Source:
http://en.wikipedia.org/wiki/File:Dallas_Meadows_Museum_Organ_by_Oldovini_1762.jpg
The organ could be envisaged as a very large panflute, while the air movement is not initiated by the lungs of the human musician, but moved mechanically, typically by the use of bellows. It consists of a series of independent pipes, each playing its own unique pitch.
At least three modes of energy could be used to trigger the air movement: air (from bellows), water (water pressure, as in water organs), and steam (such as the calliope, while even train whistles and ship horns may be regarded as variations on this theme).
Electronic organs are pipeless, and not considered here.
Chamber organ constructed by Pascoal Caetano Oldovini (1762)
Source:
http://en.wikipedia.org/wiki/File:Dallas_Meadows_Museum_Organ_by_Oldovini_1762.jpg
Organ in the Duomo in Milan with 16'000 pipes
Source:
http://en.wikipedia.org/wiki/File:Organo_sud_duomo_milano.jpg
Depending on the metrics used for historical reconstruction, the earliest instrument that might be regarded as a type of pipe organ was invented as long ago as the 300s BCE, when an instrument known as the hydraulis was used. This ancient Greek instrument was a water-powered organ that opened pipes with valves.
During the 1800s pressurized steam was used to create airflow, while this particular organ was known as the calliope.
The largest known pipe organ is the Wanamaker Grand Court Organ (Philadelphia, Pennsylvania), originally built in 1904, which has 28,543 independent pipes in 462 ranks.
http://en.wikipedia.org/wiki/File:Pipe-organ-cisnadioara.ogg
http://en.wikipedia.org/wiki/Pipe_organ