II. The mode in which I have latterly educed the rotatory movements depending upon galvanism, has been this. I have laid two discs, one of zinc, the other of copper, one on the other, having previously moistened their surfaces with salt and water. Then, as I mentioned, the ring held over the middle of the zinc disc (that being uppermost) exhibits clock-rotation. Held over the middle of the copper disc, when that is laid uppermost, versed-rotation. I mentioned, too, that if held beyond, but near the circumference of the same discs, the direction of the motion of the ring is reversed. The discs which I employ are circular, an inch and a half in diameter, and about as thick as a sovereign. Upon these I do not fail to obtain, when dried and used singly, the first series of phenomena described in the preceding letter. But it occurred to me to try what would be the result of suspending the ring over the two together, and alternately laid uppermost, when they had been well cleaned and dried. This is evidently a still simpler voltaic arrangement than when the salt and water is additionally used. The result was in the highest degree interesting. When I suspend the ring half an inch above the centre of the copper disc, (that being laid uppermost,) the first motion observed is transverse; but after a few oscillations it becomes oblique—dextrad and proximad combined, in the diagonal between the primary influences of the zinc and of the copper. This change does not last long; the transverse force again carries it, in this instance, and clock-rotation is permanently established. When the zinc is uppermost, the corresponding opposite phenomena manifest themselves; and in either case a reversed movement occurs, if the ring is held extra-marginally to the discs. III. I may say that I have now obtained positive evidence that these motions of the odometer do not depend upon my own will, or the sympathy of my will with existing conceptions in my mind; for they succeed nearly equally well when the discs are covered with half a sheet of writing-paper. In nine cases out of ten, when I thus manage to be in perfect ignorance which disc, or what combination of the two, is submitted to the odometer, the right results manifest themselves, and the cause of the occasional failures is generally obvious. Let me add upon this topic, that one day, the weather being cold and wet, and myself suffering severely with rheumatism, the odometer would not move at all in my hand. On another day, late in the evening it was, when I happened to be much fatigued and exhausted, the ring moved, indeed, but every motion was exactly reversed; thus my left hand I found now obtained exactly the results which, on other occasions, I got with the right. * * * * *