Bellows are a device used to produce a blast of air to speed the burning of a fire,
or for musical instruments (accordions and organs). Bellows usually consist
of two variously-shaped boards hinged at the nozzle end, and elsewhere joined
by pleated leather sides. As the boards are opened, air is drawn in through
a valve; as they are shut it is forced out through the nozzle. A blacksmith's
bellows has two such chambers working out-of-phase, thereby ensuring a continuous
flow of air.