Umbra Recta.-->
Yif it so be that thou wilt werke by umbra
recta-->, and
thou may come to the bas of the tour, in this maner shalt thou
werke. Tak the altitude of the tour by bothe holes, so that thy
rewle ligge even in a poynt. Ensample as thus: I see him thorw
at the poynt of 4; than mete I the space betwixe me and the tour,
and I finde it 20 foot; than beholde I how 4 is to 12, right so
is the space betwixe thee and the tour to the altitude of the
tour. For 4 is the thridde part of 12, so is the space between
thee and the tour the thridde part of the altitude of the tour;
than thryes 20 foot is the heyghte of the tour, with adding of
thyn owne persone to thyn eye. And this rewle is general in
umbra recta-->, fro the poynt of oon to 12. And yif thy
rewle falle upon 5, than is 5 12-partyes of the heyght the space
between thee and the tour; with adding of thyn owne heyghte.
