The odometer is a pendulum, formed of a ring, or other small body attached to a thread, the other end of which is wound round a finger or the thumb. The odometer employed in the following experiments was a light gold ring, having a greater mass of metal on the unattached side, and suspended to the last joint of the right forefinger, the suspending medium being either silk, or fine cotton, or the hair of a horse. The experiments were made by myself. In order to avoid the confusion resulting from a multiplicity of details, I shall state the results obtained through testing a limited number only of Od-subjects, so selected as to represent the leading divisions of the provinces of nature, and of dynamics. With some of the principal the reader will be already acquainted through Letter XII., the contents of which will have prepared him for, and probably suggested to him, the following question, as a desirable subject of preliminary consideration. I. Are the motions referred to worth examining at all? Are they more than the simple results of impulses conveyed to the pendulum by movements of the hand or wrist, or some general sway of the experimenter’s person, unintentionally going with the expectation or conception of this or that motion of the ring? Such a solution of the phenomena is not wanting in probability. It is metaphysically and physically certain, that when we maintain one and the same bodily posture or gesture, as in standing, sitting, or holding out the hand, whatever be the seeming continuousness and unity of the effort, the posture or gesture is really maintained only by a series of rapidly succeeding efforts. What is more likely than that, in such a continual renewing of voluntary actions, our fancy, or the sympathy between our will and our thoughts, should give a bias to the results, even when we most try to neutralize its influence? The fact which I propose first to mention is in complete agreement with this view. I can at will cause the odometer to move exactly as I please. Although I hold my hand as steadily as possible by leaning the arm against a table, and endeavour to keep my person absolutely still, yet I have only to form a vivid conception of a new path for the odometer, and a motion in the so-imagined direction is almost immediately substituted for that which was before going on. In like manner, I have only to conceive the cessation of motion, and the odometer gradually stops. I must farther admit that my first trials of the odometer were made under the full expectation that the results which ensued would follow. And I cannot say that it is impossible, that when other and new motions emerged, they were not often either realizations of a previous guess, or repetitions on the same principle of what occurred at first as an accident.