Sleet

Collier's New Encyclopedia (1921)

a term popularly applied to several distinct forms of cold weather precipitation, but now used exclusively by the U. S. Weather Bureau to desig nate the fall of frozen or partly frozen raindrops in the form of particles of clear ice. In England the term is commonly applied to the mixture of snow and rain falling together, while many Americans, in addition to the above meanings, use the term to describe what is now officially named glaze or glazed frost. (See GLAZE.) SMOKE PRECIPITATION. The campaign against the smoke nuisance in cities originated in the desire to rid the atmosphere of soot for reasons of health and general cleanliness, but the economic wastage and costliness involved in volumes of black smoke issuing from chimneys have gradually come to be realized not only by the community at large, but by the users of fuel themselves. Masses of laws have been enacted looking to the protection of the public health and comfort, and the growing electrification of railroads and of railroad terminals will ultimately go far toward solving the problem of locomotive smoke; but the greatest advances in regard to the question of factory smoke, etc., now seem likely to be brought about by the adoption of scientific methods of smoke precipitation, literally saving the smoke, which in fact has been found in many industries to contain valuable by-products as well as unconsumed particles of fuel. In the smelting and refining processes, for instance, where the smelters are fired with the products of the ores as well as with charcoal and other fuels, the resulting smoke commonly contains acids and alkalines, poisonous to vegetation and injurious to man if allowed to escape into the atmosphere, but very valuable in themselves, if saved. Mechanical methods of smoke precipitation were first attempted, but were only partially successful. Electrical precipitation, on other hand, which dates back only to 1912, has already proved itself highly efficient, and the by-products annually saved in many industries have exceeded in their value the total cost of installing and operating the precipitation treaters, as they are called. A high-voltage direct current ionizes or electrifies the solid particles in the smoke as they pass through pipes on their way to the stacks, causing them to be hurled against the sides of the pipes from which they are collected in various ways and gathered into receiving bins. The annual sootfall in the single city of Pittsburgh prior to the adoption of recent smoke-prevention methods was estimated at 1,031 tons per square mile.