Instrument to blow air into an accordion, organ, but usually a forge. Between the two boards is a leather bag. At the end is a mouth of leather, clay, or metal. When you widen the other side a valve goes open so that air can enter. When pressed close the valve closes and the air is blown through the mouthpiece.
A better solution to create a constant flow of air (and thus got a hotter fire) is the double (double acting) bellows. Purpose of the double effect is that every movement returns yield. An additional advantage is that no coals from the fire can be sucked in the bellows by the constant blowing force. There are several types made.
The first can you imagine as if two bellows are put against each other and are operated by pumping the mid board horizontal back and forth. Whether you take a horizontal bellows and you tighten the two outer sides in open condition. In the middle you bring a new “side plank", which you keep moving back and forth. Both parts have an exit to the fire. If a part is engorged, part 2 is blowing.
A second option is the same principle, but mounted vertically. You have to pump up and down. The air outlet is only in the upper air bag, which is weighted with stones. The bottom is used to pump air into the upper. Which remains constant air blowing through the weight of the stones.
And with a little ingenuity you can have two effects: the double pump and reservoir with pressure to merge.
Besides bellows you can also use a pump. For this, there are also double-acting air pumps. Some types can be compared to an old fire pump, a kind of seesaw (operated by two men) with under each arm a piston to create constant pressure.
To forge a rotating fan can also be used. Without additional air fire cannot be fired hot enough to forge (or melt!) iron.