NATURE 



[April io, 1913 



take one or two old experiments (see Thomson and 

 Tail's " Natural Philosophy," § 345 s et seq.), which 

 are more effectively performed with these fast-running 

 instruments. Here is a skate attachment (Fig. 4) on 

 which I place the gyrostat after its speed has been 

 adjusted to the moderate value of about 6000 revolu- 

 tions per minute. The plane of the flywheel is in- 



clined to the vertical, and you see that the top does 

 not fall down, but precesses round on the table. I 

 increase the inclination and the precession becomes 

 more rapid. Now I attempt to hurry the precession, 

 and the gyrostat stands up erect; I try to resist the 

 precession and the gyrostat falls over. 



I mount the gyrostat with its wheel horizontal over 



Fig. 6. — Motor-gyrostat balancing on stilts. 



a flexible support, in the present case a universal 

 joint (Fig. 5). Without rotation the instrument would 

 fall over at once, but you see that it stands stably 

 erect when the flywheel is spinning, and has a pre- 

 cessional motion when disturbed from the upright 

 position. 



NO. 2267, VOL. QlT 



Again, here is a two-stilt support (Fig. 6). One of 

 the stilts is held by a long socket, at one side of the 

 case, and may be regarded as rigidly attached. The 

 other stilt is simplv a bit of wire pointed at both ends ; 

 one end rests on the table, the other, the upper end, 

 rests loosely in a hollow in the under-side of this 

 projecting piece attached to the case. The gyrostat 

 is thus supported between two stilts, one fixed the 

 other quite loose, and its axis is at right angles to 

 the plane of these when the arrangement stands up- 

 right. It would be hard to devise a more unstable 

 support. You see that there is no possibility of 

 making the arrangement stand up without spin. But 

 you see, on the other hand, that there is a fair amount 

 of stability with the flywheel spinning if the_ arrange- 

 ment is allowed to oscillate, or, as one might say, 

 wriggle, backwards and forwards, horizontally. 



In the next experiment (due originally, I have been 

 told, to the late Prof. Blackburn) the gyrostat is rigidly 



Fig. 7.— Motor-gyrostat on crossed bifilar support. 



clamped to this metal bar, which, as you see, is hung 

 by two chains attached to its ends. The chains have 

 been crossed by passing one through a large ring in 

 the middle of the other. I turn the gyrostat so that 

 the chains and the rim of the case are in the vertical 

 plane. You observe that the arrangement is one of 

 instability. The gyrostat has perfect freedom to fall 

 over towards you, or towards me. Further, in conse- 

 quence of the crossing of the chains the gyrostat is 

 unstable as regards motion about a vertical axis. The 

 arrangement is thus doubly unstable without rotation. 



I now set the flywheel into rapid rotation, arrange 

 the instrument as before, and leave it to itself, when, 

 as you observe, it balances with great ease. 



I now repeat the experiment with the chains un- 

 crossed. Here there is only one instability without 

 rotation, and you observe that the gyrostat falls over. 

 An important point to be observed here is that the 

 rotation will completely stabilise two non-rotational 

 instabilities but not one. In point of fact, a system 



