g
inventor. He looked up from some drawings on the table in the office of
one of his shops. "Now I'll just show you--"

"Hold on, Tom. You know I have a very poor head for figures, even if I
do help you out once in a while on some of your work. Skip the
technical details, and give me the main facts."

The two young men--Ned Newton being Tom's special chum--were talking
together over Tom's latest scheme.

It was several days after Tom's accident in the airship, when he had
been saved by the prompt action of Mr. Peterson. That fortune-hunter,
once he had the promise of Mr. Swift to invest in his somewhat
visionary plan of locating a lost opal mine near the Panama Canal, had
left the Swift homestead to arrange for fitting out the expedition of
discovery. He had tried to prevail on Tom to accompany him, and,
failing in that, tried to work on Mr. Damon.

"Bless my watch chain!" exclaimed that odd gentleman. "I would like to
go with you first rate. But I'm so busy--so very busy--that I can't
think of it. I have simply neglected all my affairs, chasing around the
country with Tom Swift. But if Tom goes I--ahem! I think perhaps I
could manage it--ahem!"

"I thought you were busy," laughed Tom.

"Oh, well, perhaps I could get a few weeks off. But I'm not going--no,
bless my check book, I must get back to business!"

But as Mr. Damon was a retired gentleman of wealth, his "business" was
more or less of a joke among his friends.

So then, a few days after the departure of Mr. Peterson, Tom and Ned
sat in the former's office, discussing the young inventor's latest
scheme.

"How big is the biggest gun ever made, Tom?" asked his chum. "I mean in
feet, in inches, or in muzzle diameter, however they are measured."

"Well," began Tom, "of course some nation may, in secret, be making a
bigger gun than any I have ever heard of. As far as I know, however,
the largest one ever made for the United States was a sixteen-inch
rifled cannon--that is, it was sixteen inches across at the muzzle, and
I forget just how long. It weighed many tons, however, and it now lies,
or did a few years ago, in a ditch at the Sandy Hook proving grounds.
It was a failure."

"And yet you are figuring on making a cannon with a muzzle thirty
inches across--almost a yard--and fifty feet long and to weigh--"

"No one can tell exactly how much it will weigh," interrupted Tom. "And
I'm not altogether certain about the muzzle measurement, nor of the
leng