A small locomotive of two horse-power nominal is used on an i8£ inch track in the long adit of the Great Laxey lead and zinc mine in the Isle of Man ; and at Rio Tinto in Spain a much larger engine pUes in the adit on a line with a gauge of 3 feet 6 inches.
Locomotives fired with coal have the great disadvantage of poUuting the air by the products of combustion, consequently they are not available unless the ventilation is very good, nor unless there is absence of inflammable gases and freedom from the risk of setting fire to tbe timbering or to the mineral itself.  A small locomotive of two horse-power nominal is used on an i8£ inch track in the long adit of the Great Laxey lead and zinc mine in the Isle of Man ; and at Rio Tinto in Spain a much larger engine pUes in the adit on a line with a gauge of 3 feet 6 inches.  Locomotives driven by compressed air, carried in a reservoir upon a tender, improve the ventilation instead of injuring it, and are not a possible source of danger from fire ; but, except in special cases, they cannot be worked so cheaply as engines fired with coal.