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Rh 15 feet thick, enclosing a courtyard about 30 feet in diameter. The walls contain a staircase, which leads to the top of the building, several horizontal galleries, and some small conical chambers, all opening on the inside; the only external orifice being the door, which is about 7 feet high.

83.—The Burgh of Moussa, Shetland.

The absence of trees and abundance of stone probably led to this curious style of architecture. Although, moreover, so archaic in character, these burghs continued in use down to historical times, in fact until the introduction of lime, and the knowledge of the true principle of the arch enabled the natives to construct buildings of a more modern character; they are extremely numerous in Caithness, in the Orkneys, and the Shetlands; but this Moussa Burgh is one of the few that are mentioned in