A, Drain to Sewer. B, Trap in Main Drain. C, Water Closets. D, Cold Air Inlet. E, Outlet for Foul Air. F, the Tee Branch. Fresh Air entering the Soil Pipe slightly above the pavement, at D, being colder than the air already in the pipe, will cause the air therin to rise and pass out through the opening E, at the top. Thus a circulation is produced.