The song, of course, being Echos, and the instrument being the plinky thing that starts the song off. I always assumed it was a synthesizer, but I understand that they were not all that sophisticated back then.
And is it Roger that plays it?
It’s a piano through a leslie rotating speaker, which is a speaker that’s normally used on hammond organs to get that “watery” sound.
It’s just a piano, with a great deal of treatment. Rick Wright apparently hit the note and it resonated in the echo chamber (hence Echoes). I haven’t got it to hand to listen to but it may have been put through a Leslie cabinet as well.
I don’t think they got hold of a proper synth’ until Obscured by Clouds.
Rick, not Roger, if it matters.
Want to ask about the wailing seagull noises?
I’d bet anything that that’s a slide guitar.
I saw the Floyd (sans Roger) a few years ago at Tampa Stadium. Before the concert, everyone was just sitting around waiting for something to happen…and then the sound check: that one plink. And we all went crazy. 
Totally cool! Did you hold your lighter up?
Thanks for the link, Lute. That also explains the odd noise at the end.
I remember my first encounter with this song. I was hanging out with a friend, and his sisters, who were friends of my sister, and were listening to Meddle. They were talking about when their frind Fuji was over. Fuji got his nickname from the mountain. He had a wee bit of a weight problem. Actually, he didn’t have a wee anything. The gut was fargin’ huge.
Anyway, they were all a bit weeded up, and listening to Echos, and Fuji said “Turn it off! I’m scared!”
That almost happened to me at a Maritime concert, testing the synth, the characteristic “plink” on tearing up the oxygen came out, but I didn’t go wild as I was only about 10 feet from the band at the time in a club with about 100 people 