dry-built structures that are roofed, whether they be of Pagan or of Christian time, because it is the style that is best suited to the material and the manner of construction. The builders of this edifice had no stones long enough to span chambers of six feet wide, and if they had had them long enough they would have been too weak to bear the superincumbent weight of a wall forty feet in height. Therefore they made their chamber-roofs semi-vaulted, while the doors and passages, which were narrow, were simply spanned with strong flat lintels. These chambers on the ground floor are lighted by window-like openings above the doorways, which rise one over the other, and serve not only to admit light and air, but to distribute the weight to be borne by the lintels. In each of the chambers there are small ambry-like recesses in the walls, but no fireplace or chimney. They are small, dimly-lighted, dungeon-like rooms, but neither smaller, worse-lighted, or more dungeon-like than many rooms in the lime-built castles of the nobles of the Feudal ages. [Illustration: Fig. 161.—Ground Plan of the Broch of Mousa, Shetland. (From Plan by Sir Henry Dryden.)] Half-way between the chamber facing the main entrance and the one to the left of it there is a doorway placed at a height of four feet above the ground level. This doorway, which is higher and wider than those which lead into the chambers, is slightly larger than the main entrance itself, being 5 feet 4 inches high and 3 feet wide. It leads to a stair constructed like the chambers within the thickness of the wall. At the foot of the stair there is an oval chamber, from one end of which the stair rises in a steep slope, but following the curve of the wall to the top. The steps are single flat stones, varying in width from ten inches to two feet, undressed, and laid above each other so that they give a tread of about five inches and nearly the same of a rise. The upper part of the tower which is traversed by the stair is differently constructed from the lower part. To the height of about eleven feet above the ground level the wall of the tower is carried up solid except for the vacancy occasioned at intervals in its thickness by the chambers and their accesses. But above this height the wall is carried up with a vacancy in its centre (as seen in the section Fig. 162) so as to form a series of circular galleries placed one immediately over another, and crossed successively from the lowest to the highest by the rise of the stair which gives access to them. [Illustration: Fig. 162.—Section of the elevation of the Broch of Mousa. (From Plan by Sir H. Dryden.)]