"A bumping-mule?" queried Derrick, in a tone of perplexity not unmixed with disappointment. From the preceding conversation he had expected to be intrusted with something very different from mule-driving; nor had he any idea what sort of an animal the one in question might be. This time Mr. Jones not only smiled but laughed outright; for, from the boy's face and tone, he easily understood what was passing in his mind. "A bumping-mule," he explained, "is the animal that draws the loaded coal-cars from the chambers, or breasts, to where they are made up into trains. These trains are then hauled by a team of mules to the foot of the slope. Then, when the empty cars are brought back, the bumping-mule distributes them to the several places where they are required. I suppose his title comes from his causing the cars to bump together as he makes them up into trains. In attending to your duties as driver of this most important mule, I can assure you that your time will be fully occupied from the minute you go into the mine until you leave it. "I suppose," he added, with a humorous twinkle in his eyes, "that our conversation led you to think you were to be appointed 'air boss' of the mine, or placed in charge of a gang at the very least?" "No, sir," answered Derrick, a little hesitatingly; "I ain't quite such a greeny as that. But I don't see how I can help you very much by just driving a bumping-mule." "You can help me in two ways: first, by doing your duty so faithfully that I may be able to depend on you at all times; second, while I am in doubt as to whom I may trust, it will be of great assistance to me to know that there is at least one person constantly in the mine who will be true to the interests of his employers, and on the alert to detect any attempt to injure them." "I hope you don't mean that I am to be a spy in the mine, sir?" "No, my boy, I do not. I want you to attend strictly to your duties as driver of a bumping-mule. At the same time I want you to consider that your eyes and ears are acting in the place of my eyes and ears. If at any time they see or hear anything which according to your best judgment I ought to know, I hope you will be man enough to tell me of it." "Well, sir," answered Derrick, "I am glad of a chance to go into the mine and to earn five dollars a week. If you will let me do whatever I think is right about telling you things without making any promises, I will keep my eyes and ears wide open." "That is all that I want you to do, my boy." "All right, sir, then I'll do my best; and I hope I sha'n't have anything to tell you except about the bumping-mule."