A glass harp is a musical instrument made up of several wine glasses, some of which are partially filled with water. Wetting their fingers and running them around the rims of the glasses is how musicians play the glass harp. Because each glass is filled to a different level or has a different size and shape than the others, each one produces a different note. The clear tone of the glass harp has an eerie, haunting quality to it. The purity of the sound produced the glass harp and related instruments was thought to drive musicians insane in the 18th century.
The acoustic principles that underpin the glass harp are straightforward. The same way that rubbing a violin bow over a string causes it to vibrate, running a wet finger along the rim of the glass causes it to vibrate. The note is produced the vibration of the glass, which varies depending on the thickness and shape of the glass as well as whether or not it contains water. Because sound travels much more slowly through water than through glass, an empty glass resonates at a much higher frequency than a glass filled with water. The lower the frequency — and thus the lower the note — produced when the glass is rubbed, the more water it contains.
Glass harps were used in the Middle Ages in Persia, China, and other places, but the modern glass harp was invented in the 18th century Irish musician Richard Puckeridge, who developed the technique of playing with standard wine glasses partially filled with water. During the 18th century, the glass harp was popular, but it soon fell out of favor. The instrument is still played a small number of musicians today.
In 1762, inventor and statesman Benjamin Franklin created a similar instrument based on the glass harp’s principles. Franklin’s glass harmonica, also known as an armonica, was made up of 37 glass bowls on a central metal spindle that was turned a foot pedal. The player would make notes rubbing wet fingers against the rotating rims of the bowls. Each bowl was ground to a different size and thickness instead of being filled with water to produce a specific note. The glass harmonica was a popular instrument at the time, and Mozart and other well-known composers composed music for it.