The console has 3 manuals (the organ term for keyboards): swell, great and choir, plus full foot pedals. It has 37 speaking and 2 sound altering stops (these are the knobs on either side of the organ console). By drawing the stops the player engages sets of pipes, which fall into various categories of sound quality – diapasons, strings, flutes, reeds – and at various pitches and volumes. There is also an enclosed swell organ with a mechanism for opening the box, thus controlling the volume.
In 1877 the new organ for Union Chapel was completed by the firm of Henry Willis & Son. It utilized the latest in organ blowing technology in the form of a complete hydraulically powered feeder plant, to replace the man powered systems of the past. The demand for hydraulic blowing was long established and driven by ever expanding organ specifications and the need for higher wind pressures. The general development of water mains throughout London meant a ready source of power was available and so more and more hydraulic plants were coming into use with organs.
The plant comprises four hydraulic engines of reciprocating, double acting type manufactured by Watkins and Watson Ltd. of White Lion Street, Islington. Each engine is based on an original design introduced and patented by David Joy, the inventor of Joy’s valve gear applied to steam locomotives in the mid 1800’s. In the original installation of 1877 water was fed to a header tank in the roof of the chapel holding some 1500 gallons of water. This water was conveyed to the organ chamber via a four inch iron pipe, which then divided into two smaller pipes one feeding the two vertical high pressure engines and one feeding the two horizontal low pressure engines. Each of these smaller feeds was fitted with a hydraulically operated stop valve, controlled from the console, so that the engines could be switched on or off at will by the organist. With a full compliment of organ stops the water consumed by the engines could be as much as one hundred gallons a minute and hence the need to stop the supply and allow the tank to refill was necessary if a full service or recital was to be completed. From the engines the exhaust water simply fed direct into the chapels waste water system i.e. down the drain.
There are two main reservoirs within the organ each supplying a separate part of the winding system at two pressures. The Swell and Great reeds are on the highest pressure, the flue work on a lesser pressure and the feeders and engines are dimensioned to take into account the demands on each supply. The low pressure supply comes from two horizontal hydraulic engines each working double feeders and together producing nearly nine hundred cubic feet of wind per minute on full demand. The high pressure supply is from two vertical engines working four much smaller feeders and supplying two hundred and forty cubic feet per minute.
Each engine incorporates a throttle valve, mechanically linked to the reservoir it feeds, so that as the reservoir rises to full height, filling with wind under pressure, the engine gradually slows from about sixty strokes a minute to around two. The entry of water into either end of the cylinder is controlled by a mechanical linkage and rotary valve, moved by a slide rod attached to the main piston rod, that switches from one position to another at either end of the main piston stroke. This movement is adjustable so that the engine can be set to get the maximum possible movement from the feeders above and hence the greatest possible amount of air from the feeders. The rotary valve does not, however, directly control the supply of water but works yet another hydraulic piston valve, that slides in its own cylinder, opening and closing ports to alternately allow water to enter and exhaust from either end of the main cylinder. This is one area where hydraulic engines differ from their steam driven counterparts. In steam engines work is derived from expansion but with hydraulics the water cannot expand so all the work is derived from the displacement of the piston by water under constant pressure. This means water must be admitted to each side of the piston for the full stroke of the piston and hence the valve gear must switch quickly from admission to exhaust at the end of each stroke. In reality this switching is not as fast as ideally needed and with an incompressible water column forcing its way towards the engine there is then the problem of controlling the forces involved at the moment when everything comes to a halt. Stopping a large mass of water flowing at speed down a pipe leads to dangerous pressures in the pipe called ‘water hammer’. This is an apt description because the force can be like a hammer hitting an anvil. In the design of the engines this problem had to be overcome and so each engine was fitted with its own expansion vessel, a cast bronze bottle connected to the incoming water supply and filled with air slightly compressed by the water pressure. As the valves go through their transition this cushion of air compresses and so relieves any dramatic rise in water pressure.
In the 1930’s electricity was marching its way across the country, the demands upon the water supply system had grown many times and churches were being taxed on the use of these wasteful hydraulic systems. Union Chapel therefore moved over to a new electrically driven centrifugal blower to power the organ and the engines were mothballed. Subsequently a large section of the old water supply pipe from the roof tank was removed down to the organ chamber wall and the engines and remaining pipes left, still part filled with water, to decay.
When the organ was restored in 2012 the engines formed a core part of the restoration but a system was needed to revive the engines without resort to wasting water. The Duplex Pipe Organ and Blower Company Ltd. was invited to both restore the engines and devise a suitable recirculating water system to drive them.
At the start the hope was to reuse the original roof tank but the removal of so much of the old four inch pipes from the fabric of the building meant this was impractical and so the whole system had to be contained within the existing organ chamber.
The present system makes use of an electrically driven centrifugal water pump, regulated via various pressure bypass valves and safety valves, to draw and pressurize water from a large, one hundred and ten gallon, stainless steel holding tank and feed it, via the original control valves, to the engines. The exhaust water is then simply fed back to the holding tank. A centralized control system ensures safeguards against high or low water levels in the tank and also that the system cannot be used when the electric blower is in use. Since the plumbing to the engines now involved feed and return pipes, and also since the original feed pipes were corroded by over a century of hard water, an all new system of feed and return pipe work was constructed using copper pipes throughout. Small parts of the original pipe work, such as the lead feeds to the intake ports were retained, and all the original lead exhaust pipes to the drains were left in place as part of the original installation. Whilst the new system involves switching on the pump, the original organist’s control from the console was also restored and reinstated.
The organ at Union Chapel was designed and built specially for the size and acoustics of the new chapel building in 1877 by master organ builder Henry “Father” Willis. It is undoubtedly one of the finest in the world. It is one of just two organs left in the United Kingdom, and the only one in England, with a fully working original hydraulic (water powered) blowing system, which can be used as an alternative to the electric blowers.
Henry Willis was one of the most celebrated organ builders of his day. He is often referred to as "Father Willis" in recognition of his wide reputation and the fact that he has several family members who followed in his business. Our “Father Willis” is an historic organ and a fine example of its time, which enables organists to play music of the period in an authentic way using the stops that the composers would have had in mind.
The organ is deliberately hidden away behind ornate screens under the rose window. It is neither James Cubitt, the architect of the Chapel, nor Rev Henry Allom, the minister at the time, wanted the congregation to be distracted by the sight of an organ or organist. They wanted the music itself to be the focus during worship, which itself actually hints at the organ’s importance, with its depiction of eight angels all playing different musical instruments.