The precession of the equinoxes is due to the fact that the earth performs a motion of this kind about its centre, and the whole class of such motions has therefore been termed precessional.

The earths precessional motion is of this latter type, the angles being ci = .0087, ~=23 28.

The polhode and herpoihode cones are then right circular, and the motion is precessional according to the definition of 18.

We can hence deduce the condition of steady precessional motion in a top. A solid of revolution is supposed to be free to turn about a fixed point 0 on its axis of symmetry, its masscentre G being in this axis at a distance h from 0.

If we put Oso we get the condition of steady precessional motion in a form equivalent to (3).