Early in this century, the top experienced popularity in the USA under various brand names such as Whizzer, Chouen Gen, Springo, and Topsy Twirl - in addition to Diabolo. The generic name "Flying Cones" well describes the solid geometry of the object, a toy "made of two inverted cones fastened together at the apex, and manipulated by means of a cord, which is attached to two sticks, one of which is held in either hand" enabling the top to be twirled, be tossed, and otherwise manipulated. Instructions for the common rotation of the top on its cord include: "If the spool tips downward toward the spinner, he moves the right-hand wand toward him. If the spool tips away from the player, move the left-hand away." "Equal drawing in of both hands...(results in)...the outer end of the spool ...turn(ing) towards the performer's right." An added attraction is a gentle whistling sound produced when the top turns fast enough.

The primary requirement to achieve balance is that the momental ellipsoid be an ellipsoid of revolution, either prolate or oblate, that is, with the moment of inertia about the axis of symmetry either a minimum or a maximum, respectively. In other words, the plane perpendicular to the spin axis has equal moments of inertia. By establishing rotation about the axis with an extremum of inertia, gyroscopic action can be achieved. Applying the right hand rule to torques acting on the spin axis accounts for the actions and reactions described above. Consider the third (last) maneuver - drawing in both hands equally: A torgue perpendicular to the spin axis and towards the operator's left is exerted on the spool. Assuming that it was spun up with the right hand, the resulting angular momentum of the top points towards the user. Position the fingers of the right hand such that the thumb points towards the user and the index finger to the left. The middle finger, when positioned at right angles to the thumb and index finger, points downward. The direction of the middle finger is the direction of the induced angular momentum. By the right hand rule, the reaction of the spool is as described above, that is, the outer end turns towards the player. "[T]he spin axis chases, so to speak, the couple (torque) axis round...with constant precession...." The two single hand movements as described are seen to apply to a spool spun up with the left hand: The torques are up (left hand out) and down (right hand in). Applying the rule of the spin axis chasing the torque axis requires a spin vector pointing away from the operator.

Another viewpoint assists in predicting the reaction to an action, that of "hurrying or retarding the precession". The object of the game is balancing the spool on a cord. (Interestingly, Goldstein mentions "an egg standing on end" (p.243). Performing such a trick seems to have helped Christopher Columbus gain esteem in the eyes of those whose backing he needed to undertake his initial voyage of discovery, although his method of balancing seems to be in dispute.) Deviations from a preferred orientation derive from precession of the spin axis. A natural tendency to counteract, for example, a dip towards oneself is to draw both hands in, that is, retard the precession. We have seen the reaction to this torque - the spool precesses about a vertical axis. The opposite act, of hurrying the precession about the horizontal axis, invokes counterclockwise rotation of the spool about the vertical axis. Perhaps Mr. Jin is insightful when he questions if "magician" and "juggler" are synonymous in english. Both performers produce results by the use of hands - that mislead the untrained eye. Perhaps the Chinese assigned some spirit's name to the generic spinning spool, althrough not the given one in European languages.

References (besides Goldstein's textbook):
A simple description of precession has always eluded me, until reading Andrew Gray, "A Treatise on Gyrostatics and Rotational Motion". Thank you for this experience. Also cited: Steward Culin, Inez Clintock, D. W. Gould, and Larry Kettelkamp.