328 Proceedings of the Royal Society of Edinburgh. [Sess. 
nuts, to a magnalium bar provided with two rings, one at each end. Fixed 
to the rings are two chains as shown. In the middle of one of the chains 
is a large ring as shown, and through this ring passes the second chain. 
The ends of the chains remote from the bar are fastened to two points on 
the same level whose distance apart is equal to the length of the magnalium 
bar. To perform the experiment the gyrostat is placed in the upright 
position with its rim and the chains in the same vertical plane, and is then 
left to itself. Instead of falling down, the gyrostat balances on the bar. 
It is to be observed that here the gyrostat may fall over in two ways, 
either towards or away from the observer, and, further, in consequence 
of the crossed bifilar suspension the bar and gyrostat are also unstable 
azimuthally without spin. When the gyrostat is spinning rapidly the 
arrangement is one of stability, just as in the stilt experiment already 
described. 
A general explanation of the experiment just described may be given as 
follows. Starting with the bar, gyrostat rim, and chains in one vertical 
plane, we may suppose the gyrostat to fall over slightly. In consequence 
of the tilting the gyrostat precesses so that its axis turns in a plane 
which is nearly horizontal. As a result the chains get out of the 
vertical, and as soon as this takes place a couple hurrying up the pre- 
cessional motion is brought to bear on the gyrostat, which in consequence 
rears up into the vertical position. This holds for both positions in which 
it is possible for the gyrostat to fall over. Again, suppose, starting with 
the rim, bar, and chains in the same vertical plane, the chains get out of 
the vertical. A couple is brought to bear on the gyrostat tending to turn 
its axis in a horizontal plane. In consequence of gyrostatic action the 
gyrostat tilts over on the bar — in other words, it has a precessional motion 
about a horizontal axis in the plane of the flywheel. This results in a 
couple, due to gravity, being brought to bear on the gyrostat ; this couple 
is such as to hurry up the precessional motion ; the azimuthal motion is 
opposed and reversed, and with the reversal the gyrostat regains the 
upright position. This holds for both directions in which the bar tends to 
turn in consequence of the crossed bifilar. The result is complete stability. 
As performed by the aid of this motor-gyrostat the experiment is one of 
great beauty. 
In fig. 6 is shown the motor-gyrostat fitted up to show the principle 
of the gyrostatic compass. The gyrostat is mounted in two bearings pro- 
vided in a ring made of mahogany. The ring is held in the hands of the 
experimenter so that its plane is horizontal, and the arrangement is carried 
round in a horizontal circle. The gyrostat is stable, with its axis vertical, 
