"Which is as true as gospel," said the mine boss. "Well, the only other thing I can suggest is for you to go into the stable, get the harness that hangs on the peg nearest the door, and put it on him." Acting upon this hint, Derrick had hardly finished buckling the last strap of the harness when the mule began to move steadily forward of his own accord. "That's his way," said the mine boss. "In harness he knows that he is expected to work, but without it he thinks he may do as he pleases." Presently the mule stumbled slightly, and again he stopped and refused to go ahead. "Do you know what is the matter now, sir?" asked Derrick. "I think perhaps he wants his lamp lighted," replied the mine boss. A miner's lamp, attached to a broad piece of leather, hung down in front of the mule from his collar. The boy lighted this lamp, and immediately the mule began to move on, showing that this was exactly what he had wanted. "Seems to me he knows almost as much as folks," cried Derrick, highly delighted at this new proof of his mule's intelligence. "Quite as much as most folks, and more than some," answered his companion, dryly. During their long walk they passed through several doors which, as Derrick was told, served to regulate the currents of air constantly flowing in and out of the mine, and kept in motion by the great fan at its mouth. Whenever they approached one of these the mine boss called, loudly, "Door," and it was immediately opened by a boy who sat behind it and closed it again as soon as they had passed. Each of these boys had besides his little flaring lamp, such as everybody in the mine carried, a can of oil for refilling it, a lunch-pail and a tin water-bottle, and each of them spent from eight to ten hours at his post without leaving it. Finally Derrick and the mine boss came to a junction of several galleries, a sort of mine cross-roads, and the former was told that this was to be his headquarters, for here was where the trains were made up, and from here the empty cars were distributed. At the farther end of each of the headings leading from this junction two or more miners were at work drilling, blasting, and picking tons of coal from between its enclosing walls of slate. They were all doing their best to fill the cars which it was Derrick's business to haul to the junction and replace with empty ones. There were also a number of miners at work in breasts, or openings at the sides of the gangways that followed the slant of the coal vein, who expected to be supplied with empty cars and have their loaded ones taken away by Derrick. These breast miners filled their cars very quickly, as the moment they loosened the coal it slid down the slaty incline, above which it had been bedded, to a wooden chute on the edge of the gangway that discharged it directly into them.