Paul Foulkes (p.foulkes@leeds.ac.uk) posted the following on the Linguist List (Vol-8-1056, 14-JUL-1997, ISSN: 1068-4875): "The only description of this machine that we know of exists in notes in Darwin's TEMPLE OF NATURE, pp 119-20."

"...I contrived a wooden mouth with lips of soft leather, and with a vale back part of it for nostrils, both which could be quickly opened or closed by the pressure of the fingers, the vocality was given by a silk ribbon about an inch long and a quarter of an inch wide stretched between two bits of smooth wood a little hollowed; so that when a gentle current of air from bellows was blown on the edge of the ribbon, it gave an agreeable tone, as it vibrated between the wooden sides, much like a human voice. This head pronounced the p, b, m, and the vowel a, with so great nicety as to deceive all who heard it unseen, when it pronounced the words mama, papa, map, pam; and had a most plaintive tone, when the lips were gradually closed. My other occupations prevented me from proceeding in the further construction of this machine; which might have required but 13 movements, as shown in the above analysis, unless some variety of musical note was to be added to the vocality produced in the larynx; all of which movements might communicate with the keys of a harpsichord or forte piano, and perform the song as well as the accompaniment; or which if built in a gigantic form, might speak so loud as to command an army or instruct a crowd."
Erasmus Darwin (TEMPLE OF NATURE, pp. 119-120)

At the time of his posting (14-JUL-97) Foulkes wanted to know the following:

"Can anyone shed any light at all on this device? Has anyone ever seen other references to it, and even better a picture of it? Did it really work? Has it been used as a basis for other synthesizers?