“I thought there weren’t any more,” spoke Ned, soberly enough.

“Come, now, none of that!” exclaimed Tom, with a laugh.  “Why, the surface of aerial navigation has only been scratched.  The science is far from being understood, or even made safe, not to say perfected, as water and land travel have been.  There’s lots of chance yet.”

“And you’re working on something new?” asked Ned, as he looked around the shop where he and Tom were sitting.  As the young bank employee had said, he had come away from the institution that afternoon to have a little holiday with his chum, but Tom, seated in the midst of his inventions, seemed little inclined to jollity.

Through the open windows came the hum of distant machinery, for Tom Swift and his father were the heads of a company founded to manufacture and market their many inventions, and about their home were grouped several buildings.  From a small plant the business had grown to be a great tree, under the direction of Tom and his father.

“Yes, I’m working on something new,” admitted Tom, after a moment of silence.

“And, Ned,” he went on, “there’s no reason why you shouldn’t see it.  I’ve been keeping it a bit secret, until I had it a little further advanced, but I’ve got to a point now where I’m stuck, and perhaps it will do me good to talk to someone about it.”

“Not to talk to me, though, I’m afraid.  What I don’t know about machinery, Tom, would fill a great many books.  I don’t see how I can help you,” and Ned laughed.

“Well, perhaps you can, just the same, though you may not know a lot of technical things about machines.  It sometimes helps me just to tell my troubles to a disinterested person, and hear him ask questions.  I’ve got dad half distracted trying to solve the problem, so I’ve had to let up on him for a while.  Come on out and see what you make of it.”

“Sure, Tom, anything to oblige.  If you want me to sit in front of your photo-telephone, and have my picture taken, I’m agreeable, even if you shoot off a flashlight at my ear.  Or, if you want me to see how long I can stay under water without breathing I’ll try that, too, provided you don’t leave me under too long, lead the way—­I’m agreeable as far as I’m able, old man.”

“Oh, it isn’t anything like that,” Tom answered with a laugh.  “I might as well give you a few hints, so you’ll know what I’m driving at.  Then I’ll take you out and show it to you.”

“What is it—­air, earth or water?” asked Ned Newton, for he knew his chum’s activities led along all three lines.

“This happens to be air.”

“A new balloon?”

“Something like that.  I call it my aerial warship, though.”

“Aerial warship, Tom!  That sounds rather dangerous!”

“It will be dangerous, too, if I can get it to work.  That’s what it’s intended for.”

“But a warship of the air!” cried Ned.  “You can’t mean it.  A warship carries guns, mortars, bombs, and—­”