"All twists, bends, prominences, excavations, and other irregularities of form, in the covings... never fail to produce eddies in the current of air which is continually passing into, and through an open Fire-place in which a fire is burning;—and all such eddies disturb, either the fire, or the ascending current of smoke, or both; and not unfrequently cause the smoke to be thrown back into the room.—Hence... the covings... should never be made circular, or in the form of any other curve; but always quite flat."