"Now as this passage, which, in its narrowest part, I have called the throat of the Chimney, ought... to be immediately, or perpendicularly over the Fire, it is evident that the back of the Chimney must always be built perfectly upright.—To determine therefore the place for the new back, or how far precisely it ought to be brought forward, nothing more is necessary that to ascertain how wide the throat of the Chimney to be left, or what trace must be left between the top of the breast of the Chimney, where the upright canal of the Chimney begins, and the new back of the Fire-place carried up perpendicularly to that height."