Take rectangular axes, of which Oz coincides with the axis of rotation.
Their influence upon the young tsar was profoundly beneficial, and the period of their administration coincides with the most glorious period of Ivan's reign - the period of the conquest of Kazan and Astrakhan.
We have seen (~ 18) that this vector coincides in direction with the perpendicular OH to the tangent plane of the momental ellipsoid at J; also that ~ (2)
As particular cases: the mass-centre of a uniform thin triangular plate coincides with that of three equal particles at the corners; and that of a uniform solid tetrahedron coincides with that of four equal particles at the vertices.
Evidently the quadratic moment for a variable plane through 0 will have a stationary value when, and only when, the plane coincides with a principal plane of (26).
