“Our Secret Service men have been working on it for some time, not only in order to protect you, because of what you have done for the government, but because Uncle Sam wishes to protect his own property, especially the searchlight and the big cannon.  But, though our agents have worked hard, they have not been able to get any clues that would put them on the right trail.

“So we can only warn you to be careful, and this I do in all earnestness.  That was part of my errand in coming here, though, of course, I am anxious to inspect the new aerial warship you have constructed.  So watch out for two things—­your inventions, and, more than all, your life!”

“Do you really think they would do me bodily harm?” Tom asked, a trifle skeptical.

“I certainly do.  These foreign spies are desperate.  If they cannot secure the use of these inventions to their own country, they are determined not to let this country have the benefit of them.”

“Well, I’ll be careful,” Tom promised.  “I’m no more anxious than anyone else to run my head into danger, and I certainly don’t want any of my shops or inventions destroyed.  The fire in the red shed was as close as I want anything to come.”

“That’s right!” agreed Ned.  “And, if there’s anything I can do, Tom, don’t hesitate to call on me.”

“All right, old man.  I won’t forget.  And now, perhaps, you would like to see the Mars,” he said to the lieutenant.

“I certainly would,” was the ready answer.  “But hadn’t you better see those men who are waiting to find out about positions here?”

“There’s no hurry about them,” Tom said.  “We have applicants every day, and it’s earlier than the hour when I usually see them.  They can wait.  Now I want your opinion on my new craft.  But, you must remember that it is not yet completed, and only recently did I begin to solve the problem of mounting the guns.  So be a little easy with your criticisms.”

Followed by Ned and Lieutenant Marbury, Tom led the way into the big airship shed.  There, Swaying about at its moorings, was the immense aerial warship.  To Ned’s eyes it looked complete enough, but, when Tom pointed out the various parts, and explained to the government officer how it was going to work, Ned understood that considerable yet remained to be done on it.

Tom showed his official guest how a new system of elevation and depressing rudders had ben adopted, how a new type of propeller was to be used and indicated several other improvements.  The lower, or cabin, part of the aircraft could be entered by mounting a short ladder from the ground, and Tom took Ned and Lieutenant Marbury through the engine-room and other compartments of the Mars.

“It certainly is most complete,” the officer observed.  “And when you get the guns mounted I shall be glad to make an official test.  You understand,” he went on, to Tom, “that we are vitally interested in the guns, since we now have many aircraft that can be used purely for scouting purposes.  What we want is something for offense, a veritable naval terror of the seas.”