In the round arch all four forces are beautifully expressed. The upward is manifest, first, in the vertical pier, which acts very much as the column does, and, in Roman work, was often replaced by the column. The opposing downward force is expressed in the horizontal upper bound of the arch and in the line of the impost, also horizontal, which breaks the vertical line and so marks the place where the two forces come into sharpest conflict. In this conflict, the vertical is victorious; for, instead of being stopped by the impost, it is carried up throughout the entire construction by means of the upward and inward curving of the arch. The very curve of the arch shows, however, that the victory is not absolute; for its circular form is obviously determined as a compromise between an inward centripetal force, moving upward and diminishing the breadth of the arch to a mere point at its apex, and an outward centrifugal force, gradually spreading the arch downward until it reaches its greatest breadth at the impost, where it is arrested by the opposing vertical force in the pier. To the historical imagination, the round arch seems, therefore, to express the genial classical idea of a control by the higher nature which nevertheless did no violence to the demands of the lower. In the spherical dome the effect is the same, only the interplay of forces operates in three dimensions instead of two. When arches are superposed, the upward movement proceeds in stages, beginning anew at each horizontal division of the wall space. The use of entablatures applied to the wall and of engaged columns, common in Roman work, seems to involve an attempt at a fusion of two contradictory styles, and is usually condemned as such. This contradiction can be solved, however, by viewing the entablatures as mere weightless lines of division of the wall, usually marking off the different stories, and by viewing the columns in a similar fashion as having no supporting function--which is actually the case--and as simply serving the purpose of framing the arches. At most they merely indicate the direction of the chief contending forces,--the parallel lintels signalizing the force of weight, and the vertical columns, standing one upon the other, pointing the movement of the upward force. They have, therefore, a pictorial rather than a dynamic significance.