What height of chimney is required to produce a draught of \(20 \mathrm{~mm}\) of water column, if \(15 \mathrm{~kg}\) of air is required to burn \(1 \mathrm{~kg}\) of coal? The mean temperature of gas inside the chimney is \(300^{\circ} \mathrm{C}\) and that of atmospheric air is \(35^{\circ} \mathrm{C}\).