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3When arched ceilings are introduced, they must be executed as follows. Parallel ribs are set up, not more than two feet apart: those of cypress are preferable, because fir is soon injured by the rot and age. These ribs being got out to the shape of the curve, they are fixed to the ties of the flooring or roof, as the case may require, with iron nails. The ties should be of wood not liable to injury from rot, nor age nor damp, such as box, juniper, olive, heart of oak, cypress, and the like, common oak always excepted, which, from its liability to warp, causes cracks in the work whereon it is employed.
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