One feature of a coal mine that must be carefully planned is the system of ventilation. This is provided not merely for the comfort of the miners, but to prevent, as far as possible, the accumulation of poisonous and explosive gases. There are always at least two airways leading into the mine (one or both of which may also be used for hoisting or other purposes), known as the “upcast” and the “downcast,” according to the direction in which the air passes through them. A current of air is maintained either by keeping a fire burning at the bottom of the upcast or by the use of powerful fans or blowers. A system of tight trap-doors prevents the air from taking a short cut between the downcast and the upcast, and thus leaving the greater part of the mine unventilated. [Illustration: Courtesy of “Coal Age” MINING FROM THE OUTSIDE Stripping the surface of a coal-bed with steam shovel, at Pittsburg, Kans.] [Illustration: Courtesy of U. S. Bureau of Mines WATCHING THE CANARY for indications of poisonous coal gas. Reserves testing the air of a mine after an explosion] [Illustration: Courtesy of U. S. Bureau of Mines MEMBERS OF RESCUE TEAM Showing apparatus worn on entering mines after explosions. This device sustains a man for two hours] The coal in the mine constantly gives off various gases, one of which, the notorious “fire-damp” (methane or marsh-gas), is responsible for many explosions. In recent years it has been discovered that coal-dust itself, when mixed with the right proportion of air, is violently explosive. Mine explosions may be minimized by requiring the use of “safety-lamps” (oil, gasoline, or electric); by providing devices to prevent sparking in electrical apparatus; and by using for blasting operations only so-called “permissible” explosives, which give a shorter and cooler flame than black powder. Coal-dust explosions can be largely prevented by wetting the walls of the mine, or by the new process of “rock-dusting,” which consists of applying dry incombustible powdered rock to all surfaces. Unfortunately, none of these precautions are employed as generally as they should be. [Illustration: Press Illustrating Service SAFETY DEVICE IN COAL BUNKER In case of a “coal slide,” a man may be pulled out before he is buried and stifled] The elevator used for hoisting in the mine shaft is called a “cage.” After the mine cars reach the surface they pass upon an elevated structure called the “tipple.” This is generally the most conspicuous feature of a mining property above ground, and provides facilities for screening and otherwise “preparing” the coal as it passes down chutes to the railway cars underneath. The more elaborate structure used for anthracite is called a “breaker”; it includes machinery for crushing the coal and arrangements for removing “slate” and other waste rock by hand picking or otherwise.