Jules Verne is constantly referred to as one of the two fathers of modern Science Fiction. Herbert George (H.G.) Wells is the other, but whereas Wells writes about the darkness of human existence, Verne composes soaring tales that explore the best and most optimistic traits mankind has to offer.
In the 1864 novel, A Journey to the Centre of the Earth, Verne does double duty by both educating and entertaining the reader. Though his theories about what lies in the center of the earth have been discredited, this is excused because Verne places the extinct animals in their correct geological era the further he sends his travelers into the center of the earth. This goal of education and entertainment is kept alive by Verne having Professor Otto Lindenbrock explain scientific theories to his nephew Axel as they descend into the earth. At one point, Verne switches their roles, and has Axel point out rock strata to the professor as they descend.
One of the most interesting things about Verne's stories is that, ostensibly, they fall under the adventure novel category as opposed to strict Scientific Romance like Wells or Stapeldon (Last and First Men was three-quarters of a century later, but still). This story is also unusual in that many of what Verne puts forward has been discredited since the 1864 publication of the novel; we now know the temperature of space is not minus 40 degrees Farenheit for example.
To Steampunk, stories such as this give a sense of Victorian wonder about the world around them and the sense that there are great adventures to be had out in the world, so long as you follow the precepts of science. Any Steampunk tale that instills a sense of adventure in the reader owes this to Verne and his body of work, where science and technology can help mankind to previously unthought of heights of greatness and knowledge.
An addendum, that doesn't really have anything to do with this, but is interesting nonetheless: the 1871 Griffith and Farran English translation of the novel dropped a few chapters of the original French version, and changed the names of the characters -- the professor's last name went from Lindenbrock to Hardwigg, Axel was renamed Harry (or Henry) Lawson, and changed the name of Axel's sweetheart from Grauben to Gretchen. Despite these flaws, and the rewriting of and addition of material to other chapters, this edition is the most popular one. The 1877 English version printed by Ward, Lock, & Co., Ltd. is more faithful, but still has some slight rewrites.
Tomorrow: H.G. Wells's The Island of Doctor Moreau