dea is," said Tom, "to lay these down in front of the tank, by
means of cams and levers operated from inside. If we get to a ditch
which we can't climb down into and out again, or bridge with the belt
caterpillar wheels, we'll use the grippers. They'll be laid down,
taking a grip on the far side of the trench, and we'll slide across on
them."

"And leave them there?" asked Mr. Damon.

"No, we won't leave them. We'll pick them up after we have passed over
them and use them in front again as we need them. A couple of extra
pairs of grippers may be carried for emergencies, but I plan to use the
same ones over and over again."

"But what makes it go?" asked Mary. "I don't want all the details,
Tom," she said, with a smile, "but I'd like to know what makes your
tank move."

"I'll be able to show you in a little while," he answered. "But it may
be enough now if I tell you that the main power consists of two big
gasolene engines, one on either side. They can be geared to operate
together or separately. And these engines turn the endless belts made
of broad, steel plates, on which the tank travels. The belts pass along
the outer edges of the tank longitudinally, and go around cogged wheels
at either end of the blunt noses.

"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