<P> The breathing apparatus used by Nautilus divers is depicted as an untethered version of underwater breathing apparatus designed by Benoit Rouquayrol and Auguste Denayrouze in 1865 . They designed a diving set with a backpack spherical air tank that supplied air through the first known demand regulator . The diver still walked on the seabed and did not swim . This set was called an aérophore (Greek for "air - carrier"). Air pressure tanks made with the technology of the time could only hold 30 atmospheres, and the diver had to be surface supplied; the tank was for bailout . The durations of 6 to 8 hours on a tankful without external supply recorded for the Rouquayrol set in the book are greatly exaggerated . </P> <P> No less significant, though more rarely commented on, is the very bold political vision, which was revolutionary for its time, represented by the character of Captain Nemo . As revealed in the later Verne book The Mysterious Island, Captain Nemo is a descendant of Tipu Sultan, a Muslim ruler of Mysore who resisted the expansionism of the British East India Company . Nemo took to the underwater life after the suppression of the Indian Mutiny of 1857, in which his close family members were killed by the British . This change was made at the request of Verne's publisher, Pierre - Jules Hetzel, who is known to be responsible for many serious changes in Verne's books . In the original text the mysterious captain was a Polish nobleman, avenging his family who were killed by the Russians in retaliation for the captain's taking part in the Polish January Uprising of 1863 . As France was at the time allied with the Russian Empire, the target for Nemo's wrath was changed to France's old enemy, the British Empire, to avoid political trouble . Professor Pierre Aronnax does not suspect Nemo's origins, as these were explained only later, in Verne's next book . What remained in the book from the initial concept is a portrait of Tadeusz Kościuszko, a Polish national hero, leader of the uprising against Russia in 1794, with an inscription in Latin: "Finis Poloniae!" ("The end of Poland!"). </P> <P> Margaret Drabble argues that Twenty Thousand Leagues Under the Sea anticipated the ecology movement and shaped the French avant - garde . </P> <P> Jules Verne's wrote a sequel to this book: L'Île mystérieuse (The Mysterious Island, 1874), which concludes the stories begun by Twenty Thousand Leagues Under the Sea and In Search of the Castaways . While The Mysterious Island seems to give more information about Nemo (or Prince Dakkar), it is muddied by the presence of several irreconcilable chronological contradictions between the two books and even within The Mysterious Island . </P>

What is the theme of 20 000 leagues under the sea