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.
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