How much water do pour into each glass in a glass harmonica?

There's no single answer that works every time. It depends on the size and shape

of the containers, the highest and lowest note you'll need to sound, and the 'key'

or scale in which you need available notes.

Setting up a glass harmonica is a delicate process that has to be carried out before

each performance. Just the right amount of water has to be poured into each glass

to make it sound the correct pitch when struck or stroked. If the performer's ear

(sense of pitch) is good enough, he can do the whole job by ear, but it's more commonly

done by comparing the sound of each glass to a tuning fork or audio oscillator, and

adding bits of water until the note is just right. The process is a lot like the piano tuner's

painstaking job, only with fewer notes.