298 Proceedings of the Eoyal Society of Edinburgh. [Sess. 
h comes into action during yaws, and so employed the apparatus serves 
to indicate turning motion of the aeroplane. Obviously the radial motion 
of the balls of tSe system h may be caused to close electric contacts and 
light indicating lamps. 
Fig. 25 shows a convenient construction of the stabiliser in which 
two gyroscopes are employed. The cases of these are rigidly connected 
together, and the axes of spin are parallel, so that they are equivalent to 
one gyroscope. The erector e-^ is driven round slowly in the direction of 
spin of the gyroscopes (which is the same for both), and rapidly in the 
reverse direction. It is convenient to provide means whereby the speed 
of the erector may be varied. 
Theory and Action of the Further Forms of Gray Stabiliser. 
It is necessary at this stage to consider the action of the erector shown, 
in plan, in fig. 26. Three balls (in this case) are employed. These rotate 
as shown on a fixed circular track carried by the pivoted system, each ball 
being propelled by a pusher d. The pushers d form part of a spider 
which is driven round slowly in the direction of spin of the gyroscope g. 
