Before the introduction of the steam engine, the diesel engine, and the electric motor, if people wanted to build a factory or use a large tool of any sort that went beyond a hand tool, they needed something to provide the power. Engineers could and did put humans in big hamster-wheel-like affairs (treadwheels) to spin horizontal shafts. They also could have people or horses walk in circles to turn a vertical shaft. But the innovation that reliably provided a source of continuous power was the waterwheel. And the Romans appear to be the first to have exploited it in about 100 BCE. There are multiple Roman sites that show their engineering prowess, but the most impressive was the multi-wheel four mill at Barbegal in France. On a steep hillside, Roman engineers arranged two sets of eight mills with sixteen vertical overshot waterwheels. Because of the hillside arrangement, the water leaving one wheel could feed into the next wheel down. The horizontal shaft of a waterwheel would connect to a cog wheel so that: 1) the direction of the shaft rotation could switch from horizontal to vertical for the millstone and 2) the rotational speed of the millstone could be two or three times faster than the waterwheel. It is estimated that the 16 mills at this site could produce perhaps 10,000 pounds (4,500 kilograms) of four each day. A pound of flour would make a loaf of bread. The 10,000 loaves of bread per day fed the nearby Roman city of Arelate (present-day Arles), which had a population of perhaps 30,000 people. The Romans also used water to power reciprocating sawmills for wood or stone. At the start of the Industrial Revolution in America 1,700 years later, water was still the power source. Both vertical and horizontal waterwheels provided the power for the first factories. Therefore factories needed to be located where falling water was available in sufficient quantity. So, for example, the first factory of the Industrial Revolution was located at Pawtucket Falls in Rhode Island. Until steam engines became popular, every factory needed falling water to provide the power.