Jules Verne 
Secret Of The Island 

The third novel in the ‘Mysterious Island’ trilogy follows the adventures of some British ballooners who have crashed on a remote Pacific island and struggle to survive among pirates and escaped convicts. As a group they are very resourceful, but they do occasionally get a helping hand from a mysterious, unknown benefactor. This book closes the legend of Captain Nemo, the hero from Twenty Thousand Leagus Under the Sea. (Goodreads)