of the horizontal diameter of a vertical circle ABA'B', which suspended by a fine wire SA from the fixed point S. At A', the opposite extremity of the vertical diameter AA', is a pivot,
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Fig. 69.
which rests in a small hole. All the pivots are carefully polished, so that friction may be avoided as much as possible; and the dimensions of the different parts of the instrument are so adjusted that is the common centre of the disc and the rings.
Now, as the disc is symmetrical about CC', that line is a permanent axis of rotation; and as O is the common centre of gravity of the different parts of the machine, and is supported by the string SA and the reaction at A', the force of gravity will not interfere with the rotation. Thus the instrument satisfies all the conditions necessary for making the experiment. It is also clear, that the axis CC' may be placed so as to point in any direction we please by moving first the ring ABA'B', and then the ring BCB'C' into proper positions.
To make the experiment, BCB'C' is removed from its supports, and a rapid motion of rotation is impressed on the disc. The ring BCB'C' is then restored to its place. It is found, that if at any
