But how could he obtain this power? The mechanism seemed to be working at the greatest force, for, after an attempt had been made to stay the progress of the boat by reversing it, Mr. Roumann had again put on full speed ahead. But was it full speed? That was what Jack wanted to know. He examined every inch of the box. At first he saw nothing but the three knobs that had been used. Then, all at once, down underneath, he saw a small pin. It looked as if it could be moved. He took hold of it. "I wonder what will happen if I pull it out or push it in?" he asked himself. Then he happened to remember that in an electric battery, to obtain more power, you must pull out a certain pin. "Perhaps this works like an electric battery," he said. "I'll pull it out." He did so, and a surprising thing happened. The boat shot forward at enormous speed, and as Jack happened to pull the pin out at a time when the craft was high up, it began to shoot across the water at an angle to the whirlpool. He had solved the problem of how to escape. As he afterward learned, the pin was just for the purpose for which he used it--to cause a sudden increase in speed. The whirlpool did not give up without a struggle, but the boat was finally successful, and fought its way out to calm water. "How did you do it?" asked Mark, and Jack told them. "Well, we'd better start back for the city," proposed Mr. Roumann. "I guess we've had enough for one day. We'll try again, and take some other route." "There's no need of that," declared Professor Henderson. "See, we are close to the hills now. We have crossed the whirlpool. Why not go on, and see if we can't find some Cardite? Going back now will be no easier than after we have made an examination. Let's explore the hills." The boat had shot out on the farther side of the whirlpool, and there was nothing now between it and the shore. After a consultation it was decided to land. "We can be more careful coming back," said Jack. Half an hour later they had landed and started up the hills toward the summit. The place seemed to be deserted, but there were evidences that some sort of mining had been going on there, for great holes and shafts were dug in the ground, and there were remains of machinery. "They must have dug up all the Cardite in this locality," said Professor Henderson, "and they've gone to a new place." "Which we wish to find," added Mr. Roumann, "and that without being discovered. From the way in which the Martians guard this, it will go hard with us if they see us taking any." They toiled up to the top of some of the lowest hills. There did not seem to be any of the treasure there, and they went farther. They were getting tired and hungry, and they sat down to rest and eat some food which they had brought with them. "Let's try that hill," proposed Mark, pointing to a high one about a mile away. "It looks red from here."