That’s the 1895 Macombs Dam Bridge over the Harlem River. It’s honestly sort of boring: a relatively short truss swing span with a second truss span on the Bronx end to carry the roadway over the railroad. It was most famous more than half a century ago for being the shortest path between the Polo Grounds (the stadium used by the NY Giants baseball team and, after the Giants moved west, briefly used by the Mets) and Yankee Stadium. When there was a subway series between the Giants and Yankees – 1923, 1936, 1937, and 1951 – the games were alternating between two stadiums less than half a mile apart, across this bridge.
The interesting thing is what used to be at this location. Robert Macomb built a dam here in the 1810s (to power a grist mill) with a bridge nearby. The dam effectively ended navigation on the Harlem River, which is why it was eventually removed. As I’ve discussed before, the Harlem River is not a river: it’s a tidal straight connecting the Hudson River to the East River (also a tidal strait). Because of the complexity of water flow around Manhattan Island – steady flow coming south on the Hudson and the two straits reversing direction in rhythm with the tides at the south end of the harbor at the Narrows (some 9 miles south of Hell Gate) and at the entrance to Long Island Sound at Block Island Sound (some 100 miles to the east) – the direction of flow in the Harlem can only be predicted by spending a lot of time with a tide table or by flipping a coin. That does not seem like an ideal way to power a mill, but the histories say that it used an undershot waterwheel – i.e., a waterwheel above the river, where the water pushes against the lower portion of the wheel – which leaves a few possibilities. One is that it only ran half the time, when the river was flowing in the right direction. Another is that the grinding stones worked reasonably well regardless of the direction the moving stone turned. The least likely is that there was gearing to convert the alternating directions to a single direction for the moving stone.
Millstones are not particularly directional, so it’s likely that the mill ground regardless of the stone rotation direction. But in that scheme, the waterwheel had to work in both directions, which meant the fins had to be symmetrical. That’s inefficient compared to fins designed for flow in one direction, but presumably more than 50% efficiency or it would make more sense to have the mill work only when the flow was in one direction.
It’s very frustrating that there are no photos of the mill’s inner workings.