319. One of the most ordinary phenomena is that of raps in the very substance of the wood, with or without movement of the table or other object used ; this effect is one cf the easiest to imitate, either by contact of the feet, or by calling out little crackings in the furniture ; but there is a special little stratagem that may be exposed. It suffices to rest the two hands flat on the table, near enough for the thumb nails to rest strongly against each other ; then, by a muscular movement entirely imperceptible, they are rubbed to gether, which gives a little dry sound, very much like that of interior typtology. This noise resounds in the wood, and produces a complete illusion. Nothing more easy than to make as many raps heard as are asked, a drum beating, &c, to answer to certain ques tions by yes or no, by numbers, or even by indicating letters of the alphabet.
Once warned, the means of detecting this fraud are very simple. It is not possible if the hands are sepa rated, and if you are sure no other contact produces the noise. The true raps, however, have this char acteristic, that they change place and tone at will, which could not be if they were due to the cause we have mentioned, or any similar, if they go from the table to a piece of furniture no one is touching, on the walls, on the ceiling, &c, if they answer impromptu questions. (See No. 41.)
