Such surprise showed both on the face of Ned Newton and that of the man who called himself Walter Simpson that it would be hard to say which was in the greater degree. For a moment the newcomer stood as if he had received all electric shock, and was incapable of motion. Then, as the echoes of Ned's voice died away and the young bank clerk, being the first to recover from the shock, made a motion toward the unwelcome and uninvited intruder, Simpson exclaimed.

"I will not bother now. Some other time will do as well."

Then, with a haste that could be called nothing less than precipitate, he made a turn and fairly shot out of the door by which he had entered the tank.

"There he goes!" cried Mr. Damon. "Bless my speedometer, but there he goes!"

"I'll stop him!" cried Ned. "We've got to find out more about him! I'll get him, Tom!"

Tom Swift was not one to let a friend rush alone into what might be danger. He realized immediately what his chum meant when he called out the identity of the intruder, and, wishing to clear up some of the mystery of which he became aware when Schwen was arrested and the paper showing a correspondence with this Simpson were found, Tom darted out to try to assist in the capture.

"He went this way!" cried Ned, who was visible in the glare of the searchlight that still played its powerful beams over the stern of the tank, if such an ungainly machine can be said to have a bow and stern. "Over this way!"

"I'm with you!" cried Tom. "See if you can pick up that man who just ran out of here!" he cried to the operator of the searchlight in the elevated observation section of what corresponded to the conning tower of a submarine. This was a sort of lookout box on top of the tank, containing, among other machines, the searchlight. "Pick him up!" cried Tom.

The operator flashed the intense white beam, like a finger of light, around in eccentric circles. but though this brought into vivid relief the configuration of the field and road near which the tank was stalled, it showed no running fugitive. Tom and Ned were observed--shadows of black in the glare--by Mary and her friends in the tank, but there was no one else.

"Come on!" cried Ned. "We can find him, Tom!"

But this was easier said than done. Even though they were aided by the bright light, they caught no glimpse of the man who called himself Simpson.

"Guess he got away," said Tom, when he and Ned had circled about and investigated many clumps of bushes, trees, stumps and other barriers that might conceal the fugitive.

"I guess so," agreed Ned. "Unless he's hiding in what we might call a shell crater."

"Hardly that," and Tom smiled. "Though if all goes well the men who operate this tank later may be searching for men in real shell holes."

"Is this one going to the other side?" asked Ned, as the two walked back toward the tank.

"I hope it will be the first of my new machines on the Western front," Tom answered. "But I've still got to perfect it in some details and then take it apart. After that, if it comes up to expectations, we'll begin making them in quantities."

"Did you get him?" asked Mr. Damon eagerly, as the two young men came back to join Mary and her friends.

"No, he got away," Tom answered.

"Did he try to blow up the tank?" asked Mr. Nestor, who had an abnormal fear of explosives. "Was he a German spy?"

"I think he's that, all right," said Ned grimly. "As to his endeavoring to blow up Tom's tank, I helieve him capable of it, though he didn't try it to-night--unless he's planted a time bomb somewhere about, Tom."

"Hardly, I guess," answered the young inventor. "He didn't have a chance to do that. Anyhow we won't remain here long. Now, Ned, what about this chap? Is he really the one you saw up in the tree?"

"I not only saw him but I felt him," answered Ned, with a rueful look at his fingers. "He stepped right on me. And when he came inside the tank to-night I knew him at once. I guess he was as surprised to see me as I was to see him."

"But what was his object?" asked Mr. Nestor.

"He must have some connection with my old enemy, Blakeson," answered Tom, "and we know he's mixed up with Schwen. From the looks of him I should say that this Simpson, as he calls himself, is the directing head of the whole business. He looks to be the moneyed man, and the brains of the plotters. Blakeson is smart, in a mechanical way, and Schwen is one of the best machinists I've ever employed. But this Simpson strikes me as being the slick one of the trio."

"But what made him come here, and what did he want?" asked Mary. "Dear me! it's like one of those moving picture plots, only I never saw one with a tank in it before--I mean a tank like yours, Tom."

"Yes, it is a bit like moving picture--especially chasing Simpson by searchlight," agreed the young inventor. "As to what he wanted, I suppose he came to spy out some of my secret inventions--dad's and mine. He's probably been hiding and sneaking around the works ever since we arrested Schwen. Some of my men have reported seeing strangers about, but I have kept Shop Thirteen well guarded.

"However, this fellow may have been waiting outside, and he may have followed the tank when we started off a little while ago for the night test. Then, when he saw our mishap and noticed that we were stalled, he came in, boldly enough, thinking, I suppose, that, as I had never seen him, he would take a chance on getting as much information as he could in a hurry."

"But he didn't count on Ned's being here!" chuckled Mr. Damon.

"No; that's where he slipped a cog," remarked Mr. Nestor. "Well, Tom, I like your tank, what I've seen of her, but it's getting late and I think Mary and I had better be getting back home."

