Hello!  Ron Ginger wrote, "Why does it take two stages?  Why not just
let the air out of the primary relay blow the pipe directly?"

That all depends on how large the scale of the pipes! In fact, there
are several proven solutions to the 'perforated music problem':

1) Using a pressure box (as in John Smith's Busker Organ) - then the
tracker bar holes most probably will have to be graded in size
according to the scale of the pipes.

2) Using a single-relay system with round bellows (or pouches) -
tracker-bar holes all the same size, channels in wind chest accordingly
graded. Most small hand-cranked organs use this system.

3) Using a double-relay system -- providing the necessary force to move
the valves (and registers!) of a large street-organ.

Best regards
Christofer Noering
Stockholm