I know that when people think of aqueducts, they usually imagine tall bridges with giant arches. The suburban town of Arcueil I mentioned earlier was literally named after such a bridge: its name comes from "arcus", meaning arch in Latin, plus the Celtic suffix "ialo", meaning place: "arc-ialo", Arcueil, the place with the arches.
But Arcueil is much older than Eugène Belgrand, so what gives? Well, the languages should give you a hint, and a bit later I'll show you some underground ruins that will explain everything. Belgrand was an engineer and just used gravity to bring water into Paris: underground pipes are lower in altitude than tall bridges, so they allow the water to flow naturally into the city. They also take up less of that prime real estate.
Keep walking, I'll let you know when and where to turn.