Cross and Ball
so universally marked on Egyptian figures, is a circle and the letter T. The circle signifies the eternal preserver of the world, and the T is the monogram of Thoth, the Egyptian Mercury, meaning wisdom. The coronation orb is a sphere or ball surmounted by a cross, an emblem of empire introduced in representations of our Saviour. In this case the cross stands above the ball, to signify that the spiritual power is above the temporal.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894