iforium and clerestory walls are superimposed
on the nave piers; perhaps with most majesty where with greatest
simplicity, as in the old basilican types, and the noble cathedral of
Pisa.

Sec. VIII. In order to the delightfulness and security of all such
arrangements, this law must be observed:--that in proportion to the
height of wall above them, the shafts are to be short. You may take your
given height of wall, and turn any quantity of that wall into shaft that
you like; but you must not turn it all into tall shafts, and then put
more wall above. Thus, having a house five stories high, you may turn
the lower story into shafts, and leave the four stories in wall; or the
two lower stories into shafts, and leave three in wall; but, whatever
you add to the shaft, you must take from the wall. Then also, of course,
the shorter the shaft the thicker will be its _proportionate_, if not
its actual, diameter. In the Ducal Palace of Venice the shortest shafts
are always the thickest.[61]

Sec. IX. The second kind of superimposition, lightness on weight, is, in
its most necessary use, of stories of houses one upon another, where, of
course, wall veil is required in the lower ones, and has to support wall
veil above, aided by as much of shaft structure as is attainable within
the given limits. The greatest, if not the only, merit of the Roman and
Renaissance Venetian architects is their graceful management of this
kind of superimposition; sometimes of complete courses of external
arches and shafts one above the other; sometimes of apertures with
intermediate cornices at the levels of the floors, and large shafts from
top to bottom of the building; always observing that the upper stories
shall be at once lighter and richer than the lower ones. The entire
value of such buildings depends upon the perfect and easy expression of
the relative strength of the stories, and the unity obtained by the
varieties of their proportions, while yet the fact of superimposition
and separation by floors is frankly told.

Sec. X. In churches and other buildings in which there is no separation
by floors, another kind of pure shaft superimposition is often used, in
order to enable the builder to avail himself of short and slender
shafts. It has been noted that these are often easily attainable, and of
precious materials, when shafts large enough and strong enough to do the
work at once, could not be obtained except at unjustifiable expense, and
of c