For several years he had been engaged, in his own rude way, in making experiments with the fire-damp in the Killingworth mine. The pitmen used to expostulate with him on these occasions, believing his experiments to be fraught with danger. One of the sinkers, observing him holding up lighted candles to the windward of the “blower” or fissure from which the inflammable gas escaped, entreated him to desist; but Stephenson’s answer was, that “he was busy with a plan by which he hoped to make his experiments useful for preserving men’s lives.” On these occasions the miners usually got out of the way before he lit the gas. In 1815, although he was very much occupied with the business of the collieries and the improvement of his locomotive engine, he was also busily engaged in making experiments upon inflammable gas in the Killingworth pit. According to the explanation afterwards given by him, he imagined that if he could construct a lamp with a chimney so arranged as to cause a strong current, it would not fire at the top of the chimney; as the burnt air would ascend with such a velocity as to prevent the inflammable air of the pit from descending towards the flame; and such a lamp, he thought, might be taken into a dangerous atmosphere without risk of exploding. Such was Stephenson’s theory when he proceeded to embody his idea of a miner’s safety-lamp in a practical form. In the month of August, 1815, he requested his friend Nicholas Wood, the head viewer, to prepare a drawing of a lamp according to the description which he gave him. After several evenings’ careful deliberations, the drawing was made, and shown to several of the head men about the works. Stephenson proceeded to order a lamp to be made by a Newcastle tinman, according to his plan; and at the same time he directed a glass to be made for the lamp at the Northumberland Glass House. Both were received by him from the makers on the 21st October, and the lamp was taken to Killingworth for the purpose of immediate experiment.