Might be wrong here, but, the old brick built chimney's had something called feathers in them, I think (???) basically these are ledges built out from the sides at an angle upwards alternating from side to side and overlapping, this facilitates upward movement of hot air/smoke and prevents things falling down ( with the baffling exception of birds ).
Probably correct But how the hell does Santa manage to get down the chimney eh? Tell me that smart arse!