Introduction

THE circle gives one of the elemental plan forms, and it has always figured prominently in design by reason of the beauty of its curve and the endless variety which it introduces into a scheme. The theory underlying the application of the circle has been developed through the centuries in buildings of many types, ranging from small isolated structures of a commemorative nature to those of considerable size suitable for ecclesiastical and secular purposes.

The circular plan is generally accompanied by a domical section, for the dome has from early times been accepted as its logical covering because it gives the best vertical expression to the horizontal disposition. The support for a dome over a circular plan is readily obtained, and its arrangement so as to produce the most satisfactory result as seen from the interior has been one of the chief aims of builders at all times. Various arrangements of coffering have been devised by recessing panels within horizontal and vertical ribs, and when an unbroken surface for mosaic or colour decoration has not been required, one of the many varieties of coffering has invariably been used to give architectural expression to the concave surface. The properties of domical construction allow of an opening being left at the top, and advantage has been taken of this to let in light where it can be most impressively admitted through a circular “eye.”

Externally many variations are permissible: the domical form may be visible, following the curve of the inner covering as nearly as permitted by the demands of sound construction. The lower part of the dome is sometimes masked by a vertical wall above which a conical roof conceals the curve of the dome completely, or the segment of the curve remaining above the vertical surface may be exposed. When it is desired to emphasise the dome externally and to admit light just beneath its base instead of at its summit, the dome may be raised on a vertical wall or “drum.” Sections and elevations of these dispositions will be found in this series of Plates, and the theory of the setting-out of domes over rectangular sub-structures by means of “pendentives” is illustrated on Plate XII.

The impossibility of producing satisfactory expression both internally and externally from the use of a single domical covering is apparent in the domed buildings of the ancients, but it was left to the architects of the Renaissance in Italy, France and eventually in England to construct the double and triple coverings which, in their hands, were productive of such magnificent results. But compositions culminating in domes, the inner and outer coverings of which are at different levels, do not come within the scope of this book as they belong to the more complex systems.

The diagrams on the following Plates have been arranged with regard to the approximate diameter of the dome most suitable for the type of plan and section shown, and they provide a basis for further development when the student has mastered the elementary principles which they illustrate.