AND ON A PHiENOMENON OF ROTATING BODIES. 223 



axis, could be proved, whose effect was to give the same an inclina- 

 tion towards the vertical plane through the tangent, the cause 

 of the projectile's lateral deviation would be thereby also shown. 



If on the axis of a body, generated by the rotation of any 

 curve around this axis, a force acts in a direction not passing 

 through the centre of gravity, such a force, when the body does 

 not rotate, would move the axis in the plane passing through the 

 same, and the direction of the force. If the body rotate, how- 

 ever, with great velocity, the axis will not move in this plane, 

 but towards one side, describing a cone if the centre of gravity 

 be considered as a fixed point. 



The explanations given of the precession of the equinoxes and 

 nutation rest on this theorem. Bohnenberger* has, with great 

 ingenuity, invented a small apparatus to illustrate this by experi- 

 ment ; and Poisson has treated the theorem at length in a sepa- 

 rate memoir in the Journal de VEcole Poly technique, Cahier 

 xvi. p. 247. 



Bohnenberger's apparatus consists of an oblate spheroid, 

 capable of rotating with facility around its axis, and so suspended 

 within three rings that any required direction can be given to 

 the axis. 



I have had a similar apparatus constructed, which, however, 

 instead of a spheroid, contains a miniature representation, in 

 brass, of one of the above-described elongated projectiles. Its 

 cylindrical part has a diameter of 2 inches, and an equal height. 

 The height of the whole body, including its rectangular conical 

 apex, is 3*4 inches. Fixed to the axis at the apex, is a small 

 pulley, around which a fine silken thread can be wrapped. By 

 unwinding this quickly with a sharp pull, the body is made to 

 rotate, and continues so to do for some time with almost un- 

 changed velocity. The whole apparatus is shown in fig. 6, 

 Plate III. The steel axis ab of the body L turns on two pivots, 

 fixed in the inner ring EF. By means of this suspension 

 M ithin the rings, it is, of course, possible to give the axis any 

 required position ; in order, however, that it may retain the 

 same, it is necessary that the body's centre of gravity, and that 

 of the ring also, coincide with the centre of the latter. On that 



♦ Gilbert's Ann. Ix. 60. Tiibinger Blatter fur Naturwissenschaften und 

 Arzneikunde von v. Autenrieth und Bohnenberger. Bd.. III. Heft I. 



