It will readily be seen that this logical and reasonable mode of constructing arched openings would, when applied to arches carried on pillars, lead to the clustered column.
If the wall was not thick, the arches might certainly continue to be of one order, and the most natural mode of supporting them would then, as heretofore, be single columns. Where, however, the wall was so thick as to give it a clumsy look if the arch ran square through it, it would be divided into two orders, and would assume at its springing a cruciform plan. The impost must break round this figure; and though the column might still remain (and often did remain) round ([Figs. 141], [142]), the abacus only assuming the cross form, the most natural thing would be to form a complex pillar composed of four shafts united in one, each apparently supporting its own order of the arch ([Fig. 143]).
| Fig. 141. | Fig. 142. | Fig. 143. |
If the arch were divided into three orders, a more complex form suggested itself, containing eight shafts; and as the system was carried out, many other combinations arose not necessary to enumerate.
Thus we see that the adoption of the arched system of construction, unbiassed by any pre-existing laws of art, but aided only by the very rational desire to utilise the materials most abundantly provided by nature, led to two of the most important characteristics common to Romanesque and Gothic architecture, viz., the sub-ordinated arch and the clustered column, with the whole system of decoration derived from them; than which no two features can be pointed out which have been more richly fruitful of architectural forms the most original and beautiful.
Again, in the mode of bringing down the arch upon columns, the Romanesque builders exercised a sound discretion. The Greeks and Romans in their trabeated construction, reasonably enough, made their architraves only as wide as the upper diameter of their columns, so that whatever projection the capitals had beyond the shaft, they had the same beyond the architrave also. When, however, you substitute two arches for two architraves, you bring down the weight by two opposite forces; its footing on the capital, therefore, requires as much steadiment as possible.
The Romans, as many of their modern followers, were for a time so inconsistent as not only to limit the arch, like the architrave, to the thickness of the upper diameter of the column, but actually interposed, without a shadow of use, a bit of entablature between the column and the arch; thus, instead of doing all they could to give steadiness to the spring of the arch, they made it as tottering in its construction as possible. This was corrected by the Romans of the Lower Empire, and the arch was placed by them, as reason would dictate, directly upon the capital, or (still more sensibly) on a strong flat impost laid on the capital; and for this most reasonable step they have in after ages been pronounced barbarous! The Romanesque architects, taught by common sense rather than by precedent, followed their example. If they imitated or re-used the Corinthian capital, they laid upon its fragile abacus a more trustworthy impost, and to give greater steadiment to the foot of the arch they made it somewhat wider than the diameter of the column—a practice which pervades Mediæval architecture, and contributes greatly both to its good construction and its beauty.
The system of constructing doorways is directly derived from what I have already described—as many recesses being given to the jambs as the arch has rims, and these decorated with columns if thought good. The head is often filled in with a tympanum supported by corbels in the jambs, both as a field for sculptured decoration, and to make the door itself square instead of arch-formed. If this is not done, the inner arches are made to spring from a higher level, to allow the doors to open without catching against them.