drawing together, there
comes above each straight-sided compartment a round fourteen palms
high; so that there come to be eight ovals, eight straight-sided
compartments, a