The chests
The "chests", or "wind chests", to give them their full name, are the large boxes on which the pipes stand, and which contain air under pressure. They are connected by cables to the relays, and when the relays select a pipe to be sounded, the electric current activates an electro-magnet, which releases air from small pneumatic motors. These collapse, and in doing so pull down a "pallet", or sealing flap, beneath the pipe, allowing air or "wind" to enter the pipe and cause it to sound. The hammers on the percussions are also operated, via small chests, using pneumatic pressure.