in building, those pieces of timber that lie across the rafters on the inside, to keep them from sinking in the middle of their length.

By the act of parliament for rebuilding London, it is provided, that all purlins from 15 feet 6 inches to 18 feet 6 inches long, be in their square 9 inches and 8 inches; and all in length from 18 feet 6 inches to 21 feet 6 inches, be in their square 12 inches and 9 inches.