much later steam engine, when Hero built the aeolipile. With the crank and connecting rod system, all elements for constructing a steam engine (invented in 1712)—Hero's aeolipile (generating steam power), the cylinder and piston (in metal force pumps), non-return valves (in water pumps), gearing (in water mills and clocks)—were known in Roman times. Health and disease Epidemics were common in the ancient world, and occasional pandemics in the Roman Empire killed millions of people. The Roman population was unhealthy. About 20 percent of the population—a large percentage by ancient standards—lived in one of hundreds of cities, Rome, with a population estimated at one million, being the largest. The cities were a "demographic sink," even in the best of times. The death rate exceeded the birth rate and a constant in-migration of new residents was necessary to maintain the urban population. Average length of life is estimated at the mid-twenties, and perhaps more than half of children died before reaching adulthood. Dense urban populations and poor sanitation contributed to the dangers of disease. The connectivity by land and sea between the vast territories of the Roman Empire made the transfer of infectious diseases from one region to another easier and more rapid than it was in smaller, more geographically confined societies. The rich were not immune to the unhealthy conditions. Only two of emperor Marcus Aurelius's fourteen children are known to have reached adulthood. A good indicator of nutrition and the disease burden is the average height of the population. The conclusion of the study of thousands of skeletons is that the average Roman was shorter in stature than the population of pre-Roman societies in Italy and the post-Roman societies in Europe during the Middle Ages. The conclusion of historian Kyle Harper is that "not for the last time in history, a precocious leap forward in social development brought biological reverses." Daily life City and country In the ancient world, a city was viewed as a place that fostered civilization by being "properly designed, ordered, and adorned." Augustus undertook a vast building programme in Rome, supported public displays of art that expressed the new imperial ideology, and reorganized the city into neighbourhoods (vici) administered at the local level with police and firefighting services. A focus of Augustan monumental architecture was the Campus Martius, an open area outside the city centre that in early times had been devoted to equestrian sports and