"On carefully observing the countenances of the two females it is evident that they involve an effort of the will. They evidently attempted to conceal any indications of voluntary effort, but did not succeed. A voluntary effort was manifested, and it was plain that it could not be continued very long without fatigue. Assuming, then, this _positive fact_, the inquiry arises, how can the will be exerted to produce sounds (rappings) without obvious movements of the body? The voluntary muscles themselves are the only organs, save those which belong to the mind itself, over which volition can exercise any direct control. But contractions of the muscles do not, in the muscles themselves, occasion obvious sounds. The muscles, therefore, to develop audible vibrations, must act upon parts with which they are connected. Now, it was sufficiently clear that the rappings were not _vocal_ sounds; these could not be produced without movements of the respiratory muscles, which would at once lead to detection. Hence, excluding vocal sounds, the only _possible_ source of the noises in question, produced as we have seen they must be, by voluntary muscular contractions, is in one or more of the movable articulations of the skeleton, from the anatomical construction of the voluntary muscles. This explanation remains as the _only alternative_. "By an analysis prosecuted in this manner we arrive at the conviction that the _rappings_, assuming that they are not spiritual, are produced by the action of the will, through voluntary action on the joints. "Various facts may be cited to show that the motion of the joints, under certain circumstances, is adequate to produce the phenomena of the rappings; but we need not now refer to these. By a curious coincidence, after arriving at the above conclusion respecting the source of the sounds, an instance has fallen under our observation, which demonstrates the fact that noises precisely identical with the _spiritual rappings_ may be produced in the _knee-joints_.