The lack of due effect of scale in this interior has been often remarked, and it is generally attributed to the great magnitude of the structural parts. The size of these parts could not, however, well be different from what they are. Their magnitude is determined by the scale of the great dome and the width and altitude of the arms of the cross. The piers of the crossing are masses of masonry measuring on their longer sides more than fifty feet on the pavement, while the pendentive arches are one hundred and fifty feet high, and those of the arms of the cross are seventy-five feet high. But with appropriate treatment their scale might have been made more apparent. To adorn such piers and frame such arches with a classic order is to destroy the proper effect of scale, as well as to violate the true principles of architectural design by using structural members without any structural meaning. Apart from the barbarism already remarked (p. 29) of springing a vault from a classic entablature, the effect of the gigantic order is unhappy in other respects; the great salience of its cornice cuts off from view the lower part of the vaulting, and this pronounced overhanging ledge, extending around the whole interior, breaks the continuity of the upright lines into the vaulting, and diminishes the effect of altitude. [Illustration: FIG. 32.--Section of aisle of St. Peter’s.] But not only did Michael Angelo employ this incongruous and ineffective ornamental scheme for the interior of St. Peter’s, he also adopted a corresponding design for the exterior which wholly contradicts the real character of the structure and led the architect into some curious makeshifts. For this exterior he used another gigantic order surmounted with an attic story. This obliged him to carry up the enclosing walls of the aisles to a height equal to that of the nave, and led to difficulties within. For the aisle vaulting was now far down below the top of these walls, and it therefore became necessary, unless the space above this vaulting was to be left open to the sky, with the enclosing wall standing as a mere screen answering to nothing behind it,[68] to construct a flat roof at the level of the attic cornice. Figure 32, a section through this part of the structure, will explain this and some other awkward expedients to which the architect was driven by the use of this colossal external order. Of the two compartments through which the line _AB_ (plan, Fig. 31) passes, one has a barrel vault and the other a dome, and, as each of the other corresponding parts of the plan are vaulted in the same way, there are four small domes in all. The effect of four smaller domes grouped around the great central one would be happy for both internal and external effect, if they were properly related in proportions, and the scheme were carried out in a structurally consistent and rational way; but such a scheme could not be developed here.