ht in appointed places; chimneys
to carry smoke or direct currents of air; staircases; towers, which
are to be watched from or cried from, as in mosques, or to hold
bells, or to place men in positions of offence, as ancient moveable
attacking towers, and most fortress towers.

Sec. II. Protective architecture has to do one or all of three things:
to wall a space, to roof it, and to give access to it, of persons, light,
and air; and it is therefore to be considered under the three divisions
of walls, roofs, and apertures.

We will take, first, a short, general view of the connection of these
members, and then examine them in detail: endeavoring always to keep the
simplicity of our first arrangement in view; for protective architecture
has indeed no other members than these, unless flooring and paving be
considered architecture, which it is only when the flooring is also a
roof; the laying of the stones or timbers for footing being pavior's or
carpenter's work, rather than architect's; and, at all events, work
respecting the well or ill doing of which we shall hardly find much
difference of opinion, except in points of aesthetics. We shall therefore
concern ourselves only with the construction of walls, roofs, and
apertures.

Sec. III. 1. _Walls._--A wall is an even and united fence, whether of
wood, earth, stone, or metal. When meant for purposes of mere partition
or enclosure, it remains a wall proper: but it has generally also to
sustain a certain vertical or lateral pressure, for which its strength
is at first increased by some general addition to its thickness; but if
the pressure becomes very great, it is gathered up into _piers_ to
resist vertical pressure, and supported by _buttresses_ to resist
lateral pressure.

If its functions of partition or enclosure are continued, together with
that of resisting vertical pressure, it remains as a wall veil between
the piers into which it has been partly gathered; but if it is required
only to resist the vertical or roof pressure, it is gathered up into
piers altogether, loses its wall character, and becomes a group or line
of piers.

On the other hand, if the lateral pressure be slight, it may retain its
character of a wall, being supported against the pressure by buttresses
at intervals; but if the lateral pressure be very great, it is supported
against such pressure by a continuous buttress, loses its wall
character, and becomes a dyke or rampart.

Sec