Hagley sits on the original DuPont site along the Brandywine — the gunpowder works that E.I. du Pont established in 1802 and that launched what became the largest chemical company in the world. The 235-acre campus preserves the stone powder mills along the river, the workers' village above, the family residence (Eleutherian Mills), and the company's first office building.
The site is part working museum, part nature preserve, part outdoor classroom. A working scale-model of a powder mill demonstrates the milling process daily; the family house is furnished as it was in the early twentieth century. The walks along the river through the ruins of the mills are atmospheric and largely uncrowded.
A jitney bus runs between the main attractions. Allow three to four hours minimum. Don't skip the powder-mill demonstration — the sense of how dangerous and lucrative the operation was makes the rest of the visit make sense.