568 PROFESSOR MAXWELL ON A DYNAMICAL TOP. 
The smaller the difference between the moment of inertia about the axle and 
about the mean axis, the more eccentric the ellipse will be; and if, by screwing 
the bob down, the axle be made the mean axis, the path of the invariable axis 
will be no longer a closed curve, but an hyperbola, so that it will depart alto- 
gether from the neighbourhood of the axle. When the top is in this condition it 
must be spun gently, for it is very difficult to manage it when its motion gets 
more and more eccentric. 
When the bob is screwed still farther down, the axle becomes the axis of 
greatest inertia, and wa’ the least. The major axis of the ellipse described by 
the invariable axis will now be perpendicular to #2’, and the farther the bob 
is screwed down, the eccentricity of the ellipse will diminish, and the velocity 
with which it is described will increase. 
I have now described all the phenomena presented by a body revolving freely 
on its centre of gravity. If we wish to trace the motion of the invariable axis by 
means of the coloured sectors, we must make its motion very slow compared with 
that of the top. It is necessary, therefore, to make the moments of inertia about 
the principal axes very nearly equal, and in this case a very small change in the 
position of any part of the top will greatly derange the position of the principal 
axis. So that when the top is well adjusted, a single turn of one of the screws 
of the ring is sufficient to make the axle no longer a principal axis, and to set 
the true axis at a considerable inclination to the axle of the top. 
All the adjustments must therefore be most carefully arranged, or we may 
have the whole apparatus deranged by some eccentricity of spinning. The method 
of making the principal axis coincide with the axle must be studied and practised, 
or the first attempt at spinning rapidly may end in the destruction of the top, if 
not of the table on which it is spun. 
On the Earth's Motion. 
We must remember that these motions of a body about its centre of gravity, 
are not illustrations of the theory of the precession of the Equinoxes. Precession 
can be illustrated by the apparatus, but we must arrange it so that the force of 
gravity acts the part of the attraction of the sun and moon in producing a force 
tending to alter the axis of rotation. This is easily done by bringing the centre 
of gravity of the whole a little below the point on which it spins. The theory of 
such motions is far more easily comprehended than that which we have been 
investigating. ‘ 
But the earth is a body whose principal axes are unequal, and from the phe- 
nomena of precession we can determine the ratio of the polar and equatorial axes 
of the “central ellipsoid;” and supposing the earth to have been set in motion 
about any axis except the principal axis, or to have had its original axis disturbed 
