TFA uses 20 000 Leagues Under the Sea as a prototypical example of science fiction. I'm not sure that's a good example, because Jules Verne took his science and technology very seriously. As I recall, he didn't put fictional technology in his stories, but only things he had actually seen demonstrated at industrial expos. Verne used his plots more as support for descriptions of new technologies and scientific discoveries rather than the other way around.
The submarine he was inspired by never went deeper than 10 meters: https://en.m.wikipedia.org/wiki/French_submarine_Plongeur. It's still science fiction even if the speculative technology is based on very solid real-world evidence - that just makes it hard sci-fi.
As I understand the term, hard sci-fi is based on careful use of current scientific knowledge to propose plausible future technologies. Verne tried to use only technologies that had actually been implemented. So, under this definition, Verne's works were no more sci-fi than Tom Clancy.
> Verne tried to use only technologies that had actually been implemented.
No he didn't. The technology to build the submarine he imagined had not been implemented. Additionally, a cannon that could fire humans into space had not been implemented in 1865 when he wrote From the Earth to the Moon, and the interior of volcanos and the Earth's core had not been explored when he wrote Journey to the Center of the Earth.
I really think you're selling him short - he used cutting-edge scientific theories as inspiration for imaginative, speculative stories, he didn't limit himself to existing inventions. I have plenty of respect for Tom Clancy as well but they are very different writers with very different approaches.
The Nautilus said to be powered by Bunsen cell batteries in which the zinc is replaced by a sodium/mercury amalgam. The sodium is consumed in the reaction and replenished with sodium extracted from sea water (using coal power.)
As far as I can tell, this sodium/mercury modification to Bunsen cells is science fiction. Bunsen cells were real but not powerful enough to plausibly drive a ship so Verne spiced them up. Great book though.
That is what makes it hard science fiction. Everything that didn't exist could have existed if someone rich tried to build it. Or at least it seems reasonable, it is possible that they didn't know something then and so it wouldn't work. In 20000 leagues under the sea he may not have correctly accounted for water pressure, and it seems like there was more space on the sub than is reasonable.
