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.