hat no amussium is necessary. In the very centre of that spot set
up a bronze gnomon or "shadow tracker" (in Greek [Greek: skiatheras]).
At about the fifth hour in the morning, take the end of the shadow cast
by this gnomon, and mark it with a point. Then, opening your compasses
to this point which marks the length of the gnomon's shadow, describe a
circle from the centre. In the afternoon watch the shadow of your gnomon
as it lengthens, and when it once more touches the circumference of
this circle and the shadow in the afternoon is equal in length to
that of the morning, mark it with a point.

[Illustration: THE TOWER OF THE WINDS AT ATHENS]

7. From these two points describe with your compasses intersecting arcs,
and through their intersection and the centre let a line be drawn to the
circumference of the circle to give us the quarters of south and north.
Then, using a sixteenth part of the entire circumference of the circle
as a diameter, describe a circle with its centre on the line to the
south, at the point where it crosses the circumference, and put points
to the right and left on the circumference on the south side, repeating
the process on the north side. From the four points thus obtained draw
lines intersecting the centre from one side of the circumference to the
other. Thus we shall have an eighth part of the circumference set out
for Auster and another for Septentrio. The rest of the entire
circumference is then to be divided into three equal parts on each side,
and thus we have designed a figure equally apportioned among the eight
winds. Then let the directions of your streets and alleys be laid down
on the lines of division between the quarters of two winds.

8. On this principle of arrangement the disagreeable force of the winds
will be shut out from dwellings and lines of houses. For if the streets
run full in the face of the winds, their constant blasts rushing in from
the open country, and then confined by narrow alleys, will sweep through
them with great violence. The lines of houses must therefore be directed
away from the quarters from which the winds blow, so that as they come
in they may strike against the angles of the blocks and their force thus
be broken and dispersed.

9. Those who know names for very many winds will perhaps be surprised at
our setting forth that there are only eight. Remembering, however, that
Eratosthenes of Cyrene, employing mathematical theories and geometrical
method