The earliest communications were carried on by means of "raps," or, as Sir William Crookes calls them, "percussive sounds."
18 3 9) discovered that the cause of the sounds was intelligent and would make raps as requested, and, communication being established, the rapper professed to be the spirit of a murdered pedlar.
It was agreed that one rap should mean "no" and three "yes," while more complicated messages were - and are - obtained in other ways, such as calling over or pointing to letters of the alphabet, when raps occur at the required letters.
The earliest of these phenomena were the raps already spoken of and other sounds occurring without apparent physical cause, and the similarly mysterious movements of furniture and other objects; and these were shortly followed by the ringing of bells and playing of musical instruments.
For instance, it is not very uncommon to find persons who can make loud sounds by partially dislocating and restoring the toe, knee, or other joints, and some experiments made with the Fox girls in 1851 supported the view that they made raps by this method.