In deep mines, as has already been said, it is customary to build stables not far from the foot of the shaft, and to keep the mules there except when for any reason there is a long suspension of work. At many mines, however, the greater convenience of having the stables on the surface induces the operators to have the mules hoisted from the shaft every night and taken down every morning. They step on the carriage very demurely, and ascend or descend without making trouble. They are especially glad to go up to their stables at night. Where mules are fed in the mine, and especially in those mines that have stables in them, rats are usually found. How they get down a shaft is a mystery. The common explanation is that they go with the hay. But they take up their quarters in the mine, live, thrive, increase rapidly, and grow to an enormous size. They are much like the wharf rats that infest the wharves of great cities, both in size and ugliness. They are very bold and aggressive, and when attacked will turn on their enemy, whether man or beast, and fight to the death. There is a superstition among miners to the effect that when the rats leave a mine some great disaster is about to take place in it; probably an extensive fall. Rats are hardly to be credited, however, with an instinct that would lead them to forecast such an event with more certainty than human experience and skill can do. But it is not improbable that the driver boy and his mule will be superseded, at no distant day, by electricity. In one instance at least this new motive power has already been put into use. This is at the Lykens Valley Colliery of the Lykens Valley Coal Company, in Dauphin County, Pennsylvania. The duty of an outside driver boy is to take the loaded cars from the head of the shaft or slope to the breaker, and to bring the empty ones back; his work being all done in the open air. Of late this service, especially where the distance is considerable, is performed by a small locomotive, which draws trains of as many cars as can well be held together. The wages paid to inside driver boys by the Pennsylvania Coal Company in 1888 were from one dollar to one dollar and ten cents a day, and to outside driver boys eighty-eight cents a day.