The paths traced out by
the pole of this axis on the _invariable plane_ and on the _central
ellipsoid_ form interesting subjects of mathematical investigation.
But when we attempt to follow with our eye the motion of a rotating
body, we find it difficult to determine through what point of the
_body_ the instantaneous axis passes at any time,--and to determine its
path must be still more difficult. I have endeavoured to render
visible the path of the instantaneous axis, and to vary the
circumstances of motion, by means of a top of the same kind as that
used by Mr Elliot, to illustrate precession*. The body of the
instrument is a hollow cone of wood, rising from a ring, 7 inches in
diameter and 1 inch thick. An iron axis, 8 inches long, screws into
the vertex of the cone. The lower extremity has a point of hard
steel, which rests in an agate cup, and forms the support of the
instrument. An iron nut, three ounces in weight, is made to screw on
the axis, and to be fixed at any point; and in the wooden ring are
screwed four bolts, of three ounces, working horizontally, and four
bolts, of one ounce, working vertically.
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