24 



away from N ; but keeping the stretched string always parallel 

 to the perpendicular diameter NS, and to the two cross walls 

 to the right and left of the room where the experiment is made ; 

 and when the rod completes its first quadrant and reaches W, 

 the plummet will have also descrihed a quadrant for itself of the 

 same dimensions, will have crossed the orbit to the outside, and 

 will have reached a point at the angular distance of 90° further 

 from the centre of the orbit than the place it occupied in contact 

 with the centre just before starting, and at right angles with the 

 rod and its axis, showing, in my judgment, a rotation of 90° 

 round that axis. In the second quadrant, the plummet will again 

 describe a quadrant for itself, and will reach a point still further 

 distant from the primary centre by another 90°, or, altogether, 

 180°, directly outside of its pivot. But, how could it get there, 

 unless it had performed a rotary movement of 180° round that 

 pivot ? It has, unquestionably, by its regular movement under 

 the law of gravity, described a semicircle of that angular extent, 

 and of the same real dimensions as the orbit of its revolving axis; 

 and the change of their relations to the primary centre cannot 

 be ascribed to the revolving axis, for that axis has kept at pre- 

 cisely the same distance from the primary centre, and with its 

 radius-vector always subtending the same rectangle with the or- 

 bit, during the whole passage of 180°; and, as it is the plum- 

 ftiet's real and visible movement that has brought it into that si- 

 tuation, that movement must be resolved into its elements, as 

 partly progressive along with its pivot, and partly rotary about 

 that pivot, the effect being precisely the same, as if the string 

 and the plummet had lain upon the rod during its passage of 

 the quadrants, and then been set at liberty, when it would have 

 run through the angular distance of 180° at one sweej), and 

 something more, only settling at the 180th° point, after oscillat- 

 ing till tired. Through the next two quadrants, this process 

 will be repeated, in the reverse order, and when the rod returns 

 to its first position, the plummet will again be in one with the 

 centre of the orbit. The diagram, (Fig. 3,) shows the relation 

 of the orbits described by the two moving bodies, the continuous 

 circle being the orbit described by the pivot or middle point of 

 the rod, and the dotted circle being the orbit described by the 

 plummet. To prove further the reality of the plummet's revo- 

 lution round the pivot c, take a flat cylindrical disk, thick enough 

 to carry a cord on its edge, fix it by its centre on the revolving 

 pivot of the rod, mark its quadrants, N, S, W, E, and to the 

 point S, in one with the line KS of the primary circle, attach 

 the plummet by its string. Then make the rod revolve, as be- 

 fore, from west to east; the plummet will be seen again de- 

 scribing an orbit of its own, partly within and partly without the 



4 



