"How great was the temptation to use... slaves of the state as easily expendable pioneers may perhaps be judged by the fact that in an age of machinery prisoners of war have been worked to death in railway building, in Europe as well as Asia. Secondly, the city of Rome under the early empire, with perhaps 200,000 slaves in a total population of less than a million, is an outstanding instance... for example, 660 slaves maintained the 280 miles of aqueducts which gave a daily water supply of 38 gallons a head. Finally, there was the exploitation of slave labour in mining, which was used to enrich the kingdom of David and Solomon as well as the fifth-century Athenian democracy and spread as far west as Roman Spain: this won the metals, on which technical progress largely depended, by methods that stretched manpower to exhaustion. We lack the details, but Diodorus Siculus in the reign of Augustus writes that in the Spanish silver-mines the strongest workers were the most unfortunate, because they were the slowest to die."