The form of an inclosed space is also expressive. There are two chief types, the longitudinal and the radial; but since these may exist either in plan or in elevation, four possibilities result: the longitudinal-horizontal, as in an aisle; the longitudinal-vertical, as in a tower; the radial-horizontal, illustrated by every equilateral plan--triangle, square, regular polygon, and above all, the most perfect form of this type, the circle; and finally, the radial-vertical, of which domed spaces, like the Pantheon or St. Paul's, are examples. The terms used to designate them, together with the examples, afford a good idea of what these space forms are, making further description unnecessary. It is interesting to observe how different the expression of the square and the triangle is when they determine the plan of an inclosed space from what it is when they are the shapes of walls. [Footnote: Compare Fritz Hoeber: _Systematik der Architekturproportionen_, II, B, a. ] In the case of the latter, according to the analysis which we have given of them, the figures represent an interplay of antagonistic horizontal and vertical forces, about an axis drawn perpendicular to the midpoint of the base line; while as plans they express forces homogeneous in kind radiating from their centers. The feeling of longitudinal forms is one of continued movement, forward or upward as the case may be; when the distance is very great, the feeling is of infinity, either of vista, as in an aisle, or of height, as in a tower, for even when the point at the end is clearly seen and known, we continue it in the imagination. The radial forms, on the other hand, even when the axes are very long, express completeness and security, for no matter how far we go in any one direction, we have to proceed along a line which brings us back to our starting point; in following to the top the movement of the curved line of a dome or an apse, the continuation of the same line carries us down on the other side to a point corresponding to the one from which we set out; if we wander, we return home.