when you have ready access to slaves why invent labour saving devices?
In fact technology was a major factor in the demise of slavery in the 19th century. Industrial technology in the workplace requires a much higher level of commitment, attention and pride in accomplishment, and less physical strength (and much shorter working hours!), than the Iron Age technology that preceded it. It's difficult to inspire a worker to put that much dedication into his work if he's not being paid for it and if he's not being treated well during or after work. Even before the Civil War, German immigrants in Texas demonstrated that a cotton farm staffed by paid employees was more productive than one staffed by slaves. One of the conditions that prolonged slavery was the miserable weather in the South, making it difficult to recruit free men to work outside in the fields.
In fact, in the entire Western Hemisphere, from Mexico to Brazil, slavery disappeared through attrition, as plantation owners had to let their slaves go and hire employees--often simply rehiring the former slaves as freemen. Brazil's slaveowners were notorious as the cruelest in the world, and even they gave up in the 1880s. And the weather in many of those countries is even hotter than in the American South.
The only countries in which slavery was abolished by violence were the USA and Haiti, and in both places we're still paying the price for that. We have separate communities of "black" and "white" people with their own cultures and even their own dialects. In the rest of the hemisphere people just come in shades of brown.
Although not everyone agrees with me (and with many scholars), I believe that slavery would have vanished the same way in the USA if we had been more patient. Not only do we still have a rift between Afro-Americans and Euro-Americans, but the Northerners and Southerners still don't trust each other.
