(Or "Why A Duct?", with a tip o' the hat to the Marx Bros)
This house has ancient, single-pipe steam heating. From what I've been able to determine from digging in these walls over the past seven years is that it's always had steam heating. Nothing interesting there.
What's baffling is why the house also has ancient metal air ducting buried inside the walls. I discovered this shortly after I moved here when I ripped down the basement ceiling and found three vertical ducts to nowhere. Over the past hundred years, various plumbers and electricians had used them for service pulls. So did I when I ran 3/4" copper to the second floor bath, the central vac piping and various electrical branches from the basement panel.
I moved the renovation activity into the upstairs hall two weeks ago. After ripping off an old baseboard for replacement, you can see one of those ducts here.
Here's a closer look.
The ducts are a fairly heavy gauge steel wrapped in another layer of corrugated steel, which functions as plaster lathing. It's real nasty to work with. It takes quite a bit of effort to knock a hole in this stuff. Because the ducts aren't anchored to anything, you can't use a saw on them. They just flap around, loosening the surrounding plaster. And after you succeed with tin snips you're left with metal edges as lethal as a machete blade.
There used to be an old baseboard outlet here. I hate baseboard outlets. They're inconvenient and a trip hazard when anything is plugged into them. My intent was to move that outlet up the wall. But once I removed the baseboard and saw the ducting (which I'd forgotten about) I decided I liked my unlacerated flesh more than I hated baseboard outlets.