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