OF GRECIAN CONSTRUCTION, ACCORDING to the law of gravitation, all matter at rest keeps its place by its own weight, and is only to be removed by superior force, acting in a different direction. A perpendicular rock, or a solid upright wall, will preserve the same position so long as its substance endures: on this principle of perpendicular pressure, all Grecian architecture is founded [see fig. 131, a]. Hence have arisen the relative proportions and intercolumniations in the different orders, from the heaviest Doric to the most graceful Corinthian, the distances being regulated by the strength of the parts supporting and supported. Although it is probable that the first buildings were of wood, and that rude trees suggested the proportions of the Doric order, yet, the origin of Grecian architecture was, doubtless, derived from one stone laid flat upon another, and the aperture, or void, between two upright stones, was covered by a third placed across them: thus, the width of the opening was limited by the length of the cross-stone; consequently, this mode of structure required large blocks of stone, when that material was used [see fig. 132]. The difficulty of procuring such large blocks as were required for this mode of construction, suggested the idea of producing wide apertures by a different expedient; and this introduced the arch.