@katemorley
Two things:
My grandfather built a solar heater for his swimming pool that he and his son dug and built themselves. It was a grid of copper pipe along the back fence that recirculated through the pool. 1950s, not 1800s, but still... and prior to the 1965 Johnson report that laid out the risk of CO2.

I know you! (Not in person, just WWW) You taught me CSS tricks for expandable trees with neat graphics years ago.