The organ is an instrument provided with one or more keyboards, generally a set of pedals and also a number of metal or wooden pipes which are made to sound, in performance, by wind (air) pressure from bellows or other source of compressed air. Space forbids a full description of all its mechanical devices. Large organs today usually have four divisions and manuals (keyboards), besides the pedals. The manuals belong to the different departments called great, swell , choir and solo. The pedals form the pedal organ. Each department is practically a separate instrument but all are grouped so as to need but one performer. The manuals are arranged in an ascending row before the performer and the pedals are placed at the feet. Many organs have three manuals for choir, great and swell organ, and a set of pedals. The compressed air from the bellows is conveyed through a wind trunk to the wind chest, each department having its own wind chest. Attached to the top of the wind chest is the upper board, arranged to control the entrance of the wind into the pipes. When a key is pressed down a valve (pallet) is opened and the wind thus allowed to enter the groove of that key. This would cause all the pipes of that pitch to sound but for the intervention of another mechanism. There is another set of grooves at right angles to the first, and each of these is a cross-slide, which the player can move to and fro at will by means of the drawstops. When he pulls a stop out into the proper position for playing, he causes the cross-slide to move just enough so that certain holes in it will be brought opposite to the openings of the pipes. Thus the wind from the wind chest, when allowed to enter the key groove as the key is pressed down, cannot get into any of the pipes of that pitch unless their cross slides have been previously moved into position by means of the draw stops. The pipes above each cross slide, as previously noted, are of one quality, and are called a stop. Each department of the organ consists of a number of different stops, producing sounds that vary in quality. The large and powerful pipes of the great organ are generally placed in front. Back of them are the smaller pipes of the choir organ, less powerful and more suited to accompany voices. Above the latter is the swell organ, the pipes of which are enclosed in a wooden box called a swell box, with a front of louver boards – like Venetian blinds – which may be made to open and shut by means of a pedal, and thus give crescendo and diminuendo effects. Pipes are sometimes half-stopped, having a sort of chimney at the top. A reed pipe derives its tone from the vibrations of a reed instead of air fluttering, though the reed itself is set in motion by air from the wind chest. The reed is a small metal tube with its front cut away and a tongue or spring inserted, which will vibrate at the proper rate to produce the tone. If the tongue does not vibrate against the tube, the reed is called a free reed. Organ pipes differ in shape, proportion or material, though the pipes in any one stop are much alike. Among the more important stops are the open and stopped diapason, so called because they run the entire length of the manual. Also important are the instrumental stops such as the flute posaune (trombone), English horn, basset horn, oboe and viola. The mixture stops reinforce a tone with faint, high overtones and others, such a bourdon and dulciana. Also important is the vocal stops such as the vox humana. It will be seen that registration or the proper use and combination of stops in organ playing is a matter of paramount importance. The studs and pedals are so arranged that each one throws in a special combination of stops. The couplers of an organ enable to player to sound one note in more than one department of the organ at the same time or to sound the octave above or below with the note played. The origin of such grand instruments is to be sought in the antiquity almost prehistoric. The wind sounding in the hold of a broken reed first suggested to man the music of pipes Soon he fashioned a set of these pipes and mythology ascribed them to the creature “Pan.” The next step was the use of one blow-hold in a primitive wind chest below the pipes. The Romans invented hydraulic organs in which the air was compressed by water power. During the Middle Ages one organ at least was used “heated water,” possibly being run by steam pressure. The Greeks and Romans used bellows also with boys standing on them to cause the pressure. A relief showing such an organ was placed on an obelisk erected by Theodosius in 393 A.D. Pipes were then made of copper or bronze. Air was admitted by the drawing out of a rod at the base of the pipe. Organs became fairly common in Spain before 450 A.D. About 666 A.D. Pope Vitalianus introduced the organ into the church service. King Pepin introduced the instrument into France, obtaining an organ from the Byzantine Emperor. A copy of these was brought into Germany by Charlemagne and the Germans soon became expert makers. For some centuries only the “full-organ” effect was possible, so it is not surprising to read that a lady in Charlemagne’s court went crazy on hearing an organ. About 822 Charlemagne received an organ of softer tone sent by the Calif Harunal-Rashid. After the 16th century large organs became common. This was especially true in England, where the organ builders have always shown great mechanical skill. During the Protectorate the Puritans opposed organs and scattered the makers. In the US the Puritans of Boston as late as 1713 refused the gift of an organ from Thomas Brattle. The instrument went to King’s chapel and later to Newburyport and Portsmouth, N.H. Organs were made in America as early as 1745, when Edward Bromfeld Jr. copied an English model. From that time on there have been many improvements in tracker and sticker mechanism, between the key and the wind chest, in the invention of couplers, in the use of improved bellows, fans and other devices for air supply, in the applications of pneumatic and electric power – the later offering many possibilities - in the adoption of combination studs and pedals, and in many other points depending on the progress of modern manufacturing.
Organ pipes vary greatly in form and material but are divided into two chief groups: flue pipes and reed pipes. Flue pipes, having no reed mouthpiece, are further divided into stopped and open pipes. When air is admitted through the foot of the pipe, it causes various fluttering, and those which have the proper rate of speed cause vibrations in the air column of the pipe. The vibrations of a column of air consist of alternating compressions and rarefactions, acting much like pushes and pulls given to a loosely coupled freight train. In an open pipe, the push or puff travels to the end and out, creating a slight pull or suction as it emerges. This suction travels back to the lower end of the pip, where the fluttering start another puff. Meanwhile the first puff has been traveling onward, thus making the wavelength of the tone twice the length of the body of the pipe. If the top of the pipe is closed with a plug or tampion, each puff and suction has to travel up and down the tube before emerging into the air, thus making the wavelength four times the length of the pipe body, and giving a tone an octave lower than that of an open pipe of the same size. As a tone of 16 vibrations per second is the lowest one audible to man and sound travels a little over 1100 feet a second, each wavelength of the tone would be 70 feet long, needing an open pipe with a 35-foot body. A longer body than this would give, not an audible tone, but a set of rhythmic puffs like whispers. As the number of vibrations is doubled to obtain the octave of a note, it follows that the wavelength and the length of the pipe must be halved.