"But [Rumford's] chief, or perhaps only, real improvement consisted in the reduction of the size of the chimney-throat, and the rounding off of the lower edge of the chimney-breast... to afford less obstruction to the ascent of the smoke. ...This form, as given by Rumford, is, however, still defective. The smallest part of the flue should be at the bottom... so as to prevent the entrance into the flue of unburnt air from the room. ...The back of the fireplace should also incline forwards, as shown, in order to increase its radiating effect..."