As a coal mine supervisor, there were several pieces of equipment I used to detect the safety level of the environment in which we worked. First, was the barometer, which most supervisors checked before going underground. A low barometer reading meant methane gas could more easily seep out of sealed old works and accumulate in the high places on the track. Sparks from a bouncing trolley pole could ignite the gas. In the winter, such dangers were more pronounced because the dust in the mine tended to dry out creating more combustible material to feed a fire.