The harmonium can be a lovely instrument. The harmonium was a staple with traveling evangelists and missionaries in the 19th century and well into the 20th century. Bellow blown organs can be small as the one is in the Julie Fowlis video or as large as need be (thanks for the link). As Peter points out they were/are also found in many country churches. They of course fell out of the limelight as electricity took over the job of powering the pumps for these organs.
The harmonium enjoyed a revival in the late 60's and early seventies in the music of many folk rockers. The Penguin Cafe piece comes from that era. It was a way to go unplugged by avoid the cultural stigma of the accordion. Thankfully we got over that bit.
John Sebastian of the Lovin' Spoonful was a visible player of the Harmonium early on - certainly pre-Woodstock. But then again John was known for playing many somewhat obscure (to rock music) instruments at times.
Neil Young was, and still is, a proponent of the Harmonium and the larger pump organs. He has been a collector of these instruments since the early 1970's. His favorite stage pump organ can be seen in videos and photos from his annual concert for the Bridge School. His famous "unplugged" concert for MTV in 1993 featured him playing his 1885 Estey pump organ. Young's side-to-side rocking motion is due to his pumping the foot pedals of the bellows. What a vibe.
https://www.youtube.com/watch?v=G8lSGTzQrTo I've looked for a small harmonium for decades and have never found one in good condition and I don't have the stomach or deep pockets needed to revive one. Cool instrument. Melodicas and accordions have done the job for me.
Feadoggie