a fieldwork consisting of two faces, forming a salient angle, and two parallel flanks. see bastion.-a half horseshoe, which wants the sponge.-a kind of watch crystal which is more than ordinarily flattened in the center; also, a species of convexoconcave lens for spectacles.-a piece of felt to cover the eye of a vicious horse.-any surface of semicircular or segmental form; especially, the piece of wall between the curves of a vault and its springing line.-an iron shoe at the end of the stock of a gun carriage.