To err is human!!!
My brain isn't what it used to be...
Somehow these syphons kept nagging at the back of my mind - so I dived into my library and came out with the book "Big Boy" by William W. Kratville. So here are some more precise specifications of her (his?
) firebox:
The fire grate area is 235" x 96", so you could easily invite a group to have dinner inside!
And of course there were syphons, too. In the Big Boy they were called 'circulators', and their design is quite different from these in Jim's pic. The two lateral branches were used as a foundation for the brick arch which directed the flames first towards the back, then above the bricks towards the front and the boiler tubes. (These bricks are not shown in the picture below!)
Imagine the flames coming at you from the grate and then rising up and curving away above these syphon tubes. (Better not hold your head into that chamber when the engine was underway!
)
Another detail: In the side wall you can see holes - the 'secondary air openings' to deliver more oxygene to that raging fire inside. Wow! What an engine!
Ron