326 James Elliot on certain 



cording as we present the magnet to it in different positions. 

 When the disc is revolving horizontally, the magnet, pre- 

 sented above or below any point of the circumference, con- 

 verts that point into one of the nodes, the half orbit in ad- 

 vance of that point inclining upward or downward towards 

 the magnet, and the other half receding from it. Thus, in 

 the preceding diagram, the direction of rotation being from 

 west to east, as indicated by the arrow, and the magnet 

 being suspended over any point A, that side of the disc does 

 not rise towards the magnet, as we might expect, but the 

 side B does so, a quadrant in advance, while the semicircle 

 opposite B sinks. Again, when the orbit is already inclined, 

 if we place the magnet above the highest point of the disc, 

 or beneath the lowest, the effect is not to increase the obli- 

 quity of the orbit, as we should anticipate, but to produce a 

 progressive or forward motion of the nodes. If we present 

 it below the highest point, or above the lowest, it does not 

 diminish the obliquity of the orbit, but causes a retrograda- 

 tion of the nodes. If we apply it below the descending node, 

 or above the ascending, we do not draw that side towards 

 the magnet, as would appear likely beforehand, but we in- 

 crease the obliquity of the plane to the horizon : if above the 

 descending node or below the ascending, we diminish the 

 obliquity.* When a circular plane, 

 to represent the ecliptic, is fixed 

 round the revolving disc, as shown in 

 the annexed woodcut, the results are 

 more quickly made manifest, and a 

 screw upon the axis of the disc ena- 

 bles it to be adjusted beforehand, so 

 as to be free from any forward or 

 backward motion of the nodes inde- 

 pendent of that caused by the influ- 

 ence of the magnet. 



Now, the various effects just described are precisely those 



* A slight touch of the finger on the revolving disc produces exactly the 

 same effect as the magnet applied on the opposite side. The finger must be 

 kept upon the disc with a gentle pressure, rubbing sn oothly over it, so as not 

 to stop it. 



