National Geographic : 1949 Jul
Pittsburgh: Workshop of the Titans 129 "Smoke Sleuths" Work Constantly to Combat a Pittsburgh Nuisance These city inspectors study heavy clouds billowing from a steel mill's chimneys. Smoke is viewed with special glasses (right) and through a hole in a Ringelmann Chart (left). Density is graded by matching it with shades ranging from light gray to black on the card. Violators of smoke-abatement regulations may be fined (page 141).