Of digging a canal. Put this in the Book of useful inventions and in
proving them bring forward the propositions already proved. And this
is the proper order; since if you wished to show the usefulness of
any plan you would be obliged again to devise new machines to prove
its utility and thus would confuse the order of the forty Books and
also the order of the diagrams; that is to say you would have to mix
up practice with theory, which would produce a confused and
incoherent work.