AND ON A PIIiENOMENON OF ROTATING BODIES. 231 



if two equal masses rotate around the same axis with equal 

 velocity, but in opposite directions, then by the action of a force 

 which does not pass through the centre of gravity of the appa- 

 ratus, the axis is moved with equal force in two opposite di- 

 rections, therefore no lateral motion can occur, and the axis 

 deports itself exactly as if by some mechanical contrivance its 

 motion were restricted to one plane, in which it could only turn 

 on its centre of gravity. If, however, the two masses rotate 

 with unequal velocities, the opposite lateral motions are also 

 unequal, and the axis moves to one side. The fixity of its posi- 

 tion, however, decreases with the decrease of the difference 

 between the two lateral motions. 



The above-mentioned experiments are the more surprising, on 

 account of the apparent contradiction, that the axis of a rotating 

 body, when completely free, should appear fixed ; and on the 

 other hand, that when its motion is restricted to a plane, it 

 should be moveable with great facility. These experiments, at 

 the same time, illustrate the influence which a free axis exercises 

 not only upon elongated projectiles, but on rotating bodies in 

 general, whether celestial bodies or those on the earth, having 

 simultaneously both a rotatory and a translatory motion. 



