The Neptune was a submarine, believe it or not. If you're thinking it was time for an airship, you're right - we've skipped ahead a few issues. They're lost. In fact they'd make a great book: Frnak Reade's Search for the Lost Stories of NoName.
The story starts in an unusual fashion: usually someone comes to Frank and asks if he might be interested in discovering a continent or a particular fortune, and Frank says he wouldn't be interested in money but he happens to be heading just that way in his new Airship / Land runner / Submarine / Electric Cannon Shell / Whatever. This time we start it Indy-Jones style, at least in the academic sense:
Dirty, miserable Spanish! Anyway, the wonderfully named Professor Cram takes a ship to Africa, but it catches on fire and he has to jump overboard. Everyone dies except for Prof. Cram, who is rescued the next morning by Frank. Off to Guinea.
Anyway, they find some gold, and Frank refuses any money, insisting that Barney and Pomp get $10K each. Which was a hell of a lot of money. The Neptune coasts home to the usual end of the story; fora minute I thought it would actually survive to be used again.
He probably shot his horse when it started to yawn.