having said that, Babbage designed his's is computer (he didn't actually make it, I'm not sure if technology of the time would allow that) with rigid physics for the same reason most of mechanical engineering is based on rigid physics: it's a lot simpler. compare the complexity of fluid dynamics calculations with relative traditional Newtonian stuff. or check out how hard it is to make realistic 3D simulation of water - there's a reason it looks weird in older games.