The morning after Tom and his friends started on their trip in the air, the Shopton Bank was found to have been looted of seventy-five thousand dollars.Andy Foger at once told the police that Tom Swift had taken the money, and when asked how he knew this, he said he had seen Tom hanging around the bank the night before the vault was burst open, and that the young inventor had some burglar tools in his possession.Warrantswere at once sworn out for Tom and Mr.Damon, who was also accused of being one of the robbers, and a reward of five thousand dollars was offered.

Tom, Mr.Damon and Mr.Sharp sailed on, all unaware of this, and unable to account for being fired upon, until they accidentally read in the paper an account of their supposed misdeeds.They lost no time in starting back home, and on, the way got on the track of the real bank robbers, who were members of the Happy Harry gang.

How the robbers were captured in an exciting raid, how Tom recovered most of the stolen money, and how he gave Andy Foger a deserved thrashing for giving a false clue was told of, and there was an account of a race in which the Red Cloud (as the airship was called) took part, as well as details of how Tom and his friends secured the reward, which Andy Foger hoped to collect.

Those of you who care to know how the Red Cloud was constructed, and how she behaved in the air, even during accidents and when struck by lightning, may learn by reading the third volume, for the airship was one of the most successful ever constructed.

When the craft was finished, and the navigators were ready to start on their first long trip, Mr.Swift was asked to go with them.He declined, but would not tell why, until Tom, pressing him for an answer, learned that his father was planning a submarine boat, which he hoped to enter in some trials for Government prizes.Mr.Swift remained at home to work on this submarine, while his son and Mr.Sharp were sailing above the clouds.

On their return, however, and after the bank mystery had been cleared up, Tom and Mr.Sharp, aided Mr.Swift in completing the submarine, until, when the present story opens, it needed but little additional work to make the craft ready for the water.

Of course it had to be built near the sea, as it would have been impossible to transport it overland from Shopton.So, before the keel was laid, Mr.Swift rented a large cottage at a seaside place on the New Jersey coast and there, after, erecting a large shed, the work on the Advance, as the under-water ship was called, was begun.

It was soon to be launched in a large creek that extended in from theocean and had plenty of water at high tide.Tom and Mr.Sharp made several trips back and forth from Shopton in their airship, to see that all was safe at home and occasionally to get needed tools and supplies from the shops, for not all the apparatus could be moved from Shopton to the coast.

It was when returning from one of these trips that Tom brought with him the paper containing an account of the wreck of the Boldero and the sinking of the treasure she carried.

Until late that night the three fortune-hunters discussed various matters.