Rotatory Motion, the Gyroscope, ^c. 155 



only slightly diminished extent, and the resulting pressure can have no 

 other effect than that of making the top rise.* 



According to this theory the top ought to rise with the greater 

 rapidity as its rotatory speed decreases, as I find to be actually the 

 case ; because as the rotatory motion decreases, the conical motion, and 

 with it the air pressure, increases. According to Mr. Elliot's theory, 

 however, the lifting power decreases with the rotatory velocity. 



In the case of the Bohnenberger gyroscope, the phenomena are very 

 much modified by the action of the air, as in the case of the top, as 

 well as in other ways ; and also by the friction of the pivots. Thus 

 when the weight is applied to the upper end of the inclined spindle, the 

 air current produced by the rotation of the fly-wheel, and also the fric- 

 tion of the pivots, act on the rings in such a way as to tend to accele- 

 rate the precessional movement, and this has the effect of raising the 

 weight. On the other hand, when the weight is applied to the lower 

 end of the inclined spindle, the current of air, and the friction of the 

 pivots act upon the rings in such a way as to tend to retard the pre- 

 cessional movement, and this has the effect of bringing the weight 

 down. The great force acting in this way will be seen on the spindle 

 becoming vertical. In the^rs^ case, the fly-wheel and its rings all spin 

 round very rapidly in the direction of the precessional movement. In 

 the second case, the fly-wheel and rings all spin round; but in the 

 opposite direction to the precessional movement, and it is curious to 

 observe the outer ring changing the direction of its motion, the moment 

 the weight reaches its lowest position. To show this last experiment 

 satisfactorily, the apparatus must be constructed with such accuracy 

 that the ultimate spinning round of the outer ring gives no conical 

 motion to the fly-wheel. 



To return to Mr. Elliot's paper. Those who heard or have read 

 it wiU. remember that he applies precisely the same explanation to 

 the stability of Saturn's rings as to that of the top. To fully expose 

 the fallacy of the explanation of this particular phenomenon, would 

 involve tedious repetition of what I have already said. Suffice it to say 

 that Mr. Elliot attempts to show that a point of the ring which at one 

 instant is the nearest to the centre of attraction, is still the nearest 

 after a portion of a revolution has been made. Were this true, the 



* The nutation of the earth's axis is probably caused in a somewhat similar way. Ono 

 would at first expect that the change in the tilting force, caused by the change in position 

 of the moon, would merely accelerate or retard the precessional motion without producing 

 nutation ; but, as in the case of the top, the two external forces interfere with each other. 

 If the atmospheric pressure acted alternately on opposite sides of the top, a nutation of 

 the top would result. 



