Stephenson's experiments on fire-damp, and his labors in connection with the invention of the safety-lamp, occupied but a small portion of his time, which was necessarily devoted, for the most part, to the ordinary business of the colliery. From the day of his appointment as engine-wright, one of the subjects which particularly occupied his attention was the best practical method of winning and raising the coal. Nicholas Wood has said of him that he was one of the first to introduce steam machinery underground with that object. Indeed, the Killingworth mines came to be regarded as the models of the district; and when Mr. Robert Bald, the celebrated Scotch mining engineer, was requested by Dr. (afterward Sir David) Brewster to prepare the article "Mine" for the "Edinburg Encyclopædia," he proceeded to Killingworth principally for the purpose of examining Stephenson's underground machinery. Mr. Bald has favored us with an account of his visit made with that object in 1818, and he states that he was much struck with the novelty, as well as the remarkable efficiency of Stephenson's arrangements, especially in regard to what is called the underdip working. "I found," he says, "that a mine had been commenced near the main pit-bottom, and carried forward down the dip or slope of the coal, the rate of dip being about one in twelve; and the coals were drawn from the dip to the pit-bottom by the steam machinery in a very rapid manner. The water which oozed from the upper winning was disposed of at the pit-bottom in a barrel or trunk, and was drawn up by the power of the engine which worked the other machinery. The dip at the time of my visit was nearly a mile in length, but has since been greatly extended. As I was considerably tired by my wanderings in the galleries, when I arrived at the forehead of the dip, Mr. Stephenson said to me, 'You may very speedily be carried up to the rise by laying yourself flat upon the coal-baskets,' which were laden and ready to be taken up the incline. This I at once did, and was straightway wafted on the wings of fire to the bottom of the pit, from whence I was borne swiftly up to the light by the steam machinery on the pit-head." The whole of the working arrangements seemed to Mr. Bald to be conducted in the most skillful and efficient manner, reflecting the highest credit on the colliery engineer.