Smoke is injurious to health, destructive to vegetation, and fatal to architectural beauty; and, along with all this, it is enormously expensive. In the first place, a smoky chimney means imperfect combustion, and a waste of part of the heating value of fuel. Then a smoky atmosphere entails big laundry and dry-cleaning bills; frequent repainting of houses; injury to metal work; damage to goods in shops; excessive artificial lighting in the daytime. Pittsburgh was once the most famous American example of all these evils, but it has recently reformed. Before the Mellon Institute of Industrial Research carried out its elaborate smoke investigation in that city, and, in consequence, stringent smoke-abatement ordinances were adopted, the annual smoke bill of Pittsburgh was estimated at nearly ten million dollars. The city was the paradise of the laundryman, and light-colored clothing was so little worn by the inhabitants that it was known as “the mourning town.” Throughout the United States it is said that smoke causes an annual waste and damage amounting to half a billion dollars. No wonder numerous societies have been formed to mitigate this evil, and a great many laws have been enacted on the subject. With a gradual increase in the use of gas, coke and other smokeless fuels, and improved methods of stoking furnaces, the smoke nuisance is now happily abating. The pollution of the air by smoke is the subject of systematic investigation and measurement at certain places in this country and abroad. Measurements of the “soot-fall” made in Pittsburgh a few years ago indicated an annual average deposit of soot in that city amounting to 1,031 tons per square mile. London’s average is 248 tons per square mile for the whole city and 426 tons in the central districts. In the heart of Glasgow the annual soot-fall is 820 tons per square mile. In Great Britain there is a Committee for the Investigation of Atmospheric Pollution, which has installed standard measuring apparatus in sixteen English and Scotch towns. The soot is collected in a “pollution gauge,” consisting of a large cast-iron funnel, enameled on the inside. Projecting above the gauge is a wire screen, open at the top, to prevent birds from settling on the edge of the vessel. The gauge communicates at the bottom with one or more bottles for collecting rain-water, with its solid contents. The bottles are emptied once a month, when their contents are weighed and analyzed. Smoke is injurious to the respiratory organs, conducive to eye-strain and responsible for a lowering of human vitality. The gloominess of a smoke-laden atmosphere also has a depressing effect upon the minds of many people. PREPARED BY THE EDITORIAL STAFF OF THE MENTOR ASSOCIATION ILLUSTRATION FOR THE MENTOR. VOL. 6, No. 6, SERIAL No. 154 COPYRIGHT, 1918. BY THE MENTOR ASSOCIATION, INC. [Illustration: COURTESY U. S. BUREAU OF MINES RESCUE PARTY ENTERING MINE AFTER EXPLOSION] _THE STORY OF COAL_