The bellows will have a kind of sack which has air in it . Double acting piston bellows are a type of bellows , used by blacksmiths and smelters to increase the air flow going into the forge , with the property that air is blown out on both strokes of the handle ( in contrast to more common bellows that blow air when the stroke is in one direction and refill the bellow in the other direction ) . These bellows blew a stronger and more constant blast than typical bellows .