esigned to show the
precise quarters from which the winds arise; the other, how by turning
the directions of the rows of houses and the streets away from their
full force, we may avoid unhealthy blasts. Let A be the centre of a
plane surface, and B the point to which the shadow of the gnomon reaches
in the morning. Taking A as the centre, open the compasses to the point
B, which marks the shadow, and describe a circle. Put the gnomon back
where it was before and wait for the shadow to lessen and grow again
until in the afternoon it is equal to its length in the morning,
touching the circumference at the point C. Then from the points B and C
describe with the compasses two arcs intersecting at D. Next draw a line
from the point of intersection D through the centre of the circle to the
circumference and call it E F. This line will show where the south and
north lie.

[Illustration]

13. Then find with the compasses a sixteenth part of the entire
circumference; then centre the compasses on the point E where the line
to the south touches the circumference, and set off the points G and H
to the right and left of E. Likewise on the north side, centre the
compasses on the circumference at the point F on the line to the north,
and set off the points I and K to the right and left; then draw lines
through the centre from G to K and from H to I. Thus the space from G to
H will belong to Auster and the south, and the space from I to K will be
that of Septentrio. The rest of the circumference is to be divided
equally into three parts on the right and three on the left, those to
the east at the points L and M, those to the west at the points N and
O. Finally, intersecting lines are to be drawn from M to O and from L
to N. Thus we shall have the circumference divided into eight equal
spaces for the winds. The figure being finished, we shall have at the
eight different divisions, beginning at the south, the letter G between
Eurus and Auster, H between Auster and Africus, N between Africus and
Favonius, O between Favonius and Caurus, K between Caurus and
Septentrio, I between Septentrio and Aquilo, L between Aquilo and
Solanus, and M between Solanus and Eurus. This done, apply a gnomon to
these eight divisions and thus fix the directions of the different
alleys.

CHAPTER VII

THE SITES FOR PUBLIC BUILDINGS

1. Having laid out the alleys and determined the streets, we have next
to treat of the choice of building sites for