On the upper part of the
axis is placed a disc of card, on which are drawn four concentric
rings. Each ring is divided into four quadrants, which are coloured
red, yellow, green, and blue. The spaces between the rings are white.
When the top is in motion, it is easy to see in which quadrant the
instantaneous axis is at any moment and the distance between it and
the axis of the instrument; and we observe,--1st. That the
instantaneous axis travels in a closed curve, and returns to its
original position in the body. 2ndly. That by working the vertical
bolts, we can make the axis of the instrument the centre of this
closed curve. It will then be one of the principal axes of inertia.
3rdly. That, by working the nut on the axis, we can make the order of
colours either red, yellow, green, blue, or the reverse. When the
order of colours is in the same direction as the rotation, it
indicates that the axis of the instrument is that of greatest moment
of inertia. 4thly. That if we screw the two pairs of opposite
horizontal bolts to different distances from the axis, the path of the
instantaneous pole will no longer be equidistant from the axis, but
will describe an ellipse, whose longer axis is in the direction of the
mean axis of the instrument.
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