Thanks to Thomas M. Long, P.E. for scanning and sharing this one. He writes, "I don't remember where I got this, because I've had it in a file for a number of years. The reason I kept it is because we lived in a duplex about 30 years ago that had one of these gravity furnaces, although it had been converted to burn fuel oil. This was the 'central' heating system and included a square cast-iron grate in the living room floor. Somehow the warm air worked its way up the stairs and managed to keep at least part of the house reasonably warm."