_a._ Let the odometer be held a quarter of an inch away from, and over each cardinal point of a sovereign, or zinc circular disc, in succession. _Result_—Held near the distal point, its motion is _proximad_. Held near the proximal point, its motion is _proximad_. Held near the dextral point, its motion is _sinistrad_. Held near the sinistral point, its motion is _sinistrad_. (See fig. 3.) But the first two impulses thus attained correspond with the direction of the primary oscillations of the odometer, the last two with its complementary oscillations; and if the odometer be held now over different points in succession of the two diametral lines, suspended, of course, by the finger alone over the first series of points, and by the finger touched by the thumb over the second, it will be found that the primary oscillations originated over every point of the longitudinal diameter of the zinc disc are proximad; and that those obtainable over any part of the transverse diameter of the zinc disc are sinistrad. Then the forces or currents are made manifest by which the two sets of oscillations are produced; and the marvel of the prompt substitution of one for the other is at an end; for it is evident that these two forces, whether produced or only revealed by the presence of the odometer, co-exist; and that the changed Od-relations of the experimenter to the odometer, (effected by disjoining the thumb from, or joining it to, the forefinger,) simply act by giving temporary predominance to one of the two co-existent currents. If these experiments be made at the edge of the copper disc, they elicit opposite but parallel results. (See fig. 4.) They evince the existence of two currents, one _dextrad_, the other _sinistrad_, from which the same conclusions may be deduced. It is important to notice, that in all this class of the experiments, the distad and dextrad currents are manifested in combination; and in like manner the proximad and sinistrad. This combination is further exemplified in the next experiment, which I shall describe. _b._ Excite the above extra-marginal motions of the odometer held near the two plates in succession; and then apply the thumb to the finger in each experiment. _Result_—Tangential motions are manifested parallel to the diametral motions before displayed. (See figs. 5 and 6.) We cannot, however, suppose these extra-marginal tangential motions to be the lateral limits of the four great currents, inasmuch as they are obtained by the versed process to that which obtains the central motion; and the question arises, what then are the limits of the central currents?