In recent years electric traction engines for use in mines have been rapidly replacing horse-and mule-power, and have become important economic factors in mining operations. The pioneer of this type of locomotive seems to have been one built by Mr. W. M. Schlessinger for one of the collieries of the Pennsylvania Railroad about 1882, and which has remained in active use ever since. The total weight of this locomotive was five tons and it was equipped with thirty-two horse-power electric motors. The current was supplied through a trolley pole which took the current from a T-shaped rail placed above and at one side of the track. The train hauled by this locomotive consisted of fifteen cars, carrying from two to three tons of coal each. Following this first mining-locomotive a great number were quickly produced. In Pennsylvania alone something like four hundred are now in use, and in Illinois two million tons of coal were hauled in this manner in twelve mines in 1901. It was estimated at the beginning of the present century that some 3,000 electric locomotives specially built for mining were in use in the United States alone. The earlier types of mining-locomotives were much higher and bulkier than those of more recent construction, the motors being mounted above the trucks and geared downward. Very soon, however, the "turtle-back" or "terrapin-back" type was developed, with the motors brought close to the ground, so that even quite a heavy locomotive might not be much higher than the diameter of its driving-wheels. When these queer-looking machines were boxed in so that even the wheels were covered, they lost all resemblance to locomotives or vehicles of any kind, appearing like low, rectangular metal boxes placed upon the car tracks, that glided along the rails in some mysterious manner. The presence of the trolley pole helped to dispel this illusion, but in some instances this is wanting, the power being taken from a third rail. With these locomotives, some of them not more than two and a half feet high, it was possible to haul trains even in very low and narrow passages--much lower, in fact, than could be entered by the little mules used in former years. This in itself was revolutionary in its effects, as many thin veins were thus made workable. This type of low locomotive is the one that has come into general use throughout the world. Such locomotives range in size from two to twenty tons, with wheel gauges from a foot and a half wide to the standard railway gauge of four feet, eight and a half inches. Locomotives weighing more than twenty tons are not in general use on account of the small size of the mine entrances.