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inclination of this axis should produce no difference in the character 

 of the effects but merely in their intensity, since in any inclined po- 

 sition of the disc its rotation is resolvable into two others, one per- 

 pendicular to the beam, and the other, which is incapable of pro- 

 ducing any effect, in a plane containing it. When the axis of the 

 rotating disc is vertical and at right angles to the beam, no rotation 

 on the vertical axis ought to take place in any case ; but, contrary to 

 this expectation, although the beam be horizontal and in perfect 

 equilibrium, a motion round the vertical axis results, which is in 

 opposite directions according as one or the other end of the axis of 

 the disc is uppermost. It is, however, easy to see that this rotation 

 is not owing to the same cause which gives rise to the phenomena 

 hitherto considered, for whether it be accelerated or retarded no 

 change is produced in the horizontal position of the beam ; it is, in 

 fact, occasioned by the friction of the pivots of rotation dragging 

 the beam into a corresponding motion. Attention to this extraneous 

 cause of rotation will explain numerous anomalies which present 

 themselves in many of the instruments contrived to exemplify the 

 phenomena of combined rotary motions. It is one of the advantages 

 of Fessel's apparatus that the phenomena may be exhibited in their 

 more important phases without being affected by this source of error. 

 We may form a clearer conception of these phenomena by first 

 considering some simpler facts which do not appear to me to have 

 been hitherto sufficiently attended to. For this purpose let the 

 system of rings carrying the disc be removed from the rest of the 

 apparatus, and by unfastening the tightening screw let the inner 

 ring be allowed to move freely within the outer. Having set the disc 

 in rapid rotation, hold the outer ring at the extremities of the dia- 

 meter which is in the plane in which the axis of motion of the disc 

 is free to move, then giving to the outer ring a tendency to rotation 

 round that diameter, it will be observed that, in whatever position 

 the axis is, it will fly to place itself in the fixed axis thus determined, 

 and rotation will take place round it in the same direction. Consi- 

 derable resistance is felt so long as the moveable axis is changing 

 its position, but when once it coincides with the fixed axis the 

 rotation of the external ring round its diameter is effected with 

 facility. A slight alternate motion of the outer ring, tending to 

 give to it rotations in opposite directions, will occasion a continued 



