Phenomena of knockings or rappings have usually accompanied poltergeist disturbances, even before the commencement of the modern Spiritualist movement. Thus they were observed in the case of the Drummer of Tedworth, the "Cock Lane Ghost," and other disturbances of the kind, and also in the presence of various somnambules, such as Frau Frederica Hauffe, known as the Seeress of Prevorst.
With the "Rochester Rappings" —the famous outbreak at Hydesville in 1848—to which may be directly traced the beginning of modern Spiritualism —the phenomenon took on a new importance, rapidly increasing to an epidemic, remaining throughout the earlier stages of the movement the chief mode of communication with spirits.
Although it was afterward supplanted to some extent by more elaborate and complicated phenomena, it continued to occupy a place of some importance among the manifestations of the séance-room into the early twentieth century. It is apparent from descriptions furnished by witnesses that raps varied considerably both in quality and intensity, being sometimes characterized as dull thuds, sometimes as clear sounds like an electric spark, and again as deep, vibrating tones.
It has been shown that raps may be produced by the movement of various body parts (ankle-joints, knee-joints, shoulders, and other joints), and one man, Rev. Eli Noyes, claimed to have discovered seventeen different methods.
There are also instances on record where specially constructed "medium" tables were responsible for the manifestations. Besides the Spiritualist explanation and the frankly skeptical one of fraud, there have been other scientific (and pseudo-scientific) theories advanced which ascribe the raps to various forces such as od (or odyle), ectenic force, or animal magnetism.
Sources:
Brown, Slater. The Heyday of Spiritualism. New York: Hawthorne Books, 1970.
Pearsall, Ronald. The Table-Rappers. New York: St. Martin's Press, 1972.
Pond, Mariam Buckner. Time Is Kind. New York: Centennial Press, 1947.
