This was especially ineffective when the fire is first lit and the temperature and density of the smoke was closer to the ambient air. Thanks to the discontinuity produced by the brick “smoke shelf”, the flow of smoke gases up the chimney became detached from the outside wall at the lip of the shelf. This set up a counter-circulation of outside air which flowed DOWN the back side of the chimney, while a mixture of outside air and smoke flowed up the opposite side. The circulation inside the chimney, and above the smoke shelf, created a dynamic pressure in which the smoke gases were driven up one side of the chimney and cold air was pulled down the other.