A glass harp is a musical instrument that consists of a number of wine glasses, often partly filled with water. Musicians play the glass harp by wetting their fingers and running them around the rims of the glasses. Each glass produces a different note, either because it is filled with water to a different level or because the glass is of a different size and shape than the others. The glass harp produces a clear tone with an eerie, haunting quality. In the 18th century, it was believed that the purity of the sound produced by the glass harp and related instruments could drive musicians insane.
The acoustic principles underlying the glass harp are relatively simple. Running a wet finger along the rim of the glass causes it to vibrate, in the same way that rubbing a violin bow over a string causes it to vibrate. The vibration of the glass produces the note, which varies based on the thickness and shape of the glass, as well as whether it contains water. Sound travels through water much more slowly than it travels through glass, meaning that an empty glass resonates at a much higher frequency than a glass with water in it. The more water the glass contains, the lower the frequency — and thus the lower the note — it will produce when rubbed.
Instruments using the acoustic properties of glass existed in the middle ages in Persia, China and elsewhere, but the modern glass harp originated in the 18th century when Irish musician Richard Puckeridge developed the technique of playing using standard wine glasses partly filled with water. The glass harp enjoyed a period of popularity during the 18th century, but fell out of favor. Today, only a small number of musicians continue to play the instrument.
Inventor and statesman Benjamin Franklin developed a similar instrument in 1762, working from the principles of the glass harp. Franklin's instrument, called the glass harmonica or armonica, consisted of 37 glass bowls on a central metal spindle, which was turned by operating a foot pedal. The player would bring wet fingers into contact with the rotating rims of the bowls, producing notes. Instead of being filled with water, each bowl was ground to a different size and thickness in order to produce a specific note. The glass harmonica was a very popular instrument in its time; Mozart and other prominent composers wrote music for it.