Although Derrick was greatly comforted by these words, he could not help dreading to hear more of the rock explosions, which are caused by the roof, walls, and pillars of the mine giving slightly beneath the vast crushing weight of material above them. When he reached Paul Evert's station, and found that the crippled lad had heard some of the same loud snappings and crackings, but was not alarmed at them, he felt ashamed of his own fears, and casting them entirely aside, asked to see what the other was drawing. Paul was very fond of drawing with a pencil, or bit of charcoal, or anything that came to his hand, on all sorts of surfaces, and really showed great skill in his rude sketches of the common objects about him. Since coming into the mine he had found more time to indulge his taste than ever before; and though his only light was the wretched little lamp in his cap, he had produced some beautiful copies of the dainty ferns and curious patterns imprinted on the walls about him. He had also afforded Derrick great amusement by making for him several sketches of Socrates the wise rat in various attitudes. Until this time he had never hesitated before showing his friend any of his efforts, but now he did, and it was only after much urging that he reluctantly handed Derrick the sheet of paper on which he had been working. It was an outline sketch of the figures composing their underground picnic party of the day before, including Socrates, and Derrick had no sooner set eyes on it than he declared he must have it. "I was doing it for you, 'Dare,'" said Paul, using his especial pet name for Derrick, which he never did except when they were alone. "But you must let me finish it, and that will take some time; there is so much to put in, and my light is so bad." Derrick was obliged to agree to this, though he would have valued the sketch just as it was, and handing it back, he went on towards where Paul thought his father was at work. At last he found him, in a distant heading that was exhausted and about to be abandoned, engaged in the dangerous task of "robbing back." In cutting into a vein it is often necessary to leave walls and pillars of solid coal standing to support the roof, and when the workings about them are exhausted it is customary to break away these supports for the sake of what coal they contain. This is called "robbing back," and is so dangerous a job that only the very best and most experienced miners are intrusted with it. Sometimes the roof, thus robbed of its support, falls, and sometimes it does not. If it does fall, perhaps the miner "robber" gets killed, and perhaps he escapes entirely, or with only bruises and cuts.