I would thus define them:--Generally, I would say that such a style should be capable of doing all, whether structural or artistic, by means of the _arch_, which other forms of architecture had done through the use of the horizontal _beam_ or lintel. And to go more into particulars, I would add:-- 1st. That, as a rule, openings in walls and between pillars, whether taking the form of doorways, windows, gateways, or intercolumniations, should be bridged over by arches instead of by horizontal lintels or entablatures, though not descending into such purism as to reject the latter when circumstances clearly point to its adoption. 2nd. That areas enclosed by walls, or by ranges of piers or columns, and of any reasonable width, should be capable of being covered over,--and, in buildings of the highest grade, should as a rule be actually covered over,--by _vaulting_; this rule, however, not being pressed so far as to exclude level ceilings or timber roofs,--the one the most natural and economical covering for rooms, and the other for churches, halls, etc.,--where circumstances forbid the use of vaulting. 3rd. That the decorative system of the architecture should harmonise with, and result from, these prevailing structural conditions; the construction and the architectural treatment being, not only in harmony, but in the most intimate alliance the one with the other. Now, we all know that Grecian architecture almost ignored the arch, carrying the horizontal or trabeated system of covering openings to the highest artistic perfection; doing for that system just everything which the above-stated conditions would demand for an arched style. _Repose_ was the great sentiment which their architecture expressed; _vertical pressure_, the one physical condition it had to provide against; whereas arched architecture (as they say in India) “never sleeps.” It is always exerting pressure in some other direction than the mere vertical line, and the physical conditions it has to meet are the resistance of these, as well as the support of mere weight. We know, too, that Roman architecture admitted nearly all the constructive conditions we have demanded, and carried them on to a very considerable degree of practical perfection. We believe, moreover, that had not circumstances checked its progress, it would have carried out these conditions to a much greater extent. As it happened, however, it did not go so far as to make these structural conditions a leading artistic element, and the groundwork of a distinctive decorative system; but, being broken up through political convulsion before such an end was attained, it bequeathed the task to the descendants of its despoilers, and long centuries of darkness had to pass by before the work could be accomplished.