The Ten Books on Architecture
wall should be interrupted for a space the width of the tower, and have
only a wooden flooring across, leading to the interior of the tower but
not firmly nailed. This is to be cut away by the defenders in case the
enemy gets possession of any portion of the wall; and if the work is
quickly done, the enemy will not be able to make his way to the other
towers and the rest of the wall unless he is ready to face a fall.
5. The towers themselves must be either round or polygonal. Square
towers are sooner shattered by military engines, for the battering rams
pound their angles to pieces; but in the case of round towers they can
do no harm, being engaged, as it were, in driving wedges to their
centre. The system of fortification by wall and towers may be made
safest by the addition of earthen ramparts, for neither rams, nor
mining, nor other engineering devices can do them any harm.
6. The rampart form of defence, however, is not required in all places,
but only where outside the wall there is high ground from which an
assault on the fortifications may be made over a level space lying
between. In places of this kind we must first make very wide, deep
ditches; next sink foundations for a wall in the bed of the ditch and
build them thick enough to support an earth-work with ease.
7. Then within this substructure lay a second foundation, far enough
inside the first to leave ample room for cohorts in line of battle to
take position on the broad top of the rampart for its defence. Having
laid these two foundations at this distance from one another, build
cross walls between them, uniting the outer and inner foundation, in a
comb-like arrangement, set like the teeth of a saw. With this form of
construction, the enormous burden of earth will be distributed into
small bodies, and will not lie with all its weight in one crushing mass
so as to thrust out the substructures.
8. With regard to the material of which the actual wall should be
constructed or finished, there can be no definite prescription, because
we cannot obtain in all places the supplies that we desire. Dimension
stone, flint, rubble, burnt or unburnt brick,--use them as you find
them. For it is not every neighbourhood or particular locality that can
have a wall built of burnt brick like that at Babylon, where there was
plenty of asphalt to take the place of lime and sand, and yet possibly
each may be provided with materials of equal usefulness so that out of
them a faultless wall may be built to last forever.
CHAPTER VI
THE DIRECTIONS OF THE STREETS; WITH REMARKS ON THE WINDS
1. The town being fortified, the next step is the apportionment of house
lots within the wall and the laying out of streets and alleys with
regard to climatic conditions. They will be properly laid out if
foresight is employed to exclude the winds from the alleys. Cold winds
are disagreeable, hot winds enervating, moist winds unhealthy. We must,
therefore, avoid mistakes in this matter and beware of the common
experience of many communities. For example, Mytilene in the island of
Lesbos is a town built with magnificence and good taste, but its
position shows a lack of foresight. In that community when the wind is
south, the people fall ill; when it is northwest, it sets them coughing;
with a north wind they do indeed recover but cannot stand about in the
alleys and streets, owing to the severe cold.
2. Wind is a flowing wave of air, moving hither and thither
indefinitely. It is produced when heat meets moisture, the rush of heat
generating a mighty current of air. That this is the fact we may learn
from bronze eolipiles, and thus by means of a scientific invention
discover a divine truth lurking in the laws of the heavens. Eolipiles
are hollow bronze balls, with a very small opening through which water
is poured into them. Set before a fire, not a breath issues from them
before they get warm; but as soon as they begin to boil, out comes a
strong blast due to the fire. Thus from this slight and very short
experiment we may understand and judge of the mighty and wonderful laws
of the heavens and the nature of winds.
3. By shutting out the winds from our dwellings, therefore, we shall not
only make the place healthful for people who are well, but also in the
case of diseases due perhaps to unfavourable situations elsewhere, the
patients, who in other healthy places might be cured by a different form
of treatment, will here be more quickly cured by the mildness that comes
from the shutting out of the winds. The diseases which are hard to cure
in neighbourhoods such as those to which I have referred above are
catarrh, hoarseness, coughs, pleurisy, consumption, spitting of blood,
and all others that are cured not by lowering the system but by building
it up. They are hard to cure, first, because they are originally due to
chills; secondly, because the patient's system being already exhausted
by disease, the air there, which is in constant agitation owing to winds
and therefore deteriorated, takes all the sap of life out of their
diseased bodies and leaves them more meagre every day. On the other
hand, a mild, thick air, without draughts and not constantly blowing
back and forth, builds up their frames by its unwavering steadiness, and
so strengthens and restores people who are afflicted with these
diseases.
4. Some have held that there are only four winds: Solanus from due east;
Auster from the south; Favonius from due west; Septentrio from the
north. But more careful investigators tell us that there are eight.
Chief among such was Andronicus of Cyrrhus who in proof built the marble
octagonal tower in Athens. On the several sides of the octagon he
executed reliefs representing the several winds, each facing the point
from which it blows; and on top of the tower he set a conical shaped
piece of marble and on this a bronze Triton with a rod outstretched in
its right hand. It was so contrived as to go round with the wind, always
stopping to face the breeze and holding its rod as a pointer directly
over the representation of the wind that was blowing.
5. Thus Eurus is placed to the southeast between Solanus and Auster:
Africus to the southwest between Auster and Favonius; Caurus, or, as
many call it, Corus, between Favonius and Septentrio; and Aquilo between
Septentrio and Solanus. Such, then, appears to have been his device,
including the numbers and names of the wind and indicating the
directions from which particular winds blow. These facts being thus
determined, to find the directions and quarters of the winds your method
of procedure should be as follows.
6. In the middle of the city place a marble amussium, laying it true by
the level, or else let the spot be made so true by means of rule and
level that no amussium is necessary. In the very centre of that spot set
up a bronze gnomon or "shadow tracker" (in Greek [Greek: skiathêras]).
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.
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
methods, discovered from the course of the sun, the shadows cast by an
equinoctial gnomon, and the inclination of the heaven that the
circumference of the earth is two hundred and fifty-two thousand stadia,
that is, thirty-one one million five hundred thousand paces, and
observing that an eighth part of this, occupied by a wind, is three
million nine hundred and thirty-seven thousand five hundred paces, they
should not be surprised to find that a single wind, ranging over so wide
a field, is subject to shifts this way and that, leading to a variety of
breezes.
10. So we often have Leuconotus and Altanus blowing respectively to the
right and left of Auster; Libonotus and Subvesperus to the right and
left of Africus; Argestes, and at certain periods the Etesiae, on either
side of Favonius; Circias and Corus on the sides of Caurus; Thracias and
Gallicus on either side of Septentrio; Supernas and Caecias to the right
and left of Aquilo; Carbas, and at a certain period the Ornithiae, on
either side of Solanus; while Eurocircias and Volturnus blow on the
flanks of Eurus which is between them. There are also many other names
for winds derived from localities or from the squalls which sweep from
rivers or down mountains.
11. Then, too, there are the breezes of early morning; for the sun on
emerging from beneath the earth strikes humid air as he returns, and as
he goes climbing up the sky he spreads it out before him, extracting
breezes from the vapour that was there before the dawn. Those that still
blow on after sunrise are classed with Eurus, and hence appears to come
the Greek name [Greek: euros] as the child of the breezes, and the word
for "to-morrow," [Greek: aurion], named from the early morning breezes.
Some people do indeed say that Eratosthenes could not have inferred the
true measure of the earth. Whether true or untrue, it cannot affect the
truth of what I have written on the fixing of the quarters from which
the different winds blow.
12. If he was wrong, the only result will be that the individual winds
may blow, not with the scope expected from his measurement, but with
powers either more or less widely extended. For the readier
understanding of these topics, since I have treated them with brevity,
it has seemed best to me to give two figures, or, as the Greeks say,