Pl. LXXXV No. 13 (MS. B. 62a) and Pl. XCIII No. 1. (MS. B. 15a) give
a few examples of arches supported on piers._

_XIII.

Theoretical writings on Architecture.

Leonardo's original writings on the theory of Architecture have come
down to us only in a fragmentary state; still, there seems to be no
doubt that he himself did not complete them. It would seem that
Leonardo entertained the idea of writing a large and connected book
on Architecture; and it is quite evident that the materials we
possess, which can be proved to have been written at different
periods, were noted down with a more or less definite aim and
purpose. They might all be collected under the one title: "Studies
on the Strength of Materials". Among them the investigations on the
subject of fissures in walls are particularly thorough, and very
fully reported; these passages are also especially interesting,
because Leonardo was certainly the first writer on architecture who
ever treated the subject at all. Here, as in all other cases
Leonardo carefully avoids all abstract argument. His data are not
derived from the principles of algebra, but from the laws of
mechanics, and his method throughout is strictly experimental.

Though the conclusions drawn from his investigations may not have
that precision which we are accustomed to find in Leonardo's
scientific labours, their interest is not lessened. They prove at
any rate his deep sagacity and wonderfully clear mind. No one
perhaps, who has studied these questions since Leonardo, has
combined with a scientific mind anything like the artistic delicacy