It showed the tank in outline, much as appear the pictures of tanks already in service—the former simile of two wedge-shaped pieces of metal put together broad end to broad end, still holding good. From one end of the tank, as Tom drew it, there extended two long arms of latticed steel construction.
"The idea 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: