A series of holes were placed in the side of the firebox - actually small tubes were placed through the water space allowing outside air access to the firebox above the bed of burning fuel. Into this tube a small steam line was placed with a nozzle tip on the end. When the steam was turned on, it created a venturi effect drawing outside air through the tube into the firebox. The idea was to draw additonal air into the firebox above the bed of coal to create better combustion of the the volitile gasses above the firebed, thereby reducing smoke. The round cans on the outside of the firebox cover the opening - they are not tight against the firebox as they must allow air to get into the tube. They serve to keep fire from flashing out the overfire jet tube and to some extent muffle the noise of the steam jets blowing.