Arch, subst. concave and hollow structure: “as through an a. the violent roaring tide,” Lucr. 1667. “like an a., reverberates the voice,” Troil. III, 3, 120. Cor. V, 4, 50. the watery a. (rainbow) Tp. IV, 71. this vaulted a. (sc. of heaven) Cymb. I, 6, 33. “let Rome in Tiber melt, and the wide a. of the ranged empire fall,” Ant. I, 1, 33.