500 JAMES CLERK MAXWELL. 



supported at its centre of gravity and rotating freely about 

 it will move in the same manner as an ellipsoid whose centre 

 is fixed, but which rolls in such a way as always to rest 

 against a fixed plane (the invariable plane), which is, of 

 course, a tangent plane at the point of contact. The line 

 from the centre to the point of contact is the axis about 

 which the body is at the instant rotating, and this line will, 

 unless the plane touch the ellipsoid at the extremity of one 

 of the principal axes, move both in space and in the body. 

 The curve which the extremity of the axis of .rotation de- 

 scribes on the invariable plane is called a herpolhode, while 

 that which it describes on the surface of the ellipsoid is 

 called a polhode. If at any instant the axis of rotation be 

 very near either to the greatest or least axis of the ellipsoid, 

 it will always remain very near that axis, and the polhode 

 will be a small closed curve and the rotation will be stable ; 

 but if the axis of rotation be near to the mean axis of the 

 ellipsoid, the polhode will be a very large curve, the axis of 

 rotation will in time deviate very widely from its original 

 position in the body, and the motion will be unstable. 



Maxwell's top consisted of a brass bell, a long screw 

 passing through the top of the bell and terminating in a 

 steel point " finished without emery and afterwards hardened," 

 serving as the axle. The point rested in an agate cup on 

 the top of a pillar. A heavy nut could be screwed up and 

 down on the axle for coarse adjustments, and the axle could 

 be screwed through the top of the bell, to bring the point 

 to coincide with the centre of gravity of the mass. The 

 moments of inertia could be altered by means of the nut 

 above referred to, and by nine screws with massive milled 

 heads, six of which screwed horizontally into the rim of the 

 bell, while three were arranged symmetrically around the top 

 of the rim, and admitted of a vertical motion. By means of 

 these screws the axle could be made the axis of greatest, 

 least, or mean moment of inertia, or not a principal axis at 

 all. The axle terminated in a point, by which a coloured 

 disc could be fixed upon it. " The best arrangement, for 

 general observations., is to have the disc of card divided into 



