dere arter dere diamonds," volunteered the colored man.

"Perhaps," admitted Jack, with a smile. "But certainly some one has been around here who had no business to be, and we must find out who it was. Better take a look around, Wash."

"I'll help him," said Andy, and, with his rifle in readiness for any intruders, the old hunter followed the colored man outside the big shed.

Meanwhile Professor Roumann and Mr. Henderson were carefully examining the exploded motor.

"I should have looked at the breech plug before turning on the power,"

said the German, "but I had no reason to suspect that anything was wrong." He went on to explain that the explosion was something like that which occurs when the breech-block of a big navy gun is not properly in place. The force of the Cardite, instead of being directed against the piston-heads of the motor, shot out backward, and almost into the face of the professor, who was operating the machine.

"But what could be their object?" asked Mark. "Who would want to injure us, or damage the projectile?"

"Some enemy, of course," declared Jack. "But who? The crazy machinist is out of it, and as for that man who sent the note to you, he seemed too big a coward to attempt anything like this."

"Some one evidently sneaked in here and loosened the breech-plug," went on Mark, "and it was evidently done with the idea of delaying us. The enemy could not have desired to utterly disable the projectile, or else he would have tampered with the large motor, instead of the small one."

"Yes, the object seems to have been to delay us," admitted Professor Henderson; "yet, I can't understand why. Whoever did it evidently knows something about machinery."

"I hope they did not discover the secret of my Cardite motor," said Professor Roumann quickly.

"They hardly had time," declared Mark. "We have been in or around the projectile nearly every minute of the day, and whoever it was, must have watched his chance, slipped in, stayed a few seconds, and then slipped out again."

They went carefully over the entire projectile, but could find no further damage done. Nor were there any traces of the person who had so nearly caused a tragedy. Washington and Andy, after a careful search outside the shed, had to admit that they had no clews.

"Well, the only thing to do is to go to work and build a new small motor," announced Professor Roumann, after once more looking over the _debris_ of the one that had exploded.

"Will it take long?" asked Jack.

"About two weeks. Fortunately, I can use some of the parts of this one, or we would be delayed longer."

"Still two weeks is quite a while," suggested Mark. "Perhaps there'll be no diamonds left on the moon when we get there, Jack," and he smiled jokingly.

"Oh, I fancy there will. The article in the paper from Mars says there was a whole field of them."

"This brings up another matter," said Professor Henderson. "What will happen if we bring back bushels and bushels of diamonds?--which, in view of what the paper says, may be possible. We will swamp the market, and the value of diamonds will drop."

"Then we must not throw them upon the market," decided Professor Roumann. "The scarcity of an article determines its value. If we do find plenty of diamonds, it will give me a chance to conduct some experiments I have long postponed because of a lack of the precious stones. We can use them for laboratory purposes, and need not sell them. In fact, with the Cardite we brought back from Mars, we have no lack of money, so we really do not need the diamonds."

It was decided, in view of the shock and upset caused by the explosion, that no further work would be done that day, and so, after carefully locking the shed, and posting Andy on guard with his gun, the boys and the professor went into the house to discuss matters, and plan for work the next day.

"Mark," said Jack in a low voice, as they followed the two scientists, "I think it's up to us to try to find that mysterious man who sent the note. I think he did this mean trick!"

"So do I, and we'll have a hunt for him. Let's go now."

CHAPTER VI

ON THE TRACK

The two boys gazed after Professors Henderson and Roumann. The scientists were deep in a discussion of various technical matters, which discussion, it was evident, made them oblivious to everything else.

"Shall we ask them?" inquired Jack in a whisper.

"No; what's the use?" queried Mark. "Let's go off by ourselves, and perhaps we can discover something. If we could once get on the trail of the man who wrote the note, I think we could put our hands on the person responsible for the blowing up of the motor."

"I agree with you. We won't bother them about our plans," and he waved his hand toward the scientists, who had, by this time, entered the house.

"In the first place," said Mark, as he and his chum turned from the yard, and walked along a quiet country road, "I think our best plan will be to find d.i.c.k Johnson, and ask him just where it was he met the man who gave him a quarter to bring the note to me."

"What for?" asked Jack.

"Why, then, we can tell where to start from. Perhaps d.i.c.k can give us a description of the man, or tell from what direction he came. Then we'll know how to begin on the trail."

"That's a good idea, I guess. We know where he disappeared to, or, rather, in nearly what direction, so that will help some."

"Sure. Well, then, let's find d.i.c.k."

To the inquiries of the two lads from the projectile, d.i.c.k Johnson replied that, as he had a.s.serted once before, that the man was a stranger to him.

"He was tall, and had a big black mustache," d.i.c.k described, "but he kept his hat pulled down over his eyes, so I couldn't see his face very well. Anyhow, it was dark when I met him."

"Where did you meet him?" asked Mark.

"Not far from your house. He was standing on the corner, where you turn down to go to the woollen mill, and, as I pa.s.sed him, he asked me if I wanted to earn a quarter."

"Of course you said you did," suggested Jack.

"Sure," replied d.i.c.k. "Then he gave me the note, and told me where to take it, and I did. That wasn't wrong, was it?"

"No; only there seems to be something queer about the man, and we want to find out what it is," replied Mark.

"What was the man doing when you saw him?" asked Jack.

"Standing, and sort of looking toward your house."

"Looking toward our house?" repeated Jack. "Was he anywhere near the big shed where we build the machines?"

"Well, I couldn't say. Maybe he might have been."

"I guess that's all you can tell us," put in Mark, with a glance at his chum, to warn him not to go too much into details with d.i.c.k, for they did not want it known that some enemy had tried to wreck the projectile.

"Yes, I can't tell you any more," admitted the small lad.

"Well, here's a quarter for what you did tell us," said Jack, "and if you see that man again, and he gives you a note for us, just keep your eye on him, watch where he goes, and tell us. Then you will get a half-dollar."

"Gee! I'll be on the watch," promised d.i.c.k, his eyes shining at the prospect of so much money.