Source: http://en.wikipedia.org/wiki/Harmonium
The Harmonium may be regarded as a pipe instrument with reeds that act as modifying resonators. Air pressure is generated by pumping air into a reservoir (similar to the bag of bagpipes) using a bellows system. The reeds are set in a reed pan over which air is released. There are "pallets" that open holes in the reed pan. The reeds resonate as air flows over them, to escape through the specific stops that are opened.
Source: http://en.wikipedia.org/wiki/Harmonium
Source:
http://en.wikipedia.org/wiki/File:MIM_Harmonium_CN4913.jpg
Modern Indian harmonium driven with hand-bellows.
Source: http://en.wikipedia.org/wiki/Harmonium
The original concept of the Harmonium might have been invented around 1780 by Christian Gottlieb Kratzenstein (1723–1795) of Copenhagen, but the instrument was first patented in 1840 and 1843 in Paris, by Alexandre Debain.
The instrument was easy to transport and kept being in tune, unlike the piano and similar instruments that went out of tune quickly when transported. During the colonial expansion of the 1800s, it was thus an instrument of choice to be transported to other parts of the world, and was incorporated into local music, such as Indian music.
Vidyadhar Oke adapted the western harmonium to be able to play the 22 microtones (shrutis) in the octave of classical Indian music.
http://www.youtube.com/watch?v=yJg1NNyke2E