AND ON A PHiENOMENON OF ROTATING BODIES. 229 



th^. ^^use& of which have for so long a time been completely 



On a remarkable Phcenomenon of Rotating Bodies, ' ^ ^^' 



In the foregoing described experiments I had an opportunity 

 of observing a phaenomenon vv^hich, although following imme- 

 diately from the above theorems on the motion of rotating 

 bodies, deserves here to be separately described, partly because 

 it is, I believe, but little known, and partly because it illus- 

 trates, particularly well, the phaenomena which are observable 

 when forces act on a rotating body, the resultant of which passes 

 through the axis, but not through the centre of gravity. 



If the body L, in the apparatus shown by fig. 6, Plate III., 

 be made to rotate around its axis, it is well known that a some- 

 what considerable force is requisite to change the position of 

 this axis, or that of either of the two inner rings. If, however, 

 the middle ring AB, in which the pivots E and F holding the 

 inner ring are fixed, be held fast with the hand, then the axis 

 ab of th^,ro;l^tWg Ji^9^y?i(^^ ^'i^^ i^ ^^^ ^"^S ^F, can be easily 

 moved. •>,. ^..-j,,.,^ ^m r,.-- : 



If such an apparatus be constructed with only two rings, the 

 outer one being fixed, then the axis, which can only move in a 

 plane, will assume no fixed position. The smallest force will 

 be sufficient to change its direction. 



The explanation generally given of this unchangeabihty in 

 the direction of the body's axis is, that a considerable force is 

 requisite to move every single point of the body from the plane 

 in which it rotates. The above-mentioned experiments, how- 

 ever, show that this explanation is insufficient. 



In order to illustrate this remarkable phaenomenon in another 

 manner, I had the apparatus shown in fig. 7? Plate III., con- 

 structed. 



It consists of an iron axis 9 inches long, around each extre- 

 mity of which a circular brass disc, 4 inches in diameter and 

 0*25 inch thick, can rotate. Each of these discs, E and F, 

 is provided with a small pulley, around which a silken thread 

 can be wound. By unwinding this quickly, each disc sepa- 

 rately can be made to rotate. Through the centre of gravity of 

 the whole instrument, and perpendicular to the axis, a wire, CD, 



