"When both belts travel at the same rate of speed the tank goes in a straight line, though it can be steered from side to side by means of a trailer wheel in the rear. Making one belt--one set of caterpillar wheels, you know--go faster than the other will make the tank travel to one side or the other, the turn being in the direction of the slowest moving belt. In this way we can steer when the trailer wheels are broken." "And what does your tank do except travel along, not minding a hail of bullets?" asked Mr. Nestor. "Well," answered Tom, "it can do anything any other tank can do, and then some more. It can demolish a good-sized house or heavy wall, break down big trees, and chew up barbed-wire fences as if they were toothpicks. I'll show you all that in due time. Just now, if the repairs are finished, we can get back on the road--" At that moment a door leading into the compartment where Tom and his friends were talking opened, and one of the workmen said: "A man outside asking to see you, Mr. Swift." "Pardon me, but I won't keep you a moment," interrupted a suave voice. "I happened to observe your tank, and I took the liberty of entering to see--" "Simpson!" cried Ned Newton, as he recognized the man who had been up the tree. "It's that spy, Simpson, Tom!" Chapter XII Bridging a Gap Such surprise showed both on the face of Ned Newton and that of the man who called himself Walter Simpson that it would be hard to say which was in the greater degree. For a moment the newcomer stood as if he had received all electric shock, and was incapable of motion. Then, as the echoes of Ned's voice died away and the young bank clerk, being the first to recover from the shock, made a motion toward the unwelcome and uninvited intruder, Simpson exclaimed. "I will not bother now. Some other time will do as well." Then, with a haste that could be called nothing less than precipitate, he made a turn and fairly shot out of the door by which he had entered the tank. "There he goes!" cried Mr. Damon. "Bless my speedometer, but there he goes!" "I'll stop him!" cried Ned. "We've got to find out more about him! I'll get him, Tom!" Tom Swift was not one to let a friend rush alone into what might be danger. He realized immediately what his chum meant when he called out the identity of the intruder, and, wishing to clear up some of the mystery of which he became aware when Schwen was arrested and the paper showing a correspondence with this Simpson were found, Tom darted out to try to assist in the capture. "He went this way!" cried Ned, who was visible in the glare of the searchlight that still played its powerful beams over the stern of the tank, if such an ungainly machine can be said to have a bow and stern. "Over this way!"