Why get 100 slaves to do the work when you can have 5 machines running, with two slaves for each to set the things up, and two engineers to keep them going. (That's just a hypothesised situation btw) Even if the engineers demanded relatively high wages, it would still be cheaper than feeding 100 slaves, and was certainly cheaper than guarding them. And as people realise that being an engineer is profitable, more people would become engineers, driving the price for engineers down.
I'll have a shot at answering the rest tomorrow.
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