"It appearing plainly... that some of the outward air must be admitted, the question will be, How much is absolutely necessary? ...To discover this quantity, shut the door gradually, while a middling fire is burning, till... the smoke begins to come out into the room; then open it a little, till you perceive the smoke comes out no longer: there hold the door, and observe the width of the open crevice... thence... your room requires an entrance for air equal in area.. I have found ...thirty-six square inches, to... serve for most chimneys."