IV. But by what cause, then—through what mechanism, so to speak, are the movements of the odometer immediately produced? Early in the inquiry I made this experiment. Instead of winding the free end of the silk round my finger, I wound it round a cedar-pencil, and laid the latter upon the backs of two books, which were made to stand on their edges, four inches apart, with the Od-subject on the table between them, the ring being suspended half an inch above it. The ring, of course, remained stationary. Then I took hold of the pencil with my finger and thumb, at the point where the silk was wound round it; my finger and thumb rested on the silk; but no motion of the odometer ensued. Hence it follows, that the odometer is, after all, always set in motion _by the play of my own muscles_. I venture then to suppose that my sentient nerves, unknown to me, detect on these occasions certain relations of matter—let me call them currents of force—which determine in me _reflexly_ certain sympathetic motions of the very lightest, and even of an unconscious character. This idea, which I am sure affords the just solution of the matter, is highly consistent with some observations which I have before recounted. It explains how the primary delicate impression should yield to the coarser influence of a strong conception in the mind, that this or that other motion of the ring is about to follow, or even to that of a vivid and, so to say, abstract conception of another motion. It explains what I have several times verified, that on certain days a person standing behind me with his hand on my ear, or on my shoulder, can, by an effort of his will (mine not resisting,) make the odometer which I am holding move whichever way he happens strongly to image to himself, without communicating the same to me. It explains to me on what the difference consists between those who can set the divining-ring in motion, without a conscious effort, and those who cannot. The former, it will be found, are persons of so great nervous mobility, that any such motions, if their occurrence be forcibly anticipated by them, will certainly be realized by their sympathetic frames. Among this class should be sought, and would still remain to be detected by experiment, those whose _impressionability by Od should prove commensurate with their nervous mobility_. Finally, I cannot doubt that the view which I have thus arrived at respecting the mechanism of the motions of the odometer, is equally applicable to the explanation of those of the divining-rod. I see that, through its means, many before anomalous facts, with the narrative of which I have not bored the reader, which emerged in my former trials of the divining rod, made by the hands of others, lose their obscurity and contradictoriness, and leave the whole subject in the condition of an intelligible and luminous conception.