When the Fairmount Water Works was built in the early 19th century to provide clean drinking water to Philadelphia, it was a feat of modern engineering. Steam engines and a dam across the Schuylkill River powered water wheels large enough to pump millions of gallons a day uphill to a reservoir atop nearby Fair Mount. Gravity did the rest, funneling the water through a network of mains and pipes to quench the growing city’s thirst and help abate outbreaks of yellow fever.