VITRUVIUS.
218
C HA PTER VIII.
Of Sun-dials, and of the Equinoctial Shadow of Gnomons at Rome and
some other Places, with a Description of the Anallemna.
TROM these men we derive the principles of dialling, and are enabled to explain the
causes of the decrease of the days in each month, as also the depalations; for at the time of
the equinoxes, when the Sun is passing Aries and Libra, the gnomon having nine parts, its
shadow will have eight in the latitude of Rome. At Athens, the gnomon having four
parts, the shadow will have three. At Rhodes it is as five to seven. At Tarentum, as nine to
eleven. At Alexandria, as three to five: and so in all other places, according to their situations.
the equinoctial shadow of the gnomon is observed to be different. Wherever therefore sun-
dials are to be described, the equinoctial shadow, at that place, must be taken.
If it should be the same as at Rome, the shadow having eight of the nine parts
Fig. LXIX.
of the gnomon, a line is described on a plane, and from the middle, pros orthas.
is to be erected that which is called the gnomon; and from the line which is on the plane at
the end of the gnomon, a space equal to nine parts is to be measured thereon with the com¬
palies; and in that place, where the mark of the nine parts happens, as at the letter A. a
center is to be fixed. Then extending the compasses from this center to the line of the plane.
where the letter B is, a circle is described, which is called the meridian ; then of the nine
parts which extend from the plane to the center (at the top) of the gnomon, eight are taken,
and marked on the line that is in the plane, at letter G; this will be the equinoctial shadow of
the gnomon: and from the mark at letter C, through the center 4, a line is drawn, which
willbe the ray of the Sun at the equinores. Then from the center, opening the compastes
to the line on the plane, set that measure from the plane on the circumferent line, on either
('1) This word is in some manuscripts written ex¬
planationes. Some suppose it to signify the method of
finding the increase and decrease of the shadow of the
gnomon; coming from the word palor, to straggle, or
wander irregularly: or it may signify the unequal and
various divergence of the hour lines.
(2*) Perrault translates it as nine to seven-by mistake,
T suppose ; for the words are ad septem Rhodo quinque.
(3*) At right-angles, or perpendicularly.