made. In Fig. 20 (F) their width and spacing are shown. In Fig. 18 some
lines drawn across, one over the other, are shown at H. These are the
stairs, of which in this section we see only the fronts, or risers, so
that they appear merely as lines (showing the edge of each step) drawn
one over the other. At H on the plan, Fig. 21, we again see them
represented as a series of lines, but here we are looking down on the
top of them, and see only the upper surfaces, or "treads," the edges
again appearing as a series of lines. At H on the longitudinal section,
we see the same steps in section, and consequently their actual slope,
which, however, could have been calculated from Figs. 18 and 21, by
putting the heights shown in section with the width shown in plan. The
plan, Fig. 21, shows the thickness and position on the floor of the
pillars, G G. Their height is shown in the sections. The plan of a
building is merely a horizontal section, cutting off the top, and
looking down on the sectional top of the walls, so as to see all their
thicknesses. I have drawn (Fig. 24) a perspective sketch of one end of
the plan (Fig. 22) of the building, on the same principle as was done
with the section (Fig. 23), in order to show more intelligibly exactly
what it is that a plan represents--the building with the upper part
lifted off.

Returning for a moment to the subject of the relation between the plan
and the exterior design, it should be noted that the plan of a building
being practically the first consideration, and the basis of the whole
design, the latter should be in accordance with the principle of
disposition of the plan. For example, if we have an elevation (shown in
diagram) showing two wings of similar design on either side of a center,
designed so as to convey the idea of a grand gallery, with a suite of
apartments on either side of similar importance--if the one side only of
the plan contains such a suite, and the opposite side is in reality