Organ. This is the largest and most harmonious of all wind instruments. It may be more properly termed a collection of instruments. The invention of the organ is very ancient, though it is agreed it was little used till the eighth century. It. seems to have been borrowed from the Greeks. Vitru-vius describes one in his tenth book. The Emperor Julian has an epigram in its praise. St. Jerome mentions one with twelve pairs of bellows, which might be heard a thousand paces, or a mile; and another at Jerusalem, which might be heard at the mount of Olives. The structure of the modern organ may be conceived as follows: The organ is an assemblage of several rows of pipes. Its size is usually expressed by the length of its largest pipes; thus we say an organ of 32 feet, of 1G feet. of 8 feet, and of 2 feet. The organ 1 at least one set of keys, when it has only one body; and two or three, when otherwise. The large organs have four, sometimes five sets. Besides, the pedals or largest pipes have their key, the stops or touches whereof are played by the feet; The keys of an organ are usually divided into four octaves. Ctesibius of Alexandria, who lived in the reign of Ptolemy Ever-iretes, is said to have first "invented organs that, played by compressing the air with water, which may still be practised. Archimedes and Vitruvius have left us descriptions of the hydraulic organ. Among celebrated organs may be mentioned those of Haarlem and of Rotterdam, in Holland; the organ at Ulm, in Germany; and in England, those of York Minster, the Birmingham Town Hall, and the 'Temple Church of London. The great Haarlem organ, built by Christian Muller early in the last century, contains nearly 5,000 pipes. That at Ulm is 90 feet in height. York Minster organ has 4,089 pipes, and is famous for the beautiful softness of its tone. Willis's organ at the Great Exhibition of 1851 attracted much attention; it contained more than 4,400 pipes, and was built on the continental plan.