"Whether the new back and covings are constructed of stone, or... bricks, the space between them, and the old back and covings of the Chimney ought to be filled up... with loose rubbish, or pieces of broken bricks, or stones, provided the work be strengthened by a few layers or courses of bricks laid in mortar; but it will be indispensably necessary to finish the work where these new walls end... at the top of the throat of the Chimney, where it ends abruptly in the open canal of the Chimney by a horizontal course of bricks well secured with mortar."