"I'd like to know what all this machinery is for," spoke Jack.

"And what is behind that door," Mark went on, indicating the much-locked
portal.

"I knew it!" exclaimed the old man. "I knew it! Now if I tell you will
you promise to keep it a secret until I give you leave to speak?"

Of course the boys promised eagerly.

"Do you think you have rested enough now to take a look inside?" the
inventor asked, nodding toward the locked door.

"Sure!" exclaimed Jack.

"Then put on your coats and trousers and I'll introduce you to my pet."

Wonderingly, the boys followed him. It took nearly a minute to unfasten
the various bolts and bars, but at last the portal swung open. The place
was dimly lighted by a single big lamp, but in the glare of it the boys
caught sight of a strange, weird object. It looked like an immense
cigar, and swayed slowly back and forward. It seemed to be covered with
a net-work of cords. On the ground beneath it was what seemed to be a
good-sized boat, with a large cabin amidships.

"What in the world is it?" cried Jack.

"It's my airship!" exclaimed Professor Henderson. "The only successful
airship ever invented. It is the electric _Monarch_!"