"We'll be ready to start in a little while," Tom said, after a brief consultation with one of his men. "Still, perhaps it would be just as well if you didn't ride back with me. She may go all right, and then, again, she may not. And as it's dark, and we're in a rough part of the field, you might be a bit shaken up. Not that the tank minds it!" the young inventor hastened to add "She's got to do her bit over worse places than this--much worse--but I want to get her in a little better working shape first. So if you don't mind, Mary, I'll postpone your initial trip."

"Oh, I don't mind, Tom! I'm so glad you've made this! I want to see the war ended, and I think machines like this will help."

"I'll ride back with you, Tom, if you don't mind," put in Ned. "I guess a little shaking up won't hurt me."

"All right--stick. We're going to start very soon."

"Well, I'm coming over to-morrow to have a look at it by daylight," said Mr. Damon, as he started toward his car.

"So am I," added Mary. "Please call for me, Mr. Damon."

"I will," he promised."

Mr. Nestor, his daughter, and Mr. Damon went back to the automobile, while Ned remained with Tom. In a little while those in the car heard once more the rumbling and roaring sound and felt the earth tremble. Then, with a flashing of lights, the big, ungainly shape of the tank lifted herself out of the little ditch in which she had come to a halt, and began to climb back to the road.

Ned Newton stood beside Tom in the control tower of the great tank as she started on her homeward way.

"Isn't it wonderful!" murmured Mary, as she saw Tank A lumbering along toward the road. "Oh, and to think that human beings made that To think that Tom should know how to build such a wonderful machine!"

"And run it, too, Mary! That's the point! Make it run!" cried her father. "I tell you, that Tom Swift is a wonder!"

"Bless my dictionary, he sure is!" agreed Mr. Damon.

Along the road, back toward the shop whence it had emerged, rumbled the tank. The noise brought to their doors inhabitants along the country thoroughfare, and some of them were frightened when they saw Tom Swift's latest war machine, the details of which they could only guess at in the darkness.

"She'll butt over a house if it gets in her path, knock down trees, chew up barbed-wire, and climb down into ravines and out again, and go over a good-sized stream without a whimper," said Tom, as he steered the great machine.

There was little chance then for Ned to see much of the inside mechanism of the tank. He observed that Tom, standing in the forward tower, steered it very easily by a small wheel or by a lever, alternately, and that he communicated with the engine room by means of electric signals.

"And she steers by electricity, too," Tom told his friend. "That was one difficulty with the first tanks. They had to be steered by brute force, so to speak, and it was a terrific strain on the man in the tower. Now I can guide this in two ways: by the electric mechanism which swings the trailer wheels to either side, or by varying the speed of the two motors that work the caterpillar belts. So if one breaks down, I have the other."

"Got any guns aboard her--I mean machine guns?" asked Ned.

"Not yet. But I'm going to install some. I wanted to get the tank in proper working order first. The guns are only incidental, though of course they're vitally necessary when she goes into action. I've got 'em all ready to put in. But first I'm going to try the grippers."

"Oh, you mean the gap-bridgers?" asked Ned.

"That's it," answered Tom. "Look out, we're going over a rough spot now."

And they did. Ned was greatly shaken up, and fairly tossed from side to side of the steering tower. For the tank contained no springs, except such as were installed around the most delicate machinery, and it was like riding in a dump cart over a very rough road.

"However, that's part of the game," Tom 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 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."

"Have you got her in shape again?"

"Yes, everything is all right. I've taken out the weak part in the steering gear that nearly caused us to run you down, and we're safe in that respect now. And I've got the grippers made. It only remains to see whether they're strong enough to bear the weight of my little baby," and Tom affectionately patted the steel sides of Tank A.

While his men were getting the machine ready for a test out on the road, and for a journey across a small stream not far away, Torn told his chum about conceiving the idea for the tank and carrying it out secretly with the aid of his father and certain workmen.

"That's the reason the government exempted me from enlisting," Tom said. "They wanted me to finish this tank. I didn't exactly want to, but I considered it my 'bit.' After this I'm going into the army, Ned."

"Glad to hear it, old man. Maybe by that time I'll have this Liberty Bond work finished, and I'll go with you. We'll have great times together! Have you heard anything more of Simpson, Blakeson and Scoundrels?" And Ned laughed as he named this "firm."

"No," answered Tom. "I guess we scared off that slick German spy."

Once more the tank lumbered out along the road. It was a mighty engine of war, and inside her rode Tom and Ned. Mary and her father had been invited, but the girl could not quite get her courage to the point of accepting, nor did Mr. Nestor care to go. Mr. Damon, however, as might be guessed, was there.

"Bless my monkey wrench, Tom!" cried the eccentric man, as he noted their advance over some rough ground, "are you really going to make this machine cross Tinkle Creek on a bridge of steel you carry with you?"

"I'm going to try, Mr. Damon."

A little later, after a successful test up and down a small gully, Tank A arrived at the edge of Tinkle Creek, a small stream about twenty feet wide, not far from Tom's home. At the point selected for the test the banks were high and steep.

"If she bridges that gap she'll do anything," murmured Ned, as the tank came to a stop on the edge.