[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.)]
These galleries, situated in the heart of the wall, are six in number. Each begins about 3 feet 9 inches in front of the stair, and goes round the whole tower on the level till it comes against the back of the stair, which closes it at that end, so that entrance to the gallery or exit from it can only be obtained by stepping across the space intervening between the end of the gallery floor and the steps of the stair. The floors of the galleries are formed of flat undressed slabs, the end of which reach into the walls on both sides. These slabs are about 6 inches thick, and those whose under surfaces form the roof of the first gallery present their upper surfaces as the floor of the second, and so throughout. None of the galleries exceed 5 feet 6 inches in height or 3 feet 2 inches in width, and some of the upper spaces are now much narrower; but as the position of the upper walls has evidently shifted, the original dimensions of the upper galleries cannot be ascertained. Four of the galleries that now remain (for the tower is incomplete at top) are lighted by four vertical ranges of windows all looking into the interior court. One range of fourteen openings is over the main entrance. Another of eighteen openings is over the entrance to the stairs. The third set has seventeen openings, and the fourth is imperfect, many of the lintels having been broken out. The peculiarities of these windows are—(1) that they are placed close to each other, vertically, with merely the thickness of a lintel between each opening; (2) they are wider than they are high, the greatest width being 2 feet 9 inches, and the greatest height not exceeding a foot; (3) they diminish in size gradually from the lowest to the highest; and (4) they do not range so far upwards as to include the two upper galleries, which are windowless.