It is the first Verne novel in which he perfected the "ingredients" of his later work, skillfully mixing a plot full of adventure and twists that hold the reader's interest with passages of technical, geographic, and historic description. The book gives readers a glimpse of the exploration of Africa, which was still not completely known to Europeans of the time, with explorers traveling all over the continent in search of its secrets.
Public interest in fanciful tales of African exploration was at its height, and the book was an instant hit; it made Verne financially independent and got him a contract with Jules Hetzel's publishing house, which put out several dozen more works of his for over forty years afterward.
A good deal of the initial exploration is to focus on the finding of the source of the Nile, an event that occurs in chapter 18 (out of 43). The second leg is to link up the other explorers.
There are numerous scenes of adventure, composed of either a conflict with a native or a conflict with the environment. Some examples include:
In all these adventures, the protagonists overcome by continued perseverance more than anything else. The novel is filled with coincidental moments where trouble is avoided because wind catches up at just the right time, or the characters look in just the right direction. There are frequent references to a higher power watching out for them, as tidy of an explanation as any.
The balloon itself ultimately fails before the end, but makes it far enough across to get the protagonists to friendly lands, and eventually back to England, therefore succeeding in the expedition. The story abruptly ends after the African trip, with only a brief synopsis of what follows.
The novel has several themes and motifs central to European exploration: scientific achievement, the otherworldliness of the region explored, and the question of how much shared humanity there is between the explorers and the natives. The balloon is a straight allegory of scientific achievement overcoming the wild, as well as overcoming the limitations of the Western world. Most of the Africans are contrasted as being superstitious and quick to worship any object cast down from the balloon, though Verne does not generalize this to all religion. The treatment of animals is in line with the image of the Great White Hunter. This is most obvious by Dick's statement, upon seeing a herd of elephants, "Oh, what magnificent elephants! Is there no way to get a little shooting?" These aspects are both tied into the explorers being above, quite literally in this novel, the region they are traveling across, and Verne makes them worthy of their status through their technological achievements.
As one scene where the explorers confuse baboons for black men illustrates, Africa is approached as an alien place. The explorers do not, and maybe cannot, fully understand the people they are interacting with(or, as the case may be, avoiding). Only later in the novel do they comment on the similarities between themselves and the people they have flown over, when they hold that the Africans' ways of war are not one whit worse than white men's, only filthier. In most scenes, neither the Africans nor the explorers show much compassion for the other.
In Chapter 16, the Doctor equates Africa to the "Last Machine", which will serve as the place of human growth after the Americas are dry. His depiction is of an Africa tamed and cultivated over years to come.
Though the novel goes into great detail with much of the calculations involving the lift power of the hydrogen balloon, and how to obtain the proper amount of volume through changes in temperature; there are gaps in the logic. The balloon rises up when heated, and lowers as it is allowed to cool. This pattern is used as numerous plot points and is shown to be a somewhat quick process of cooling. At night, however, there is little mention of them maintaining the temperature through the night. Another gap in the scientific logic is the lack of reference to atmospheric temperature on the balloon itself, though the temperature is referenced as affecting the heating coil.
In Chapter 26, it says the doctor takes the balloon up to five miles. Later, in Chapter 29, in order to get over Mount Mendif, the doctor "by means of a temperature increased to one hundred and eighty degrees, gave the balloon a fresh ascensional force of nearly sixteen hundred pounds, and it went up to an elevation of more than eight thousand feet" which is noted as being "the greatest height attained during the journey." If this is to imply that the doctor went eight thousand feet above Mount Mendif, at a height greater than five miles; Jules Verne would have greatly underestimated the drop in temperature and how much heat would have been required to keep the balloon at that height for any length of time.
At the time when the book was first written, lands to the north and northwest of Lake Victoria were still poorly known to Europeans. Jules Verne makes a few mistakes here, such as placing the source of the Nile river at 2°40′N (instead of 0°45′N ); claiming that this source is just over 90 miles from of Gondokoro (the actual distance is closer to 300 miles ); not mentioning Lake Albert at all (it was not discovered by Europeans until after the publication of the book ). Much of the geography described further in the book is completely fictional. For example, coordinates given for the "desert oasis" in chapter 27 correspond to a location in a savanna region of southern Chad, less than twenty miles from a big river.
Also, neither novel deals directly with the French, but with (generally positive) stereotypes of other countries.