The hallmark and byword of Steampunk is highly advanced steam technology. Tanks, cannons, airships, auto-carriages and the like populate the worlds of novels such as The Affinity Bridge by George Mann, The Difference Engine by William Gibson and Bruce Sterling, and Soulless and Changeless by Gail Carriger.
The inclusion of these innovations in literature had to start somewhere, and that was with the Scientific Romances. A "Scientific Romance" is distinct from its direct heir -- Science Fiction -- in that many writers of the period took an evolutionary perspective, theorized mankind as inherently flawed, and had little interest in space travel. They also sometimes took a very, very bleak view of the future.
There are two sides to the Scientific Romance coin -- the Light and Optimistic side of the genre was exemplified by French writer Jules Verne in such seminal works as Twenty Thousand Leagues Under the Sea, Around the World in Eighty Days, From the Earth to the Moon, and A Journey to the Centre of the Earth, whereas the Dark and Pessimistic worldview was held by Herbert George Wells in his novels The Sleeper Wakes, The Time Machine, The Island of Doctor Moreau, and War of the Worlds.
Verne and Wells both detailed exquisite inventions for their period. The Nautilus submarine of Twenty Thousand Leagues is powered by electricity from sodium/mercury batteries, and described as a "masterpiece containing masterpieces." It carries Captain Nemo and his crew through the depths of the ocean in a self-contained environment. Verne published this novel in 1869. To craft this astounding innovation even in fiction showed a grasp of mechanics any Steampunk author would love to have.
In Doctor Moreau, Wells deals with the concept of vivisection and some genetic science. This forerunner of the genetic science so popular in modern Steampunk details what can happen when science divorces itself from morality. The 1996 movie of the same name, starring Val Kilmer and Marlon Brando, shows the blend of animal and man that the eponymous Doctor achieves through the course of the novel. This is the sort of dark science that dystopian Steampunk thrives on. One could even argue that China Mieville's Remade creatures in Perdido Street Station, draws a direct parallel to Wells's Beast-men.
Scientific Romance would continue as a genre up through the 1950s, when the term Science Fiction became more prominent in the lexicon due to the influence of American authors after World War II. One of the late, great stories of Scientific Romance was Olaf Stapledon's Last and First Men (1930), which describes humanity over the course of 5 billion years and 18 distinct species. It's depressing, but at the same time fascinating in the breadth of the storytelling.
To Steampunk, the Scientific Romance gives its pessimistic, dystopian viewpoint of mankind, and offers up the highly advanced Victorian-era steam technology that is so integral to the subgenre. Looking at the writing inherent in many of the Scientific Romances of the late Victorian period or early Edwardian period is also a good way to get the aesthetic and language down. If you're writing Victorian Steampunk of course (The Affinity Bridge by George Mann seeks to faithfully replicate the style of these novels).
Thursday: Jules Verne's A Journey to the Centre of the Earth