What is draft

Draft (d) at any point in the gas flow system is

d=[ Absolute pressure in the system -
                                        Absolute pressure of the surrounding atmosphere at the same level]
Figure shows the chimney or stack in which hot gases are flowing at temperature T. We mount two water manometers at positions  and. One leg of the manometer is fixed with the chimney whereas the other leg is open to the atmosphere as shown in the figure.