Sentence:  [The dome was] turned upon a centre laid without any standard from below to support it. Every story of the scaffolding being circular, and the ends of the <WSD>ledgers</WSD> meeting as so many rings, and truly wrought, it supported itself; ... 
Meaning of the word marked with <WSD> tags:  A board attached to a wall to provide support for attaching other structural elements (such as deck joists or roof rafters) to a building.