"Then I guess we'll lock up and turn in," decided Tom. "Come over to-morrow, Ned."
"I will," promised the young bank clerk. "I want to see more of what makes the wheels go round." And he laughed at his own ingenuousness.
The next day Tom showed his friends as much as they cared to see about the workings of the tank. They inspected the powerful gasolene engines, saw how they worked the endless belts made of plates of jointed steel, which, running over sprocket wheels, really gave the tank its power by providing great tractive force.
Any self-propelled vehicle depends for its power, either to move itself or to push or to pull, on its tractive force—that is, the grip it can get on the ground.
In the case of a bicycle little tractive power is needed, and this is provided by the rubber tires, which grip the ground. A locomotive depends for its tractive power on its weight pressing on its driving wheels, and the more driving wheels there are and the heavier the locomotive, the more it can pull, though in that case speed is lost. This is why freight locomotives are so heavy and have so many large driving wheels. They pull the engine along, and the cars also, by their weight pressing on the rails.
The endless steel belts of a tank are, the same as the wheels of a locomotive. And the belts, being very broad, which gives them a large surface with which to press on the ground, and the tank being very heavy, great power to advance is thus obtained, though at the sacrifice of speed. However, Tom Swift had made his tank so that it would do about ten miles and more an hour, nearly double the progress obtained up to that time by the British machines.
His visitors saw the great motors, they inspected the compact but not very attractive living quarters of the crew, for provision had to be made for the men to stay in the tank if, perchance, it became stalled in No Man's Land, surrounded by the enemy.
The tank was powerfully armored and would be armed. There were a number of machine guns to be installed, quick-firers of various types, and in addition the tank could carry a number of riflemen.
It was upon the crushing power of the tank, though, that most reliance was placed. Thus it could lead the way for an infantry advance through the enemy's lines, making nothing of barbed wire that would take an artillery fire of several days to cut to pieces.
"And now, Ned," said Tom, about a week after the night test of the tank, "I'm going to try what she'll do in bridging a gap."