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@Wrzlprmft Indeed, that's in the title of your question "Considered a breakthrough at its time". In my mind a breakthrough does not depend on the epoch it was made but I see what you mean... although I have no answer to suggest!
@Wrzlprmft I think you can not mix "no relevant technological application" and "[not] relevant (point)". A lot of discoveries in maths or astrophysics don't (won't?) have any technological application (=technologically not relevant) but are in textbooks. If you remove this last thing too, the result probably never was a real "breakthrough", by definition.
Interesting. I didn't think this question would bring me back to the 9th century! ...although I will keep the 18th century as the best answer. And thank you for extracting the suitable references.
Interesting, I didn't realize that such a fundamental idea (the Earth is not solid) was yet to be established at that time... And the point on Wegener having difficulties because he was not a specialist of the field makes me wonder if it's really different today!
