The fellow looked at him stupidly, as if his nerve had failed, or he thought the order too risky to obey. There was only one thing to be done, and since it must be done at once, Dick must undertake it himself. The engine was now running down the line after the truck, which had not gathered much speed yet, and he climbed across the coal and dropped upon the rear buffer-frame. Balancing himself upon it, he waited until the gap between him and the truck got narrower, and then put his hand on top of the concrete and swung himself across. He got his foot upon the side of the car and made his way along, holding the top of the block, while the dust rolled about him and he thought he would be jolted off. Indeed, there was only an inch-wide ledge of smooth iron to support his foot, which slipped once or twice; but he reached the brake-gear and screwed it down. Then, crawling back, he hooked on the spare coupling and returned, breathless and shaky, to his engine. A minute or two later he brought it to a stop and had got down upon the line when somebody called him. Looking round, he saw Fuller standing near, and knew him as the man who had given him the dollar in the American town. He had heard that his employer had come out to see what progress was being made, but had not yet encountered him. He did not notice Ida, who was sitting in the shadow of the rock. "You were smart," said Fuller. "There'd have been an ugly smash if the blocks had got away down the grade. But why didn't you stick to the throttle and send your fireman?" "I don't think he understood what he ought to do, and there was no time to explain." Fuller nodded. "So you did it yourself! But why didn't you push the car? You could have held her up better then." "I couldn't get behind it. The loop-track down at the switches has caved in." "I see. But it's a stiff grade and you didn't seem to be hustling your engine much." "The boiler was priming and I was afraid of the cylinders." "Just so. You pumped up the water pretty high?" "No; it was at the usual working level," said Dick, who paused and resumed thoughtfully: "I can't account for the thing. Why does a boiler prime?" There are one or two obvious reasons for a boiler's priming; that is to say, throwing water as well as steam into the engine, but this sometimes happens when no cause can be assigned, and Fuller saw that Dick did not expect an answer to his question. It was rather an exclamation, prompted by his failure to solve a fascinating problem, and as such indicated that his interest in his task was not confined to the earning of a living. Fuller recognized the mind of the engineer. "Well," he replied, "there's a good deal we don't know yet about the action of fluids under pressure. But do you find the grade awkward when she's steaming properly?"