The doorways which gave entrance to the Acropolis, are five in number, and placed between piers, towards the back of the edifice. The inclined plane gave access to the central one, the others were approached by steps, they bear some marks of having been adorned with architraves of metal. There is a small sinking, apparently formed to receive it, 1 foot 9.8 inches in width, to the larger opening: and one of 13.7 inches in width to the two adjacent ones, yet there are no cramp-holes, except in the upper part; the smaller side doorways are filled up with earth. Over each opening a stone is placed edgewise, occupying in consequence the height of two common courses. This plan seems to have been uniformly adopted above the doorways of the Athenian buildings, and will guide us in several instances to determine what openings were originally intended, and what were not. On the eastern side of this wall, the traces of a metallic architrave are much more certain, since besides the change of colour in the marble, there is a regular series of cramp-holes; yet how it was managed is still a subject of some difficulty. Taking the central opening as my example, (and the two others are similarly designed) we find on the edge of the opening a band, not of a very uniform width, in which the surface of the marble has not been finished, and then one in which the surface made quite smooth, is of a whiter colour than elsewhere; apparently from having been sheltered from the air; both these must have been covered by the architrave, and their united width is 1 foot 10.4 inches. On the highest stone of the jamb there is a deep and double sinking, which seems to have been intended to receive a strong block, projecting from the face of the architrave; my notion is, that this block had a hole on the under side to receive the pivot of the door, which worked in a corresponding hole in another block of bronze at the base, and consequently lay when shut, against the wall, without being let into it. I should perhaps hardly have ventured to express this opinion, if it were not strengthened by the appearance of the grooves receiving the metal plate on which the doors opened, at the entrance of the opisthodomus of the Parthenon. The centre from which these grooves are drawn is not within the thickness of the wall. The jambs of these doorways, viewed laterally, exhibit several sinkings about two inches deep, which doubtless were made with some object at present unknown. The height of the centre opening was equal to fifteen courses,[28] each of nearly 1 foot 7.4 inches in thickness, making in all 24 feet 1.9 inch, on a width of 13 feet 4.75 inches. The secondary openings are of eleven courses, or 17 feet 8.5 inches, on a width of 9 feet 3.6 inches. The side doors are of six courses, 9 feet 7.8 inches, with a width of 4 feet 8 inches. All the piers rest on a continued course of black marble.