Pipes are sometimes half-stopped, having a kind of chimney at the top. The reed-pipe consists of a reed placed inside a metallic, or occasionally a wooden pipe. This reed is a tube of metal, with the front•part cut away, and a tongue or spring put in its place. The lower end of the spring is free. the upper end attached to the top of the reed; by the admission of air into the pipe, the spring is made to vibrate, and in striking either the edge of the reed or the air, produces a musical note, dependent for its pitch on the length of the spring, its quality being determined to a great extent by the length and form of the pipe or bell within which the reed is placed. When the vibrating spring does not strike the -edge of the reed, but the air, we have what is called the free reed, similar to what is in use in the harmonium (q.v.). To describe the pitch of an organ-pipe, terms are used derived from the standard length of an open mouth-pipe of that pitch. The largest pipe in use is the 32-feet C, which is an octave below the lowest C. of the modern piano-forte, or two ocaves below the lowest C on the manuals and pedal of the organ: any pipe pro ducing this note is called a 32-feet C pipe, whatever its actual length may be. By a .32-feet or 16-feet stop, we mean that the pipe which speaks on the lowest C on which• that stop appears, has a 32-feet or a 16-feet tone.
The stops of an organ do not always produce the note properly belonging to the key -struck; sometimes they give a note an octave, or, iu the pedal-organ, even two octaves lower, and sometimes one of the harmonics higher in pitch. Compound or mixture stops have several pipes to each key, corresponding to the different harmonics of the ground tone. There is an endless variety in the number and kinds of stops in different organs; some are, and some are not continued through the whole range of manual or pedal. Some of the more important stops get the name of open or slapped diapason (a term which implies that they extend throughout the whole compass of the clavier); they are for the most part 16feet, sometimes 32-feet stops; the open diapason chiefly of metal, the close chiefly of wood. The dulciana is au 8-feet manual stop, of small diameter, so called from the sweetness of its tone. Among the reed-stops are the clarion, oboe, bassoon, and vox humana, deriving their names from real or fancied resemblances to these instruments and to the human voice. Of the compound stops the most prevalent in Britain is the sesguialtera, consisting of four or five ranks of open metal pipes, often a 17th, 19th, 22d, 26th, and 29th from the ground-tone. The resources of the organ are further increased by appliances called couplers, by which a second clavier and its stops can be brought into play, or the same clavier can be united to itself in the octave below or above.
Organs are now generally tuned on the equal temperament. See TEMPERAMENT. The notation for the organ is the same as for the pianoforte, in two staves in the treble and bass clefs; hut in old compositions the soprano, tenor, and alto clefs are used.
Instruments of a rude description, comprising more or less of the principle of the organ, seem to have existed early. Vitruvius makes mention of a hydraulic organ, but his description is not very intelligible. The organ is said to have been first introduced! into church music by pope Vitalian I. in 666. In 757 a great organ was sent as a present. to Pepin by Byzantine emperor, Constantine Copronymus, and placed in the church of St. Corneille at Compiegne. Soon after Charlemagne's time organs became common. In the 11th c. a monk named Theophilus wrote a curious treatise on organ-building. But it was not till the 15th c. that the organ began to be anything like the noble instru ment which it now is. The family of the Antignati, in Brescia, had a great name as organ-builders in the 15th and 16th centuries. The organs of England were also in high repute, but the puritanism of the civil war doomed most of them to destruction; and when they had to be replaced after the restoration, it was found that there was no longer a sufficiency of builders in the country. Foreign organ-builders were therefore invited to settle in England, the most remarkable of whom were Bernhard Schmidt (generally called father Smith) and his nephews, and Renatus Harris. Christopher Schreider, Snetzler, and Byfield succeeded them; and at a later period Green and Avery, some of whose organs have never been surpassed in tone. The largest English organs are those of York cathedral, Birmingham town hall, Christ Church, London; and a gigantic and exceedingly perfect instrument, completed in 1870 for the ball Primrose Hill, London. The latter surpasses in size the famous Haarlem organ, long rectIoned the largest in the world, which is 103 ft. high and 50 broad. The German organs are remarkable for preserving the balance of power well among the various masses, but in mechanical con trivances they are surpassed by those of England.
For a full account of the structure of the organ see Hopkins and Rimbault, The Organ, its History and Construction (Lond. 1855). Rink's Praldische Orgeischule, Leipzig, v. y., is the best work on organ playing. See also Dr. Staiuer's The Organ (1877).