326 Proceedings of the Royal Society of Edinburgh. [Sess. 
its axis horizontal. If the axis becomes inclined to the horizontal, a couple 
tending to overbalance the gyrostat, that is, to produce angular momentum 
about a horizontal axis at right angles to the axis of spin, comes into play. 
The gyrostat in consequence precesses round on the skate. If the preces- 
sional motion is delayed, the inclination of the axis to the horizontal in- 
creases ; if it is hurried, the gyrostat returns to the position in which its 
axis is horizontal and the centre of gravity is vertically above the support. 
With this motor-gyrostat such experiments are very striking. With 
the armature going at full speed — 12,000 to 20,000 revolutions per minute 
— the balancing power is extraordinary; if the gyrostat is supported 
Fig. 4. — Motor- Gyrostat on stilts. 
on the skate with its axis inclined to the horizontal it precesses round 
very slowly ; a slight force tending to hurry up the precessional motion 
suffices to cause the instrument to stand up with its plane vertical. 
The spin can be readily altered by connecting up a rheostat in the 
circuit, and the dependence of the precessional motion upon the spin 
(that is, the angular momentum of the flywheel) demonstrated with 
the utmost ease. 
In fig. 4 the motor-gyrostat is shown performing the two-stilt experi- 
ment invented by Lord Kelvin. To adapt the gyrostat for this experiment 
two pieces of magnalium, cast in the form shown in the figure, are screwed 
on the rim of the gyrostat at extremities of a diameter. The left-hand 
extremity is pierced with a cylindrical socket whose length is parallel to 
the plane containing the rim, and into this fits tightly one end of a 
rigid stilt ; the other end of the stilt is pointed. The second magnalium 
