observed.

Tank A reached her "harbor" safely--in other words, the machine shop
enclosed by the high fence, inside of which she had been built.

Tom and Ned made some inquiries of Koku and Eradicate as to whether or
not there had been any unusual sights or sounds about the place. They
feared Simpson might have come to the shop to try to get possession of
important drawings or data.

But all had been quiet, Koku reported Nor had Eradicate seen or heard
anything out of the ordinary.

"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